CA1077752A - Impact wrench with joint control - Google Patents

Abstract

IMPACT WRENCH WITH JOINT CONTROL
John T. Boys A B S T R A C T

An impact wrench having an adaptive control system for determining the yield point or some similarly significant point of a fastener assembly by detecting a signal representative of the peak deceleration of the hammer, one embodiment of which is the peak recoil value of the hammer after impacting with the anvil of the wrench, and a signal representative of the angular dis-placement of the output shaft of the wrench. Yield of the fastener is determined when the respective magnitudes of successive deceleration signals do not exceed the magnitude of a previously stored maximum deceleration signal by a predetermined fixed amount.
Upon attaining the yield point or other similarly significant point, the wrench may be allowed to rotate the fastener an additional preselected number of degrees before shutting off.

Description

S P E C I F I C A T I O N
This invention relates generally to the field of tool driving or impacting, and more particularly to an impac~ type wrench having a control system for accurately controlling the tension in a fastener of a joint.
It is well known in the prior art that tightening a O fastener 'o its yield point produces optimum joint efficiency.
A fa~ten~d joint havins a grea_er preload value up to the yield poi~t o f the ma~erial of the joint is more r~eliable and insures bet~er ~as~ener performance. ~ish fastener preload further .

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increas~s fatigue resistance due to the fastener feeling less added stress from external joint loading, and dynamically loaded joints have less tendency to slip and loosen.
The prior art reveals various types of impact wrench S control system3 for controlling the a unt of preload in a fasten-er. On~ commonly used type employs som~ for~ of torque control, in which the impact wrench tightens a fastener ~o a maximum pre-determined v~lue of torqu~ and ther~upon shuts off. Example~ of impact wranches utilizing torque control can be found in United State~ P~t~nts to ~ ; Hall, No. 3,833,068;
Schoep~, M~, 3,703,933; Vaughn~ ~o. 3,174,559; Elliott et al, No.
3,018,866 and Maurer, No. Z,543,979. Another means of controlling impact wrancho~ found in the prior art is commonly known as a ~turn-of-tho-nut" system, in which a fastener is tightened to some pre8elected initial condition, such as a predetermined torque value or spin~le speed, and thereupon rotated an additional pre-determined nu~ber of degrees before shutting off. Examples of vario~s turn-of-the-nut impact wrench systems are found in United States Patents to Allen, No. 3,623,557; Ho2a et al, No. 3,318,390 and Spyradakis et al, No. 3,011,479. Another type of control comprises i~parting a constan~ angular momentum of each lmpulse :
blow, such as found in the United States Patent to Swanson, No.
3,}81,672.
As can be seen from the numerous existing prior art 3ystems, the problem is not a novel one. The ultimate desired r~sult is to achieve preload of the fastener into the yield region.
The common problem which each of the prior art sys~ems attempts to solve is determining when the yield point of the fastener has been reached. In all of the contsol systems described in the above-no~ed pa~ents, prior knowledge of the fastener and joint character-i~tic~ ~ust be known or assum~d in order to d~termine either the . - 2 '.' ~

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exact predetermined final torque, the exact amount of additional rotation or the amount of constant angular momentum of each impact blow. It is well known that tightening to a predetermined preload condition, such a~ the yield point~ is a function of many variables, ~m~ng them beir.g joint stiffness9 fastener stiffness, surface friction between mating threads ancl thread form. Therc-fore, in aach of ~he prior art systems the yield point cannot ~lways be accurately determined ~ecause the conditions of each fasten~r and joint vary and may not be known in advance. Thi~
consequence can lead to uneven tightening from joint to joint in a ~tructure, which can in turn result in loosening of the fastener ~ in the joi~t and premature fatigue failure.
- It is known from the characteristics of fasteners that a yield phenomena occurs in the applied moment and the preload simultaneously, so that preload can be controlled by stopping the tightening process when the applied moment suggests that yield is occurri~g. Because of the nature of operation of certain types of wrenches,a contin~ous moment is not applied. For example, in an impact wrench a series of pulsed impacts of a hammRr onto an anvil advances the fastener into a workpiece. During each impact, when the fastener has been tightened unt11 it presents maximum re-; sistance to further rotation, the anvil which is coupled thereto, also presents maximum resistance to further rotation and the peak torque or maximum moment applied by the hammer is r~ached. At this point, the hammer is subjected to its maximum deceleration which is proportlonal to its maximum applied moment, and experiences a recoil, the magnltude of which has been found to be proportional ;
; to the maximum deceleration of the hammer and thus of the maximum applied moment. In the present preferred embodiment of an impact wrench in accoxdance with this invention, the deceleration of the ., .;

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hammer in the form of its rotary motion i5 sensed by a recoil or bounce back mechanism. The magnitude of the recoil, either its duration, force, velocity or total distance of travel, give a measure of the deceleration of the hammer and, hence, the maximu~
applied moment. However, it has been fow~d to be relatively easy to mea~ure duration of recoil. Thu~ the recoil timR and the angle of rotation can be ~onitored simult2lneou~1y, but a graph showing one as a unction of the o~her is so~ewhat hypothetical a~ recoil~ only occur at the end of a blow while angular dis-placement occurs during a blow. ~y convention, therefore, th~graphs are plotted as angular displacement at constant moment followed by a change of moment at constant angle.
, , SUMMARY OF THE INVENTION
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? Accordingly, it is a general purpose and object of the : 15 present invention to provide apparatus for tightening a fastener to the yield point or to some similarly significant point in a ,, joint. It is another object of the invention to provide a control system for tightening a fastener to its yield point and which is particularly useful with a wrench that applies its tightening -~ 20 moment periodically. It is another object of the lnven~ion to provide an impact wrench having an adaptive control system for accurately tightening a fastener to a predetermined preload con-dition and which utilizes measured characteristics of the fastener and joint being tightened. It is still a further object of the invention to provide an adaptive control system in an impact wrench for accurately ti~htening a fastener to a predetermined preload with minimum prior knowledge of the fastener and joint . .:
` characteri~tics. It is yet another object to provide an impact . . .
wrench having an adaptive control system which determines the , . . .

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D777~2 yield point of the fastener by measuring the magnitude of de~
celeration of the hammer after engagement with the anvll, and issuing a stop control signal when no subsequent deceleration values exceed a previous peak deceleration value by a predeter-mined additional amount. It is still a further object to pro-vide an impact wrench having an adaptive control system which ; measures the magnitude of recoil of the hammer after engagement with the anvil, measures the angular displacement of -the output shaft, and issues a shutoff signal to the wrench after a pre-determined additional number of degrees of rotation subsequent to measuring a peak recoil value which is not exceeded by sub-sequent recoil values by more than a fixed or variable additional amount.
These and other objects are accomplished according to a preferred embodiment of the present invention by providing a wrench such as an impact wrench having a control system including means for developing a signal representative of the deceleration of the hammer after engagement with the anvil which signal is also representative of the applied moment, means responsive to the deceleration signal for determining the yield point or some similarly significant point of a fastener assembly and means for producing a control output signal when the fastener assembly is tightened to the yield point or similarly significant point.
In a further aspect of this invention there is provided in an impact wrench including a hammer impacting with an anvil to rotate an output shaft operative to tighten a fastener assembly ' to its yield point or some similarly significant point by applying torque thereto, a control system comprising: means for developing . a signal representative of the deceleration of the hammer after engagement thereof with the anvil; calculator means responsive to said deceleration signal for determining the yield point or some similarly significant point of the fastener assembly; and control . .

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means responsive to said calculator means for produclng a control signal when the fastener assembly is tightened to said point.
In a still further aspect of this invention there is provided an impact wrench for tightening a fastener assembly comprising: a motor; a hammer assembly adapted to be driven by said motor; an anvil adapted to be rotatingly impacted by said hammer assembly; wrench means operatively attached to said anvil and adapted to drive the fastener assembly to applying torque thereto; means for developing a signal representative of the recoil of said hammer after engagement thereof with said anvil;
calculator means responsive to said recoil signal for determining ,~ the yield point or some similarly significant point of the fast-:~, ener assembly; and control means responsive to said calculator ' means for producing a control signal when the fastener assembly is tightened to said point.
In a still further aspect of this invention there is provided a method of tightening a fastener assembly -to its yield point or some similarly significant point by applying torque thereto with an impact wrench of the type including a hammer impacting with an anvil to rotate an output shaft operatively coupled to the fastener assembly comprising the steps of:
developing successive signals representative of the recoil of the hammer after engagement thereof with the anvil; determining ~.
the yield point or some similarly significant point of the fastener assembly based upon a predetermined relationship of said recoil signals; and producing a control signal when the fastener assembly is tightened to said point.
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In a still further aspect of this invention there is provided apparatus for tightening a fastener, said apparatus comprising: wrench means for periodically applying a tightening .
:` moment to a fastener in a joint assembly; first means for measur-ing the moment applied to the fastener during each per:iod and ~ . ~
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for developing a signal representa-tive of the peak moment applied - during each period; and control means responsive to said peak !` moment signals for de-termining when a peak moment signal has not increased by more than a predetermined amount and for developing ; a control signal.

BRIEF DESCRIPTION OF THE DRAWI~GS

Fig. 1 is a side elevational view of an impact wrench contructed according to the invention partially cut away and in cross-section, showing an angle encoder and sensing means;
; 10 Fig. 2 is a front elevation view of the angle encoder ; shown in Fig. l;

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' '. ' ,- : ' 77t7S~2 Fig. 3 is a transv~rse sectional ~iew taken along the line 3-3 of Fig. 1 looking in the direction of the arrows, showing the recoil detection apparatus;
Fig. 3A is a partial transverse sectional view schematically illustra~ing another embodiment of a recoil detection apparatus usable with this invention;
Fig. 4 is a block diagram of the control circuit of the impact wrench of Fig. 1, and Fig. 5 is a graph showing the various parameters during the operation of the wrench.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Before proceeding with a description of an apparatus in accordance with this invention, a brief explanation of a method in accordance with this invention will be explained.
Referring briefly to Fig. 5 of the drawing there is disclosed a curve (PRELOAD IN FASTENER) illustrating the relationship between the preload induced in a fastener tightened by a periodically or cyclically operated tool such as an impact wrench and elapsed time during the tightening cycle. From the noted curve it can be seen that initially the preload increases rapidly and eventually levels of~ so that only small a~ditional preload is induced in the fastener. This leveling off occurs at about the yield point and continues through the remainder of the tightening cycle. Similar phenomena are observable in the relationship between applied moment and time as illustrated in curve L (RECOIL TIME) and curve O (PEAK VALUE). It is merely noted here that recoil time is representative of the applied moment.
In accordance with this invention a fastener is tightened to its yield point by applying a tightening moment to the fastener and periodically measuring the applied moment.

Preferably the moment is applied periodically and the pea}c moment applied 7~75Z

during each period is determined. ~y ~pe~ mvment~ is meant the largest moment applied during each period. The instantaneous peak moment is compared with the largest pe~k moment which has been applied previously during the tightening cycle to determine if the instantaneous peak moment exceeds the previous la~ges' peak moment by more tha~ a predetermined am~)u~t. The predetermined amount may vary slightly for fastener~ of different types but it ha~ b~en d~term~ned that the predetermlned amount i3 normally about

2~ of the previous large~t peak moment in which case the predeter-~ned amount i~ variable. It hAs al~o been determined that the 2~
ca~ be approximated and an absolute value can be u-~ed, ior example, 2~ of the peak moment expected to be applied at the yield point.
If the instantaneous peak moment exceeds the previous largest peak moment by the predetermined amount, the application of the tightening moment continues and the instantaneous peak ment is ~tored for comparison with the next instantaneous peak moment; if the instantaneous peak moment does not exceed the pre-vious largest peak moment by more than the predetermined amount the application of the tightening moment can be discontinued since this indicates that the fas~ener has been tightened to its yield point as 8hould be understood from the explanation of the relatio~ships between preload and time and between moment and time.
Referring briefly to curve L(RECOIL T~ME) in Fi~. S of the drawing it can be ~een that during some periods before the fastener has been tightened to its yield point the instantan~ous peak moment is less than the largest previous peak moment. These occu~rencas are random in the sense that they are not predict-able and it is possible that the application of the tightening moment could be discontinued before the yield poin~ is reached.
Accordingly, it is desira~le to not discontinue the application of the tightening moment until the instantaneouq peak amount has not 3777~'~

exceeded the previous largest peak mom~nt by the predetermined ~mount for a predetermined number of successive period~ during which the moment is applied. While t~o such detections are sufficient, three to five is preferable. It h~s been found most preferable to meaRure a second tightening charasteristic related to tha period during which the moment is applied, for example, to mea~ure angul~r xotation o~ the fa3tener during the tightening cycle, and to ~ot di~continue the application o~ the tightening moment until the insta~taneou~ peak moment has not exceeded the previou~ large~t peak ~oment by the predetermlned amount during a predetermin~d rotation of the fas~tener, for example, during 15 tO 25 degre~. In this way, it can be assured that the applied moment i9 op~rative to cause rotation of the fastener even though the torque i9 levelling off. It should be understood that other characteristic~ could be measured instead of rotation so long a~
these other characteristics are related to the moment in the same general way as rotation. That is, any characteristic related to the moment such that the momentlevel~ of~ with respect to that characteri~tic can be used in place of rotation. Time, for example, can be used.
. The described metho~ could be performed by hand, but an apparatus performing the method will be described. While any type o~ wrench system applying torque periodically can operate t perform the method, the preferred emb~diment disclosed herein is an impact wrench.
Referring to Figs. 1, 2 and 3, an impact wrench lO is shown, which may be any one of many conventional types that in-clude an external source of compressed air suitably connected to the wrench in order to ~uccessively impact a hammer onto an anvil.
An ~nvil 12 i~ rotatably secured within the forward portion of the - a -. . ~
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wrench housing 11 by a bearing 13. The forward end 14 of anvil 12 comprises, for example, a ~quare drive for attachment to a drive socket or some other suitably shaped wrenching member for driving a fas~ener. A hammer assembly 15 connected to and dxiven by a conven~ional air motor (not shown~ surrounds and contacts anvil 12 imparting impact blows thereto to rotate the anvil and drive a fastener (not shown)~ Wrench 10 a:Lso includes a con-ventional trigger 22 which, when depressed, allows air from the external source ~not shown) to enter wrench 10 at an inlet por~
23 connected to an air ~otor ~not shown) driving hammer 15 to rotate anvil 12.
A bidirectional incremental encoder 16 used in a sys~em :~ for measuring angular rotation of the fastener is suitably fixed to anvil 12 for rotation therewith within the forward portion of wrench housing 11, such as, for example, by key 17 mating with a corresponding recess 18 in anvil 12. Since the anvil 12 drives the wrenching mem~er driving the fastener, the encoder 16 rota~es with the fastener as the fastener is tightened. ~etween i.mpacts of the hammer 15 against the anvil 12, the anvil and encoder 1~
- recoil, but the fastener does not. Thus the rotation measuring system in which the encoder 16 i~ used should be ca~able of de-; tecting and di~regarding the recoil of the encoder. Holes 21 are each located at a fixed radius on encoder 16. A pair of sensors 19 and 20 are suitably mounted in the forward end of housin~ 11, each : at a fixed radius from the center line of anvil 12 so that they line up radially with holes 21. Sensors 19 and 20 are preferably of a ma~netic type, that is, could include an induction coil whose ~utput varies due to the presence or absence of metal, but any other suitable proximity type sensor may be used to detect the passage of ~uccessive ones of hole~ 21 during operation of the ~ _ g wrench. As can be seen in Fig. 2, encod~r 16 in the prefexred embodiment contains eighteen ~18) equally spaced holes, the center lines of each hole being twenty ~20~ degrees apart at a fix~d radiu~ from the centar line of the encoder. As will be explained later the outpu~ signals of the sensors 19 and 20 are ninety ~90) degrees out of phase ~o the sensors are spac~d a~art to provide that result. Thus, the sensor~ 19 an~ 2n could be sp~ced apart a distance equal to the sum of five ~53 degreas plu8 ~ome whole numbe~ multiplied by twenty ~20~) degr,ees, for example twenty-five (25~, forty-five t45), sixty-five ~65), etc. degrees. Resolution with this encoder i~ 72 counts per revolution as will also be explained late~. It should be under ,, stood that the encoder could contain any reasonable number of hole~ depending on the degree of accuracy desired, the only L5 requirement being that the holes are spaced equally apart from each other. A proximity type sensor 24, which also can include . ~:
an induction coil similar to sensors 19 and 20, is mounted at the bottom rear portion of the wrench housing for measurinq decelera-tion in the form of recoil or bounce back o f the hammer. As noted previously the deceleration of the hammer is proportional to the pea~ moment applied during each impact.
Referring now to Fig. 3, the bounce back or recoil indicating mechanism is shown. An output shaft 30 from the air motor (not shown) is connected t~rough a conventional one-way clutch 31, to a rotatable cannister 32 having an arm 33 extending from the surface thereof. The arrows on clutch 31 indicate that the normal direction of ro~ation of shaft 30 is clockwise when viewed in a direction opposite the arrows on line 3- 3. Clutch 31 transmit~ rotational force to cannister 32 when hammer 15, whi~h is suitably connected to rotate shaft 30, rebounds off of ~ ;lo . -- 10 ";;' ' ;: ,..
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anvil 12 in the counter-clockwise direction when viewed in a direction opposite the arrows on line 3 3 after imparting a blow thereto. Cannister 32 is located inside of cutout 34 at the rear portion of wrsnch housing 11. A spring 35 i9 at~ached at one o its ends in some suitable manner at a point 36 adjacent the di~tal end o~ arm 33, and at its other end at a point 37 adjacent the bottom of w~ench lO. Spring 35 i~ typically an elongated coil spri~gt but may be any oth~r suitable elastic te~3ioni~g d~vice for exerting a downward force on arm 33. An end stop 38 i9 ~ounted at the bottom of wrench lO and extend~
upwardly at an angle with its distal end 39 proxim~te the sensing end of sensor 24.
; Operation of the bounce back or recoil detection apparatus will now be described. On each successive impact of the hammer onto the anvil, as the fastener i9 rotated the energy stored in the hammer a~d anvil drops to a point where resistance to furth~r rotation caused by tightening of the fastener in the workpiece begins to occur. Upon further tightening, a deceleration oi the hammer at the end of a blow in the form of a recoil occurs, the duration, total displacement, velocity and force of the re- ?0 coil being proportional to the applied moment. The force of the recoil is transmitted through shaft 30 and clutch 31 to cannister 32~ which is initially in a position indicated by the dotted lines in Fig. 3 with arm 33 resting on distal end 39 of end stop 38.
~- ~he force of the recoil causes shaft 30 and cannister 32, coupled together by clutch 31, to rotate in a counter-clockwise direction looking forward ~clockwise as seen in Fig. 3), causing arm 33 to move upwardly off o end 39 of stop 38 against the restoring force of sprinq 35. This restoring force causes arm 33 to return to it~ initial position resting on distal end 39 of stop 38 after ~o some finite duration of time which is proportional to the recoil energy, and thus the deceleration of hammer lS. Sensor 24 measures the duration of time it takes arm 33 to complete its ~ cycle. The duration of time is, as mentioned hereinabove, 5 dependent upon the amount of recoil energy tran~mitted ro~
hammer 15 to ~haft 30, the maximum amount of recoil snergy occurring at approximately th~ maximum preload in ~he fastener, ~ at or n~ar the fastener yi~ld point. It sho~ld be understood ~hat either the di~tanc~ t~av~lled or veloclty of arm 33 travel, or force exerted by spring 36 o~ pin 37 could also be measured, .. a~ they ~re all proportional to hammer deceleration and thus the ,,.^
applied momsnt. Other parameters proportional to the applied moment can al80 be measured, for example, the rotation of the ~ fastener.
In another embodiment of the recoil detection apparatus " shown in Fig. 3, clutch 31 could be replaced ~y a viscous Newtonian fluid 31A suitably co~tained between shaft 30 and cannister 32 as shown in Fig. 3A. Viscous drag force of the fluid would then transmit the recoil force of the hammer which is coupled to sh~ft 30, to cannister 32 in the same manner as clutch 31 illustrated in Fig. 3. For a more complete descxiption of a ne-way fluid clutch, reference is made to United States Patent, No. 2,521,11~, issued to G. B. Du~ois et al. Measurement of the . total duration of the recoil woul~ be exactly as described above.
: Referring to ~ig. 4, a control system is shown for controlllng the tightening cycle of wrench l0. The coils of sensors 19 and ~0 are supplied with a suitable voltage and as the encoder 16 rotates, the sensorsoutputs vary depending on whether ~ a hole 21 or the l~tal between holes is adjacent their ends. ~or : example, the sensors 19 and 20 can be arranged ~o pro~ide a high . ~o . .

' " 1~7~'752 output when metal is detected and a low output when it is not.
The output signal from sensor l9 is fed into an amplifier 40, and the output signal from sensor 20 is similarly fed into an am-plifier 42, in order to amplify the respective angle signals to a magnitude at which they are compatible with the rest o~ the control sy~tem. Signal A from amplifier 40 .is characteristically 90 out of phase (~) with signal B from amplifier 42, the signals having a ch~racteri3tic ~quare wave shape the pulse width of which are proportional to th~ radian spacing between hol~s 21.
The square wave shapa of signals A and ~ can be a~sured by using Schmitt triggers in the amplifier circuits. Output signal A
from amplifier 40 i~ fed concurrently into a first nostable multivibrator 44 having a positive trigger, a second monostable multivibrator 46 having a negative trigger, and pulse sorting logic 48 which separates pulses produced by forward and reverse recoil rotations of angle encoder 16. Logic 48 will be described in greater detail hereinbelow. Output signal B from amplifier 42 is fed concurrently into a first monostable multivibrator 50 having a positive trigger, a second monostable multivibr~tor 52 having a negative trig~er and pulse sorting logic 48. Output signal C from multivibrator 44 is characteristically a sharp pulse corresponding to the positive going portion of signal A, and output signal D from multivibrator 46 is a pulse corresponding to the negative going portlon of signal A. Similarly, o~tpu~
s.ignal E from multivibrator 50 lS a pulse corresponding to the po~itive going portion of signal B, and output signal F f~om :
multivibrator 52 is a pulse corresponding to the neyative going portion of signal B. Signals C, D, E and F are each introduced into pulse sorting logic 48 along with sign~ls A and B. The pulsas produced by forward a~d reverse rotations of anyle encoder . ., .

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16 are separated in logic 4~, which yields output signals G, each .representing an increment of forward rotation of the encoder 16, and H, each representing an increment of reverse rotation of the encoder 16. Signals G and H are fed into a counter/storage unit S0 which counts the number of forward and reverse rotation pulse~
and stoxes this information. Unit 50 may typically comprise a synchronous 8-bit up~down binary coun~er w:hich include~ two 4-bit binary c:oun~era in cascade. Counter/storage unit 51 act~ as an inhibi'cor of forward rotation pulse~ G through a NAl!ID gate 53 and i9 arranged to count up forward rotation pulses G and count down reverse rotation pulses H. Counter/storage unit 51 is further arranged so that it provides a low input signal to NAND gate 5 when it i~ set to zero or is counting up from zero and so that it provides a high input signal to the NAND gate when it is count-ing down from zero or counting up to zero. These inputs to NAND
gate S3 are preferably provided by placing a signal inverter between the output of counter/storage unit 51 and NAND gate 53 and by havin~ the counter/storage unit output a high signal when it is set at 2ero or counting up from zero and ouput a low signal : when it is counting down or c~nting up to zeroO The signal inverter, a~ is conventional, inverts the output of counter/storage unit 51 before it is fed to N~ND gate 53. Thus, signals G cause NAND gate 53 to dischar~e only when unit 51 is set at zero or is counting up from zero.

In addition a second NAND gate can be placed between 25 the output of NAND gdte 100 and the input of signal G to counter/
storage unit Sl so that signals G are fed to unit 51 through this second NAND gate. For its other input the second NAND gate receives the output signal from counter/storage unit 51 before :~ that signal is inYerted.

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Operation of this preferred arrangement will now be explained. When tightening of the fastener commences, forward -~ rotation pul~es G are discharged by NAND gate 100 and provide inputs to the s~cond NAND gate and N~ND gate 53. The output from counter/~torage u~t 51 is high since the unit i~ set at zero and thi3 high ~ign~ receiv~d a~ the second input to the ~econd NAND gate. Thu~ pulses G are not fed to count~r~storage unit 80 it remaln~ set at ~exo. The output from counter/s~orage unit 51 i~ inve~ted by the inverter so that the secor~d input to NAND

gato 53 is low. Thus, pu~ses G cause NAND gate 53 to output a high ~ignal to mono~table multivibrator 54 causing i~ to outpu~ a ~ignal.
If encoder 16 recoils between an impact of hammer 15 against anvil 12, NAND gate 100 does not output ~ignal G and NAND

gate 98 output~ signals H which are fed to counter~storage unit 51 and counted down. The output of unit 51 i5 now a low signal which is fed to the second NAND gate and which is inverted and `~ fsd to NAND gate 53 as a high signal. When forward rota~ion pul~es G are provided by NAND gate 100 indicating forward rotation, the second NAND gate discharges to counter/storage unit 51 and are counted up. At the same time the pulses G canno~ feed past NAND gate 53 because of the high input signal from the inverter.
When the forward rotation p~lses G equal the reverse rotation pulses H counter/storage unit 51 counts zero and its output goes high. As noted previously, when unit 51 outputs a high siqnal, signals G are not counted up and are fed thro~gh NAND ga~e 53 to monost~ble multivibrator 54. From the preceding it should be understood that recoil pulses are made up and slgnal I is representative of an increment of fastener rotation. Signal I

i~ characteri~tically a single step function. The output from gate 52 i~ fed into a ,,., -~

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monostable multibibrator 54 whose output signal J is fed into a selectable ring counter 56, which produces an output signal R after a predetermined number of forward rotation pulses between 1 and 10 has been received, as will be more fully explained hereinafter.
Counter 56 ~ay also be referred to as a divide-by-lO counter/
divider with ten decoded outputs, and is typically a pair of 5-bit shift registers connected serially. Ou~put signal J from multivibrator 54 is thus a pulse repre~enting an increment o~ net orward angular rotation of encoder 16.

~he output signal from sensor 24 is fed into an amplifier 58 which yields an output signal K representative of the magnitude of the total time for arm 33 (Fig. 3) to move off of, and return to re~t upon end 39 of stop 38. It should be understood that force, velocity or distance of recoil could al50 be used with equally successful results as they are each similarly proportional to the applied mome~t. Since the rotation of the fastener is proportio~al to the applied moment, another technique for develop-ing a ~ignal representative of the applied moment of eaci- lmpact ; would be ~o measure the rotation of the fastener during each im-pact. The coil of sensor 24 is supplied with a suitable voltage and its output varies depending on whether arm 33 is seated on the end 39 of stop 38. For example, sensor 24 and amplifier 58 can be arranged to provide a high output when no metal is detected and a output when metal is detected. Signal K, which is a s~uare wave whose width is proportional to total recoil time, is fed into 25 a ramp generator 60 which produces a characteristic ram~ function output signal L whose amplitude is proportional to the duration of signal K. Signal L is then fed into a peak value detector and storage u~it 62 which stores the maximum or peak value of recoil tLm~ from sensor 24. Peak value detector and storage unit ' ~
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` 62 is generally conventional and includes an amplifier (not shown) - for detecting whethsr an instantaneous signal L has increased and a storage unit (not shown) Eor storing the largest signal L plus a pradetermined increment as will ba explained. The storage unit can be in the form of a capacitor arrangement. The amplifier :~ receives input signal L ~rom ramp generator 62 and also the ~ignal ~tored in the storage unit so that it can ~etermine whether the instantaneous signal L is larger than the stored signal. If it i~ not the amplifier provide~ no output; if it i5 the amplifier ~ 10 outputs th~ larger signal to the storage unit and provides ano~her output to a pea~ value increase detector 64, which i5 typically a monostable multivibrator, producing an output pulse M. Output ~ignal M from detector 64 is characteri5tically a sharp pulse and is fed simultaneously into an exclusive NOR gate 66 and a step generator 68 which outputs a signal N which increases the instantaneous signal L stored in the storage unit of peak value detector and storage unit 62 by a fixed or variable amount for each input pulse M received. Output signal N from step generator 68 i5 a square wave of short duration and fixed amplitude. As will be more fully explained in the description of the operation 20 of the control system, a fixed ~alue of voltage may be added ; (lOOmv, for example), or a fixed percentage of the maximum stored peak recoil value may be added ~2%, for example). The increased peak recoil value output signal from the storage unit is fed back into the amplifier for comparison with incoming signal L. : 25 The storage unit o~ the peak value lncrease and storage unit 62 is also fed as an ~utput signal 0, indicative of the increased peak value, into a voltage comparator 70, which is typically an operational ampli~ier, receiving a second input signal from a snug :' torque setting unit 72. Signal O has a characteristic stepped .' .

, ,~ .

:. .
.: , ~77~i2 r~mp function profile. Unit 72 may be any suitable variable volt-age producing device, such as a potentiometer, in which a voltage proportional to so~ne determinable snug torque is generated. By snug torque i9 meant the torque at which the fastener has pulled the joint part~ to~ether and wherein prelo,~d ~s being induced.
; The v~ltage l~vels from detector and stora~e unit 62 and ~etting unit 72 are compared in comparator 70, and when the first i5 at lea~t equal to the ~econd, an output signal P from comparator 70, i fed into NOR gate 66 which also receive~ a~ a ~cond inpu~
the signal M from detector 64. Signal P has a characteristic signle step function shape. As is conventional, NOR gate 66 will provide a high output signal Q only when it has two low input ~ignaLs or two high input signals. Thus t before the fastener ha~ been tightened to its snug torque and with no ; 15 increased peak v~lue signal from the storage unit in unit 62, that i~, with both input~ low, MOR qate 66 outputs signal Q which reset3 co~nter 56 to zero. When the snug torque is reached, signal P i9 fed f~om comparator 70 so that the NOR gate does not output :. signal Q and counter 56 can now count. If, after the snug torque '~ i9 reached, a signal L exceeds the previous maximum signal L
. 20 by the predetermined amount added by signal N, monostable multi-. vibrator 64 outputs signal M to NOR gatè 66 so that signal Q is again ~ed to counter 56 resetting the counter to zero. Thus, if the instantaneous peak applied moment does not exceed the prevlous maximum peak applied ~oment by the predetermined amount . over a~ interval of rotation equal to a predetermined number of count~ multiplied by the predetermined increm~nt of rotatlon sensed by the encoder, then counter 56 will output a signal R which is a ~:
single step function amplifed in amplifier 74 and fed to the coil , ..
; of a conventional solenoid valve 76 for shifting the spindle of , - 18 :' .
~ .

the valve to its closed position. Solenoid valve 76 is placed in the air supply line to the impact wrench so that when the spindle is shifted to its closed position, the air Cupply to ~ort 23 of wrench 10 is closed.
Still .ef~rring to F~g. 4, pul~e sorting logic 48 will b~ doscribed in greater detail. Logic 48 includes a plurality of N~ND gatos 78, ~0, 82, 84, 86, ~8, 90, 92v 94 and 96, each havi~
: two inputs a~d ~ single output, and 4-input NAND gates 98 and 100, each havl~ four input8 and a single Outpllt. Gate 78 receivc~ a ~ign~l C ~t ~ first input terminal and sicJnal B at a second input ter~inal. Gate 80 ~eceives gi~nal B at both input terminal~.

Gate 82 receive~ signal B at a first input terminal and ~ignal D
; at a second input terminal. Gate 84 receives signal E at a fir~t input terminal and sig~al A at a second input terminal. Gate B6 receives signal A at both input terminal~. Gate 88 receives lJ signal F at a first input terminal and signal A at ~ second input ~erminal. Gate 90 receives signal C at a fir~t input terminal and a signal ~A, representing the output signal from gate 80, at a second input terminalO Gate 92 receives signal D at a first input ~erminal and ~ignal AA from gate 80 at a second input terminal.
Gate 94 receives signal E at a first input terminal and a signal ,~ BB, repre~enting the output from gate 86, at a second input terminal. Gate 96 receives signal F at a first input terminal and signal B8 from gate 86 at a 5econd input terminal. Gate 98 receives a signal CC, representing the output from gate 78, at a fir~t input terminal, a signal DD, representing the output from gate 92, at a second input terminal, signal EE, representing the output fro~ gate 94, at a third input terminal, and signal FF, repxes~nting the output from qate 88, at a fourth input terminal.
Ou~put ~ignal H from g~te g8 is representative ~ the reverse ~, .-~v a f 0 ~

rota~ion pul~os only of encoder 16. Ga~e 100 receives an input signal GG~ representing the output from gate 90, at a first input terminal, a signal HH, representing the output from gate 82, at a ~econd input t~rminal, a signal II represen~ing the output from gate 8~, at a third input ter~in~l, and a ~ignal U, repre~enting the output fro~ ~ate 96, at a ourth input terminal.
Output signal G from gate 100 i~ representativa of the forward rotation pulses only of encoder 16.
A~ should be clear from the preceding description, in the circuit comprising the pulse sorting logic 48, each transition from high to low or from low to high of each signal A and B is oper~tive to cause either of the NA~D gates 98 or 100 to provide ~ s~gnal ~ndicating the encoder 16 has experienced a predetermined increment of rotation. Since two transitions occur in each of :
~ two encoders, each hole 21 causes four tr~n~itions per revolution.
:: 15 ; Since there are eighteen (18) holes in the encoder 16, the encoder ~- has a resolution of seventy-two counts per t~rn (four multiplied by eighteen) which in turn means that each signal G and H
`;: repre~ents five (5) degrees of rotation (360 ~ 72)- For each five ; (5) degrees of forward rotation of the encoder, NAND gate 100 out- - 20 puts the pulse G and for each five ~5) degrees of reverse rotation ox recoil of the encoder, NAND ga~e 98 outputs the pulse H.
; Operation of the pulse sorting logic should be clear :
from the preceding description, but will be explained briefly.

- Assume that encoder 16 is rotating in the ~orward direction, that 25 is, that the fa~tener is being tightened by the impact of hammer :~ 15 on anvil 12. Assume further that signal ~ is eXperiencing a :;

low to high tr~nsition and signal B, ninety degrees out of phase, is low. Undex these conditions, pulse C is produced by monostable multivibrator 44, and monostable multivibrators 46, 50 and 52 . :

, ~:~7~75;~:

have no output. NAND gate 78 receives high input pulse C and signal B which is at its low level so output signal CC is high;
NAND gate 80 receives the low input signals ~ so output signal AA is high; NAND gate 82 receives a low input siganl B and low input signal D so output signal H~ is high; NAND ga~e 84 receives the low inpu~ aignal E and high input signal A so output signal II is high; NAND gate ~6 receives ~kae high input signals A so out-put ~ign~l BB is low; and NAND gate 88 receive~ high input signal : . A and low input signal F so output signal FF i~ high. NAND gate 90 receiYes high input pulse C and high input signal AA so output signal GG i8 low; NAND gate 92 receives high input sicJnal AA and low input signal D so output signal DD is high; NAND gate 94 ; receive~ low input signal E and low input signal ~B so output slgnal EE is high; and NAN~ gate 96 receives low input signal and low input signal F 90 that output signal U is high. NAND
gate 98 receives high signal CC, high signal DD, high signal EE
and high ~ignal FF so ther~ i5 a low output signal. NA~D gate ". :
100 receives low signal GG, high signal ~H, high signal JI and ,- high signal U so there is provided a pulse G representative of an increment of forward rotation.
" 20 At the instant signal ~ e.YperienceS a low to high if ~; encoder 16 is rotating forward, signal A is still high so that monostable multivibrator 50 provides output pulse E while the output of monostable multivibrators 44, 46 and 52 remain low.
Working the logic through the various NAND gates it can be seen that NAND gate 98 receives high input signal CC, high input signal DD, high input signal EE and hiyh input signal FF so there i5 a low output signal. NAND gate 100 receives high input signal GG, .~ high input signal HH, low input signal II and high input signal ~ so outpue pulse G is provided.

"' . , :

77~S2 At the instant signal A experiences high to low transi-tions, if encoder 16 is still rotating forward, signal B is still high so that monostable multivibrator 46 provides output pulse D
while th~ output of mono~ta~le multivibratoss 44, 50 and 52 remaln low. Working the logic through the various NAND gates it can be seen that NAND gate 98 receives high input signal CC, high inp~t signal DD, high input signal EE and high inp~t signal FF so th~re is a low ou~put ~ignal. NAND gate 100 receiv~ high input qignal GG, low input ~ignal H~, h~gh input signal II and high input signal 0 U 50 output pulse G is provided.
At the instant signal B experiences a high to low transi-tion, if encoder 16 is still rotating forward, signal A is low .qo that mono9table multivibrator 52 provides output pulse F while the output of monostable multivibrators 44, 46 and S0 remains low.

. 15 Working the logic through the various N~ND gates it can be seen - that NAND gate 9~ receives high input signal CC, high input signal DD, high input signal EE and hiqh input signal FF so there is a ~ :
low output signal. NAND gate 100 receives high input signal GG, .~ high input signal HH, high input signal II and low input signal U

. so output pulse G is provided.
; Assume now that encoder 16 is rotating in the reverse ; direction, that i5, that the encoder is recoiling between impact~
of hammer lS on anvil 12. Assume further that signal B is e~-- periencing a low to high transition and signal A, ninety degrees out of phase is low. Under these conditions, pulse E is produced by monosta~le multivibrator 50 and monostable multivibrators 44, ~6 and 52 have no output. NAND gate 78 receives low input signal C and sig~ B which is high so output signal CC is high; NAND
gate 80 receives the high input signals B so output signal AA is : low; NAND gate 82 seceives high input signal B and low input signal . ~

, . . . .
. - . . .

~L~77752 D so output signal HH is high; NAND gate 84 réceives the high ln-~ put pulse E and low input signal A so output signal II is high;
.; NAND gate 86 receives the low input signals A so output signal B~
is high; and NAND gate 88 receives low input signal A and low in-put signal F iso output signal FF is high. NAND gate 90 receives low input signal C and low input signal AA so output signal GG
: i8 high; NAND gate 92 reGeives low input ~ignal AA and low input ~ signal D iso that ou put signal D iis high; NAND gate 94 receives .: the hlgh input pulse E and high input si~nal BB so output signal ~E is low; and NAND gate 9~ receives high input signal B~ and low . input signal F so output signal U is high. NAND gate 98 receivQs .. ~ high input ~ignal CC, high input signal DD, low input signal EE
~ and high input signal DD so there is provided a pulse H representa-- tive of an increment of reverse rotation. NAND gate lOO receives high input signal GG, high input signal HH, high input signal II
': 15 ~r. and high input siynal U so there is a low output signal.
......
At the instant signal A experiences a low to high transi-.~:; tion, if enc3der 16 is rotating in the reverse direction, ~ignal . . .
i5 still high so that monostable multivibrator 44 provides outpu~
; pulse C while the output of monostable multivibrators 46, 50 and 52 . remain low. Working the l~ic through the various NAND ~ates it .
, . . .
. can be seen that NAND gate 98 receives low input signal ~C, high ~ .
;,~, input signal DD, high input signal EE, high input siynal FF so out-, ~
i`~ put pulse H is provided. NAND gate lOO receives high input signal ii~.

: GG, high input sicJnal HH, hi~h input siynal II and high input .~.......... signal~u~so there is a low output signal.
At the instant signal B experiences a high to low transi-tion, if encoder 16 is still rotating in the reverse clirection, signal A ii~ still high so that monostable multivibrator 52 provides output pu}se F while the output o~ monosta~le multi-vibrator~ 4~, g6 and 50 remain low. Working the loyic through th~
. .
: - 23 . ~

~"

~7775Z

various NAND gates it can be seen that N~ND gate 98 receives hlgh input signal CC, high input signal DD, high input signal EE and 1OW input signal FF so output pulse H is provided. NAND gate l00 recaives high input siganl GG, high input signal HH, h.igh input 3ignal II and high input signal U so there i~ a low output ~ignal.
At the instant ~ignal A experiences a high to low transi-tion, if ancoder 16 i8 still rota~ing in the reverse direction, signal ~ i~ still low so that. monostable multiYibrator 46 provide~
output pul~e D while ~he output monostable multibibrators 44, 50 and 52 re~ains low. Working the logic through the variou~ NAND
gates it can be seen that NAND gate 98 receives high input signal CC, low input signal DD, high input signal EE and high input ~ignal FF ~o output pulse H is provided. NAND gate 100 receives high input signal GG, hiyh input signal HH, high input signal lI

and high input signal ~ so there is a low output signal.
Operation of the control system will now be described with ref~rence to all of the figures and particularly with refer-ence to Figs. 4 and 5. As the impact wrench begins to tighten a fastener, sensors 19 an~ 20 detect the passage of holes ~1 of encoder 16 and provide si~na1s A and H which are processed to provide pulses G representative of anguldr increments of rotation as explained previously. Pulses G are fed to the NAND gate 53 which also receives the signal from the inverter between the out-put of up/down counter storage unit 51. Since no reverse rotation slynals have been produced, the output of unit 51 is high and of : the inverter is low. Thus, with the low input from the illverter, each pulse G applied to the NAND gate 53 causes a high ou~put signal which ires the monostable multivibrator 54 which produces output ~ignal J similarly representative of the predetermined incre~ent of rotation. As previously explained ~ignal J is fed .
- 2~ -1`'' ~

: ~7~7~;Z

to the ring counter 56. After a preset number of pulses have been counted in counter 56 it produces output signal R. During the initial tightenlng impacts, counter 56 is continually reset to zero by signal Q so that it cannot count the preset number of pulse~ and, of course, so that signal R cannot be provided. Re-ferring particularly to Fig. 5, initial tightening produces a.~
steady increa~e in the angle of forward rotatio~ of encoder 16 t a~ ~hown by curve J at 102, with no corresponding increase in `;- either fastener preload or recoil time as indicated by curve ~.

Ai should also be clear from curve L, snug torque has not yet been applied to the fastener nor has the applied moment increased by more than the predetermined amount so that comparator 70 and peak value increase detector 64 have low output signals. Thus , exclu~ive NOR gate 66 outputs signal Q. It should be noted that ; successive pulses 5hown in curve J each denote a 5 increase in forward rotation of encoder 16 in the paxticular oscillographic record shown here for illustrative purposes. Actually the amount of forward rotation between pulses can be set at any deslred value , . . .
~epending on the degree o~ accuracy desired. When the fastener has been tightened sufficiently, causing it to contact a mating workpiece (not shown), a preload beglns to build up in the fastener as shown by the preload curve at 104 in Fig. 5. The preload was obtained by well known external instrumentation means (not shown) for purposes of explaining this invention, but it should be under- -stood that usually such instrumentation means is not utilized. At this point in the tightening cylce no measurable recoil of the hamner against the anvil in the wrench occurs. Upon furthertightening, sufficient resistance to further rotation is encountered causing the hammer to recoil upon striking ~he anvil, as shown by ~urve L at 106. It should be understood that recoil time is ~- - 25 .....
:.,.
. '! , ,~. .

~`
dependent on the residual strain energy ~tored in the im~act wrench driving shaft sockets and couplings, and this strain energy is dependent on the moment being applied, which moment varles with the instantaneous coefficient of friction as the fastener ~tops rotating. If signal L is equal to or exceeds some electrically equivalent predetermined snug torque value, which may be experimentally d~termined a~d set ~y adjusting ~he output from unit 72, si~nal P is fed to NOR gate 66 so that output signal Q
which reset~ counter 56 to zero is discontinued and the counter starts counting forward angle rotation signals J. It has been determined that the s~lection of a snug torque value from unit 72 is not critical to the operation of the wrench. The cri~eria used in selecting a snug torque value is that it be set hlgh enough to assure that preload is beginning to build up in the fastener, but that it not be set too high in the event that a maximum recoil value might occur before counter 56 is allowed ~o count forward rotation pulses J. In the pre~ent preferred embodi-ment, the snug torque value was set at the level of the first peak :
recoil value in stor~ge unit 62 and in practice is an ap~roxima-; tion of the torque required to build preload in the fastener.
Signal L representative of the peak recoil value at 106 is stored in the storage unit of peak value detector/storage unit - 62 and the amplifier unit in unit 62 outputs to peak value increas~
detector unit 64 providing output pulse M which is fed t~ ~tep generator 68 and NOR gate 66 causing signal Q to res~t counter . 56 to zero. Step generator 68 in Fig. 4 causes the previously highe~t recoil pulse L stored in unit 62 to be increased ~y a preset fixed or variable amount, thus building into the system ; successively higher recoil values than the previously highest stored value. For example, for the system shown by curve L
: 30 :`;
. .

~L~777~

of Fig. S, an incremental fixed amount of abcut 100mv is added for a peak value store of approximately 6 volts. This incremen~al i value may be varied depending on the accuracy desired. The practical cons~ralnts on this incremental value are that it be small enough ~o that subscquent higher peak recoil values are detected, but that it be large enough so that subsequent peak recoil value~ ju~t slightly greater than the previou~ly stored highe~t peak recoil value do not continue to reset counter 56.
lt should ~lso be u~d~rstood that a fixed percen~age of the previously ~tored highest peak recoil value could be added, such as two percent (2%), for example, with equally effective : result~. It can be seen from Fig. S that the initial peak recoil value of curve L at 106 causes curve O to increase to a first 3tored peak value at 108. The peak value at 108 of curve O is ; 15 exceeded by the recoil 110, that is the applied moment exceeds the applied moment at 106 by the previously described predeter-mined fixed amount. As described signal M ~see 114 curve M~ is produced causing NOR gate 66 to discharge signal Q resettin~
counter 56 to zero and causing step generator 68 to increase the value of ~he ignal L at 110 to be increased by the predetermined amount. This increased peak value is then stored in unit 62~ as indicated by curve O at 112. Counter 56 then must be~in counting forward rotation pulses J again. The next peak recoi1 value at . 116 exceeds the previous peak value at 110 by the predetermined : fixed amount and in the manner described causes peak value curve O to increase as shown at 118 and produce reset pulse 120 on curve :- M. Peak value 118 is stored in unit 62 until the next peak recoil . . .
value lZ2 of curve L occurs, which value exceeds previously highest peak recoil value 116 by the prede~ermined amount. A new peak value ~hown at 12~ of curve N occurs and a reset pulse 126 , 30 ;.
~ - 27 ~ , .:, ,~:

77~

on curve M is produced. Once again counter 56 is reset to ~ero and ~tarts counting forward rotation pulses J. Subsequent recoil .~ignals 128, 130, 132 and 134 do not excee~d prevlously highest recoil value 122 by the predetermined amou,nt, so that no higher . 5 p~ak value of curve N occurs after 1~4, no.r does a reset pulse :~ on curv~ M occur after 126. Counter 56 i~ then allowed to count successive forward ro~ation pulses 136, 138, 140, 142 and 144 of curVe J without interruption. In the present preferred em-bodiment repre~entated by Fig. 5, the prese~ number of pulses prGgrammed into counter 56 is five (5), thus causing a st~p signal 146 of curve R to be ~enerated. Stop 5ignal 146 is then fed into the control coil of solenoid value 76 to shut off the air supply to port 23 of the impact wrench. The number of angle pulses before shutoff of the wrench after the ~reviously highest stored peak recoil value can be varied by adjusting the preset programmed .: value of counter 56. As shown by the fastener preload curve, no significant further preload is induced in the Pastener beyond approximately the third angle pulse 140 after the previously highest stored peak recoil value 124. Thus the optimum shutoff point for the present preferrad embodiment occurs between angle :. 20 .. pulses 140 and 144 (i.e. 15 25 degrees of rotation after the last re~et pulse 126), but the counter is set at five (S) pulses to insure that the fastener has reached the yield point.
~ Having thus described the structure and operation of a -~ preferred embodiment of an impact wrench contxol system, somP of ::: 25 the many advan~ages of the present invention should now be readily ,, .
apparent. The control system provides a highly accurate Ind reliable means for ti~htening a joint ~o the yield point, that is, Por providing maximum preload in a fastener tightened by an impact-`~. type wrench, that is, a wrench wherein the tightening momenc is ,~'- 30 - 2~ -'~
..,,. ~

~ al7~

.
applied periodically. Since the control system is adaptive!
only minimal prior ~nowledge of the joint and fastener character-i~tics being tightened need be known in orc!er to insure tighteninq to the maximum attainable preload of the fastener, namely the yield point. As previously stated, tighteniny to maximum preload at the yield point of the fastener material insures a joint of max$~um efficiency with greatest resistance to loosening due to vibration and fatigue failure. The tightening cycle is very rapid, making the wrench ideally 5uitable for rapid dsselnbly line use. In addition to tightening fastenerto the yield point it should be understood that the method and apparatus according to this invention can be used to tighten fasteners to a similarly significant point, for example, preloads other than the yield point, by bullding into the fastener system a confiquration causing the fastener to deform at a predetermined preload such that the applied toxque levels out.
Obviously, many modifications and variations of the present invention are possible in light of the above teachings.
It i5 therefore understood that within the scope of the appended claims, the invention may be practiced otherwise than as specific-ally described.

. ., ,' ., i:, . . .
;,, ;-..

'

Claims (68)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In an impact wrench including a hammer impacting with an anvil to rotate an output shaft operative to tighten a fastener assembly to its yield point or some similarly significant point by applying torque thereto, a control system comprising:
means for developing a signal representative of the deceleration of the hammer after engagement thereof with the anvil;
calculator means responsive to said deceleration signal for determining the yield point or some similarly significant point of the fastener assembly; and control means responsive to said calculator means for producing a control signal when the fastener assembly is tightened to said point.

2. A control system in accordance with Claim 1 further comprising:
means for developing a signal representative of the angular displacement of the output shaft.

3. A control system in accordance with Claim 2 wherein said calculator means is also responsive to said angular displace-ment signal.

4. A control system in accordance with Claim 3 wherein said calculator means determines the largest deceleration signal during a first angular displacement of the output shaft and said control means produces said control signal only if a larger de-celeration signal is not developed during a second angular dis-placement of the output shaft.

5. A control system in accordance with Claim 4 wherein said first angular displacement occurs prior to reaching the largest deceleration signal and said second angular displacement occurs subsequent to reaching the largest deceleration signal.

6. A control system in accordance with Claim 5 wherein said control means produces said control signal after a predeter-mined number of degrees of said second angular displacement.

7. A control system in accordance with Claim 4 wherein said predetermined number of degrees of said second angular dis-placement is no greater than about 25 degrees.

8. A control system in accordance with Claim 4 wherein:
said calculator means includes means for storing the largest deceleration signal developed, and means for successively adding an incremental value to each of said previously stored largest deceleration signals; and said control means produces said control signal only if a larger deceleration signal equal to the previously stored largest deceleration signal plus said incremental value is not developed.

9. A control system in accordance with Claim 8 wherein said incremental value is a fixed percentage of the previously stored largest deceleration signal.

10. A control system in accordance with Claim 9 wherein said percentage is no greater than about 2%.

11. A control system in accordance with Claim 8 wherein said incremental value is a signal having a fixed value.

12. A control system in accordance with Claim 11 wherein said fixed value is no greater than about 100 millivolts for a deceleration signal having an amplitude of about 6 volts.

13. A control system in accordance with Claim 1 wherein said signal representative of the deceleration of the hammer is proportional to the duration thereof.

14. A control system in accordance with Claim 1 wherein said signal representative of the deceleration of the hammer is proportional to the displacement thereof.

15. A control system in accordance with Claim 1 wherein said signal representative of the deceleration of the hammer is proportional to the velocity thereof.

16. A control system in accordance with Claim 1 wherein said signal representative of the deceleration of the hammer is a signal proportional to the recoil of the hammer after impacting the anvil.

17. An impact wrench for tightening a fastener assembly comprising:
a motor;
a hammer assembly adapted to be driven by said motor;
an anvil adapted to be rotatingly impacted by said hammer assembly;
wrench means operatively attached to said anvil and adapted to drive the fastener assembly by applying torque thereto;
means for developing a signal representative of the recoil of said hammer after engagement thereof with said anvil;
calculator means responsive to said recoil signal for determining the yield point or some similarly significant point of the fastener assembly; and control means responsive to said calculator means for producing a control signal when the fastener assembly is tightened to said point.

18. An impact wrench in accordance with Claim 17 further comprising:
means for developing a signal representative of the angular displacement of the output shaft.

19. An impact wrench in accordance with Claim 18 wherein said calculator means is also responsive to said angular displace-ment signal.

20. An impact wrench in accordance with Claim 19 wherein said calculator means determines the largest recoil signal during a first angular displacement of the output shaft and said control means produces said control signal only if a larger recoil signal is not developed during a second angular displacement of the output shaft.

21. An impact wrench in accordance with Claim 20 wherein said first angular displacement occurs prior to reaching the largest recoil signal and said second angular displacement occurs subsequent to reaching the largest recoil value.

22. An impact wrench in accordance with Claim 21 wherein said control means produces said control signal after a predeter-mined number of degrees of said second angular displacement.

23. An impact wrench in accordance with Claim 22 wherein said predetermined number of degrees of said second angular dis-placement is no greater than about 25 degrees.

24. An impact wrench in accordance with Claim 20 wherein said calculator means includes means for storing the largest recoil signal developed, and means for successively adding an incremental value to each of said previously stored largest recoil signals; and said control means produces said control signal only if a larger recoil signal equal to the previously stored largest recoil signal plus said incremental value is not developed.

25. An impact wrench in accordance with Claim 24 where-in said incremental value is a fixed percentage of the previously stored largest recoil signal.

26. An impact wrench in accordance with Claim 25 where-in said percentage is no greater than about 2%.

27. An impact wrench in accordance with Claim 24 where in said incremental value is a signal having a fixed value.

28. An impact wrench in accordance with Claim 27 wherein said fixed value is no greater than about 100 millivolts for a recoil signal having an amplitude of about 6 volts.

29. An impact wrench in accordance with Claim 17 wherein said signal representative of the recoil of the hammer is propor-tional to the duration thereof.

30. An impact wrench in accordance with Claim 17 wherein said signal representative of the recoil of the hammer is propor-tional to the displacement thereof.

31. An impact wrench in accordance with Claim 17 wherein said signal representative of the recoil of the hammer is propor-tional to the velocity thereof.

32. An impact wrench in accordance with Claim 17 wherein said signal representative of the recoil of the hammer is propor-tional to the deceleration thereof.

33. A method of tightening a fastener assembly to its yield point or some similarly significant point by applying torque thereto with an impact wrench of the type including a hammer impacting with an anvil to rotate an output shaft operatively coupled to the fastener assembly comprising the steps of:
developing successive signals representative of the recoil of the hammer after engagement thereof with the anvil;
determining the yield point or some similarly significant point of the fastener assembly based upon a predetermined rela-tionship of said recoil signals; and producing a control signal when the fastener assembly is tightened to said point.

34. A method of tightening a fastener assembly in accordance with Claim 33 further comprising the step of develop-ing a signal representative of the angular displacement of the output shaft.

35. A method of tightening a fastener assembly in accor-dance with Claim 34 wherein said yield point or similarly signifi-cant point is determined further with respect to said angular dis-placement signal.

36. A method of tightening a fastener assembly in accor-dance with Claim 35 wherein said largest recoil signal is deter-mined during a first angular displacement of the output shaft and wherein said control signal is produced only if a larger recoil signal is not developed during a second angular displacement of the output shaft.

37. A method of tightening a fastener assembly in accor-dance with Claim 36 wherein said first angular displacement occurs prior to developing said largest recoil signal and said second angular displacement occurs subsequent to developing said largest recoil signal.

38. A method of tightening a fastener assembly in accor-dance with Claim 37 wherein said control signal is produced after a predetermined number of degrees of said second angular displace-ment.

39. A method of tightening a fastener assembly in accor-dance with Claim 38 wherein said predetermined number of degrees of said second angular displacement is no greater than about 25 degrees.

40. A method of tightening a fastener assembly in accor-dance with Claim 36 wherein said largest recoil signal developed is stored and an incremental value is successively added to each of the previously stored largest recoil signals, and wherein said control signal is produced only if a larger recoil signal equal to the previously stored largest recoil signal plus said incremental value is not developed.

41. A method of tightening a fastener assembly in accor-dance with Claim 40 wherein said incremental value is a fixed percentage of the previously stored largest recoil signal.

42. A method of tightening a fastener assembly in accor-dance with Claim 41 wherein said percentage is no greater than about 2%.

430 A method of tightening a fastener assembly in accor-dance with Claim 40 wherein said incremental value is a signal having a fixed value.

44. A method of tightening a fastener assembly in accor-dance with Claim 43 wherein said fixed value is no greater than about 100 millivolts for a recoil signal having an amplitude of about 6 volts.

45. A method of tightening a fastener assembly in accor-dance with Claim 33 wherein said signal representative of the recoil of the hammer is proportional to the duration thereof.

46. A method of tightening a fastener assembly in accor-dance with Claim 33 wherein said signal representative of the recoil of the hammer is proportional to the displacement thereof.

47. A method of tightening a fastener assembly in accor-dance with Claim 33 wherein said signal representative of the recoil of the hammer is proportional to the velocity thereof.

48. Apparatus for tightening a fastener, said apparatus comprising, wrench means for periodically applying a tightening moment to a fastener in a joint assembly;
first means for measuring the moment applied to the fastener during each period and for developing a signal represen-tative of the peak moment applied during each period; and control means responsive to said peak moment signals for determining when a peak moment signal has not increased by more than a predetermined amount and for developing a control signal.

49. Apparatus in accordance with Claim 48 wherein said control means includes second means for determining when a plural-ity of peak moment signals have not increased by more than a predetermined amount and wherein said control signal is developed when said second means has made said determination.

50. Apparatus in accordance with Claim 48 wherein said control means includes second means for determining when a plural-ity of successive peak moment signals have not increased by more than a predetermined amount during a predetermined period in which peak moment signals are developed and wherein said control signal is developed when said second means has made said determination.

51. Apparatus in accordance with Claim 50 wherein said predetermined period is a predetermined rotational displacement of the fastener being tightened.

52. Apparatus in accordance with Claim 50 further inclu-ding third means for measuring the rotation of the fastener being tightened and for developing a signal representative thereof and wherein said control means is responsive to said rotation signal for measuring said predetermined period.

53. Apparatus in accordance with Claim 50 further inclu-ding third means for developing signals representative of increments of a second tightening characteristic related to the periods during which the tightening moment is applied and wherein said predeter-mined period is a predetermined number of said increments.

54. Apparatus in accordance with Claim 53 wherein said second tightening characteristic is rotational displacement of the fastener being tightened and wherein said third means measures increments of said rotational displacement.

55. Apparatus in accordance with Claim 48 wherein said control means includes storage means for storing a peak moment signal and comparator means for comparing the stored peak moment signal with an instantaneous peak moment signal for determining the difference therebetween.

56. Apparatus in accordance with Claim 55 including second means for increasing the stored signal by a predetermined amount.

57. Apparatus in accordance with Claim 48 wherein said control means includes comparator means and storage means, said comparator means receiving an instantaneous peak moment signal and a stored signal from said storage means and outputting an indicator signal when said instantaneous peak moment signal exceeds said stored signal, said comparator means also outputting said instan-taneous peak moment signal to said storage means, signal generator means responsive to said indicating signal for increasing said instantaneous peak moment signal in said storage means by a pre-determined amount.

58. Apparatus in accordance with Claim 48 wherein said control signal is operative to discontinue the application of the tightening moment.

59. A control system usable with a wrench for controlling the tightening of a fastener, said system comprising:
first means for periodically developing a signal represen-tative of the instantaneous moment applied to a fastener; and control means responsive to said instantaneous moment signals for determining when an instantaneous moment signal has not increased by more than a predetermined amount and for developing a control signal.

60. A system in accordance with Claim 59 wherein said control means includes second means for determining when a plural-ity of instantaneous moment signals have not increased by more than a predetermined amount and wherein said control signal is developed when said second means has made said determination.

61. A system in accordance with Claim 59 wherein said control means includes second means for determining when a plural-ity of successive instantaneous moment signals have not increased by more than a predetermined amount during a predetermined period in which instantaneous moment signals are developed and wherein said control signal is developed when said second means has made said determination. 38

62. A system in accordance with Claim 61 wherein said predetermined period is a predetermined rotational displacement of a fastener being tightened.

63. A system in accordance with Claim 61 further inclu-ding third means for measuring the rotation of a fastener being tightened and for developing a signal representative thereof and wherein said control means is responsive to said rotation signal for measuring said predetermined period.

64. A system in accordance with Claim 61 further inclu-ding third means for developing signals representative of incre-ments of a second tightening characteristic related to the periods over which the instantaneous moment signals are developed and wherein said predetermined period is a predetermined number of said increments.

65. A system in accordance with Claim 64 wherein said second tightening characteristic is rotational displacement of a fastener being tightened and wherein said third means develops signals representative of increments of said rotational displace-ment.

66. A system in accordance with Claim 59 wherein said control means includes storage means for storing an instantaneous moment signal and comparator means for comparing the stored sig-nal with an instantaneous moment signal for determining the dif-ference therebetween.

67, A system in accordance with Claim 66 including second means for increasing the stored signal by a predetermined amount.

68. A system in accordance with Claim 59 wherein said control means includes comparator means and storage means, said comparator means receiving an instantaneous moment signal and a stored signal from said storage means and outputting an indicator signal when said instantaneous moment signal exceeds said stored signal, said comparator means also outputting said instantaneous moment signal to said storage means, signal generator means res-ponsive to said indicating signal for increasing said instantaneous moment signal in said storage means by a predetermined amount.