GB1591352A - Tension control of fasteners - Google Patents

Description

PATENT SPECIFICATION ( 11) 1 591 352

> ( 21) Application No 2821/80 ( 22) Filed 3 Aug 1977 ( 19)( ( 62) Divided Out of No 1591351 ( 31) Convention Application No 712554 ( 32) Filed 9 Aug 1976 in tv <>> ( 33) United States of America (US) tn ( 44) Complete Specification Published 17 Jun 1981 ( 51) INT CL 3 G 05 D 15/01 B 23 P 19/06 ( 52) Index at Acceptance G 3 N 292 393 404 DX ( 54) TENSION CONTROL OF FASTENERS ( 71) We, ROCKWELL INTERNATIONAL CORPORATION a corporation of the State of Delaware, having a place of business at 600 Grant Street, Pittsburgh, Pennsylvania, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly

described in and by the following statement: 5

TENSION CONTROL OF FASTENERS Background of the invention

This invention relates to a technique for tightening threaded fasteners The function of threaded fasteners is, of course, to unite two or more pieces into a typically rigid part called 10 a joint For purposes of convenience, the term fastener pair may be used to designate male and female threaded members, e g a nut and bolt, bolt, and internally threaded hole of a joint part, threaded stud and nut, and the like The connected pieces of a joint should be so tightened as to remain in contact during vibration, static and/or dynamic loading of the part.

In many applications where several threaded fasteners are used, it may be of substantial 15 importance to assure that the contact pressure between the pieces created by the fasteners is uniform since non-uniform deflection of the pieces may create unacceptable joint conditions Proper assembly should produce uniform contact pressures from joint to joint in accordance with design requirements This can be achieved only by assembly procedures that produce uniform joint preload or clamping load Although it is conceivable to 20 determine joint preload or clamping load in terms of compression of a nut, it is more practical to deal in terms of bolt tension There is, unfortunately, no direct technique for measuring bolt load externally without instrumenting the bolt or using a load washer which are either impractical or uneconomic for assembly line production Accordingly, all practical techniques of bolt tension control in production quantities are inferential 25 There are a number of well known techniques for tightening threaded fasteners based on information available from external instruments such as torque and angle sensors as contrasted to specially designed fasteners or load washers Included in these techniques are torque control, turn-of-the-nut method, the yield point method, acoustic measuring, overrunning schemes and torque rate methods 30 One of the present techniques in wide use is torque control in which a constant final torque is applied to the fasteners Final torque is typically produced by a stall air tool and the degree of torque control depends on the uniformity of air pressure, motor performance and the hardness of the joint The intention is to achieve tension scatters in the range of 10-20 % about the mean The actual scatter limits can only be verified by instrumenting the 35 bolts in a laboratory environment Opinions vary on what tension scatters are actually present in large quantities of fasteners tightened with torque control methods It would not be surprising to learn that total tension scatter in production quantities are on the order of % of mean which can be caused by 41 % scatter in friction alone.

Torque is, of course, related to tension but the relationship is subject to large 40 uncertainties resulting from a first order dependence on thread and head friction In the simplest theoretical consideration, the following equation describes the relation of torque and tension:

T = (fhrh + f thrh)F( ( 1) 45 2 1 591 352 2 where T is torque, fh is the coefficient of friction between the fastener head and the abutting piece, rj, is the effective radius of head friction, fth is the coefficient of friction between the threads of the fastener, rth is the effective radius of thread friction and F is bolt tension Although the mean value of the coefficients of friction can be substantially reduced by lubricants and coatings, the relative scatter about the mean value cannot be substantially 5 affected Combining the friction uncertainties with the variations in applied torque,' the tension control actually achieved in practice is quite poor Accordingly, in order to minimize fastener failures during assembly, the mean torque must be designed at unreasonably low levels as compared with the strength of the bolt Even with unreasonably low mean torque values, a significant proportion of the fasteners are woefully understressed 10 while many have been stressed past the elastic limit.

Discussion of torque control methods of tightening threaded fasteners are found in Assembly Engineering, October, 1966, pages 24-29; Hydrocarbon Processing, January 1973, pages 89-91; Machine Design, March 6, 1975, pages 78-82; The Engineer, London, May 26, 1967, pages 770-71; The Iron Age, February 24, 1966, page 66; Machine Design, 15 February 13, 1964, pages 180-85; Power Engineering, October 1963, page 58; and United States patents 3,555,938 and 3,851,386.

Another widely used technique for tightening threaded fasteners is called the turn-of-the-nut method which makes use of the applied torque as well as the angle of threading advance In its simplest form, the technique is to advance the fasteners until a 20 predetermined torque value is reached, for example snug torque, and then turn the nut an additional constant predetermined angle The concept is that the relation of the turn of the fastener to the strain of the bolt will eliminate the influence of friction on the final desired tension value If the clamped pieces were purely elastic and contact between them were immediate and perfect, one would expect the bolt tension to increase linearly with unit 25 angle of advance starting with the value of zero at the onset of contact In theory, tension control would be as accurate as the uniformity of the joint tension rate which is the slope of the curve obtained by plotting tension against angle of advance.

In practice, the tension rate is not exactly a constant from joint to joint nor is it uniform as a function of angle for any single joint The reasons are related to microplasticity which is 30 the yield of surface irregularities in the moving fastener components, lubricant squeeze film, and the fact that contact is gradual rather than immediate The turnof-the-nut method is customarily considered to be substantially superior to the torque control technique although data developed during the investigation of this invention suggests that this method is substantially overrated, at least at low to moderate tension values The turn-of-the-nut 35 method does have the disadvantage of partly relying on torque which is subject to the large uncertainties previously discussed The selection of the threshold torque is a critical decision If threshold torque is too high, the theoretical advantage over the torque control method is substantially reduced If threshold torque is too low, final bolt tension will fluctuate greatly from joint to joint, since at low torque values, both the torque-angle and 40 tension-angle curves have varying curvature The combination of uncertain tension at the threshold torque and nonuniformity of tension rate in a large angle span will more than offset the theoretical advantage gained The turn-of-the-nut method, being essentially a strain approach to tightening has the advantage of reducing substantially the rate of bolt failure during assembly because very large strains can be sustained by the bolt material in 45 the plastic zone During the investigation of this invention it has been learned that the difference between low torque rate fasteners and high torque rate fasteners from the same sample can develop a scatter in the final desired tension value of 50 % at tension values in the range of 3000 pounds for a 5/16 " 24, grade 8 bolt using the turn-ofthe-nut method As the final tension value increased, the scatter reduces as a percentage of final tension 50 Another difficulty with turn-of-the-nut methods is that recalibration is required when the final desired tension value is changed.

Discussion of turn-of-the-nut methods of tightening threaded fasteners are found in Hydrocarbon Processing, January 1973, pages 89-91; Machine Design, March 6, 1975, pages 78-82; Journal of the Structural Division, Proceedings of the American Society of 55 Civil Engineers, April 1966, pages 20-40; Machine Design, February 13, 1964, pages 180-85; and United States patent 3,851,386.

As pointed out in some detail in United States patents, 3,643,501 and 3, 693,726, and Design Engineering (London), January 1975, pages 21-23, 25, 27, 29, another approach for tightening threaded fasteners is known as the yield point method In this approach, an 60 attempt is made during tightening to sense the onset of plastic elongation of the bolt and terminate tightening in response thereto The yield point, which is the boundary between the elastic and plastic deformation zones of a metal in a uniaxial state of stress, is quite difficult to determine precisely Accordingly, the yield point is customarily defined in terms of an offset strain, typically 1 29 %, which is arbitrarily chosen 65 3 1 591 352 3 It is apparent that a joint is made up of the clamped pieces as well as the fasteners The design is usually such that yielding occurs in the bolt although it could conceivably occur in the bolt head or nut The bolt is also subject to shear as a result of torsion created by the turning moment or torque Accordingly, the bolt is in a complex state of stress Thus, at high torque values, the stress in the bolt is due to both torque and tension and can 5 substantially alter the tensile strength of a particular specimen Additional errors may be introduced when the goal is bolt tension control due to natural scatters in the material yield point Other errors involved in yield point methods are the result of noise and other uncertainties in consistently sensing the yield point The main objection to the yield point method is the concern over the fatigue strength and reusability of the bolt Although the 10 matter is subject to some controversy, it appears clear that one time application and release of an external load will cause relaxation of the joint and accordingly reduce the clamping force applied by the bolt below the original clamping force In extreme cases, the bolt may lose all tension and be loose.

In a technique, used to detect galled threads or cross threaded members, disclosed in 15 U.S patents 3,368,396 and 3,745,820, a warning signal is produced when a predetermined torque is developed before a given number of turns has been effected which may be indicative of galled threads A different warning signal is produced when a larger number of turns are effected before the development of a desired higher torque is obtained which is suggestive of cross threading It will be apparent that these approaches are not designed to 20 control bolt tension.

Another approach for controlling bolt tension involves acoustic devices which attempt to measure the elongation in a bolt caused by tension Such devices are discussed and illustrated in U S patents 3,306,100; 3,307,393; 3,650,016, 3,759,090 and 3,822,587.

Another technique related to yield point methods is found in United States patent 25 3,939,920 This technique basically is to measure a tightening parameter, e g torque, at the yield point, conduct certain calculations and back off the nut until the final desired axial stress is achieved and terminate tightening There are several defects to such a technique:

( 1) yield point is not a closely reproducible characteristic of most production fasteners, ( 2) the yield point is difficult to consistently sense by the disclosed technique, ( 3) the joint 30 properties,, i e tension rate and torque rate, are not the same during the first advance toward the yield point as during a subsequent advance toward the yield point, and ( 4) the properties of the fastener pair, particularly adjacent the threads, have been permanently altered at the yield point.

Another group of prior art techniques which have been suggested involve a consideration 35 of the rate of torque increase relative to the angle of threading advance as disclosed in Assembly Engineering, September 1974, pages 42-45; Design Engineering (London), January 1975, pages 21-23, 25, 27, 29; Iron Age, April 28, 1975, page 44; and Machine Design, Vol 47, January 23, 1975, page 44 These techniques monitor the torque-angle curve during the tightening process in order to terminate tightening in response to 40 conclusions derived from the torque-angle relationship In the Design Engineering disclosure, tightening is terminated upon sensing a significant drop in the torque rate, which occurs at the yield point In the remaining articles, tightening is apparently terminated when a predetermined torque range is attained within a fairly narrow angle range These disclosures are thus similar to the overruning schemes mentioned above 45

The goal of inferential tightening techniques is not merely to achieve a predetermined clamping load on one set of fasteners, since this can be readily done in the laboratory by instrumenting the bolt The goal is to achieve consistent and reproducible clamping loads or final tension values in large loss of fasteners at a low cost per fastener Thus, the major fallacy in prior art inferential tightening techniques has been to select a fixed tightening 50 parameter, such as torque or angle in the torque control and turn-of-thenut methods respectively, or a fixed range of a particular tightening parameter and terminate tightening in response to the attainment of the fixed tightening parameter or the range thereof This broad approach of the prior art has several major difficulties.

First, the critical item in tightening is clamping load as may be measured by final bolt 55 tension With the possible exception of some of the acoustic methods, no one has apparently heretofore been able to inferentially determine final bolt tension in production quantities Second, because of the selection of some parameter other than tension, there is introduced such widely variable factors as friction coefficients, speed related losses, and the like which grossly affect the relationship between the fixed tightening parameter or the 60 fixed range thereof and the only important result in tightening, which is clamping load or bolt tension.

It is accordingly an object of this invention to provide a technique for tightening threaded fasteners which is substantially more consistent than the prior art.

According to the present invention there is provided an apparatus which tightens 65 1 591 352 1 591 352 threaded fasteners, comprising a powered tool which imparts rotation to each fastener, thereby tightening said fasteners; means which turn the tool off after the tool is started; control means which delivers a shut off signal to the turn off means; calculating means which predict the amount of fastener rotation, variable from one 5 fastener to the next, occurring after the shut off signal is generated; and means which utilize the predicted amount of fastener rotation occurring after the shut off signal is generated so that each fastener is tightened to a desired amount of tension.

Also according to the present invention there is provided a method of tightening threaded fasteners, comprising imparting rotation to each fastener with a powered tool to 10 thereby tighten said fasteners; turning-off said tool after the tool is started by means of a shut-off signal delivered by a control means; calculating a predicted amount of fastener rotation, variable from one fastener to the next, occurring after the shut-off signal is generated; and utilizing the predicted amount of fastener rotation so that each fastener is tightened to a 15 desired amount of tension.

The present invention will be described further, by way of example, with reference to the accompanying drawings, in which:Figure 1 is an illustration of typical torque-angle and tension-angle curves generated during the continuous tightening of a fastener pair far beyond the elastic limit; 20 Figure 2 is an enlarged illustration of the low end of a typical torqueangle curve where tightening is temporarily suspended at the upper end thereof; Figure 3 is an illustration of a typical torque-speed relationship of an air powered tool; Figure 4 is an illustration of a tension-angle curve representative of the relaxation of a typical joint at the termination of continuous tightening; 25 Figure 5 is an illustration of typical tension-angle curve representative of the relaxation of the joint at a mid-point stop during tightening to a higher tension value; Figure 6 is an illustration similar to Figure 1 graphically explaining another facet of the invention; Figure 7 is an enlarged illustration of torque and tension curves graphically explaining 30 another facet of the invention; Figure 8 is a schematic view of the mechanism of this invention; Figure 9 is a side view of a component of the mechanism of Figure 8; Figures JOA and JOB are circuit diagrams of another component of Figure 8; Figure 11 is a front view of a typical operator's console; and 35 Figure 12 is a block diagram of another mechanism of this invention.

Referring to Figure 1, there is illustrated a typical torque-angle curve 10 and its corresponding tension-angle curve 12 which are developed during the continuous threading of a fastener pair to a point far beyond the elastic limit of the bolt, as may be measured in the laboratory from suitable equipment In the torque curve 10, there is typically a free 40 running region or period 14 where only a small torque is required to advance the nut and no appreciable bolt tension exists This is followed by an engagement period or region 16 where the contract between the surfaces of the fastener and the clamped pieces are being established while the rate of angle advance is gradually being reduced according to the torque-speed characteristics of the tool employed The tension rate FR, in the region 16 is 45 less than the ultimate tension rate FR 2 but is rather well defined The engagement region 16 appears to cover an approximate tension range of about ten percent to about fifty percent of the elastic limit of the bolt Above the engagement region is a final tensioning region or period 18 which exhibits the increased tension rate FR 2 Fortunately, FRI, FR 2 and the location of the bend therebetween are well defined and reproducible properties of 50 the joint and are not related to friction or other variable factors which may develop in the course of tightening.

The torque rate TR is initially quite low in the free running region 14 and begins to rise substantially during the engagement period 16 Due to the existance of speed-dependent losses such as lubricant squeeze film and microplasticity of the surface irregularities 55 between the fastener parts and clamped pieces, a linear approximation TR of the torque curve 10 in the region 16 does not intersect the angle axis at the point of origin of the tension curve 12 An offset angle a, exists which is proportional to such speeddependent losses.

Because of the torque-speed curve of the tool employed, it can be shown that (X, is torque rate dependent so that the offset torque TO, is the appropriate joint property and T 05 is the 60 product of the offset angle ac,, and the torque rate TR.

The elastic limit 20 occurs at a point beyond which strain is not recoverable upon unloading and appears toward the upper end of the final tightening region 18 as is well known in classical mechanics Somewhere in the yield region 22, the bolt commences to deform plastically rather than elastically As alluded to previously, the normal definition of 65 1 591 352 the yield point is in the range of 1- 2 % strain which is somewhat arbitrary The proportional limit occurs substantially below the yield point 20 and occurs where the stress/strain ratio is no longer constant.

In order to implement the hereinafter disclosed method of torque control, one needs to determine FR 1, FR 2, T 0, and other parameters as discussed more fully hereinafter This is 5 conveniently accomplished by selecting a reasonably large sample of the fasteners that ultimately will be tightened by the technique of this invention and empirically determining the values in the laboratory It will normally be experienced that scatters in FR 1, and either FR 2 or r, the ratio of FR 2/FR 1, will be quite small In new bolts, FR 2 is normally 5-15 % higher than FR 1 In fasteners that have been tightened previously, FR 2 is normally quite 10 close to FR 1 The conclusion is that the difference between FR 1 and FR 2 is related to the microplasticity of surface irregularities between the mating faces of the joint As is true in all torque measurements, TO, will have much larger scatters Fortunately, the offset torque correction is normally quite small so that its lack of consistency has a quite minimal effect on the final tension values One exception is in the use of so-called "prevailing-torque" 15 fasteners which usually comprise a bolt or nut having the threads intentionally deformed for various reasons Another exception involves the use of a bolt or nut in which the threads are unintentionally deformed In such situations, the normal value of Ts should be increased by the addition of the measured "free-running" torque.

Broadly, the preferred technique is to periodically or continuously sense the torque 20 applied to the fastener pair and the angle of advance corresponding to the sensed torque, determine the tension appearing at least at one point 24, calculate a value of a tightening parameter sufficient to achieve a final desired tension value FD and instruct a tool to advance the fastener pair until the attainment of the tightening parameter.

During a study of torque-tension-angle relationships, it was discovered that the inverse of 25 the rate with respect to angle of the logarithm of torque is theoretically a measure of bolt tension irrespective of joint friction.

Defining, 30 da F x, a>ap ( 3) P 35 ap is the angle where P achieves a maximum value and conceivably could be used as the origin for the turn-of-the-nut method thereby totally eliminating the influence of joint friction In practice, it is difficult to detect a single meaningful peak which can be labeled ap because of the noise inherent in the actual torque-angle signal Although the concept expressed in equation ( 2) is valid, it requires a different procedure for processing the 40 torque-angle data to achieve a practical solution As will be apparent to those skilled in the art, the solution may be analog, rather than digital, as hereinafter disclosed The theoretical basis for equation ( 2) can be derived from equation ( 1), as follows:

T = (fhrh+ fthrth)F ( 1) 45 differentiating equation ( 1) relative to angle, da = (fhrh + fthrth) da ( 4) 50 dividing equation ( 4) by equation ( 1), 5 d T/da _ d F/da ( 5) T F Since d T/T is the definition of d log T, 60 d F/da d LOG T F d o GT ( 6) 1 591 352 if d F the joint tension rate, is a constant, then:

F =(d) (d LOG T) 1 = FR/P ( 7) 5 d a da Equation ( 7) shows that the constant of proportionality in equation ( 3) is the tension rate FR.

Several assumptions have been made in the above derivation: 10 1) The tension rate is a constant This not precisely true throughout the tightening range The more precise assumption would have been that tension at any angle of advance after the angle of origin, where the tension rate commences, is a unique function of the joint and therefore that the tension rate at any angle after the angle of origin is a unique function of the joint 15 2) Torque is not a function of the turning speed This is not strictly true and for accurate application, it should be accounted for.

3) Joint friction (f Ifth) is not load dependent for any one sample This is a good assumption except when non-metallic (molybdenum disulfide, Teflon, etc) coatings are utilized Even in the case of non-metallic coatings, any changes in a finite tension 20 range should be small.

For purposes of convenience, the preferred tightening technique of this invention may be referred to as the logarithmic rate method.

The importance of equations ( 5) and ( 7) should now be appreciated It has been demonstrated in the laboratory that the value of tension rate d F/da is a function of the joint 25 having small scatters and is independent of friction The torque rate d T/da can be determined from torque and angle measurements taken during the tightening of each fastener pair by suitable torque and angle sensors on the tightening tool The torque value T is, of course, measured by the same torque transducer It will accordingly be apparent that the friction dependent parameters, i e torque rate and torque, are determined for each 30 fastener while tightening, which is here defined as the time frame commencing with the onset of threading and stopping at the termination of tightening Since tension rate d F/da is a function of the joint which is determined empirically prior to the tightening of production fasteners, it is a simple matter to solve equation ( 5) for tension.

Referring to Figure 1, it may be assumed that the fasteners are threaded together with 35 measurements being taken of both torque and angle with tightening being advanced to the point 24 The average torque rate TR is calculated, as by the use of the least squares method Since the tension rate FR, is known from empirical measurements of the joint in question, the tension in the joint can be calculated at the point 24 from equation ( 5).

Graphically, the angle required to advance the fasteners from the tension value calculated 40 at the point 24 to the final desired tension value FD can be easily done since the tension rate FR 2 has likewise been determined empirically After determining the additional angle Qfji,,,, the tool may be instructed to so advance the fasteners thereby attaining the desired final tension value F) In a similar fashion, the additional torque AT or the final desired torque TD can be calculated 45 There are substantial difficulties in applying these principles to production line operations It will be apparent that the calculations being made are being done while tightening It will be apparent that the duration of tightening should be minimized so far as practicable commensurate with the attainment of consistent results In any event, it will be apparent that long tightening times, for example two minutes, would render the technique 50 unsuitable for many production line operations although some suitability may remain for special purpose applications such as in the fabrication or reactor vessels, aircraft and the like where precision is paramount It is accordingly, evident that the use of electronic computation techniques is highly desirable for processing the data obtained from measurements taken during tightening Even with the use of electronic computation 55 techniques, it is desirable to advance the fasteners from some initial distance, suspend tightening momentarily and then resume tightening to the final desired tension value The momentary stop allows time to complete lengthy calculations and has the additional benefit of allowing the joint to relax at this point rather than at the final tension value attained As will be more fully apparent hereinafter, many of the calculations are done while the tool is 60 running as well as when the tool is momentarily stopped It will, however, be evident that simplified computations may be utilized thereby eliminating the necessity for a momentary pause in the tightening operation.

More specifically, the following steps may be taken to attain a consistent bolt tension utilizing an instructable tool equipped to measure torque and angle information only, after 65 1 591 352 the acquisition of certain empirical information: 1 Engage the fasteners, start the tool and record torque at equal angle

increments.

2 Shut the tool off in a tension range of 4- 5 of elastic limit A turn-ofthe-nut approach may be used to estimate the initial tool shut off, or a simplified logarithmic rate method may be applied 5 3 Calculate the torque rate from the torque and angle measurements by a suitable smoothing technique, e g least squares Calculate the torque at the midpoint of the range from which the torque rate was calculated, by averaging the torque value along this range.

Accordingly, the intersection of the average torque rate with the angle axis is established.

Since the offset torque T 05 is largely a function of the joint, the intersection of the tension 10 curve with the angle axis is established.

4 The tension curve is then a straight line emerging from the origin or intersection determined in 3 above with the initial slope FR 1 This is valid up to about 5 of elastic limit at which point the tension curve has a slope of FR 2 The location of the bend in the tension-angle curve is determined empirically when determing the value of FR 1, FR 2, and 15 T.

Calculate the tension value appearing in the fasteners at some location, for example, point 24 Given the tension value at point 24, calculate the additional angle afinal or the additional torque AT necessary to tighten the fasteners to the final desired tension value FD 20 6 Instruct the tool to resume tightening and advance the fasteners through the angle afina,1 or for the increased torque AT.

For purposes of discussion, the implementation of the technique of this invention may be broken down into four segments: ( 1) determining the mid-point stop, ( 2) calculating the torque rate and the angle of tension origin, ( 3) calculating the final shut off parameter 25 including a prediction of tool overrun and ( 4) determining the occurrence of yield and calculating the final attained tension value in the event yield occurs prior to the attainment of the final shut off parameter.

The intent of the mid-point stop determination is for the joint to be tightened to an angular location corresponding to the break in the tension-angle curve for reasons pointed 30 out more fully hereinafter In order to get to the mid-point, one may use a torque approach, a turn-of-the-nut approach, or a logarithmic rate method Using turn-ofthe-nut as illustrative, one technique for arriving at the mid-point is to empirically determine the average angle beyond snug torque where a predetermined tension value, e g at least about 4 of yield strength, occurs In use, an account should be taken of tool overrun For 35 example, if the empirical determinations show that 4 yield strength occurs, on the average, at 450 past 2 foot pounds and one experiences an average of 8 overrun, one should turn off the tool at 37 past 2 foot pounds in order to arrive at the mid-point.

Although the turn-of-the-nut method can be used to determine the midpoint stop, it is preferred to use a simplified logarithmic rate method Referring to Figure 2, the tool is 40 turned on with torque values being recorded and stored at fairly small equal angle increments ha which may be, for example, in the range of 1-5 With fasteners of the type studied, a selection in the range of 2-3 seems preferable In getting to the mid-point, torque and angle measurements obtained in the lower part of range 16 are used The torque value T 1 is an empirically determined value and is the first torque value utilized to calculate a 45 preliminary torque rate T 1 is on the order of about 20 % of the average final torque value obtained in running the samples to empirically determine FR 1, FR 2 and T 05 When running torque is first sensed to be equal to or greater than T 1, such as at the location 26, the angular position of the location 26 is noted and stored When the tool passes the point 28 which is one ak degrees beyond the location 26, the torque value T 2 is sensed and stored The value 50 of ak is preferably large enough to give a rough approximation for a preliminary torque rate, which is calculated as (T 2 T 1) /ak If ak were very large, e g 200, the tool would not be stopped until late, leaving little or no additional room to resume tightening If ak were very small, e g 3, the value of torque rate calculated from (T 2 T 1)/ak would be so influenced by noise in the torque sensings that it would be unreliable A compromise of 90 55 for ak has proved acceptable although other compromise values are obviously acceptable.

The data processor then calculates a,, in accordance with the following equations:

1 591 352 a, = c + a T 2 ( 7 a) C(OrT or Tos c= ad{l (laakr) (;)} _ eor (l-y) K 2-( 2 Ko + Nk k) ( 7 b) 1 orad a ak To +T NkÂKo-T} ( 7 c) 10 ad is the desired angle from tension origin to midpoint and is FM 15 FR, or slightly greater where FM is the tension value at the junction of the two tension regions indicated by FR,, and FR,, 'or is the tool overrun at idle due to actuation delay, To is the stall torque of the tool, K, is a typical torque rate for the particular fasteners involved and is 20 determined empirically, and Nk is a correction factor necessitated by the inaccurate algebraic expansion of a more precise equation, which expansion substantially reduces calculation time for equation ( 7 a) It will be apparent that, in a production line situation involving the same size bolts and the same size tools, every value in these equations, except T 2, can be reduced to numbers before starting Thus, the computations are actually easier 25 and quicker than appears.

It might be questioned why the value of ak is of any importance since neither equation ( 7 a), ( 7 b) or ( 7 c) appear to contain a value for preliminary torque rate Equations ( 7 a), ( 7 b) and ( 7 c) constitute an application of the logarithmic rate method to achieve a mid-point tension value of F Rc ad with provisions made for tool overrun due solely to the time delay 30 between the shut off command and exhaustion of air from the tool The mathematical complexities have, by design, been transferred from equations ( 7 a) to equations ( 7 b) and ( 7 c) so that computation of equation ( 7 a) during tightening requires the least possible elapsed time Equations ( 7 b) and ( 7 c) can be computed manually either prior to system installation or computed by the microprocessor when in a dormant portion of the tightening 35 cycle, for example, prior to the initiation of tightening Although the preliminary torque rate T 2 T.

ak 40 does not appear in equations ( 7 a), ( 7 b) or ( 7 c) as written, if one were to substitute the equations for a and c into equation ( 7 a), one would find that the preliminary torque rate appears Accordingly, the reasons why ak should not be too large or too small are as previously discussed 45 As will be recognized by those skilled in the art, equations ( 7 b) and ( 7 c) do not include a tool overrun prediction due solely to the inertia of the rotating parts of the tool For moderate and high torque rate bolts, the amount of angular overrun due solely to inertia is rather insignificant The reason, of course, is that the tool is not rotating very fast With low torque rate bolts, which the tool is able to turn faster, the amount of overrun due solely to 50 inertia is still modest For applications where maximum accuracy is desirable, equation ( 7 b) and/or ( 7 c) may be modified to incorporate a measure of overrun prediction based on inertia.

The determination of the mid-point stop is of some importance as may be visualized from an appreciation of Figure 1 It will be recollected that it is desired to calculate the average 55 torque rate TR If the mid-point stop occurs, for example, in the lower part of the region 16, the average torque rate will be substantially too low If the midpoint stop is too late and well into the region 18, two difficulties are presented: ( 1) the calculated average torque rate TR may be substantially too high although some calculations can be done to disregard some of the later data in order to shift the range where torque rate calculations are actually being 60 conducted, and ( 2) there may be little or no additional room available to resume tightening to the final desired tension value considering allowance for tool overrun.

Referring to Figure 2, the tool is commanded to shut off at a point 30 which is al degrees beyond point 26 which was where the torque value T 1 was first equalled or exceeded.

Because of the time delay in the tool from the shut of command until the tool actually stops, 65 1 591 352 which is represented by the point 32, the tool has overrun by an angle a The mid-point stop 32, typically falls in the range of about 4- 6 of the elastic limit For any given application, the empirically determined values act to establish the mid-point 32 at a given fraction of the elastic limit which is not changed until new empirical data is developed, as for example may occur when a different type fastener is selected 5 In order to calculate the average torque rate TR, a decision must be made of which torque and angle measurements are to be used It has been learned that the torque value at the stopping point 32 is somewhat unreliable because of speed dependent variables.

Accordingly, the highest torque value used in the torque rate calculations is at a location 34 which is one ha backward from the point 32 The torque value at the point 34 is T 3 The 10 total number of values used in torque rate calculations, designated N for more general purposes, may vary widely and is subject to considerable compromise A total of fourteen consecutive data points with the point 34 being the highest torque value has proved quite acceptable The mean torque Rm and the average torque rate TR are then calculated using the following summations where i is a designation for each point selected for the torque rate 15 calculations and Ti is the torque value there sensed:

Tm = 1 N Ti;( 8 n i;( 8) i= 1 20 TR = -6 N (n+ 1-2 i)Ti i== (Aa) N (n+ 1) (n-1) 25 Equation ( 8) will be recognized as merely adding the torque values occuring at each of the points i and dividing this sum by the total number of data points n.

Equation ( 9) will be recognized as a least squares fit for the data points i.

It is desirable to assure that the mean torque Tm and the average torque rate TR are 30 taken over substantially the same tension range during the tightening of each fastener pair.

This may be accomplished by checking to determine how close the angular position of the stopping point 32 is to the break in the tension-angle curve 12 The angular position of the mean torque Tm along an abcissa T 05 may be calculated from the equation:

35 F = Tmr Tos where a F > O ( 10) TR The angular distance from the point of origin of the tension curve 12 to the stopping point 40 32 may be calculated from actual data derived from the fastener being tightened from the equation:

-origin1 Aca(n+ 1) a F where a Origin < 0 ( 11) 45 For calculation purposes, it is desirable that aorigin be negative.

From empirically determined information done prior to the tightening of production fasteners, the start of the second tension region may be calculated from the equation: 50 Ct FM = FM where a FM > O ( 12) FR, 55 where FM is the tension value at the break The difference between a Origin and CFM may be obtained from the equation:

X = aorigin a F, ( 13) 60 If X 3 0, this means that the mid-point stop 32 is too late and consequently that the largest torque value T 3 in the torque rate calculations is too large Without revising the value for TR, TR will tend to be too high as previously discussed Accordingly, one needs to shift the range of torque rate calculations downwardly on the torqueangle curve presented in Figure 2 Thus, 65 1 591 352 n H =-4 ( a) + 1 ( 14) n 1 = N ( 15) 5 From the stopping point 32, one moves downwardly along the torque-angle curve by n H angle increments of Ac to define a new point 35 as the upper limit of the range through which torque rate will be calculated The symbol 4 means that any fractional value is dropped so that the number used is the next lowest integer from the calculated value The total number of data points N remains the same 10 If X< 0, this means that the stopping point 32 occured too soon which would tend to give a value for torque rate that is too low Since one cannot move upwardly on the torque-angle curve to obtain an additional area of measurement, the practical solution is to accept fewer data points for torque rate calculations thereby, in effect, lopping of the lower end of the range Accordingly, 15 n Hl = 1 ( 16) Fo FL) nj = |( FL) + (x) ( 17) 20 Where n H, indicates the point or location where the largest torque value used in the torque rate calculations occurs Since the largest torque value will remain the same, n H 1 = 1 so that the torque T 3, being one ha removed from the stopping point 32, is the largest torque value 25 used The new value for nj, which is the total number of data points used, is based on the assumption that the tension rate in the first region is substantially linear above a minimum tension value FL, determined empirically, and that the tension F( in the joint at the stopping point 32 lies in the first tension range The symbol AF is the additional tension in the first tension range per angle increment ha and may be expressed mathematically as: 30 AF = FR Act ( 18) The tension F, in the joint at the stopping point 32 is 35 Fo = -FR, origin ( 19) It is conceivable that nj may be too small, e g two or three points, to give good results with the least squares equation ( 12) Accordingly, a check is made to determine if nj is less than one half of n In this event, 40 = 2 F -F F, ( 20) 45 and N 2 is used as the total number of data points.

Accordingly, a new summation is performed for mean torque Tm and torque rate TR in accordance with equations ( 8) and ( 9) utilizing the new starting place in the event that X 20 or starting with the same highest torque value but using fewer number of data points in the event that X< O 50 With revised values for mean torque Tm and torque rate TR, a revised value may be obtained for the angle of origin of the torque-angle curve using equation ( 10) and a revised value may be obtained for the origin of the tension-angle curve using equation ( 11) A calculation is again made to determine whether the tool has overshot or undershot the break in the tension curve in accordance with equations ( 12) and ( 13) If X 0, the tension 55 appearing in the joint at the stopping point 32 is calculated to be:

Fo = FM + r FR 1 x ( 21) where FM is the empirically determined tension value at the break in the tension curve and r 60 is the ratio of FR 2/FRI If X < 0, the tension appearing in the joint at the stopping point 32 is obtained from equation ( 19).

It will be apparent that the values of mean torque Tm, torque rate TR, u F, origin and the like may be revised as many times as deemed desirable.

One of the defects in the technique heretofore described is the assumption that the 65 1 591 352 empirically determined tension rate FR 1 correctly describes the elastic properties of the fastener actually being tightened For good quality joints, the tension rate FR 1 does not vary widely There are, however, a number of relatively common situations, e g galled threads, misaligned fasteners, poor contact surfaces, dirt or other foreign particles between the contact surfaces, and the like, where the actual tension rate for the fasteners being 5 tightened is significantly below the empirically determined tension rate FR 1 In such poor quality joints, the actual final tension value will be significantly below the desired tension value FD and significantly below the final calculated tension value Ffinal To determine the significance of such poor quality joints, two 5/16 " 24, SAE grade 8 nuts and bolts were tightened with a shim, 015 inches in thickness inserted from one end in order to simulate 10 poor contact due to misalignment The final desired tension value FD was 5500 pounds The actual measured final tension value was 2400 pounds and 1700 pounds for the two fasteners, a percentage variation of -56 % and -69 % from desired It will accordingly be apparent that the occurrence of such poor quality joints can have a major effect on the scatter seen in fasteners tightened by the technique of this invention It will also be evident, upon 15 reflection, that such poor quality joints will have a like effect on the scatter in fasteners tightened by a turn-of-the-nut method.

It has been learned that poor quality joints of the type exhibiting abnormally low tension rates can readily be detected by the data developed during the course of tightening a fastener pair with this invention In such poor quality joints, the torque rate is not constant 20 in the upper part of the region 16 where the average torque rate TR is calculated, as contrasted to the showing in Figure 2 Instead, the torque is arcuate and, if plotted, is upwardly concave Thus, it is a relatively simple matter to measure or calculate and then compare the average torque rates in the upper and lower parts of the range where the torque rate TR is calculated For example, in a situation where thirteen data points are 25 being used to calculate TR, with the point 34 being the highest torque value used, the torque rate T Ra over an angle of six Aa increments backward from the point 34 would be calculated The calculations may, of course, be a two point or a least squares technique.

Next, the torque rate T Rb over an angle commencing with six Aa increments backward from the point 34 and ending twelve increments backward from the point 34 is calculated by 30 a two point or least squares technique Then, the ratio of T Ra/T Rb is computed If the ratio of T Ra/T Rb is near unity, e g 1 + 10, the conclusion is that the joint has an acceptable tension rate If the ratio of T Ra/T Rb diverges significantly from unity, e g T Ra/T Rb'1 10, the conclusion is that the joint has an abnormally low tension rate FR 1 and, if tightened by the technique of this invention or by a turn-of-the-nut method, will result in a fastener 35 stressed substantially below the desired tension value FD A suitable signal may be displayed at the operator's station and the joint rejected and the parts replaced.

It will now be appreciated that the location 32 of calculated tension F, appearing in the joint corresponds to the point 24 illustrated in the more general showing of Figure 1 The only determination yet to be made is the additional angle afinal or the additional torque AT 40 required to achieve the final desired tension value FD Compared to the manipulations used to assure consistently reliable values for torque rate TR and the angle of tension origin, these calculations are relatively straight forward.

One tightening parameter that may be selected to attain the final desired tension value FD is the additional angle afinal 45 If x 0, finl FR Fo, ( 22) 50 If x < 0, Cfinal = -x + 1 FR ( FD FM) ( 23) F, is, of course, obtained from equation ( 19) or ( 21) while FM is the tension value at the break in the tension-angle curve and is determined empirically 55 It will be appreciated that the tool overran an angle ba when stopping at the point 32 If is equally apparent that some amount of tool overrun will occur approaching the final desired tension value FD A typical torque-speed curve for an air powered tool is shown in Figure 3.

Since the tool will be slowing down during tightening, it will be apparent that the tool overrun approaching the final desired tension value FD will be less than the overrun 60 approaching the point 32 Defining, Ta TR To-& ( 24) 65 1 1 12 1 591 352 12 where T 4 is the torque value at the point 30 where the initial shut off command was given prior to reaching the stopping point 32, To is the stall torque of the tool, TR is the calculated torque rate and &a is the measured angle overrun approaching the point 32 The expected tool overrun approaching the final desired tension value FD is:

5 da = bet ( 1 _ final) ( 25) caa 10 It has become apparent that a typical joint will relax, i e lose tension without unthreading of the fasteners, at the mid-point stop 32 and/or at the termination of tightening If the fasteners were continuously tightened, i e without a mid-point stop, the relaxation at termination of tightening can be rather significant while, with a mid-point stop, the relaxation at termination of tightening is quite modest By stopping at the 15 mid-point 32, the bulk of joint relaxation occurs prior to the resumption of tightening.

Thus, the stopping at the mid-point 32 provides greater consistency in final joint tension although this phenomenon complicates the determination of a final angle shut off parameter.

If the joint did not relax at the mid-point stop 32, the tool would be instructed to go an 20 additional angle aetfi,, -da beyond the mid-point stop 32 where the final shut off command is given As shown in Figure 1, the final shut off command would occur at about the point 36 whereby the tool overruns to tighten the fastener pair through an angle da until stopping at the final desired tension value FD.

The phenomenon of joint relaxation is illustrated in Figure 4 where the curve 38 25 represents the tension-angle relationship during continuous tightening to a location 40 below the elastic limit of the fastener When tightening stops, the joint relaxes as suggested by the tailing off of tension along a constant angle line 42 The final tension appearing in the fastener is accordingly at the point 44 A typical value for joint relaxation along the line 42 is 7 % of joint tension within twenty-one hours 30 Referring to Figure 5, the curve 46 represents the tension-angle relationship during tightening to the mid-point stop 32 Because the joint relaxes, tension in the fasteners tails off along a constant angle line 48 to a tension value at the point 50.

Instead of instructing the tool to go an additional angle cfina 1 -da from the mid-point stop 32, the instruction is to advance the fasteners an additional angle O final -da after the 35 running torque equals or exceeds TP where TP = T 3 + TR (hac) ( 26) It will be recollected that the torque value T 3 is located at the point 34, which is one La 40 backward from the mid-point stop 32 By advancing the tool until running torque equals or exceeds TP, the torque and tension values at the mid-point stop 32, before relaxation occurs, are essentially reproduced This is indicated in Figure 5 where the point 52 designates the location where running torque is equal to or greater than T Tightening will then be done correctly, regardless of prevailing tension in the bolt at t&e time the tool 45 resumes tightening As shown in Figure 5, the final shut off command occurs at the point 54 whereby the tool overruns to tighten the fastener pair through an angle da until stopping at the final desired tension value FD In order to shift the bulk of joint relaxation from the final stopping point to the mid-point stop 32, the mid-point stop 32 is at least 4 of yield strength and conveniently is in the range of 4 6 of yield strength With the midpoint stop 32 so 50 located, typical joint relaxation at the final stopping point is on the order of 1/2 2 % of final bolt tension within one hour It should be clear that this amount of joint relaxation is the relaxation of a good quality joint rather than a joint suffering from misaligned parts, compressed gaskets and the like.

Another tightening parameter that may be selected to attain the final desired tension 55 value FD is the additional torque or the final torque TD (Figure 1) The final torque TD is preferred since the joint may relax at the mid-point stop 32 Because the tool instruction is to achieve an absolute torque value TD, any relaxation in the joint is automatically accomodated In using a torque-governed shut off parameter, even a possible tightening of the joint at the mid-point stop will also be automatically compensated for 60 In using a torque-governed shut off, an interesting phenomenon has been noted for which no simple explanation appears Referring to Figure 1, it will be noted, as previously mentioned, that the tension rate FR, is greater than the tension rate FR 1, typically by 5-15 % depending mainly on the value selected for FM This would lead one to believe that the torque rate in the region 18 would be greater by a similar amount than the torque rate in 65 13 1 59 35 _ 13 _A the region 16 Laboratory investigations indicate that the torque rate in the region 18 typically exhibits a slightly smaller increase over torque rate in the region 16 Fortunately, the ratio of the torque rates in the regions 16, 18 to the ratio of the tension rates FR 1, FR 2 is more nearly constant for a single type fastener pair In calculations for a final torque shut off command, this factor is taken into account 5 TMC = Tos + TR FM ( 27) FR 1 10 T TD TMC R(TR) (F O FM) ( 28) FRI where TMC is a calculated value for the torque at the break in the tension curve, R is defined as TR 2/r TR, TR 2 is the torque rate in the region 18, TR is the torque rate in the region 16, r 15 is the ratio of FR 2/FR 1.

As is the case in the angle governed final shut off calculations, the tool will overrun after the final shut off command is given Defining, 6 T TR (bct) ( 29) 20 Ta = To T 4 6 T, then ( 30) d T = T A Ta D) ( 31) 25 After tightening is resumed, the final shut of command is given when running torque T 3 TD d T As shown in Figure 1, the final shut off command will occur at about the point 36 whereby the tool overrun continues to tighten the fastener pair for an additional torque d T 30 until stopping at the final desired tension value FD.

It is apparent that tightening of the fastener pair can be terminated in response to calculated tension which is derived by the techniques of this invention Upon anlysis, it will be evident that termination tightening in response to calculated tension is in reality the same as terminating tightening in response to either angle or torque, depending on how the 35 calculations of tension are conducted.

It will also be apparent that tightening may be terminated in response to a combination of torque and angle, for example, a linear combination of torque and angle Assuming that one wished to equally weigh the calculated advance derived from the torque and angle computations, the appropriate equation is generically: 40 FD Fo + /2 r FR, lCfina 1 + (TD Tsp)l ( 32) TR 2 45 where F, is the calculated tension value at the mid-point stop 32 as may be calculated fromequation ( 19) or ( 21) depending on whether X 0 or X > 0, and TP is the calculated torque value at the mid-point stop 32 as may be calculated from equation ( 26) The calculations for afinal will depend on whether X 0 or X < 0 as pointed out in equations ( 22) and ( 23).

Calculations for TD are made using equations ( 27) and ( 28) 50 As with the use of other tightening parameters, it is desirable to provide an overrun correction It is apparent that the angle overrun correction of equation ( 25) may be incorporated as an overrun prediction, as follows:

For = r(FRI) da ( 33) 55 where For is the increase in tension due to overrun It may also be desirable to use an equally weighted linear combination of torque and angle in determining the predicted tool overrun The tension produced in the bolt during overrun may be calculated as 60 For = /2 r FR, (da + d T) ( 34) It will be apparent that one cannot merely instruct the tool to proceed an additional angle 1 591 352 131 14 1 591 352 1 or until a desired torque level is reached in order to stress the bolt to the desired tension value FD when using a mixed parameter of torque and angle Instead, one may calculate the tension appearing at any angular position a 3 beyond the point 32 as If x 0, Fa 3 =Fo + /2 r FR, la 3 + TC 3-TSPR l ( 35) If x < 0, Fa 3 = FM + /2 r FR, la 3 + x + TU 3 TMCl ( 36) 10 TR, where Ta 3 is the sensed torque value at the angular position ct 3, Tp is the calculated torque value at the mid-point stop 32, and TMC is the calculated torque value at the location of FM according to equation ( 27) 15The calculated tension value at the point of shut off is:

Fs( = FD For ( 37) where FD is from equation ( 32) and For is from equation ( 33) or ( 34) By comparing the 20 value of Fa 3 at angle increments, such as Aa, 1 or the like, with F,0, as soon as Fct 3 F 00, the shut off command is given In this fashion, tightening may be terminated in response to a linear combination of torque and angle.

Referring to Figure 6, another feature is illustrated When tightening to the final desired tension value, it is highly desirable to assure that the yield point is not reached or is at least 25 not substantially exceeded This may be done graphically as shown in Figure 6 by drawing a line 56 parallel to the torque curve 10 in the region 18 or parallel to the tension curve 12 and spaced therefrom by an angle "Y The value of ay can be correlated with an acceptable amount of strain in the bolt since the amount of nut rotation in this region of the torque curve can be calculated into a percentage of bolt elongation because of the known pitch of 30 the threads When the running torque value T intersects the line 56 at the point 58, the tool is given a shut off command and ultimately comes to rest at a point 60 because of tool overrun.

In order to implement this technique, the torque value sensed by the tool is monitored after the tool is turned on again after the mid-point stop 32 One difficulty arises since the 35 restarting torque applied to the fastener in order to resume tightening typically is relatively substantially larger than the running torque immediately prior to the midpoint stop 32 as is caused by the difference between the static and dynamic coefficients of friction and complicated dynamic factors When the sensed value of running torque T first equals or exceeds the value of TM where: 40 TM = T 3 + TR (Aa x) ( 38) this location is marked and two Act increments beyond this location, which is location 62, the running torque value T is sensed and stored as T 5 TM will be recognized as a calculated 45 torque value which appears at the location on the torque-angle curve corresponding to the break in the tension curve.

As is apparent from Figure 6, the calculations being done to detect yield or non-linear strain occur in the region 18 where the value of torque rate is somewhat different than the torque rate value calculated in the region 16 The torque rate in the region 18 can be 50 expressed:

u r R(TR) ( 39) Since R has been defined as TR 2/r TR, it will be apparent that equation ( 39) reduces to the 55 proposition that u = TR 2.

Yield or non-linear strain calculations can be conducted periodically during tightening in the region 18 as often as is deemed desirable Although the calculations can be done at every angle increment Acc, results are quite satisfactory if done every other angle increment ha Accordingly, 60 AT, = 2 u (La) ( 40) 1 591 352 ( 41) ATY = ucty 1 591 352 where ay is the angle corresponding to a desired strain level which can either be elastic but non-linear or plastic, AT 1 is the incremental torque over the incremental angle 2 Aa and ATY is the incremental torque over the incremental angle ay By selecting small values for ay, the shut off command will tend to be in the elastic but non-linear range If ay is selected to be a large value, the shut off point will appear in the yield range It is thus apparent that 5 the detection of non-linear strain can encompass both elastic and plastic strain The only difficulty in selecting very small values for ay is that noise in the torque curve 10 in the range 18 might create a premature and false yield signal At a point 64, which is two Ac degrees after the occurrence of T 5, the value of running torque T is compared with 10 Ty, = T 5 A Ty + AT 1 ( 42) It is apparent that Ty 1 is a torque value on the curve 10 at the point 64 If T > Ty 1, tightening continues At a point 66, which is two Aa degrees beyond the point 64, the value of running torque T is compared with 15 Ty 2 = Ty, + AT 1 ( 43) 0 (T A TY + A T 1) + A T 1 ( 44) 20 If T > TY 2, tightening continues This procedure continues by adding an additional torque value AT 1 to the preceding value of T at angle increments of two Aa In the event that T S TY before the occurrence of the shut off command derived from the normal tightening parameter of torque or angle, a shut off command is given to the tool It will be apparent that the actual shut off command from detection of non-linear strain or the actual 25 detection of non-linear strain will not occur at exactly the point 58 since comparisons are being made every two Aa Thus, the actual yield detection will probably occur later, e g at the point 68 as shown in Figure 6.

Thus, tightening is normally terminated in response to a torque-governed, an angle-governed or a mixed shut off command, but in the case of yield detection, a 30 premature shut off command is given It will accordingly be apparent that the upper end of the scatter band is eliminated by a secondary yield point shut off Thus, the total scatter will be reduced It will also be apparent that the danger of bolt failure during assembly is eliminated It will also be apparent that the detection of non-linear strain may be conducted as disclosed in United States patents 3,643,501 or 3,693,726, although the technique herein 35 disclosed is deemed preferable.

It is preferred that the selection of FD will be low enough so that the cutoff due to detection of non-linear strain will be rare, e g 1 % In the event that the percentage of non-linear strain detection rises substantially during a production run, this indicates that the fasteners, i e bolts or/and threaded parts, employed to not meet design specifications 40

Thus, a high percentage of non-linear strain detections is a signal that quality control investigations need to be conducted on the fasteners employed For example, if the normal occurrence of non-linear strain is on the order of 1 %, and a running average of non-linear strain detections is 10 %, it is likely that the fasteners being run do not meet specifications.

To this end, a running count of the number of joints tightened is maintained and a 45 running count of the number of joints exhibiting non-linear strain is detected When CY A ( 45) where Cj is the number of joints tightened, Cy is the number of yielded joints and A is some 50 fraction acceptable to the user From present information, it appears that the value of A should be in the range of 10 20, e g 15 The ratio of Cy/Cj is preferably a running ratio, rather than a cumulative ratio, as by storing, on a first-in, first-out basis, a finite number of joints tightened Cj, e g 30, and any instances of yield detection Cy When the running ratio of Cy/Cj exceeds the selected value A, a suitable signal may be provided indicating that the 55 frequency of non-linear strain is much too high The investigations to be conducted normally include analysis of the strength and material composition of the fasteners, a technique well known in the prior art.

It is highly desirable to calculate and store the final tension appearing in a fastener which has been stopped prematurely because of non-linear strain detection It may be that the 60 final tension value achieved is well within an acceptable range In this event, it would be disadvantageous to require removal of the fastener pair and replacement with a new pair if the problems associated with marginally yielded fasteners are not material if the fasteners are sufficiently stressed to assure acceptable joint conditions.

Accordingly, when using an angle approach, the value of final tension may be calculated 65 is is 1 591 352 as follows:

Ffinal = FD r FRI ( afinal + a Y a 2) ( 46) where a 2 is the angle from the stopping point 32 to the location where yield detection is 5 sensed It will be appreciated that any calculated value of Ffinal is somewhat of an approximation since the tension rate well above the proportional limit is unknown and perhaps unknowable with any degree of accuracy Figure 7 graphically illustrates the difficulty If the final tension value were calculated:

10 Ffinal = FD r FR 1 ( afinal a 2) ( 47) the tension actually being calculated would be at the point 70 which is at the same angular position a 2 from the stopping point 32 as the yield detection point 68 It will be appreciated that the difference in tension values between the points 68, 70 may be significant in some 15 circumstances Since it is known that the tension rate falls off substantially immediately prior to the point 58, it is safe to calculate the tension value at the point 72 which is spaced downwardly along the slope FR 2 by an angular distance ay Thus, the rationale for the equation ( 47) is apparent It will be appreciated that the actual final tension appearing in the joint is that at the point 68 which differs from the calculated tension value appearing at 20 the point 72 It will be seen, however, that the tension value at the point 72 is a substantially better estimation of actual final tension than is the tension that would be calculated at the point 70 This is particularly true since the tension rate in the range 74 is known to be quite low The final tension value Ffinal along with a notation that the bolt has yielded may be displayed at the tool location, printed or otherwise recorded for further use or analysis 25 In the event the torque-governed final shut off parameter is being used, when T S Ty non-linear strain is detected and a shut off command is given the tool The final tension value may be calculated from a torque approach, as follows:

F = F FR 1 (TD Tf) ( 48) Ffinl D R(TR) ( 8 Where Tf is the highest value of torque sensed within one or two Aa increments before the final stopping point 60 This is likewise illustrated in Figure 7 The detection of yield occurs 35 at point 68 on the torque curve 10 with the point 60 being the final stopping point The torque at the point 60 is unreliable for the same reasons that the torque reading at the mid-point stop 32 is unreliable Accordingly, the torque value Tf is taken as the peak within one or two ha increments backward from the point 60, such as at the point 76 The effect of this graphically, is shown by the horizontal line 78 terminating on the torque slope TR 2 at 40 the point 80 and the vertical line 82 terminating at the point 84 on the tension slope FR 2.

Thus, the final tension value Ffinal is the calculated tension at the point 84.

It is also desirable to calculate and store the final tension appearing in a fastener, the tightening of which is terminated normally, i e in response to torque and/or angle rather than yield When using a torque approach, equation ( 48) gives the value for Ffinai regardless 45 of whether yield has occurred or not When using an angle approach, the final achieved tension value may be calculated from:

Ffinal =FD r FR 1 (a final factual) ( 49) 50 where aact,,tl is the actual angle increment between the mid-point stop 32 and the final stopping point.

It may also be desirable to calculate and store final tension appearing in a fastener in other unusual circumstances, such as when the tool stalls Tool stall may occur before the mid-point stop 32 or after the mid-point stop 32 Before the mid-point stop 32, 55 Ffinal =FO ( 50) After the mid-point stop 32, the final achieved tension value Ffinal may be calculated, using a torque approach, as: 60 Ffinal =FO + FR, (Tf) ( 51) 1 591 352 where TS is the calculated torque at the mid-point stop by equation ( 26).

After the mid-point stop 32, the final desired tension value Ffina, may be calculated, using an angle approach, as; for instance Ffinal = FO + r FR, (lactual, X > 0 ( 52) 5 where aactual is the actual measured angle from the mid-point stop 32 to the final stopping point.

In the event the tool continues to run far beyond any reasonable angle of advance, the conclusion is that the tool was never coupled to the fastener or that the bolt has failed 10 without yield detection, as may occur before the mid-point stop 32 Thus, no appreciable tension appears in the bolt and Ffinal O ( 53) 15 Another approach is to normally terminate tightening in response to one parameter, e g.

torque, and check this shut off parameter against another shut off parameter, e g angle If the results compare closely, this is an indication that the assumptions made, the empirically determined joint parameters and the like are reasonably correct If the comparisons are 20 significantly different, this is an indication that something is amiss and that the operation should be stopped or investigations instituted to determine the cause When using torque as the tightening parameter, FD has been placed in the calculations for the final torque value TD by equation ( 27) or ( 28) depending on whether X 3 0 or X< 0 The calculated value of final tension Ffina, using an angle approach at a final stopping angle of is: 25 Ffina=FD r FR, (Cfinal aactual) ( 54) where Cactual is the angle of advance from the mid-point stop 32 to the final stopping point.

If the difference between FD and Ffinal is small, e g 5-10 %, it is apparent that substantial 30 confidence may be placed in the technique If the difference between FD and Ffinal is larger, e.g 20 %, it is apparent that something is amiss and that the tightening operation should be stopped or investigations instituted to determine the cause.

In any circumstance where Ffinal is calculated, it may be desirable to compare it with the final desired tension value FD In this event, if 35 Ffinal FD S B FD 40 where B is a fraction deemed acceptable to the user, a suitable signal may be displayed to indicate that calculated tension is substantially below desired tension From present information, it appears that the magnitude of B should be greater than the expected scatter from use of this invention and preferably should be 3-4 normal deviations Thus, an exemplary value of B is 17 It will be seen that only when B is negative is there a difficulty 45 with the joint because if Ffinal is too high and the bolt has not yielded, there is obviously nothing wrong with the joint in a normal situation One very useful advantage of this technique is to detect cross-threaded fasteners.

A feature of this invention resides in the use of the tool overrun prediction to indicate tool malfunction Although the tool overrun at the termination of the tightening may be 50 used to determine tool malfunction, this operation more conveniently and accurately monitored during overrun adjacent the mid-point stop 32 Let 55Y = To + a, TR ( 56) Z = ( 57) 55 T 2 aor E = 100 y + 2 z -1 % ( 58) 1 z 60 It will be apparent that Y is a dimensionless number and basically is the ratio of T 4/T O As shown in Figure 2, T 4 is the existing torque value at the mid-point shut off command location 30 while To is the normal stall torque It will be seen from Figure 3 that Y is an inverse function of tool speed If the time delay between the giving of the shut off command 65 17 ' 18 1 591 352 18 and the closing of the valve remains constant, Y is a prediction of tool overrun Since oc is the measured tool overrun, it will be seen that Z is a function of measured tool, overrun while ar is the normal angular overrun of the tool under no torque conditions E will be recognized as a percentage change in tool and control performance.

If E is low, for example < 10 %, the deduction is that actual stall torque has decreased 5 significantly, such as from a loss or decline in air pressure, lack of lubrication, worn or broken parts, or the like In such an event, a signal may be displayed at the tool location to indicate that the tool requires inspection, maintenance, repair or replacement It is conceivable, but quite unlikely, that a significant decrease in E could be caused by a decrease in the time delay between the shut off command and the air valve closing 10 If E is positive, i e greater than zero, complications arise It appears that Z, which is a simplification of a more complex equation, loses accuracy The more complex equation indicates that if E is positive, Z should be reevaluated as oa 15 Z 1 =a( 59) 1 aor Accordingly, E should be reevaluated for greater accuracy, when positive, as:

20 El = 100 11 1 % ( 60) If E,, is high for example > + 10 %, the deduction is that the time delay between the shut 25 off command and the air valve closing has increased significantly or that air pressure supplied to the tool has increased This normally indicates that the valve control solenoid is beginning to stick or that air pressure is too high In such event, a signal may be displayed at the tool location to indicate that the air control system requires inspection, maintenance, repair or replacement It is conceivable, but quite unlikely, that a significant increase in E 1 30 could be caused by increased tool efficiency.

As will be apparent to those skilled in the art, the prediction of tool overrun embodied in equation ( 56) does not include a measure of overrun based on inertia, but instead is based solely on time delay As mentioned previously, inertial overrun is rather insignificant with moderate to high torque rate fasteners although accuracy can be improved somewhat for 35 low tension rate fasteners In the event that is desirable, a measure of inertial overrun can be incorporated into equation ( 58) through one or both of equations ( 56) or ( 57).

It is apparent that a single indication of tool malfunction is probably not significant but that an abnormal frequency of tool malfunction is significant Thus, a running ratio of 40 CTL/CJ 3 C ( 61) is maintained where CTL is the number of times that Es -10 %, Cj is the number of joints tightened and C is a fraction acceptable to the user As with the ratio Cy/CJ, of equation ( 47) a running ratio is preferable to a cumulative ratio From present information, it 45 appears that C should be in the range of 1 2, for example 15.

Similarly, a running ratio of CTC/CJ D ( 62) 50 is maintained where CTC is the number of times that E B + 10 % and D is a fraction acceptable to the user, for example, 15.

Another approach for predicting tool overrun and thereby detecting tool malfunction is pointed out by:

55 oep = ( 1 T 4) aw ( 63) Tor where ba is the predicted tool overrun from the shut off command point 30 where the 60 torque value T 4 appears The measured value of overrun from the point 30, which has been previously designed 8 a, can be compared against &i%, as follows:

19 1 591 352 19 F < oa 3 G ( 64) op where F and G are values acceptable to the user, such as 85 and 1 15 respectively When 5 measured overrun ba is too small, this indicates a motor malfunction while if ba is too small, this indicates a motor malfunction while if ba is too large, it indicates a control system malfunction.

When tightening seriatim a multiplicity of fasteners comprising part of a single joint using a conventional technique, it is well known that the first tightened fasteners will lose at least 10 some tension by the time the last fasteners are tightened This is, of course, related to joint relaxation and alignment of the joint parts In accordance with this invention, one powered instructable tool as disclosed more fully hereinafter may be used for each fastener of a joint and used in the following manner:

The tools for each fastener of a single joint are started simultaneously, when all of the 15 tools have stopped at the mid-point 32, all the tools are restarted simultaneously to accomplish the final advance In this manner, the alignment of all the fasteners and all joint relaxation occurs at the mid-point 32 Each tool would then compensate for any relaxation that may have occurred adjacent the fastener coupled thereto It will be apparent that the control mechanism for the tools would be interconnected electronically in a fashion that will 20 be apparent to those skilled in the art following the more complete description of the tool hereinafter.

Referring to Figure 8, there is illustrated a schematic showing of a mechanism 86 for performing the previously described technique The mechanism 86 includes an air tool 88 connected to an air supply 90 and comprising an air valve 92, an air motor 94 having an 25 output 96 coupled to the fastener pair comprising part of the joint 98, a torque transducer and an angle transducer 102 The torque transducer 100 is connected to a signal conditioner 104 of a data processing unit 106 by a suitable electrical lead 108.

The signal conditioner 104 is designed to receive electrical signals from the transducers 100,102 and modify the voltage and/or amperage thereof into a form acceptable to an 30 analog-to-digital converter 112 through a suitable connector 114 The converter 112 changes the signal received from the conditioner 104 into digital form for delivery to an interface logic unit 116 through a suitable connection 118 The angle transducer 102 is connected to the interface logic unit 116 by a suitable electrical lead 110.

The interface logic unit 116 comprises an interface logic section 120 designed to handle 35 information and is connected through suitable connections 122, 124 to a microprocessor unit 126 which is in turn connected to a data memory unit 128 and an instruction memory and program unit 130 through suitable connections 132, 134, 136, 138 The interface logic section 120 is also designed to receive input parameters such as T O s, FR 1, r, FD and the like.

The interface logic unit 116 also comprises an amplifier section 140 controlling a solenoid 40 (not shown) in the air valve 92 through a suitable electrical connection 142 The amplifier section 140 also controls a display panel 144 having suitable signal lights through an electrical connection 146 as will be more fully explained hereinafter.

The air tool 88 may be of any type desired but is conveniently a Rockwell model 63 W which has been modified to reduce the amount of overrun It has been surprising to learn 45 that the bulk of the tool overrun occurs between the time the shut off command is given through the electrical connection 142 and the time that high pressure air downstream of the valve 92 is exhausted through the motor 94 while the amount of overrun attributable to inertia of the air tool 88 is rather insignificant at high running torque values because tool speed is rather slow 50 The data processor 106 is shown in greater detail in Figure 9 and conveniently comprises a Rockwell microprocessor model PP 58 For a more complete description of the data processor 106, attention is directed to publications of Rockwell International pertaining thereto.

The data processor 106 comprises a chassis 147 having a power source 149 mounted 55 thereon along with the signal conditioner 104, the instruction memory and program unit 130, the data memory unit 128, the microprocessor unit 126, the interface logic section 120, the converter 112 and the logic interface amplifier section 140 The signal conditioner 104, the interface logic section 120, the microprocessor unit 126, and the data memory unit 128 are not modified in order to equip the data processor 106 to handle the calculations 60 heretofore described.

The instruction memory and program unit 126 is physically a part of the data processor 106 and is physically modified to the extent that a suitable program has been placed therein.

The interface logic and amplifier circuits 116 and 140, illustrated schematically in Figures 10 A and 10 B, serve to provide interfacing of data and control signals between the 65 1 591 352 microprocessor unit 126, a conventional teletype console (not shown), the torque and angle transducers 100, 102, and the air valve 92 controlling tool operation.

Interfacing between the teletype console and the microprocessor 126 is necessitated by the fact that the console receives and transmits data in a serial format while the microprocessor 126 receives and transmits in a parallel format The interface logic and 5 amplifier circuits 116, 140 include a universal asynchronous receiver transmitter circuit 148 which receives input data, such as a desired torque value FD, from the teletype console over the lines 150 in a serial or one bit at a time format, temporarily stores the data, and then transmits the data in parallel format over the lines 152 to the microprocessor 126 Thus a teletype console or other suitable means may provide an input 154 (Figure 8) for variable 10 empirical paramets, desired bolt tension and the like Likewise, data from the microprocessor 126, which is to be printed out by the teletype console, is converted from the parallel format in which it is received from the microprocessor 126 over the lines 152 into the serial format for reception by the teletype console.

Timing pulses for the control of the universal asynchronous receiver transmitter 148 as 15 well as other components of the interface logic and amlifier circuits are provided from the microprocessor 126 over line 156, the pulse train being supplied to a conventional divider circuit 158 to produce a timing signal on the line 160 which is a pulse train of lesser but proportional rate to that supplied by the processor 126 Timing pulses are also provided to other components of the interface logic and amplifier circuit over the line 162 The 20 microprocessor 126 also provides signals over the lines 164 which signals are generated in response to the program to control the transmission of data to and from the microprocessor 126 Thus, for example, when the microprocessor 126 is in condition to input data, such as the final desired torque value TD, a signal is transmitted from the microprocessor 126 over the lines 164 to a gating circuit 166 to furnish control inputs at 168, 170 to the universal 25 asynchronous receiver transmitter 148 Control and status indication signals for the teletype console are also provided over the lines 172 and, via signal conditioner circuits 174, over the lines 176.

Figure 10 B schematically illustrates that portion of the circuit which provides interfacing between the microprocessor 126, the torque and angle transducers 100,102 and the air valve 30 92 Torque data from the torque transducer 100 (Figure 8) is converted by the analog to digital converter 112 into twelve digit binary signals transmitted on the lines 118 The particular microprocessor employed is, however, only capable of receiving an eight digit input In order to permit transmission of torque data to the processor, a multiplexing arrangement is provided Thus, the twelve digit output of the analog to digital converter 112 35 is sup plied, through logic level buffers 178,180 to a pair of steering gates 182, 184, the first four digits being supplied to the first inputs a of the gate 182 while the second four digits are supplied to the corresponding first inputs a of the gate 184 The final four digits are supplied to the second inputs b of the gate 182 The corresponding second inputs b of the gate 184 are connected to ground, supplying a constant zero input The eight line output 186 of the 40 steering gates 182, 184 provide the torque data input to the microprocessor 126 The gates 182, 184 are controlled by signals on the lines 188, 190 to first pass the a input signals, i e.

the first eight bits of the torque signal, to the output lines 186 followed by the b input signals, i e the final four bits and four zeros In addition to being supplied to the steering gates 182,184, the torque data transmitted on lines 118 is also temporarily stored in the 45 registers 192, 194, 196 These registers 192, 194, 196 normally store the current torque value received from the analog to digital converter 112 A hold signal furnished by the microprocessor 126 over the line 198 actuates a latching circuit 200 to temporarily freeze the registers 192, 194, 196 permitting the torque value stored therein to be read over the lines 202 This arrangement permits reading of the torque data into the microprocessor 126 while 50 updated torque data is being supplied from the analog to digital converter 112 without the danger of inadvertently reading into storage a data value which is a mixture of old and updated values.

The analog to digital converter 112 supplies an end of conversion signal over line 204which signal is supplied to the latching circuit 200 over the line 206 to reset the circuit 200 55 when transmission of a torque value has ended permitting updating of the registers 192, 194, 196 It should be noted that the analog to digital converter 112 is under the control of the microprocessor 126 Thus the microprocessor 126 provides an enable signal over the line 208 and a convert signal over the line 210 to a gate 212 which also received, over a line 214, a tool rotation indicating signal, the origin of which will be described below It will be 60 understood that the enable and convert signals on lines 208,210 are generated in response to the program controlling the microprocessor 126 The output of the gate 212 provides a start conversion signal to the analog to digital converter 112 over the line 216.

As mentioned previously, the steering gates 182, 184 receive control signals over the lines 188, 190 These control signals are generated by a pair of gating circuits 218, 220 The gating 65 1 591 352 circuit 218 is responsive to the end of conversion signal from the analog to digital converter 112 on the line 204 and an enable signal on the line 222 which signal is derived from the enable signal supplied by the microprocessor 126 over the line 208 The gating circuit 218 provides an input to the gating circuit 220 which also receives a signal over the line 224 from the microprocessor 126 in the form of a response back signal indicating that the previous 5 data has been loaded into the microprocessor memory In addition to controlling the steering gates 182, 184, the gating circuit 220 furnishes a data ready signal on the line 226 to the microprocessor J 26 A further input 228 is provided for the logic gating circuit 218 The function of this input is to supply an event marker to memory.

The circuitry of Figure 10 B also provides interfacing between the angle transducer 102 10 and the microprocessor 126 The output signals of the angle transducer 102, in the form of sine and cosine signals over the line 110 are supplied to a converting circuit 230 which, in response to the transducer signals, generates an output pulse for each degree of rotation of the tool This pulse signal on the line 232 provides the tool rotation indicating signal on the line 214 and also provides an input to a gating circuit over a line 236 The gating circuit 234 15 also receives an input signal from the microprocessor 126 over the line 238 This latter signal is present during the tool on period and goes off simultaneously with the tool off signal The output 240 of the gating circuit 234 provides an input to the microprocessor 126 in the form of a pulse train with one pulse for each degree of tool rotation The portion of this signal occurring after the input signal on the line 238 has been removed is a measure of the degree 20 of tool overrun.

Also included in the interface logic and amplifier circuits is a reset circuit 242 connected at 244 to a reset switch and providing output signals 246, 248 which serve to reset various of the circuit components when the system is turned on -Signal conditioner circuits are also provided, the circuits 250 providing interfacing between the microprocessor 126 and 25 external controls for reset, gain, internal calibration and external calibration while the circuit 252 serves to interface the tool on signal from the processor 126 over the line 254 with a solid state relay controlling the air valve 92, the output signal being provided over the line 256 A further circuit 258 is connected to a single pole double throw external switch serving as an emergency or panic switch The output 260 of the circuit 258 supplies an 30 interrupt signal to the microprocessor 126.

In Figures 10 A and 10 B, the number adjacent each resistor is the resistance in ohms,and all resistors except 33, 620 have 1/4 watt ratings The number adjacent each capacitor is the capacitance in microfarads The symbol "v' is used to designate that the particular lead is connected to a 5 volt buss through a resistor, e g of 1000 ohm capacity, to prevent damage 35 to the component The symbol "POR" is used to designate "power on reset" which means that power stays on about 1/2 second.

Although the computer program and the circuitry of the interface amplifier section 140, previously described, are designed to activate a conventional teletype console in order to enter different values for the empirically determined parameters and to obtain a printed 40 readout of certain calculated values such as the tension at the mid-point stop 32, it is apparent that the details thereof can be adapted to manipulate a display panel 144 as shown in Figure 11 The display panel 144 is preferably located within view of the tool operator and comprises a base section 262 supported in any suitable fashion having a first group of signal lights 264, 266, 268, 270 indicating features of the joint 98 The signal light 264 45 indicates that the final desired tension value FD has been reached or that the final calculated tension value Ffinal is within an acceptable range The signal light 266 indicates that the joint has experienced non-linear strain The signal light 268 indicates that the final calculated tension value Ffinal is in an unacceptable range With the lights 264, 266 lit, the deduction is that non-linear strain has occurred but that Ffina, is acceptable With the lights 266, 268 lit, 50 the deduction is that non-linear strain has occurred but that Ffinal is not acceptable The light 270 is energized when the fastener exhibits a low tension rate as pointed out by the ratio of T Ra/T Rb.

The display 144 also provides another group of lights 272, 274, 276 indicating quality control features The light 272 is normally energized when the frequency of non-linear 55 strain detection is minimal while the light 274 is energized when the frequency of non-linear strain detection is too high as pointed out in equation ( 45) The light 276 is energized when the final calculated tension Ffinal differs significantly from the final desired tension value FD as pointed out by equation ( 55).

The display 144 also comprises a third group of lights 278, 280, 282 indicating tool 60 operating features The light 278 indicates that the tool is functioning normally The light 280 is energized when the ratio ba/&iap is too small or when the frequency of low ratio values becomes significant Similarly, when the ratio of bet/ha, is too large, or when the frequency cf high ratio values becomes significant, the light 282 is energized.

N A typical fastener system for use with this invention may comprise 5/16 ", 24 threads/inch, 65 1 591 352 SAE grade 8 nuts and bolts With this fastener pair and the modified Rockwell 63 W air tool, the following values were found for the empirically determined parameters:

= 47 lb/degree 1 12 = 2900 lb FL = 1000 lb T 1 = 5 ft-lb = 4 ft-lb = 20 degrees ay = 12 Degrees = 9 degrees n To Nk R Ko = 14 = 54 ft-lb = 68 degrees = 0 80 = 0 93 = 21 ft-lb/degree = 3 degrees Using these parameters and 5/16 "-24, grade 8 fasteners, with a grip length of 2 44 ", and having a cadmium dichromate coating, the following data was developed using part of the technique here disclosed The stiffness of the load washer used to measure tension directly was a 5 x 106 lb/in and the clamped pieces were hardened steel In running the tests reported in the following table, the angle option was used and execution was within + 2 to 1 degrees, which corresponds to + 104 to 52 pounds in tension The overall instrumentation repeatability and linearity, including the tension probe and the torque transducer, is estimated at 4 %.

FR 1 r FM Tos oor TABLE II

LRM Set for 6,200 Ffinal, lb 6355 6179 6517 6356 6274 6147 6221 6205 6151 6742 6377 6706 6407 6103 6045 6030 5634 5891 6618 6381 6267 Observed + 7 8 deviation from -10 1 Avg % Tfinal, ft-lb 28.91 32.33 34.00 27.61 28.23 30.65 28.91 30.77 28.85 31.02 16.38 18.05 16.81 12.16 15.14 16.00 14.64 15.20 17.68 16.56 21.31 + 59 5 -42 9 Final Angle From ft-lb a T 1, deg 108 109 107 107 108 106 108 102 109 96 88 87 84 83 91 97.5 + 11 9 -28 2 One std.

deviation, % 4 3 of Avg.

Exact a Tl for 6,200 lb Tension 109 101 104 104 109 106 108 103 87 87 96 72 91 89 83 96.3 + 13 2 -25 2 10.6 8.4 on tension Exact Torque for 6,200 lb Tension 28.22 32.44 32.18 26.93 27.90 30.91 28.81 30.75 29.08 28.37 15.93 17.18 16.27 12.35 15.53 16.45 15.59 15.73 16.88 16.09 Condition As received As received As received As received As received As received As received As received As received As received Lubricated Lubricated Lubricated Lubricated Lubricated Lubricated Lubricated Lubricated Lubricated Lubricated with with with with with with with with with with 22.68 + 43 0 -45 5 31.9 Run No.

1 2 3 4 6 7 8 9 11 12 13 14 16 17 18 19 Average ,) t Oi SAE 10 oil SAE 10 oil SAE 10 oil SAE 10 oil SAE 10 oil SAE 10 oil SAE 10 oil SAE 10 oil SAE 10 oil SAE 10 oil The tension value reported in the second column was recorded approximately 15 seconds after the tool stopped This is believed to involve a relaxation in the joint amounting to 1-2 % of the recorded tension value.

A statistical analysis of the data gathered on the twenty fasteners reported in Table II shows that the partial technique of this invention acts to control tension to within 11 1 % 5 of the desired value in 99 out of 100 cases, or within 2 58 standard deviations It should be thoroughly understood that the above data was taken with a program which does not include a number of features disclosed herein, including (I) the use of a second calculation of TR and aorigin; ( 2) the provision of yield detection and shut off in response thereto; and ( 3) the use of a curvature check of torque rate in the region where TR is calculated in order 10 to identify and reject low tension rate fasteners The effect of these additions to the program is, of course, somewhat speculative It is believed, however, that the inclusion thereof will reduce scatter still further.

With the same joint and tool, the use of a torque control method would have to produce an average final torque 22 68 ft-lbs to achieve an average final tension value of 6267 15 pounds The observed deviations from average is 43 0 to -45 5 % Thus the torque control method would have produced a tension scatter of 82 3 % of the desired value in 99 out of 100 cases, assuming that the bolts would have been capable of accepting any tension.

In reality, 10 4 % of the bolts would have ruptured, producing no tension at the termination of tightening Another 14 7 % of the bolts would terminate in the plastic zone, i e past the 20 yield point.

With the same joint and tool, the use of a turn-of-the-nut method would have to advance the nut 96 30 from a threshold torque of 5 ft-lbs to achieve an average final tension value of 6267 pounds The observed deviation is + 13 2 to -25 2 % Thus, a turn-ofthe-nut method would have produced a tension scatter of 21 7 % of the desired value in 99 out of 100 25 cases It is interesting to note that the selection of 6200 pound tension for a bolt having an elastic limit of 6950 pounds appears to be optimum because only about 6 % of these bolts would end up in the plastic zone.

Referring to Figure 12, there is illustrated another device 298 for implementing the technique of this invention The basis of this approach is equation ( 7) where the value of 30 d F/da indicates the tension rate Rewriting equation ( 7), d lot T = d F/da ( 65) da F 35 If d log T can in some fashion be determined, F in equation ( 65) can become the final desired tension value FD or the tension value Fso at the point of shut off command while d F/da is an empirically determined tension rate FR 3 which is an appropriate average FR 1 40 and FR 2 over the angle interval in question It will be apparent that d d log T -dolog T =-t 45 da dt ( 66) As suggested in Figure 12, the analog device 298 includes an angular speed pickup 300 of any suitable type, such as a tachometer, for continuously sensing a value for da/dt, which is 50 the speed the fastener is being tightened.

A torque transducer 302 continuously senses the value of running torque T The transducer 302 may be of the same type as the transducer 100 A logarithmic amplifier 304, such as is available from Analog Devices, Inc, Norwood, Massachusetts, under the designation of Logarithmic Amplifier, Model 755, is connected to the torque transducer 302 55 by a suitable connection 306 The logarithmic amplifier 304 continuously converts the sensed value of running torque T into a continuous signal representative of log T.

A time differentiating device 308 is connected to the logarithmic amplifier 304 by a suitable lead 310 and continuously differentiates the signal from the logarithmic amplifier with respect to time in order to obtain the differential of the logarithm of running torque 60 d d -log T.

1 591 352 1 591 352 The time differentiating device 308 may be of any suitable type, such as an operational amplifier 312 in parallel with a capacitor 314 A suitable operational amplifier is available from Analog Devices, Inc, Norwood, Massachusetts, under the designation Operational Amplifier, Model 741.

The signal from the time differentiating device 308 is delivered through a lead 316 to a 5 low pass filter 318 which acts to smooth out the signal from the time differentiating device 308 thereby removing some of the noise inherent in the torque signal from the transducer 302.

The angular speed pickup 300 and the low pass filter 318 are connected by suitable leads 320,322 to an analog divide device 324 such as may be obtained from analog Devices, Inc, 10 Norwood, Massachusetts under the designation Divide Module 436 B The leads 320, 322 are connected to the divide device to produce an output signal along a lead 326 consisting of the ratio d 15 dt g As dax det 20 indicated in equation ( 66), this signal is representative of d log T.

da 25 When the value of d log T: FR 3 when Th TI ( 67) 30 det Fs ' where F,0 is the tension value in the bolt at the time of shut off, and T 1 is threshold torque, e.g about 20 % of average final torque, the tool is commanded to shut off It will be evident that the threshold may be measured in terms of angle, where Gx>c, rather than torque 35 Because the tool will overrun after shut off, the value of F,, is selected so that average tool overrun after shut off, the value of F,0 is selected so that average tool overrun advances the fasteners to the final desired tension value FD The average tool overrun may be determined empirically or from 40 AFSO = (d-t)s (At) FR 3 ( 68) where ( d-a 45 is the average speed of the tool at shut off, A Fsc, is the average additional tension due to overrun, and At is the time delay between the giving of the shut off command and the closing of the air valve Thus, 50 Fs = FD AF,0 ( 69) Because Fs, and FR 3 are assumed to be a constant, the ratio of FR 3/F 10 is obviously constant Thus, a constant signal representative of FR 3/F 1 S is placed on a lead 328 The 55 leads 326, 328 are connected to another divide device 330 When the output signal from the divide device on a lead 332 becomes unity, an amplifier 334 is triggered to energize a solenoid catch 336 to allow the solenoid spring (not shown) to close the air valve.

Although the analog device 298 of Figure 12 is not believed to have the accuracy of the digital device 86, it is apparent that it has the advantage of simplicity, both physical and 60 operational The analog device 298 operates closer to the theoretical basis of the invention and contains fewer assumptions and simplifications Some of the disadvantages of a simple analog device, such as the inability to vary the overrun prediction and the noise reduction in the filter 318, are capable of being surmounted by more sophisticated analog techniques as will be apparent to those skilled in the art 65 26 1 591 352 26 As will be apparent to those skilled in the art, the technique of this invention can be used to monitor other tightening strategies thereby determining the accuracy thereof in tightening fasteners to a final desired tension value This may readily be accomplished by modifying the amplifier section 144 in order not to manipulate the air valve solenoid in response to the tightening parameter 5 The present application has been divided out of out co-pending Application No 28244/77 (Serial No 1591351) in which there is described and claimed apparatus which tightens seriatim a multiplicity of substantially identical joints including at least one threaded fastener pair to substantially the same final desired stress value, below the yield point appearing in the fastener pairs, the apparatus comprising: a powered instructable tool, 10 capable of terminating tightening in response to an instruction which varies from one fastener pair to the next, which tightens the fastener pairs; microprocessor means which includes means which determine, while tightening below the yield point, a tightening parameter value, variable from one fastener pair to the next, sufficient to tighten each fastener pair to the final desired stress value; and means which instruct the tool to terminate 15 tightening of each fastener pair in response to the determined tightening parameter value.

In our co-pending application No 2822/80 (Serial No 1591353), there is described and claimed apparatus which simultaneously tightens threaded fastener pairs of a multiplicity of joints tightened seriatim, the apparatus comprising: a powered instructable tool, of the type which continuously delivers torque when the tool is on, which temporarily suspends and 20 then resumes tightening of the fastener pairs; means which instructs the tool to tighten each fastener pair; means which calculates the existence of stress in each fastener pair of at least 4 yield strength and below the yield point; means which then suspends threading movement temporarily thereby allowing the joint to relax; and means which instruct the tool to resume tightening after the joint relaxation 25 In our co-pending application No 2823/80 (Serial No 1591354), there is described and claimed apparatus which tightens a threaded fastener pair, comprising:

means which apply torque to the fastener pair, means which sense applied torque at various angles of threading advance, means which temporarily suspend tightening of the fastener pair to enable the fastener to 30 relax, means which resume tightening of the fastener pair after relaxation, means which calculate the torque applied to the fastener pair at an angle location adjacent to the angle location where tightening was temporarily suspended, and means which subsequently terminate tightening at a calculated final tightening angle increment 35 measured from the said adjacent angle location at which location the sensed torque is at least equal to the calculated torque.

In our co-pending application No 2824/80 (Serial No 1591355), there is described and claimed apparatus which tightens a threaded fastener at least partially into a region of increasing fastener stress where the torque rate, as herein defined, exhibited by the fastener 40 is expected to be linear, comprising means which apply torque to the threaded fastener, means which stop the torque applying means in the region of increasing fastener stress, and means, operating independently of a decision to terminate tightening, which conduct a quality control procedure after the termination of tightening which monitors the curvature of the torque rate to thereby enable detection of defective fasteners as indicated by an 45 upwardly concave torque rate.

Claims (1)

1. WHAT WE CLAIM IS:-

   1 Apparatus which tightens threaded fasteners, comprising a powered tool which imparts rotation to each fastener thereby tightening said fasteners; means which turns the tool off after the tool is started; control means which delivers a shutoff signal to the turn-off 50 means; calculating means which predict the amount of fastener rotation, variable from one fastener to the next, occurring after the shut-off signal is generated; and means which utilize the predicted amount of fastener rotation occurring after the shut-off signal is generated so that each fastener is tightened to a desired amount of tension.

   2 The apparatus of claim 1 wherein a tightening parameter is established and wherein 55 the turn-off means comprises means for terminating tightening in response to a function of the tightening parameter and said predicted amount of fastener rotation.

   3 The apparatus of claim 2 wherein the turn-off means comprises means terminating tightening in response to the difference between the tightening parameter and said predicted amount of fastener rotation 60 4 The apparatus of claim 1 wherein the calculating means comprises means for suspending tightening and measuring the amount of fastener rotation following suspension of tightening; means causing resumption of tightening; and means calculating the amount of fastener rotation at the termination of tightening as a function of measured fastener rotation at the suspension of tightening 65 1 591 352 The apparatus of claim 1 further comprising means determining, during tightening, the torque rate of the fastener and wherein the calculating means comprises means which predict the amount of fastener rotation following the shut-off signal from the determined torque rate.

   6 The apparatus of claim 1 wherein the tool exhibits a predetermined stall torque and 5 the calculating means comprises means for predicting the amount of fastener rotation following the shut-off signal as a function of stall torque.

   7 The apparatus of claim 1 wherein the tool exhibits a predetermined idle speed and the calculating means comprises means for predicting the amount of fastener rotation following the shut-off signal as a function of idle speed 10 8 The apparatus of claim 1 further comprises means for measuring the amount of fastener rotation following the shut-off signal and wherein the utilizing means comprises means for comparing the measured fastener rotation with said predicted fastener rotation.

   9 The apparatus of claim 8 wherein the utilizing means further comprises means for signalling in the event that the measured fastener rotation following the shut-off signal, 15 diverges significantly from the predicted fastener rotation.

   A method of tightening threaded fasteners, comprising imparting rotation to each fastener with a powered tool to thereby tighten said fasteners; turningoff said tool after the tool is started by means of a shut-off signal delivered by a control means; calculating a predicted amount of fastener rotation, variable from one fastener to the next, occurring 20 after the shut-off signal is generated; and utilizing the predicted amount of fastener rotation so that each fastener is tightened to a desired amount of tension.

   11 The method of claim 10 further comprising establishing a tightening parameter and wherein the turn-off step comprises terminating in response to a function of the tightening parameter and predicted tool overrun 25 12 The method of claim 11 wherein the turn-off step comprises terminating tightening in response to the difference between the tightening parameter and predicted fastener rotation following the shut-off signal.

   13 The method of claim 10 wherein the calculating step comprises suspending tightening and measuring the amount of fastener rotation following suspension of 30 tightening; resuming tightening; and predicting the amount of fastener rotation at the termination of tightening as a function of measured fastener rotation at the suspension of tightening.

   14 The method of claim 10 further comprising determining, during tightening, the torque rate of the fastener and wherein the predicting step comprises predicting the amount 35 of fastener rotation, following the shut-off signal, from the determined torque rate.

   The method of claim 10 wherein the tool exhibits a predetermined stall torque and the predicting step comprises predicting the amount of fastener rotation, following the shut-off signal, as a function of stall torque.

   16 The method of claim 10, wherein the tool exhibits a predetermined idle speed and 40 the predicting step comprises predicting the amount of fastener rotation, following the shut-off signal, as a function of idle speed.

   17 The method of claim 10 wherein the utilizing step comprises measuring the amount of fastener rotation following the shut-off signal and comparing measured fastener rotation with predicted fastener rotation 45 18 The method of claim 17 wherein the utilizing step further comprises signalling in the event that measured fastener rotation diverges significantly from predicted fastener rotation.

   POTTS, KERR, & CO, 50 Chartered Patent Agents, 27, Sheet Street, Windsor, Berkshire SL 4 1 BY.

   and 15, Hamilton Square, 55 Birkenhead, Merseyside L 41 6 BR.

   Printed for Her Majesty's Stationery Office, by Croydon Printing Company Limited, Croydon, Surrey, 1981.

   Published by The Patent Office, 25 Southampton Buildings London, WC 2 A IAY, from which copies may be obtained.