CA1054826A - Apparatus for and method of determining rotational or linear stiffness - Google Patents

Abstract

A B S T R A C T
A meter for determining stiffness or torque gradient including, in one embodiment, a deflectable coupling connected between an output shaft and a power input shaft, sensing means for developing two separate series of signals representative of the displacement of the input and output shafts respectively, a comparator for determining from the two series of signals a function of the stiffness or torque gradient and, in another embodiment including a single shaft having an input end and an output end, transducer means associated with the shaft for developing a signal representative of the torque being applied through the shaft, sensing means for developing a signal re-presentative of the displacement of the shaft, and comparator means for determining from the two signals a function of the stiff-ness or torque gradient. Either embodiment of the stiffness meter may be used for tightening a fastener by including an additional circuit responsive to the comparator output for sequentially storing information representative of the maximum stiffness developed up to any given point during the tightening operation and for producing a stop signal when the instantaneous stiffness has dropped to a predetermined ratio of the maximum stiffness.

Description

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The invention relates to apparatus for and a method of determining rotational or linear stiffness and is particularly, but not exclusively, concerned with the tightening of screw~
threaded or other axially-loaded fasteners. This application is concerned with improvements in the invention described in , our copending Canadian Patent Application No. 170,642 filed May 8, 1973.
The precise clamping load of a fastener is extremely important in determining whether or not a joint including the fastener will fail in service. It is therefore desirable that fasteners should be tightened to a consistent preload. An object of the present invention is to provide apparatus for and a method of tightening a screw-threaded fastener to a predetermined preload, for example, the maximum preload attainable without plastic deformation of the fastener or joint.
Another object of the invention is to provide an improved stiffness meter or torque gradient meter not only for use in tightening fasteners but for other applications where stiffness or torque gradient is required to be determined.
According to one broad aspect, the invention relates to apparatus for determining torque gradient comprising means for developing a signal representative of the instantaneous torque applied to a rotary member, storage means including a series of sample and hold means for storing a series of signals representative of instantaneous torque and gradient register means for sequentially subtracting a stored signal frsm an instantaneous torque signal and developing a signal representative of the torque gradient.
Features of the invention will be described with reference to the accompanying drawings, in which:-Figure 1 is a curve of torque plotted against angle of ~ - 2 -1~548'~ti turning for a screw-threaded fastener in a typical application, Figure 2 is an elevation of the first form of the apparatus;
Figure 3 is a diagram illustrating a logic device to be used in conjunction with the apparatus shown in Figure 2;
Figure 4 is a particular part of the logic device of Figure 2 shcwn in greater detail;
Figure 5 is a derivative curve obtained for Figure 1, that is the curve of ~ plotted against ~2;
Figure 6 is an alternative form of the apparatus shown in elevation and part section, Figure 7 is another alternative form of the apparatus including the logic device; and Figure 8 i8 a section view taken along the line 8-8 of Figure 7.
The torque required to tighten a screw-threaded fastener is a function of several variables, namely the joint stiffness,- the fastener stiffness, the surface friction and the thread form. The general characteristic showing the relationship between torque and angle of turning of a fastener is shown by the curve in Figure 1 of the accompanying drawings.
The maximum gradient of the curve at a point A is a function of the variables set out hereinbefore. The gradient can vary widely even with a fastener of given diameter and thread configuration. The clamping load at which the gradient begins to fall below the maximum value has been found to be relatively independent of friction and joint stiffness and to be primarily dependent upon the yield

- 2~ -11~548'~
strength of the fastener and/or the joint. The point indicated at X in Fig. 1 represents a point on the torque/angle of turning curve to which the fastener is tightened to a consistent tensile stress close to its yield stress. Further rotation of the fastener would cause the torque to approach a maximum value and the tensile stress in the fastener to approach its ultimate value. Depending on the ductility and dimensions of the fastener and/or the joint, fracture would finally occur at a point representative at Y.

The general objective in controlling the tightening of a screw-threaded fastener is to achieve a consistent joint preload close to the maximum that the fastener can apply without yielding commencing. One common method is to use a torque control by which a specific maximum torque is applied in an attempt to attain a desired preload for particular thread and frictional conditions. This method has the disadvantage that as the exact frictional conditions are not known under practical fastener assembly conditions, there will be variations in the torque/tension relationship. This leads to varying tensile loads in the fastener for a given applied torque.
Another known method which is not dependent upon frictional conditions involves measuring the elongation of the fastener.
In most cases direct measurement of elongation is impossible and therefore a modificat~on of the method employs angle-controlled tightening in which an estimated elongation is effected by tightening to a precise angle of tightening.
The disadvantage of this method is determining where to start measuring angles when operating on torque angle of turning curVes which ha~e different initial and maximum slopes. For preloads in th~ elastic range this is extremely difficult as the deviation from a typical curve may be large.

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In the plastic range of the fastener, however, an error in angle does not change the preload appreciably and the deviation is small; but for very short fasteners this angle ~rror may cause overstrain and consequent fracture. Angle controlled tightening is therefore only acceptable in the plastic region for long fasteners (for example those having more than six free threads) where some plastic deformation will not cause structural damage in the fastener.

It is therefore desirable that the opt~mum point at which tightening is to cease is the point where the gradient of the torque/angle of turning curve has just started to fall from its value in the generally linear region or its maximum value, that is the point at which the fastener is just begin-ning to yield. Taking X in Fig. 1 as a point where the gradient is a predetermilled ratio of the gradient in the generally line~r region or ma~imum achieved for the fastener, this point determines a practical position at which tightening should be stopped to give clamping loads which are essentially independent of the fastener ge~metry and the conditions of friction. A further object of the present invention is to provide a method of an apparatus- for enabling the point X to be identified during a tighten~ng operation as soon as it occurs so that the tightening process can be immediately stopped at that point and furthermore to enable the point to be identified independently of the joint or fastener characteristics, that is without foreknowledge of the joint or fastener characteris-tics or calibration.

According to the lnvention, apparatus for deter-mining rotational or linear stiffness, i.e., a stiffness meter, compris-es means for developing a changing si~nal representative of the instantaneous torque appl~ed to a rotary member, storage means for storlng a series of signals representative of instan-lOS~
taneous torque and gradient register means for sequentiallycomparing a stored signal with an instantaneous torque signal ancl developing a signal representative of the torque gradient.

The invention also provides a stiffness meter for determining the yield point or similarly significant change in slope of a torque-rotation curve, the meter comprising means for developing a signal representative of the gradient of the torque-rotation curve; means responsi~e to said gradient signal for storing an information signal representative of the gradient developed in the generally linear portion of the torque-rotation curve; and comparator means for comparing said gradient signal and said information signal and developing a control signal when said gradient has changed to a predetermined rela,tionsh,ip with said information sign~l.

The sti,ffness meter may also include means for determining the largest gradient measured and the information signal is then representati~e of the largest gradient measured up to any point along the torque-rotation curve. Preferably, the control signal is developed when the gradient signal is about 50% of th.e information signal.

The invention also provides apparatus for tightening a fas-tener to the ~i.eld polnt or a similarly significant point including wrench, ~e.ans for applying torque and rotating the fastener; means for developing a signal representatiVe of the instantaneous gradient of the torque-rotation curve t~rough which the fastener is being tightened;

and, means responslve to the gradient signal for determining ~s~
the yield point or other similarly significant point on the torque-rotation curve through which the fastener is being tightened and for developing a control signal when the fastener is tightened to the determined point.
The fastener tightening apparatus determines the yield point or similarly significant point by storing an information signal representative of the gradient in the generally linear portion of the torque-rotation curve, preferably the largest gradient, and comparing the instantaneous gradient signal with the information signal and by developing the control signal when the compared signals have a predetermined relationship, preferably when the instantaneous gradient signal is 50% or less of the information signal.
~ ccording to certain embodiments of the inven-tion, apparatus for determining rotational or linear stiff-ness/ i.e., a stiffness meter, co~prises output means for imparting movement to a member; input means for receiving power; deflectable coupling means between said output means and said input means for transmitting power from said input means to said output means; first sensing means responsive to said input means for developing a first series of signals representative of the displacement of said input means;
second sensing-means responsive to said output means for developing a second series of signals representative of the displacement by said coupl~ng means OI said output means, and comparator means responsive to said fi`rst and said second series of signals for determining the difference of the number ~f signals in s-aid first and said second series of signals during a datum number of consecutive signals in said second series of signals as power is transmitted by said coupling means from sa~d input means to said output means, 11~54~

said difference being a function of the stiffness.

The stiffness meter may also include circuit means responsive to said comparator means for sequentially storing information representative of the maximum stiffness or torque gradient developed up to any given point during the driving of said member.

The stiffness meter may be such as to be capable of being used as a wrench, torque input to said input means being stopped when said difference reaches a predetermined ratio of said maximum gradient stored by said circuit means.

Each of said first and second sensing means is conveniently an incremental encoder mounted on the respective input or output means and capable of emitting a precise number of equally-spaced signals during a complete rotation of the respective input or output means.

Certain embodiments of the invention also provide apparatus, for tightening a fastener, comprising fastener engaging means for imparting tightening movement to a fastener;
power input means; deflectable coupling means between said fastener engaging means and said power input means for transmitting power to said fastener engaging means; first sensing means responsive to movement of said power input means for developing a first series of signals representative of the displacement of said power input means; second sensing means responsive to movement of said fastener engaging means for developing a second series of signals representative of the displacement by said coupling means of said fastener engaging means; comparator means responsive to said first and second signals for determining the difference of the number of signals in said first and second Zf~

series of signals during a datum number of consecutive signals in said second series of signals as power is transmitted by said cou.pling means from said power input means to said fastener engaging means, said difference being a function of the in-stantaneous stiffness; circuit means responsive to said com-parator means for sequentially storing information representative of the maximum stiffness developed during the tightening of the fastener and for producing a control signal when said instantaneous stiffness has dropped to a predetermined ratio of the maximum stiffness developed during the tightening of the fastener, and control means responsive to said control signal for controlling power input by said power input means.

~ pparatus in accordance with certain embodiments of the invention may particularly, but not exclusively, be used for tightening a screw-threaded fastener, in which case the apparatus comprises fastener engaging means for imparting rotary movement to a threaded fastener; rotary power input means;
torsionally deflectable coupling means between said fastener engaging means and said rotary power input means for transmitting rotary power to s-aid fastener engaging means, first sensing means responsive to movement of said rotary power input means for developing a first series of signals representative of the rotary displacement of said rotary power input means, second sensing means responsive to said fastener engaging means for developing a second series of signals representative of the dis-placement by- said coupling means of said fastener engaging means;
comparator means responsive to said first and second signals for determining the difference of the number of signals in said first and second series o~ signals as rotary power is transmitted by said coupling means from said rotary power input means to said ~054~2~
fastener engaging means, said difference being a function of the instantaneous torque gradient of the fastener; circuit means responsive to said comparator means for sequentially storing information representative of the maximum torque gradient develop-ed during the tightening of the fastener and for producing a control signal when said instantaneous torque gradient has dropped to a predetermined ratio of the maximum torque gradient developed during the tightening of the fastener, and control means responsive to said control s-ignal for controlling power input by said rotary power input means.

Certain embodiments of the invention may also provide a method of determining stiffness using deflectable coupling means for transmitting power from input means to output means or imparting movement to a member, the method consisting of the steps of developing a first series of signals representa-tive of the displacement of sa;d input means~ developing a second series of signals representative of the displacement by said coupling means of said output means, and comparing said first and second series of signals by counting th.e difference of the number of signals in said first and second series of signals during a datum number of consecutive signals in said second series of signals as power is transmitted by said coupling means from said input means to said output means, said difference being a function of the stiffness.

Certain embodiments of the invention may also provide a method of tightening a fastener by power input means driving fastener engaging means through deflectable coupling means, the method consisting of the steps of developing a first series of signals representative of the displacement of said power input means; a second series of signals representative of the displacement by said coupling means of said fastener engaging means;

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comparing said first and second series of signals by counting the difference of the number of signals in said first and second series of signals during a datum number of consecutive sig:nals in said second series of signals as power is transmitted by said coupling means from said power input means to said fastener engaging means, said difference being a function of the instantaneous stiffness of the coupling means, sequentially storing information representative of the maximum stiffness developed during the tightening of the fastener and producing a control signal controlling said power input means when said instantaneous stiffness has dropped to a predetermined ratio of the maximum stiffness developed during the tightening of the fastener.

Particularly, but not exclusively, the method according to certain embodiments of the invention ~ay be for tightening a screw-threaded fastener by rotary power input means driving fastener engaging means through torsionally deflectable coupling means, the method consisting of the steps of developing a first series of si-gnals representative of the rotary displace-ment of said rotary power input means; a second series of signals representative of the displacement by satd coupling means of said fastener engaging meansi comparing said first and second series of signals by counting the difference of the number of signals in said first and second series of signals duxing a datum number of consecuti-ve signals in said second series of signals as power is transmitted by said coupling means fxom said rotary power input means to said fastenex engaging means, said difference being a function of the instantaneous torque gradient of the fastener;
sequentially storing information representative of the maximum torque gradient developed ~uring the tightening of the fastener ~OS4t~Z6 and producing a control signal controlling said rotary power input means when said instantaneous torque gradient has dropped to a predetermined ratio of the maximum torque gradient de-veloped during the tightening of the fastener.
Desirably, the control signal may be produced when said instantaneous torque gradient has dropped to substantially 50~ of the maximum torque gradient developed during the tight-ening of the fastener.
The control signal would usually be employed to stop the power input means; but in some applications it may be used to maintain the power input means to hold the instantaneous torque gradient at the aforesaid predetermined ratio of the maximum torque gradient.
The theory involved in the method and apparatus pro-vided by the certain embodiments of the invention noted above is as follows:
Assuming that the coupling between the input and out-put means is or is equivalent to a torsionally-flexible shaft with a substantially linear characteristic having an incre-mental encoder mounted at each end thereof, if the torsionalstiffness of the shaft is K and ~1 and ~2 are the angular dis-placements of the two ends of the shaft, the torque or turning moment M transmitted by the shaft = k (~ 2 ) - If the end of the shaft having the angular displace-ment ~2 iS connected to the fastener to be tightened and the other end is attached to a motor or other driving means, the gradient of the torque/angle of turning characteristic being followed by the coupling, and therefore by the fastener, is given by:-1054~2~
dM K (~

This equation may be evaluated in two alternative ways; the first of ~hich is to expand the equation, as follows:

~d~l \
d~2 ( d~2 - 1) \~ ~

= K ' - ~ 2 where ~ 2 are the angular velocities of the two ends of the shaft.

If the times between successive signals or pulses from the increment~l shaft encoders are tl and t2 , then 1 0 t I cc 1 t 2 '~ --) 2 and thus, dM = K t2 - t ~2 tl The times tl and t2 can be measured electronically by means of the ~ignals or pulses produced by the incremental encoders and after the necessary calculations by the gradient-determining means, an output signal proportional to dM can d~2 be obtained.

Alternatively, the equation dM K (d~, _ l) may be evaluated by employing small incremental values, as ~ollows:

~M A~l Q~ 2 = K ~-- -~S4~Z~

If high resolution incremental encoders giving of the order of four to five thousand pulses per revolution are em]?lYed~ ~2 and ~1 are directly measurable by counting puLses. For example, if ~ 2 iS determined by counting say 100 pulses by the encoder at the fastener end of the torque-trans-mitting coupling, the gradient is directly determined by count-ing the number of additional pulses produced by the encoder at the other end of the coupling during the production of the said 100 pulses by the encoder at the fastener end. If, for exam-ple, 6 extra pulses are counted at the encoder at the torque input end of the coupling dM ~ 6 Thus the gradient determining means either has to determine ~ t 1 by being responsive to the time intervals between pulses produced by the two encoders or it has to de-termine ~ merely by counting the numbers of pulses produced by the two encoders or by measuring the number of extra pulses produced by the encoder at the torque input end while the encoder at the fastener end produces a given number of pulses, e.g., 100.
According to certain other embodiments of the inven-tion, a tightening apparatus is provided that includes a wrench for tightening a fastener, torque transducer means for develop-ing a changing signal representative of the instantaneous torque being applied to the fastener and sensing means develop-ing signals representative of fixed increments of rotation of the fastener. As in the certain embodiments discussed above, there is alsG provided shift register means receiving signals 1054~2~

representative of instantaneous torque being applied to the fastener and which is clocked by signals from the sensing means for sequentially feeding signals to gradient register means, in the form of comparator means, subtracting torque siynals outputted from the shift register means from the in-stantaneous signals outputted from the transducer means and for developing a signal representative of the instantaneous gradient of the torque-rotation curve which could be plotted for the particular fastener being tightened. Also as in the certain embodiments discussed above, the instantaneous gradient signal is compared with a gradient signal determined to be representative of the gradient of the torque-rotation curve in its generally linear portion and when the compared signals have a predetermined relationship, a control signal is de-veloped.
The transducer means may be in the form of a torque cell associated with the wrench to measure the reaction torque on the wrench and the sensing means includes a proximity probe which is operatively mounted adjacent the rotary vanes of the motor input for the wrench for developing signals correspond-ing to the passage of the vanes and the rotation of the fasten-er.
Turning now to Fig. 1 of the accompanying drawings, which as already stated is a typical Torque or Turning Moment versus Angle of Turning characteristic, the curve followed by the torque-transmitting coupling and hence by the fastener may be divided into three regions:-I. An initial pre-tightening region;
II. A tightening region, and III. A region of yield and subsequent failure of the fastener.
~ - 14 -lOS'~Z~
In region I the effect of burrs and irregularities in ~the threads of the fastener must be ignored and therefore the gradient-determining means of the torque-transmitting apparatus - 14a -~os~z~

must first of all determine or be informed that region I has been left and the fastener is being tightened in region II. Conven-ien1:1y entry into region II~is by a torque measurement derived frorn the encoder signals. Normally the gradient of the character-istic will be substantially constant in region Il, i.e., the curve wil:L approximate to a straight-line; but if the characteristic is curved in region II it will reach a typical maximum value A.
Accordingly, region II can be considered as the generally linear portion or region of the curve. When the fastener is tightened beyond region II, region III is reached, the gradient of the characteristic commences to decrease as the fastener begins to yield and ultimately the fastener would fail at point Y. The transition point X between regions II and III may be the point at which the gradient has been reduced by a certain proportion of the maximum gradient at A. It is desirable that a fastener should be tightened to point X and therefore the gradient-determining means must be capable of determing that region II has been reached and then of determining the instantaneous gradient during tightening and continuously comparing it with the gradient in region II, preferably, the maximum value A, to determine when the point X has been reached. The gradient-determining means may be any suitable logic circuit or circuits responsiYe to the signals received from the two encoders. When the gradient-determining means has detected that point X has been reached, it can issue a stop command so that the operator will stop the tightening of the fastener. Alternatively the stop command may be used to stop the driving motor or other means automatically.

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Referring I~Ow to the drawings, and specifically to Figure 2, a screw-threaded fastener is shown at 1, and this is engaged by a driver or other tool 2 for turning the fastener and having~
a shaft mounted for rotation in a bearing 3. The bearing 3 together with a companion bearing 4 supports a shaft 5 which is drivingly engaged with the tool 2 and carries a first incremental encoder 6. The apparatus also includes another bearing 7 supporting a shaft 8 carrying a second incremental encoder 9.
The shaft 8 is arranged to be driven by a torque-applying motor 10. The shafts 8 and 5 are interconnected by a helical spring 11. When the motor 10 is driven, it will rotate the shaft 8 and this in turn will rotate the helical spring 11 which will drive the shaft 5. The shaft 5 will turn the tool 2 which will tighten the fastener 1.
The encoders 6 and 9 may be of an optical, electro-magnetic or other kind capable of producing signals in association with fixed light-responsive or other stationary receiver devices (not shown) which will produce a series of signals effected at ~54~
precise intervals corresponding to the angular turning of the encoders or the angular velocities thereof and hence of the respective shafts 3 and 8. The frequency of the signals pro-duced by the respective encoders 6 and 9 determines their re-spective angular velocities or a function thereof. The inter-val!3 between the signals produced by the encoders 6 and 9 can be ~neasured, for example, by electronic means. From the time intervals between signals produced by the respective encoders or by counting the number of signals, as aforesaid, the gradient of the torque/angle of turning curve can be determined as explained herein and hence when the gradient has fallen to a value indicating that point X in Fig. 1 has been reached, the operator would stop the motor 10. The signals produced by the operation of the encoders 6 and 9 are fed into a logic de-vice which produces a "stop" signal informing the operator that the motor 10 should be stopped, or the "stop" signal could be used to stop the motor automatically. Alternatively, instead of the signal produced by the logic device being a "stop"
signal it may be used to hold the fastener under a substan-tially constant load, in which case the signal could be acontrolling signal which prevents additional input power being suppl~ed.
Referring now to Fig. 3, the logic device is illus-trated in block diagram form. Commencing at the top of the diagram, references 6 and 9 indicate the two encoders. The left-hand encoder, i.e., the one nearer to the fastener, issues pulses ~2 and the right-hand encoder, i.e., the one nearer to the motor 10, issues pulses ~1. The two streams of pulses are supplied to a circuit illustrated by block 20. This counts the number of pulses ~l and ~2 or determines the time intervals ~054t~2~
between the pulses of the respective streams of pulses or computes the gradient from measured angular velocities. A
signal from block 20 passes to block 21 which is a logic cir-cuit which decides whether region II has been reached. If the answer given by block 21 is affirmative, signals pass to block 22 which is a logic circuit which calculates the func-tion of the gradient by determining ~ 2 2 t as aforesaid. Output signals from block 22 then pass to block 23, This is another logic circuit which compares the gradient function with a stored maximum function previously determined by block 22 and stored in a circuit indicated by block 24. Block 23 gives affirmative or negative output sig-nals depending upon whether or not point X in Fig. 1 has been reached. If the output signal from block 23 is affirma-tive, this is the aforesaid stop command and tightening is immediately stopped, as aforesaid. If the output signal from block 23 is negative a signal is fed back to block 22 and the circuit thereof continues to continuously up-date the gradient function and feed an output signal to block 23.
Fig. 4 is a schematic block diagram showing the main co,,~ponents of the electronic circuit indicated by the block 22 in Fig. 3. As already stated block 22 determines the gradient ~1 ~ or aM . The electronic circuit shown in Fig. 4 calculates the expression ~ by measuring the number of ~, pulses received fr~m the input encoder 9 during a given value a~ 2 . The value of a~ 2 chosen is referred to as the chord length because the measurement is equivalent to taking the difference between the readings of torque (M) across a chord of substantially constant length which is continuously moved along the M_~ curve as indicated by a~ bl and a2 b2, in Fig.

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1 the projection of the chord length on the ~ axis is fixed by the characteristic or setting of a chord length shift register 27 which is operated each time it receives an ac-tuating pulse, as hereinafter explained.
Pulses (~1) received from the input encoder 9 at the motor end of the spring 11 are gated directly into a residue register 25 with due regard to the sense of rotation of the rotor. The function of the residue register 25 is to store ~1 pulses.
Pulses (~ 2 ) from the encoder 6 at the output or fastener end of the spring 11 are checked by a checking de-vice 28 to determine whether the fastener is rotating in the correct direction and are then used to decrement the residue register 25. If the encoder 6 at the fastener end were to rotate backwards the amount of backwards twist is counted and must be restored by an equal forward twist before any ~ 2 pulses are gated to the register 25. As soon as a ~ 2 pulse has de-cremented the residue register 25, the value shown thereby is examined. If it is greater than zero there have been more pulses than ~2 pulses and as a result a signal is passed through a gate 29 to both a gradient register 26 and the chord length shift register 27 and also to decrement the re-sidue register 25. The chord length shift register 27 is ac-tuated each time it receives a ~ 2 pulse passed by the checking device 28, as indicated by arrow 30, and if the shift register 27 also receives a plalse through the gate 29 it registers '1' at its input. If it does not receive a pulse through the gate 29 it registers '0'. Each time the output of the shift re-gister ~7 registers '1' it decrements the gradient register by one pulse. When the shift register output registers '0' it 1054~
it does not affect the gradient register. Thus each time the shift register is operated on by the production of a ~2 pulse in the correct sense, the gradient register either changes by one pulse or does not change depending upon whether there are positive "readings" by the residue register 25 and the output of the shift register 27. The chord length shift register 27 h~s a characteristic or a setting such that the gradient re-gister reading is taken over the effective chord length. The chord length must be sufficiently long to average out the effect of "noise", that is unwanted signals superimposed on the basic signals. An angle of twist of 1 corresponding say to one ~2 pulse is too small for this purpose. It has been found that 20 is a suitable value of angle of twist over which to measure the torque; but if readings were only taken say every 20, the Torque/Angle of Twist Curve could not be fol-lowed~ Therefore the chords, such as al bl and a2 b2 in Fig.
l are overlapped and measurement of ~M over a 20 chord are taken on each ~2 pulse that is about every 1 of twist. When the gradient register is operating normally, the indicated reading of the gradient register is ~M/~ for the fastener being tested. Signals from the gradient meter 26 are fed to the maximum gradient store, i.e., to block 24 and to the com-parator in block 23 (see Fig. 3).
The signal given by block 23 to stop the motor driving the fastener is produced when the instantaneous gra-dient has fallen to 50% of the maximum gradient attained, that is, the gradient in the generally linear region of the M-~ 2 curve. The reason for this is that the derivative of the M-~ 2 curve, that is the ~ ~ ~2 curve shown in Fig. 5, has a point of inflexion at or near to the 50% of maximum value as 1~354~
indicated at X in Fig. 5. This point is also the steepest part of the curve and thus the point where the curve passes most quickly through a superimposed "noise" curve. Still referring to Fig. 5, it can be seen that point X lies within a range of about 25~ to about 75~ of the maximum value and a pre!set relationship utilizing a value within this range could be utilized to stop the motor driving the fastener. The point X is therefore the point on the curve which will give maximum immunity to noise, i.e., spurious signals.
Because the apparatus, in its fastener-tightening form, is essentially a torque gradient or torsional stiffness meter and it can compare the instantaneous torque gradient with the torque gradient in the generally linear region of the M-~2 curve and, preferably with the maximum torque gradient, the limitations of existing systems for controlling the tightening of fasteners are overcome. These require relatively accurate pre-knowledge of the torque/angle characteristic, either by knowing the approximate value of the maximum gradient or by knowing the torque to provide a given clamping load or by knowing the angular rotation to ensure optimum clamping.
The apparatus described herein does not require pre-knowledge of a particular torque/angle of turning character-istic because the deviation of the instantaneous gradient from the gradient in the approximately linear region of the curve, preferably the maximum gradient, will be determined automati-cally and the tightening will be stopped automatically when the point X at which the instantaneous gradient is any preset fraction, e.g., 50%, of the maximum gradient has been reached.
It should also be clear that the instantaneous gradient may never exactly equal the present fraction and, therefore, 1054~
the apparatus should stop tightening when the instantaneous gradient is a preset fraction or less than that fraction of the maximum gradient. The invention, therefore, provides app,aratus and a method preferable to known tightening systems.
Using a technique analogous to that applied in the fastener-tightening form of the invention, the aforesaid torque gradient or torsional stiffness meter may be converted to a linear stiffness meter either by converting the output rota-tion into linear movement, for example with a rotary-to-linear converter such as a screw-thread or rack-and-plnion device.
Alternatively linear encoders and a linear spring, or other linearly resilient element, connected between shaft means carrying the encoders may be employed. In this way, the point of yield in a linear system can be determined in a similar way to the determination of the point of yield in the afore-said torsional system. Hence, for example, the yield point of tensile samples in a tension testing machine or a creep testing machine, can be identified without the measurement of gauge lengths and without the necessity to measure separately force and displacement.
Instead of using the helical spring 11 as the coup-ling means between the input and output shafts a spiral spring may be employed. Alternatively, the torque transmitting coupling may be a torsion bar or tube connected between the input and output shaft means or ha~ing end portions forming said input and output shaft means. The torsion bar or tube may be of metal, rubber, plastics or it may be a composite bar or tube formed from any of these materials.
Fig. 6 shows an alternative form of the apparatus which can be used instead of that shown in Fig. 2. In Fig.

~054~Z~
2, the spring 11 is rotated about its longitudinal axis in addition to being twisted as the fastener follows the Torque/
Angle of Turning Curve shown in Fig. 1. In some instances this may be undesirable and instead the apparatus shown in Fig. 6 could be employed in which a non-rotatable helical spring 31 transmits torque between an input shaft 32 driven by a motor 33 to an output shaft 34 by which a fastener or other member to be turned is arranged to be driven. The input shaft 32 is connected through a gear box 35 containing a gear drive, shown for diagrammatic purposes as a simple differen-tial gear train 36, to the output shaft 34. When the input shaft 32 is driving the output shaft 34 at the same speed, i.e., when the torque M is constant, there will be no bodily movement of the gear train 36; but when the input shaft starts to move faster than the output shaft, that is as in region II
in Fig. 1, the gear train will swing about the common axis of the shafts 32 and 34 and transmit the swinging movement to the housing of the gear box 35. One end of the spring 31 is attached to an end wall 37 of the gear box 35 which is mounted for swinging about the common axis of the shafts 32 and 34 on a bearing 42. The other end of the spring is attached to a fixed mounting plate 38 in which the output shaft 34 is freely rotatable in a bearing 39. The input shaft 32 carries an incremental encoder 40, equivalent to the encoder 9 in Fig.
2, and the output shaft 34 carries an incremental encoder 41, equivalent to the encoder 6 in Fig. 2. When the point X is approached and there is a change in the rate of differential rotation between the input and output shafts, the spring 31 will transmit torque in a similar manner to the spring 11 in Fig. 2; but the spring 31 does not rotate, it only twists and lOS4~Z~
therefore does not have to be dynamically balanced. The angular deviation between the shafts will be measured by the encoders 40 and 41 and their signals will be transmitted through and handled by the logic devices shown in Figs. 3 and 4 in the way already described. Another advantage of the arrangement shown in Fig. 6 is that the axial length of the apparatus can be reduced because the gear box 35 can be made short and housed within the spring 31 as shown. Also the en-coders may be positioned within the spring. Alternatively, the encoder 40 can be positioned at the input end of the gear box, as shown, and so a low resolution encoder can be used, thereby saving cost.
In either arrangement (i.e., Fig. 2 or Fig. 6) of the apparatus, the logic devices may include switch means to enable the apparatus to be used as a normal tightening device o~ wrench in whi~h the torque is transmitted from the motor to the fastener without indication of deviation from a linear Torque/Angle of Turning relationship and without automatic stopping of the driving motor.
Although in Fig. 2, a helical spring 11 has been used as the coupling means between the input and output shafts, that is the apparatus is employed as a wrench to which input torque is applied continuously during tightening, the apparatus could be used as an impact wrench, that is a wrench of the kind to which input torque is applied intermittently in steps, by using in place of the spring 11 a coupling means which has a non-resilient, substantially linear M-~ characteristic. The coupling means would in such a case be effectively a non-return type of spring to prevent recoil of the apparatus be-tween the periods during which input torque is applied.

- ~4 -lOS4~'~,ti Another embodiment of the apparatus is illustrated in Figs. 7 and 8 and includes certain means reducing the cost of the apparatus relative to the embodiment shown in Figs. 2-4. As shown in Fig. 7, the apparatus comprises a wrench 50 inc:Luding an air motor 52, the operation of which is controlled by a suitable solenoid valve 54, and which drives an output shaft 56 through a speed-reducing gear box 58 so that the out-put shaft does not rotate at the same high speed as the motor.
Output shaft 56 carries an adapter 57 for attachment with a driver bit 59 and is mounted in a suitable rotary bearing assembly 60 facilitating rotation of and taking up any bend-ing stresses in the output shaft. Bearing assembly 60 may be mounted on a rigid frame 62, but use of the frame is not necessary for the practice of the invention. At this point it should be noted that while motor 52 has been described as an air motor, it may be of any suitable type, for example elec-tric, hydraulic or any combination of pneumatic, electric or hydraulic. It should also be noted that the apparatus thus far described is generally conventional and need not be ex-plained in greater detail.
Located between gear box 58 and bearing assembly 60is transducer means in the form of a torque cell 64 which de-velops a signal representative of the instantaneous torque being applied to the fastener. Torque cell 64 includes a first mounting base 66 securing the cell to gear box ~8 and a second mounting base 68 securing it to bearing assembly 60. Extend-ing axially of the wrench between mounting bases 66 and 68 are a plurality of strut members 70 which are somewhat deform-able, that is, are relatively rigid members capable of twisting somewhat about the axis of the wrench. When wrench 50 is 1054~
operative to tighten a fastener, the reaction torque acting thereon causes strut member 70 to twist about the axis of the wrench, the amount of twist being proportional to the reac-tion torque which of course, is equal to and opposite the torque being applied to the fastener. Each strut member 70 carries a strain gauge 72 which is connected in a wheatstone bridge circuit (not shown) to develop an electric signal representative of the instantaneous torque being applied to the fastener. Instead of strain gauges, contacting or proxi-mity displacement gauges could be used to develop the elec-tric signal. Thus, with the torque cell arrangement dis-closed in this embodiment, one of the encoders and the spring arrangement disclosed in the apparatus illustrated in Figs.
2-4 have been replaced by a relatively inexpensive transducer.
The exact form of torque cell 64, of course, may vary some-what. For example, struts 70 could be replaced by a somewhat deformable cylindrical member, if desired.
Since one encoder in the Figs. 2-4 embodiment of the invention has been replaced by torque cell 64, it should be clear that only one encoder need be utilized. However, in accordance with this embodiment of the invention, it can be replaced by a relatively inexpensive sensing means further reducing the cost of the apparatus. Accordingly, further dis-closed in this embodiment of the apparatus is a proximity probe 74 mounted through the housing of motor 52 adjacent to and radially spaced from rotary vanes 76 in the motor, as illustrated most clearly in Fig. 8. Proximity probe 74 can be in the form of an induction coil which develops an electrical signal when metal passes through its magnetic f ield. Thus, as vanes 76 rotate when the fastener is being tightened, signals ~054~

are provided by proximity probe 74 which represent fixed in-crements df rotation of the fastener. The size of the incre-ments depends on the number of vanes 76 in motor 52 and the gear ratio of gear box 58. It should be understood of course, that proximity probe could cooperate with one of the gears in gear box 58 in a similar manner.
Still referring to Fig. 7, a control system func-tionally equivalent to that illustrated in Figs. 3 and 4 of the drawing is illustrated and further reduces the cost of the system by utilizing certain analog circuitry. The output sig-nal from torque cell 64 representative of the instan~aneous torque being applied to the fastener is fed through a torque amplifier 78 which amplifies the torque signal to a magnitude wherein it is compatible with the rest of the control system.
From amplifier 78, the torque signal is fed through shift register means which, since the circuit is analog, comprises a series of charge coupled devices in the form of sample and hold circuits 80, 82, 84 and 86. As in the circuits illus-trated in Figs. 3 and 4, the gradient shift register means is clocked by signals representative of fixed angular increments of displacement of the fastener. Accordingly, signals from proximity probe 74 which are in the form of spike shaped pulses are fed through a square wave generator 88 which shapes the signals and feeds the shaped signals through a chord length divider 90 to an analog switch driver 92 which sequentially clocks the sample and hold circuits. Chord length divider 90 is a suitable divider circuit which electronically divides the pulses from square wave generator 88 by 1, 2, 4, 8, 16 or 32 so that every pulse, or every second pulse, or every fourth pulse, etc., is utilized to clock the shift reglster. By 10541~
selecting the appropriate division to be made in chord length divider 90, it is possible to adjust the chord length on the Torque-Angle of Turning curve over which the torque gradient is measured, that is, chord len~th a and b in Fig~ 1.
Analog switch driver 92, although not necessary, assures that each sample and hold circuit has discharged its stored signal before receiving a new signal. Accordingly, analog switch driver 92 sequentially clocks the sample and hold circuits first clocking circuit 86, then circuit 84, then cir-cuit 82, and finally circuit 80. Accordingly, sample and holdcircuit 86 has discharged its stored signal prior to receiving a new signal from sample and hold circuit 84, etc. The output from sample and hold circuit 86 is representative of torque a fixed increment of rotation prior to that particular instant and is fed through a gradient register or comparator circuit 94 in the form of a differential amplifier which also receives an input signal representative of the instantaneous torque being applied to the fastener from torque amplifier 78. As in the circuits illustrated in Figs. 3 and 4, comparator 94 sub-tracts its input signals and has an output signal representa-tive of the instantaneous torque gradient for the particular fastener being tightened. The gradient signal from comparator 94 is fed through a suitable gradient signal ampli~ier 96 which amplifies it to a magnitude compatible with the rest of the control system.
From gradient signal amplifier 96, the instantaneous gradient signal is fed to means for determining the maximum gradient and also to means for comparing the maximum and in-stantaneous gr~d~ent signals. Looking first at the means for determining the maximum gradient, there is included a maximum 1~54t~Z~i gradient comparator 100 receiving input signals from gradient signal amplifier 96 and from a sample and hold circuit 102 which also receives signals from gradient signal amplifier 96.
As will be made clear hereinafter, sam~le and hold circuit 102 stores a signal representative of the maximum gradient en-countered up to any point in the tightening cycle prior to the instantaneous output from the gradient signal amplifier.
Comparator 100 determmnes whether the instantaneous gradient signal from gradient signal amplifier 96 or the previously stored signal from sample and hold circuit 102 is larger. If the instantaneous gradient signal is larger, comparator 100 feeds an output signal to an AND gate 104 which also receives signals from analog switch driver 92 when the switch driver outputs a clocking signal to sample and hold circuit 84. When both signals are received by AND gate 104, it outputs a clock-ing signal to sample and hold circuit 102 which allows the sample and hold circuit to receive a new signal from gradient signal amplifier 96 representative of the larger gradient.
If the instantaneous gradient is smaller, comparator 100 pro-vides no output, nor does AND gate 104, so that sample and holdcircuit 102 cannot accept a new gradient signal. By utilizing the clocking signal from analog switch drive 92 ~o sample and hold circuit 84, a time lag is provided which allows the com-parison to be made before a clocking signal can be fed through :^= AND gate 104 and before a new gradient signal can be developed.
Looking now at the means for comparing the maximum and instantaneous gradient signals, it can be seen that as the signal representative of the maximum gradient is fed from sample and hold circuit 102 to comparator 100 it is split and fed to a division circuit 106 which is operative to divide the signal 1t)~4~
by the preset relationship utilized to determine the point X
on the curve illustrated in Fig. 1 or Fig. 2. If the preset - xelationship is 50%, as is preferred as noted above, dividing circuit 106 splits the maximum stored gradient signal in half and feeds the signal to a control comparator 98 so that it may be compared with an instantaneous gradient signal from gradient signal amplifier 96 which is also fed to the control comparator.
When the input signals to control comparator 98 are equal, or when the gradient signal is smaller than the divided maximum gradient signal, the control comparator provides an output sig-nal which is fed to another AND gate 108. At this point, it should be noted that the output signal rom comparator 98 could be fed directly through a valve drive amplifier 110 which would amplify the signal to a suitable magnitude to close the solenoid in v~lue 54 and stop motor 52. However, to assure that comparator 98 does not inadvertently provide an output signal in Region 1 of the curve illustrated in Fig. 1, AND
gate 108 is utilized and receives an additional input signal from a snug torque comparator 112. Instantaneous torque sig-nals are fed from torque amplifier 78 to snug torque compara-tor 112 which also receive6 an input signai from a preset sn~g torque signal generator 114 which, or course, could be in the form of a suitable potentiometer for providing a predetermined input signal representative of the torque approximately cor-responding to that at the point marking the transition fromRegion 1 to Region 2 on the curve illustrated in Fig. 1. The noted point is commonly referred to as the snug point or snug torque point. The setting in snug torque signal generator 114 need not be exactly representative of the snug point and may be an approximation, for example, a signal representative of ~054~
about 20% of the torque value expected at the yield point would suffice. When the instantaneous torque signal from amplifier 78 exceeds that generated by snug torque signal gen~erator 114, comparator 112 provides an output signal to AND gate 108 which allows the feeding of the signal from control comparator 98 to valve drive amplifier 110. The out-put of valve drive amplifier 110 is ~ed to control valve 54, closing same and stopping motor 52. Thus, any si~nals inad-vertently developed by control comparator 98 in the pretight-ening region, that is Region 1, of the curve illustrated inFig. 1 would not close control valve 54.
Finally, a reset switch 116 is provided which can be utilized to clear the circuits and prepare the tool for a new tightening operation with another fastener. One other point that should be noted involves the fact that various pre-determined relationships can be utilized to determine when to stop the tightening cycle depending on the characteristic of the torque-rotation curve. For example, if the curve included a temporary flattening at a ~nown load less than the load at the yield point, the tool could be utilized to stop tightening at that point. Such a temporary flattening of the curve could be caused by a particular fastener configuration.
In the foregoing, there has been disclosed several embodiments of the invention and it should be obvious to one ~- skilled in the art that various modifications and changes can be made without departing from the true spirit and scope of the invention as recited in the appended claims.

Claims (2)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Apparatus for determining torque gradient comprising means for developing a signal representative of the instantaneous torque applied to a rotary member, storage means including a series of sample and hold means for storing a series of signals representative of instantaneous torque and gradient register means for sequentially subtracting a stored signal from an instantaneous torque signal and developing a signal representative of the torque gradient.

2. Apparatus for determining torque gradient in accordance with claim 1 wherein said sample and hold means are clocked sequentially.