GB1560529A - Wrench having a pulsed output - Google Patents

Description

PATENT SPECIFICATION

an ( 21) Application No 20619/76 ( 22) Filed 19th May 1976 (>' ( 31) Convention Application No ( 32) Filed 19th May 1975 in 579110 0 ( 33) United States of America (US) Uj'; ( 44) Complete Specification Published 6th February 1980 ( 51) INT CL 3 B 25 B 23/00 ( 52) Index at Acceptance:

B 3 N2 A 2 2 A 4 2 E 2 3 JX G 3 N 287 292 371 DB ( 54) WRENCH HAVING A PULSED OUTPUT ( 71) We, SPS TECHNOLOGIES, INC.

(formerly known as Standard Pressed Steel Co.), a Corporation duly organised and existing under the laws of the Commonwealth of Pennsylvania, United States of America, of Jenkintown, Pennsylvania 19046, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:-

This invention relates to wrenches having pulsed outputs and more particularly to such wrenches having control systems for accurately controlling the tension in a fastener of a joint.

It is well known in the prior art that tightening a fastener to its yield point produces optimum joint efficiency A fastened joint having a greater preload value up to the yield point of the material of the joint is more reliable and insures better fastener performance High fastener preload further increases 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 control systems for controlling the amount of preload in a fastener.

One commonly used type employs some form of torque control, in which the impact wrench tightens a fastener to a maximum predetermined value of torque and thereupon shuts off Examples of impact wrenches utilising torque control can be found in U S Patents to Schoeps et al No.

3,835,934, Hall, No 3,833,068, Schoeps, No 3,703,933, Vaughn, No 3,174,559, Eilliot et al No 3,018,866 and Maurer, No.

2,543,979 Another means of controlling impact wrenches found in the prior art is commonly known as a "turn-of-the-nut" system, in which a fastener is tightened to some preselected initial condition, such as a predetermined torque value or spindle speed, and thereupon rotated an additional predetermined number of degrees before shutting off Examples of various turn-ofthe-nut impact wrench systems are found in U.S Patents to Allen, No 3,623,557, Hoza et al, No 3,318,390 and Spyradakis et al, No.3,011,479 Another type of control comprises imparting a constant angular momentum of each impulse blow, such as found in the U S Patent to Swanson, No.3,181,672.

As can be seen from the numerous existing prior art systems, the problem is not a novel one The ultimate desired result is to achieve maximum preload of the fastener into the yield region The common problem which each of the prior art systems attempts to solve is determining when the yield point of the fastener has been reached In all of the control systems described in the abovenoted patents, prior knowledge of the fastener and joint characteristics must be known or assumed in order to determine either the exact predetermined final torque, the exact amount of additional rotation, the amount of constant angular momentum of each impact blow or the predetermined amplitude of hammer rebound Such systems are known as open loop control systems It is well known that tightening to a predetermined preload condition, such as the yield point, is a function of many variables, among them being joint stiffness, fastener stiffness, surface friction between mating threads and thread form Therefore, in each of the prior art systems the yield point cannot always be accurately determined because the conditions of each fastener and joint vary and may not be known in advance This consequence can lead to uneven tightening from joint to joint in a structure, which can in turn result in loosening of the fastener in the joint and 1) 1 560 529 1 560 529 premature fatigue failure.

It is known from the characteristics of fasteners that yield related phenomena occur 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 occurring Because of the nature of operation of certain types of wrenches, a continuous moment is not applied For example, in an impact wrench a series of pulsed impacts of a hammer onto an anvil advances the fastener into a workpiece.

During each impact, when the fastener has been tightened until it presents maximum resistance 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 reached At this point, the hammer is subjected to its maximum deceleration which is proportional to its maximum applied moment, and experiences a recoil, the magnitude 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 accordance with this invention, the deceleration of the hammer in the form of its rotary motion is sensed by a recoil or bounce back mechanism The magnitude of the recoil, either its duration, force, velocity or total distance of travel, gives a measure of the deceleration of the hammer and, hence, the maximum applied moment However, it has been found to be relatively easy to measure duration of recoil Thus the recoil time and the angle of rotation of the anvil can be monitored simultaneously, but a graph showing one as a function of the other is somewhat hypothetical as recoils only occur at the end of a blow while angular displacement occurs during a blow By convention, therefore, the graphs are plotted as angular displacement at constant moment followed by a change of moment at constant angle.

According to the invention, apparatus for tightening a fastener comprises in combination wrench means having a pulsed output for periodically applying a tightening moment to a fastener in a joint assembly and a control system, said system comprising first means responsive to the moment applied to the fastener during each tightening pulse and for developing a signal representative of the peak moment applied during each application and calculator means responsive to said peak moment signals for determining when an instantaneous peak moment signal has not exceeded a previous largest instantaneous peak moment signal by more than a predetermined amount and control means responsive to said calculator means for developing a control signal when said calculator means has determined that said instantaneous peak moment signal has not exceeded said previous largest instantaneous 70 peak moment signal by more than said predetermined amount.

According to another aspect of the invention, an impact wrench includes a hammer arranged to impact an anvil to 75 rotate an output shaft operative to tighten an assembly, including a fastener, to the yield point or to a similarly significant point in a tightening cycle by applying torque to the fastener, and a control system for the 80 wrench, the control system comprising means for developing a signal representative of the deceleration of the hammer after engagement thereof, with the anvil; calculator means determining the largest deceleration 85 signal developed up to any instant in the tightening cycle and determining when a larger deceleration signal is not developed subsequently during the tightening cycle and hence determining that the yield or said 90 similarly significant point has been reached, and control means responsive to said calculator means for producing a control signal when the assembly is tightened to the yield or said similarly significant point 95 According to yet another aspect of the invention, an impact wrench includes a hammer arranged to impact an anvil to rotate an output shaft operative to tighten an assembly, including a fastener, to the 100 yield point or to a similarly significant point in a tightening cycle by applying torque to the fastener and a control system for the wrench, the control system comprising means for developing a signal representative 105 of the deceleratiion of the hammer after engagement thereof with the anvil; means for developing a signal representative of the angular displacement of the output shaft:

calculator means responsive to said deceler 110 ation signal and said angular displacement signal and for determining the yield or similarly significant point in a tightening cycle of the assembly, said calculator means determining the largest deceleration signal 115 during a first angular displacement of the output shaft; and control means responsive to said calculator means for producing a control signal when the assembly is tightened to said yield or similarly signifi 120 cant point, said control means producing said control signal only if a larger deceleration signal is not developed during a subsequent second angular displacement of the output shaft 125 The impact wrench may further include apparatus for measuring, or developing a signal representative of, the recoil of the hammer, the apparatus comprising first means operatively coupled to the hammer 130 1 560 529 for movement in the direction of recoil therewith; second means juxtapositioned with respect to said first means and being rotatably movable between a first and a second position; force transmitting means disposed between said first means and said second means for transmitting force therebetween due to recoil of the hammer, biasing means attached to said second means for exerting a force thereupon toward said first position, said force being in a direction opposite to the direction of recoil of the hammer; and measuring means for measuring a characteristic of the movement of said second means between said first and second positions.

The invention also includes a method of tightening an assembly including a fastener to its yield or similarly significant point in the tightening cycle by applying torque thereto with an impact wrench, according to the immediately preceding paragraph, the method comprising the steps of developing successive signals representative of the recoil of the hammer after engagement thereof with the anvil; developing a signal representative of the angular displacement of the output shaft; determining the yield point of the assembly based upon a desired relationship between said recoil signals and further with respect to said angular displacement signal, said largest recoil signal being determined during a first angular displacement of the output shaft; and producing a control signal when the assembly is tightened to said yield point, said control signal being produced only if a larger recoil signal is not developed during a subsequent second angular displacement of the output shaft.

By way of example an impact wrench constructed in accordance with the invention is now described with reference to the accompanying drawings, in which:Figure 1 is a side elevational view of the impact wrench partially cut away and in cross-section, showing an angle encoder and sensing means; Figure 2 is a front elevation view of the angle encoder shown in Figure 1; Figure 3 is a transverse sectional view taken along the line 3 3 of Figure 1 looking in the direction of the arrows, showing the recoil detection apparatus; Figure 4 is a block diagram of the control circuit of the impact wrench of Figure 1, and Figure 5 is a graph showing the various parameters during the operation of the wrench.

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 Figure 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 70 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 off so that only small additional preload is induced in 75 the fastener This levelling 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 80 time as illustrated in curve L (RECOIL TIME) and curve N (PEAK VALUE) It is merely noted here that recoil time is representative of the applied moment.

In accordance with this invention a 85 fastener is tightened to its yield point by periodically applying a tightening moment to the fastener and determining the peak moment applied during each period By "peak moment" is meant the largest 90 moment applied during each period The instantaneous peak moment is compared with the largest peak moment which has been applied previously during the tightening cycle to determine if the instantaneous peak 95 moment exceeds the previous largest peak moment by more than a predetermined amount The predetermined amount may vary slightly for fasteners of different types but it has been determined that the pre 100 determined amount is normally about 2 % of the previous largest peak moment in which case the predetermined amount is variable It has also been determined that the 2 % can be approximated and an 105 absolute value can be used, for 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 110 more than the predetermined amount, the application of the tightening moment continues and the instantaneous peak moment is stored for comparison with the next instantaneous peak moment; if the instan 115 taneous peak moment does not exceed the previous largest peak moment by more than the predetermined amount the application of the tightening moment can be discontinued since this indicates that the fastener 120 has been tightened to its yield point as should be understood from the explanation of the relationships between preload and time and between moment and time.

Referring briefly to curve L (RECOIL 125 TIME) in Figure 5 it can be seen that during some periods before the fastener has been tightened to its yield point the instantaneous peak moment is less than the largest peak moment The occurrences are random in the 130 1 560 529 sense that they are not predictable and it is possible that the application of the tightening moment could be discontinued before e the yield point is reached Accordingly, it is desirable not to discontinue the application of the tightening moment until the instantaneous peak moment has not exceeded the previous largest peak moment by the predetermined amount for a predetermined number of successive periods during which the moment is applied While two such detections are sufficient, three to five is preferable It has been found preferable to measure a second tightening characteristic related to the period during which the moment is applied, for example, to measure angular rotation of the fastener during the tightening cycle, and not to discontinue the application of the tightening moment until the instantaneous peak moment has not exceeded the previous largest peak moment by more than the predetermined amount during a predetermined rotation of the fastener, for example, during 15 to 25 degrees In this way, it can be assured that the applied moment is operative to cause rotation of the fastener even though the torque is levelling off It should be understood that other characteristics could be measured instead of rotation so long as these other characteristics are related to the moment in the same general way as 6 rotation That is, any characteristic related to the moment such that the moment levels off with respect to that characteristic can be used in place of rotation Time, for example, can be used.

While any type of wrench system applying torque periodically can operate to perform the method, the preferred embodiment disclosed herein is an impact wrench.

Referring to Figures 1, 2 and 3, an impact wrench 10 is shown, which may be any one of many conventional types that include an external source of compressed air suitably connected to the wrench in order to successively impact a hammer onto an anvil An anvil 12 is rotatably secured within the forward portion of the wrench housing 11 by a bearing 13 The forward end 14 of anvil 12 comprises, for example, a square drive for attachment to a drive socket or some other suitably shaped wrenching member for driving a fastener A hammer assembly 15 connected to and driven by a conventional air motor (not shown) surrounds and contacts anvil 12 imparting impact blows to rotate the anvil and drive a fastener (not shown) Wrench 10 also includes a conventional trigger 22, which, when depressed, allows air from the external source (not shown) to enter wrench at an inlet port 23 connected to an air motor (not shown) driving hammer 15 to rotate anvil 12 A bidirectional incremental encoder 16 used in a system 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 70 mating with a corresponding recess 18 in anvil 12 Since the anvil 12 drives the wrenching member driving the fastener, the encoder 16 rotates with the fastener as the fastener is tightened Between impacts of 75 the hammer 15 against the anvil 12, the anvil and encoder 16 recoil Thus the rotation measuring system in which the encoder 16 is used should be capable of detecting and disregarding the recoil of the 80 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 housing 11, each at a fixed radius from the centre line of anvil 12 85 so that holes 21 will move into registration with each sensor Sensors 19 and 20 are preferably of a magnetic type, that is, could include an induction coil whose output varies due to the presence of absence of 90 metal, but any other suitable proximity type sensor may be used to detect the passage of successive ones of holes 21 during operation of the wrench As can be seen in Figure 2, the encoder 16 in the preferred embodiment 95 contains eighteen ( 18) equally spaced holes, the centre lines of each hole being 200 apart at a fixed radius from the centre line of the encoder As will be explained later the output signals of the sensors 19 and 20 are 10 l ninety ( 900) degrees out of phase so the sensors are spaced apart to provide that result Thus, the sensors 19 and 20 could be spaced apart a distance equal to the sum of five ( 50) degrees plus some whole number 10 ' multiplied by twenty ( 200) degrees, for example twenty-five ( 250), forty-five ( 450), sixty-five ( 650), etc degrees Resolution with this encoder is 72 counts per revolution as will be explained later It should be 11 understood that the encoder could contain any reasonable number of holes depending on the degree of accuracy desired, the only requirement being that the holes are spaced equally apart from each other A proximity 11 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 indicating deceleration in the form of recoil or bounce back 12 of the hammer As noted previously the deceleration of the hammer is proportional to the peak moment applied during each impact.

The bidirectional incremental encoder is 12 the subject of our Patent Application No.

7901523, Serial No 1 558 560.

Referring now to Figure 3, the bounce back or recoil indicating mechanism is shown An output shaft 30 from the air 13 :1 S 1 560 529 motor (not shown) is connected through a conventional one-way clutch 31, to a rotatable sleeve 32 having an arm 33 extending from the surface thereof The arrows on clutch 31 indicate that the normal direction of rotation of shaft 30 is clockwise when viewed in a direction opposite to that of the arrows 3 3 in Figure 1 Clutch 31 transmits rotational force to sleeve 32 when hammer 15, which is suitably connected to be rotated by shaft 30, rebounds off anvil 12 in the counter-clockwise direction when viewed in a direction opposite to that of the arrows 3 3 in Figure 1 after imparting a blow thereto Sleeve 32 is located inside cutout 34 at the rear portion of wrench housing 11 A spring 35 is attached at one of its ends in some suitable manner at a point 36 adjacent the distal end of arm 33, and at its other end at a point 37 adjacent the bottom of wrench 10 Spring 35 is typically an elongated coil spring, but may be any other suitable elastic tensioning device for exerting a downward force on arm 33 An end stop 38 is mounted at the bottom of wrench 10 and extends upwardly at an angle with its distal end 39 proximate 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 is rotated, the energy stored in the hammer and anvil drops to a point where resistance to further rotation caused by tightening of the fastener in the workpiece begins to occur Upon:n further tightening, a deceleration of the hammer at the end of a blow in the form of a recoil occurs, the duration, total displacement, velocity and force of the recoil being proportional to the applied moment the force of the recoil is transmitted through shaft 30 and clutch 31 to sleeve 32, which is initially in a position indicated by the dotted lines in Figure 3 with arm 33 resting on distal end 39 of end stop 38 The force of the recoil causes shaft 30 and sleeve 32, coupled together by clutch 31, to rotate in a counter clockwise direction looking forward (i e clockwise as seen in Figure 3), causing arm 33 to move upwardly off end 39 of stop 38 against the restoring force of spring 35.

This restoring force causes arm 33 to return to its initial position resting on distal end 39 of stop 38 after some finite duration of time which is proportional to the recoil energy, and thus the deceleration of hammer 15.

Sensor 24 measure the duration of time it takes arm 33 to complete its cycle The duration of time is, as mentioned hereinabove, dependent upon the amount of recoil energy transmitted from hammer 15 to shaft 30, the maximum amount of recoil energy occurring at approximately the maximum preload in the fastener, at or near the fastener yield point It should be understood that either the distance travelled or velocity of arm 33 travel, or force exerted by spring 35 on pin 37 could alternatively be measured, as they are all proportional to 70 hammer deceleration and thus the applied moment.

In another embodiment of the recoil detection apparatus, clutch 31 could be replaced by a viscous Newtonian fluid suit 75 ably contained between shaft 30 and sleeve 32 Viscous drag force of the fluid would then transmit the recoil force of the hammer which is coupled to shaft 30, to sleeve 32 Measurement of the total 80 duration of the recoil would be exactly as described above.

Referring to Figure 4, a control system is shown for controlling the tightening cycle of wrench 10 The coils of sensors 19 and 20 85 are supplied with a suitable voltage and as the encoder 16 rotates, the sensors' outputs vary depending on whether a hole 21 or the metal between holes is adjacent their ends.

For example, the sensors 19 and 20 can be 90 arranged to provide a high output when metal is detected and a low output when it is not The output signal from sensor 19 is fed into an amplifier 40, and the output signal from sensor 20 is similarly fed into an 95 amplifier 42, in order to amplify the respective angle signals to a magnitude at which they are compatible with the rest of the control system Signal A from amplifier is characteristically 90 out of phase (+) 100 with signal B from amplifier 42, the signals having a characteristic square wave shape the pulse widths of which are proportional to the radian spacing between holes 21.

Output signal A from amplifier 40 is fed 105 concurrently into a first monostable 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 110 and reverse rotations of angle encoder 16.

Logic 48 will be described in greater detail hereinafter Output signal B from amplifier 42 is fed concurrently into a first monostable multivibrator 50 having a 115 positive trigger, a second monostable multivibrator 52 having a negative trigger and pulse sorting logic 48 Output signal C from multivibrator 44 is characteristically a sharp pulse corresponding to the positive-going 120 portion of signal A, and output signal D from multivibrator 46 is a pulse corresponding to the negative-going portion of signal A Similarly, output signal E from multivibrator 46 is a pulse corresponding to the 125 positive-going portion of signal B, and output signal F from multivibrator 52 is a pulse corresponding to the negative-going portion of signal B Signals C, D, E, and F are each introduced into the pulse sorting 130 1 560 529 logic 48 along with signals A and B The pulses produced by forward and reverse rotations of angle encoder 16 are separated in logic 48, 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 51 which counts the number of forward and reverse rotation pulses and stores this information Unit 51 may typically comprise a synchronous 8-bit up/down binary counter which includes two 4-bit binary counters in cascade Counter/ storage unit 51 acts as an inhibitor of forward rotation pulses G through a NAND gate 53 and is 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 53 when it is 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 counting down from zero or counting up to zero Signal G representing the forward rotation pulses and output signal I from unit 51 are fed into a conventional NAND gate 53 which only allows the net forward pulses to pass when there are no reverse rotation pulses stored in unit 51 Signal I is characteristically a single step function The output from gate 53 is fed into a monostable multivibrator 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 may also be referred to as a divide-by-10 counter/ divider with ten decoded outputs, and is typically a pair of 5-bit shift registers connected serially Output signal J from multivibrator 54 is characteristically a pulse representing net forward angular rotation of encoder 16.

The 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 (Figure 3) to move off, and return to rest upon end 39 of stop 38 It should be understood that force, velocity or distance of recoil could also be used with equally successful results as they are each similarly proportional to the applied moment Signal K, which is a square wave whose width is proportional to total recoil time, is fed into a ramp generator 60 which produces a characteristic ramp 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 unit 62 which stores the maximum or peak value of recoil time from sensor 24 A first output from unit 62 is fed into a peak value increase detector 64, which is typically a monostable multivibrator, producing an output pulse M Output signal M from detector 64 is characteristically a sharp pulse and is fed 70simultaneously into an exclusive NOR gate 66 and a step generator 68 which provides an output signal N which increases the instantaneous signal L stored in the storage unit of peak value detector and storage unit 75 62 by a fixed or variable amount for each input pulse M received Output signal N from step generator 68 is a square wave of short duration and fixed amplitude As will be more fully explained in the description of 80 operation of the control system, a fixed value of voltage may be added (l O Om V, for example), or a fixed percentage of the maximum stored peak recoil value may be added ( 2 %, for example) The increased peak re 85 coil value output signal from the storage unit is fed back for comparison with incoming signal L The storage unit of the peak value detector and storage unit 62 provides an output signal 0, indicative of the 90 increased peak value into a voltage comparator 70, which is typically an operational amplifier, receiving a second input signal from a snug torque setting unit 72 By snug torque is meant the torque at which the 95 fastener has pulled the joint parts together and wherein preload is being induced.

Signal O has a characteristic stepped ramp function profile Unit 72 may be any suitable variable voltage producing device, such 100 as a potentiometer, in which a voltage proportional to some determinable snug torque is generated The voltage levels from detector and storage unit 62 and setting unit 72 are compared in comparator 70, and 105 when the first is at least equal to the second, an output signal P from comparator 70, is fed into NOR gate 66 which also receives a second input signal M from detector 64.

Signal P has a characteristic single step 110 function shape As is conventional NOR gate 66 will provide a high output signal Q only when it has two low input signals or two high input signals Thus, before the fastener has been tightened to its snug 115 torque and with no increased peak value signal from the storage unit in unit 62, that is, with both inputs low, NOR gate 66 provides an output signal Q which resets counter 56 to zero When the snug torque is 120 reached, signal P is fed from comparator 70 so that the NOR gate does not give the output signal Q and counter 56 can now count.

If, after the snug torque is reached, a signal L exceeds the previous maximum signal L 125 by the predetermined amount added by signal N, monostable multivibrator 64 outputs signal M to NOR gate 66 so that signal Q is again fed to counter 56 resetting the counter to zero Thus, if the instan 130 1 560 529 taneous peak applied moment does not exceed the previous maximum peak applied moment by the predetermined amount over an interval of rotation equal to a predetermined number of counts multiplied by the predetermined increment of rotation sensed by the encoder, then counter 56 will give an output signal R which is a single step function amplified in amplifier 74 and fed to the coil of a conventional solenoid valve 76 for shifting the spindle of 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 supply to port 23 of wrench 10 is closed.

Still referring to Figure 4, pulse sorting logic 48 will be described in greater detail.

Logic 48 includes a plurality of NAND gates, 78, 80, 82, 84, 86, 88, 90, 92, 94 and 96, each having two inputs and a single output, and 4-input NAND gates 98 and 100, each having four inputs and a single output.

Gate 78 receives a signal C at a first input terminal and signal B at a second input terminal Gate 80 receives signal B at both input terminals Gate 82 receives signal B at a first input terminal and signal D at a second input terminal Gate 84 receives signal E at a first input terminal and signal A at a second input terminal Gate 86 receives signal A at both input terminals Gate 88 receives signal F at a first input terminal and signal A at a second input terminal.

Gate 90 receives signal C at a first input terminal and a signal AA, representing the output signal from gate 80, at a second input terminal Gate 92 receives signal D at a first input terminal and signal AA from gate 80 at a second input terminal Gate 94 receives signal E at a first input terminal and a signal BB, representing the output from gate 86, at a second input terminal.

Gate 96 receives signal F at a first input terminal and signal BB from gate 86 at a second input terminal Gate 98 receives a signal CC, representing the output from gate 78, at a first input terminal, a signal DD, representing the output from gate 92, at a second input terminal, signal EE, representing the output from gate 94, at a third input terminal, and signal FF, representing the output from gate 88, at a fourth input terminal Output signal H from gate 98 is representative of the reverse rotation pulses only of encoder 16 Gate 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 second input terminal, a signal II representing the output from gate 84, at a third input terminal, and a signal U, representing the output from gate 96, at a fourth input terminal Output signal G from gate 100 is representative of the forward rotation pulses only of encoder 16.

As 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 70 signal A and B is operative to cause either of the NAND gates 98 or 100 to provide a signal indicating that the encoder 16 has experienced a predetermined increment of rotation Since two transitions occur in each 75 of two encoders, each hole 21 causes four transitions per revolution Since there are eighteen ( 18) holes in the encoder 16, the encoder has a resolution of seventy-two counts per turn (four multiplied by 80 eighteen) which in turn means that each signal G and H represents five ( 5) degrees of rotation ( 360 72) For each five ( 5) degrees of forward rotation of the encoder, NAND gate 100 provides output pulse G and for 85 each five ( 5) degrees of reverse rotation or recoil of the encoder, NAND gate 98 provides output pulse H.

Operation of the pulse sorting logic should be clear from the preceding descrip 90 tion, but will be explained briefly Assume that encoder 16 is rotating in the forward direction, that is, that the fastener is being tightened by the impact of hammer 15 on anvil 12 Assume further that signal A is 95 experiencing a low to high transition and signal B, ninety degrees out of phase, is low Under these conditions, pulse C is produced by monostable multivibrator 44, and monostable multivibrators 46, 50 and 52 100 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 B so output signal AA is high; 105 NAND gate 82 receives a low input signal B and low input signal D so output signal HH is high; NAND gate 84 receives the low input signal E and high input signal A so output signal II is high; NAND gate 86 110 receives the high input signal A so output signal BB is low; and NAND gate 88 receives high input signal A and low input signal F so output signal FF is high NAND gate 90 receives high input pulse C and high 115 input signal AA so output signal GG is low; NAND gate 92 receives high input signal AA and low input signal D so output signal DD is high; NAND gate 94 receives low input signal E and low input signal BB so 120 output signal EE is high; and NAND gate 96 receives low input signal BB and low input signal F so that output signal U is high.

NAND gate 98 receives high signal CC, high signal DD, high signal EE and high 125 signal FF so there is a low output signal.

NAND gate 100 receives low signal GG, high signal HH, high signal II and high signal U so there is provided a pulse G representative of an increment of forward 130 1 560 529 rotation.

At the instant signal B experiences a low to high value 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 high input signal FF so there is a low output signal NAND gate 100 receives high input signal GG, high input signal HH, low input signal II and high input signal U so output pulse G is provided.

At the instant signal A experiences high to low transitions, if encoder 16 is still rotating forward, signal B is still high so that monostable multivibrator 46 provides output pulse D while the output of monostable multivibrators, 44, 50 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 high input signal FF so there is a low output signal NAND gate 100 receives high input signal GG, low input signal HH, high input signal II and high input signal U so output pulse G is provided.

At the instant signal B experiences a high to low transition, if encoder 16 is still rotating forward, signal A is low so that monostable multivibrator 52 provides output pulse F while the output of monostable multivibrators 44, 46 and 50 remains 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 high 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 is, that the encoder is recoiling between impacts of hammer 15 on anvil 12 Assume further that signal B is experiencing a low to high transition and signal A, ninety degrees out of phase is low Under these conditions, pulse E is produced by monostable multivibrator 50 and monostable multivibrators 44, 46 and 52 have no output NAND gate 78 receives low input signal C and signal 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 receives high input signal B and low input signal D so output signal HH is high; NAND gate 84 receives the high input 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 BB is high; and NAND gate 88 receives low input signal A and low input signal F so output signal FF is high NAND gate 90 receives low input signal C and low input signal AA so 70 output signal GG is high; NAND gate 92 receives low input signal AA and low input signal D so that output signal D is high; NAND gate 94 receives the high input pulse E and high input signal BB so output signal 75 EE is low; and NAND gate 96 receives high input signal BB and low input signal F so output signal U is high NAND gate 98 receives high input signal CC, high input signal DD, low input signal EE and high in 80 put signal DD so there is provided a pulse H representative of an increment of reverse rotation NAND gate 100 receives high input signal GG, high input signal HH, high input signal II and high input signal U so 85 there is a low output signal.

At the instant signal A experiences a low to high transition, if encoder 16 is rotating in the reverse direction, signal B is still high so that monostable multivibrator 44 pro 90 vides output pulse C while the outputs of monostable multivibrators 46, 50 and 52 remain low Working the logic through the various NAND gates it can be seen that NAND gate 98 receives low input signal 95 CC, high input signal DD, high input signal EE, high input signal FF so output pulse H is provided NAND gate 100 receives high input signal GG, high input signal HH, high input signal II and high input signal U so 100 there is a low output signal.

At the instant signal B experiences a high to low transition, if encoder 16 is still rotating in the reverse direction, signal A is still high so that monostable multivibrator 105 52 provides output pulse F while the output of monostable multivibrators 44, 46 and 50 remain low Working the logic through the various NAND gates it can be seen that NAND gate 98 receives high input signal 110 CC, high input signal DD, high input signal EE and low input signal FF so output pulse H is provided NAND gate 100 receives high input signal GG, high input signal HH, high input signal II and high input signal U 115 so there is a low output signal.

At the instant signal A experiences a high to low transition, if encoder 16 is still rotating in the reverse direction, signal B is still low so that monostable multivibrator 120 46 provides output pulse D while the output monostable multivibrators 44, 50 and 52 remains low Working the logic through the various NAND gates it can be seen that NAND gate 98 receives high input signal 125 CC, low input signal DD, high input signal EE and high input signal FF so output pulse H is provided NAND gate 100 receives high input signal GG, high input signal HH, high input signal II and high input signal U 130 1 560 529 so there is a low output signal.

Operation of the control system will now be described with reference to all of the Figures and particularly with reference to Figures 4 and 5 As the impact wrench begins to tighten a fastener, sensors 19 and detect the passage of holes 21 of encoder 16 and provide signals A and B which are processed to provide pulses G representative of angular increments of rotation as explained previously Each pulse G applied to the NAND gate 53 causes a high output signal which fires the monostable multivibrator 54 which produces output signal J similarly representative of the predetermined increment of rotation As previously explained signal J is fed 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 tightening impacts, counter 56 is continually reset to zero by signal Q so that it cannot count the preset number of pulses and, of course, so that signal R cannot be provided.

Referring particularly to Figure 5, initial tightening produces a steady increase in the angle of forward rotation of encoder 16, as shown by curve J at 102, with no corresponding increase in either fastener preload or recoil time as indicated by curve L As 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 exclusive NOR gate 66 outputs signal Q It should be noted that successive pulses shown in curve J each denote a 50 increase in forward rotation of encoder 16 in the particular oscillographic record shown here for illustrative purposes.

Actually the amount of forward rotation between pulses can be set at any desired value depending on the degree of accuracy desired When the fastener has been tightened sufficiently, causing it to contact a mating workpiece (not shown), a preload begins to build up in the fastener as shown by the preload curve at 104 in Figure 5 The preload measurement was obtained by well known external instrumentation means (not shown) for purposes of explaining this invention, but it should be understood that usually such instrumentation means is not utilized At this point in the tightening cycle no measurable recoil of the hammer against the anvil in the wrench occurs Upon further tightening, sufficient resistance to further rotation is encountered causing the hammer to recoil upon striking the anvil, as shown by curve L at 106 It should be understood that recoil time is dependent on the residual strain energy stored in the impact wrench driving shaft, sockets and couplings, and this strain energy is dependent on the moment being applied, which moment varies with the instantaneous coefficient of friction as the fastener stops rotating If signal L is equal to or exceeds 70 some electrically equivalent predetermined snug torque value, which may be experimentally determined and set by adjusting the output from unit 72, signal P is fed to NOR gate 66 so that output signal Q which 75 resets counter 56 to zero is discontinued and the counter starts counting forward angle rotation signals J It has been determined that the selection of a snug torque value from unit 72 is not critical to the operation 80 of the wrench The criteria used in selecting a snug torque value is that it be set high enough to ensure that preload is beginning to build up in the fastener, but that it not be set too high in the event that a maximum 85 recoil value might occur before counter 56 is allowed to count forward rotation pulses J In the present preferred embodiment, the snug torque value was set by unit 72 at the level of the first peak recoil value in storage 90 unit 62 and in practice is an approximation 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 95 peak value detector/storage unit 62 gives an output to peak value increase detector unit 64 providing output pulse M fed to step generator 68 and NOR gate 66 causing signal Q to reset counter 56 to zero 100 Step generator 68 in Figure 4 causes the previously highest recoil pulse L stored in unit 62 to be increased by a preset fixed or variable amount, thus building into the system successively higher recoil values than 105 the previously highest stored value For example, as shown by curve L of Figure 5, an incremental fixed amount of about Om V is added for a peak value store of approximately 6 volts This incremental 110 value may be varied depending on the accuracy desired The practical constraints on this incremental value are that it be small enough so that subsequent higher peak recoil values are detected, but that it be 115 large enough so that subsequent peak recoil values just slightly greater than the previously stored highest peak recoil value do not continue to reset counter 56 It should also be understood that a fixed percentage 120 of the previously stored highest peak recoil value could be added, such as two percent ( 2 %), for example, with equally effective results It can be seen from Figure 5 that the initial peak recoil value of curve L at 106 125 causes curve N to increase to a first stored peak value at 108 The peak value at 108 of curve N is exceeded by the recoil 110, that is the applied moment exceeds the applied moment at 106 by the previously described 130 1 560 529 predetermined fixed amount As described, signal M (see 114 curve M) is produced causing NOR gate 66 to discharge signal Q resetting counter 56 to zero and causing step generator 68 to increase the value of the signal L at 110 to be increased by the predetermined amount This increased peak value is then stored in unit 62, as indicated by curve N at 112 Counter 56 then must begin counting forward rotation pulses J again The next peak recoil value at 116 exceeds the previous peak value at 110 by the predetermined fixed amount and in the manner described causes peak value curve N 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 122 of curve L occurs, which value exceeds previously highest peak recoil value 116 by the predetermined amount A new peak value shown at 124 of curve N occurs and a reset pulse 126 on curve M is produced Once again counter 56 is reset to zero and starts counting forward rotation pulses J Subsequent recoil signals 128, 130, 132 and 134 do not exceed previously highest recoil value 122 by the predetermined amount, so that no higher peak value of curve N occurs after 124, nor does a reset pulse on curve M occur after 126 Counter 56 is then allowed to count successive forward rotation pulses 136, 138, 140, 142 and 144 of curve J without interruption In the present preferred embodiment represented by Figure 5, the preset number of pulses programmed into counter 56 is five ( 5), thus causing a stop signal 146 of curve R to be generated Stop signal 146 is then fed into the control coil of solenoid valve 76 to shut off the air supply to port 23 of the impact wrench The number of angle pulses before shut off of the wrench after the previously 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 fastener beyond approximately the third angle pulse after the previously highest stored peak recoil value 124 Thus the optimum shut off point for the present preferred embodiment occurs between angle pulses 140 and 144 (i.e 15 -25 degrees of rotation after the last reset pulse 126), but the counter is set at five ( 5) 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, some of the many advantages of the present invention should now be readily apparent The control system provides a highly accurate and reliable means for tightening a joint to the yield point, that is, for providing maximum preload in a fastener tightened by an impacttype wrench, that is, a wrench wherein the tightening moment is applied periodically.

Since the control system is adaptive, only minimal prior knowledge of the joint and fastener characteristics being tightened need 70 be known in order to ensure tightening to the maximum attainable preload of the fastener, namely the yield point As previously stated, tightening to maximum preload at the yield point of the fastener 75 material ensures a joint of maximum efficiency with greatest resistance to loosening due to vibration and fatigue failure The tightening cycle is very rapid, making the wrench ideally suitable for rapid assembly 80 line use In addition to tightening fasteners to 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 85 example, preloads other than the yield point, by building into the fastener system a configuration causing the fastener to deform at a predetermined preload such that the applied torque levels out 90 Obviously, many modifications and variations of the present invention are possible in light of the above teachings It is therefore to be understood that within the scope of the appended claims, the invention 95 may be practised otherwise than as specifically described.

Claims (9)

WHAT WE CLAIM IS:-

1 Apparatus for tightening a fastener, said apparatus comprising in combination 10 ( wrench means having a pulsed output for periodically applying a tightening moment to a fastener in a joint assembly and a control system, said system comprising first means responsive to the moment applied to 10 the fastener during each tightening pulse and for developing a signal representative of the peak moment applied during each application and calculator means responsive to said peak moment signals for determin 11 ( ing when an instantaneous peak moment signal has not exceeded a previous largest instantaneous peak moment signal by more than a predetermined amount and control means responsive to said calculator means 11, for developing a control signal when said calculator means has determined that said instantaneous peak moment signal has not exceeded said previous largest instantaneous peak moment signal by more than said pre 12 ( determined amount.

2 Apparatus in accordance with claim 1 wherein said control means includes second means for determining when a plurality of peak moment signals have not increased by 12 more than a predetermined amount and wherein said control signal is developed when said second means has made said determination.

3 Apparatus in accordance with claim 1 13 ) D LO 1 560 529 wherein said control means includes second means for determining when a plurality 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.

4 Apparatus in accordance with claim 3 wherein said predetermined period is determined by a predetermined rotational displacement of the fastener being tightened.

Apparatus in accordance with claim 3 further including 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 determining said predetermined period.

6 Apparatus in accordance with claim 3 further including third means for developing signals representative of increments of a second tightening characteristic related to the pulses during which the tightening moment is applied and wherein said predetermined period is related to a predetermined number of said increments.

7 Apparatus in accordance with claim 6 wherein said second tightening characteristic is rotational displacement of the fastener being tightened and wherein said third means measures increments of said rotational displacement.

8 Apparatus in accordance with any preceding claim 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.

9 U Agents for the Applicants Printed for Her Majesty's Stationery Office by MULTIPLEX techniques ltd, St Mary Cray, Kent 1980 Published at the Patent Office, 25 Southampton Buildings, London WC 2 l AY, from which copies may be obtained.

9 Apparatus in accordance with claim 8 including means for increasing the stored signal by a predetermined amount.

Apparatus in accordance with any one of claims 1 to 7 wherein said control means includes comparator means and storage means for storing a peak moment signal, said comparator means receiving an instantaneous peak moment signal and a stored signal from said storage means and giving an output indicator signal when said instantaneous peak moment signal exceeds said stored signal, said comparator means also giving an output instantaneous peak moment signal to said storage means and signal generator means responsive to said indicator signal for increasing said instantaneous peak moment signal in said storage means by a predetermined amount.

11 Apparatus in accordance with any preceding claim wherein said control signal is operative to discontinue the application of the tightening moment.

12 An impact wrench including a hammer arranged to impact an anvil to rotate an output shaft operative to tighten an assembly, including a fastener, to the yield point or to a similarly significant point 70 in a tightening cycle by applying torque to the fastener, and a control system for the wrench, the control system comprising means for developing a signal representative of the deceleration of the hammer after en 75 gagement thereof with the anvil; calculator means determining the largest deceleration signal developed up to any instant in the tightening cycle and determining when a larger deceleration signal is not developed 80 subsequently during the tightening cycle and hence determining that the yield or said similarly significant point has been reached, and control means responsive to said calculator means for producing a control signal 85 when the assembly is tightened to the yield or said similarly significant point.

13 An impact wrench in accordance with claim 12 in which the calculator means determines when said larger deceleration 90 signal is not developed during a predetermined period subsequent to the development of the largest deceleration signal.

14 An impact wrench in accordance 95 with claim 13 wherein said calculator means includes means for adding an incremental value to each of said previously stored largest deceleration signals and said control means produces said control signal only if a 100 larger deceleration signal equal to the previously stored largest deceleration signal plus said incremental value is not developed during said predetermined period.

An impact wrench in accordance 105 with claim 13 wherein said signal representative of the deceleration of the hammer is proportional to the duration of the recoil of the hammer during deceleration.

16 An impact wrench in accordance 110 with claim 13 wherein said signal representative of the deceleration of the hammer is proportional to the displacement of the hammer during the recoil.

17 An impact wrench in accordance 115 with claim 13 wherein said signal representative of the deceleration of the hammer is proportional to the velocity of the hammer during the recoil.

18 An impact wrench in accordance 120 with claim 13 wherein said signal representative of the deceleration of the hammer is a signal proportional to the recoil of the hammer after impacting the anvil.

19 An impact wrench including a 125 hammer arranged to impact an anvil to rotate an output shaft operative to tighten an assembly, including a fastener, to the yield point or to a similarly significant point in a tightening cycle by applying torque to 130 1 1 1 560 529 the fastener, and a control system for the wrench, the control system comprising means for developing a signal representative of the deceleration of the hammer after engagement thereof with the anvil; means for developing a signal representative of the angular displacement of the output shaft; calculator means responsive to said deceleration signal and said angular displacement signal and for determining the yield or similarly significant point in a tightening cycle of the assembly said calculator means determining the largest deceleration signal during a first angular displacement of the output shaft; and control means responsive to said calculator means for producing a control signal when the assembly is tightened to said yield or similarly significant point, said control means producing said control signal only if a larger deceleration signal is not developed during a subsequent angular displacement of the output shaft.

An impact wrench in accordance with claim 19 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.

21 An impact wrench in accordance with claim 20 wherein said control means produces said control signal after a predetermined number of degrees of said second angular displacement.

22 An impact wrench in accordance with claim 21 wherein said predetermined number of degrees of said angular displacement is not greater than 25 degrees.

23 An impact wrench in accordance with claim 19 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 greater than the previously stored largest deceleration signal plus said incremental value is not developed.

24 An impact wrench in accordance with claim 23 wherein said incremental value is a fixed percentage of the previously stored largest deceleration signal.

An impact wrench in accordance with claim 24 wherein said percentage is substantially 2 %.

26 An impact wrench in accordance with claim 23 wherein said incremental value is a signal having a fixed value.

27 An impact wrench in accordance with claim 26 wherein said fixed value is substantially 100 millivolts for a deceleration signal having an amplitude of substantially 6 volts.

28 An impact wrench in accordance with any preceding claim 19 to 27 wherein said signal representative of the deceleration of the hammer is proportional to the duration of the recoil of the hammer during deceleration 70 29 An impact wrench in accordance with any preceding claim 19 to 27 wherein said signal representative of the deceleration of the hammer is proportional to the displacement of the hammer during recoil 75 An impact wrench in accordance with any preceding claim 19 to 27 wherein said signal representative of the deceleration of the hammer is proportional to the velocity of the hammer during the recoil 80 37 An impact wrench in accordance with any preceding claim 19 to 27 wherein said signal representative of the deceleration of the hammer is a signal proportional to the recoil of the hammer after impacting the 85 anvil.

32 An impact wrench in accordance with any preceding claim 19 to 31 wherein said control signal is operative to discontinue operation of the wrench 90 33 An impact wrench in accordance with any preceding claim 19 to 32 further including apparatus for measuring, or developing a signal representative of, the recoil of the hammer, the apparatus comprising first 95 means operatively coupled to the hammer for movement in the direction of recoil therewith; second means juxtapositioned with respect to said first means and being rotatably movable between a first and a 101 second position; force transmitting means disposed between said first means and said second means for transmitting force therebetween due to recoil of the hammer; biasing means attached to said second means 10 for exerting a force thereupon toward said first position, said force being in a direction opposite to the direction of recoil of the hammer; and measuring means for measuring a characteristic of the movement 11 of said second means between said first and second positions.

D 34 An impact wrench in accordance with claim 33 wherein said force transmitting means is a mechanical coupling 115 An impact wrench in accordance with claim 33 wherein said force transmitting means is a fluid coupling.

36 An impact wrench in accordance with any one of claims 33 to 35 wherein said 120 measuring means measures the duration of time for movement of said second means between said first and second positions.

37 An impact wrench in accordance with any one of claims 33 to 35 wherein said 125 measuring means measures the distance travelled of said second means between said first and second positions.

38 A method of tightening an assembly including a fastener, to its yield or similarly 130 1 560 529 significant point in the tightening cycle by applying torque thereto with an impact wrench according to claim 33, the method comprising the steps of developing successive signals representative of the recoil of the hammer after engagement thereof with the anvil; developing a signal representative of the angular displacement of the output shaft; determining the yield point of the assembly based upon a desired relationship between said recoil signals and further with respect to said angular displacement signal, said largest recoil signal being determined during a first angular displacement of the output shaft; and producing a control signal when the assembly is tightened to said yield point, said control signal being produced only if a larger recoil signal is not developed during a subsequent second angular replacement of the output shaft.

39 A method of tightening a fastener assembly in accordance with claim 38 wherein said first angular displacement occurs prior to developing said largest recoil signal and said angular displacement occurs subsequent to developing said largest recoil signal.

A method of tightening a fastener assembly in accordance with claim 39 wherein said control signal is produced after a predetermined number of degrees of said angular displacement.

41 A method of tightening a fastener assembly in accordance with claim 40 wherein said predetermined number of degrees of said second angular displacement is not greater than 25 degrees.

42 A method of tightening a fastener assembly in accordance with claim 38 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 greater than the previously stored largest recoil signal plus said incremental value is not developed.

43 A method of tightening a fastener assembly in accordance with claim 42 wherein said incremental value is a fixed 50 percentage of the previously stored largest recoil signal.

44 A method of tightening a fastener assembly in accordance with claim 43 wherein said percentage is substantially 2 10 55 A method of tightening a fastener assembly in accordance with claim 42 wherein said incremental value is a signal having a fixed value.

46 A method of tightening a fastener 60 assembly in accordance with claim 45 wherein said fixed value is substantially millivolts for a recoil signal having an amplitude of substantially 6 volts.

47 A method of tightening a fastener 65 assembly in accordance with claim 38 wherein said signal representative of the recoil of the hammer is proportional to the duration thereof.

48 A method of tightening a fastener 70 assembly in accordance with claim 38 wherein said signal representative of the recoil of the hammer is proportional to the displacement thereof.

49 A method of tightening a fastener 75 assembly in accordance with claim 38 wherein said signal representative of the recoil of the hammer is proportional to the velocity thereof.

A method of tightening a fastener 80 assembly in accordance with any one of claims 38 to 49 wherein said control signal is operative to discontinue operation of the impact wrench.

51 An impact wrench constructed and 85 arranged substantially as described herein and as shown in Figures 1 4 and operable in accordance with Figure 5 of the accompanying drawings.

WALFORD & HARDMAN BROWN, Chartered Patent Agents, Trinity House, Hales Street, Coventry, West Midlands.