JPS628732B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a stiffness meter.

The precise clamping load of a fastener is critical to determining whether a joint containing a fastener will fail in use. Therefore, it is desirable that the fasteners be tightened to a consistent preload. One object of the invention is to provide a device for tightening threaded fasteners to a predetermined preload, which is, for example, the maximum preload that can be reached without plastic deformation of the fastener or joint. The second object is to provide an improved stiffness meter or torque gradient that is used not only for tightening fasteners, but also for other applications where stiffness or torque gradients are required to be measured. The aim is to provide a measurement system.

The torque required to tighten a threaded fastener depends on several variables: fitting stiffness,
It is a function of stiffness, surface friction, and thread morphology. A general characteristic illustrating the function between torque and angle of rotation of a fastener is shown by the curve in FIG. 1 of the accompanying drawings, which shows torque plotted against angle of rotation for a threaded fastener in a typical application. The maximum slope of this curve at point A is a function of the variables mentioned above. Torque gradients can vary widely for a given diameter and thread configuration fastener. It is recognized that the clamping load at which the gradient begins to decrease below the maximum value is largely independent of friction and joint stiffness and is primarily determined by the descending strength of the fastener and/or joint. ing. The point marked X in FIG. 1 represents the point on the torque-rotation angle curve at which the fastener is tightened to a harmonic tensile stress close to its yield strength. Further rotation of the fastener will cause the torque to approach its maximum value and the tensile stress within the fastener to approach its ultimate value. Depending on the ductility and dimensions of the fastener or joint, failure will eventually occur at the point designated Y.

The general purpose of controlling the tightening of threaded fasteners is to achieve a harmonic joint preload that is close to the maximum that can be applied without the fastener starting to yield. One common method is to use a torque control device that applies a specific maximum torque in an attempt to obtain the desired preload for a specific thread and friction condition. This method is
Under actual fastener assembly conditions, the exact frictional state is not known, so it has the disadvantage that fluctuations occur in the torque-tension relationship. This results in a variation in tensile load within the fastener for a given applied torque. Another method that is independent of friction conditions involves measuring the elongation of the fastener. In most cases, direct measurement of elongation is not possible, so an alternative to this method utilizes angle-controlled tightening, where the estimated elongation is achieved by tightening to a precise tightening angle. The disadvantage of this method is
When operating on torque-rotation angle curves with different initial slopes and maximum slopes, the point is to determine where to start the angle measurement. For preloads within the elastic range, this is extremely difficult as the deviations from the typical curve can be large. However, within the plasticity range of the fastener, the angular error does not noticeably change the preload, although the deviation is small; however, for very short fasteners, this angular error can lead to excessive strain and resultant failure. may occur. Therefore, angle-controlled tightening is only acceptable within the plastic zone for long fasteners (e.g. fasteners with more than 6 free threads) where plastic deformation does not result in structural damage to the fastener. .

Therefore, it is desirable that the optimal point at which tightening should stop is the point where the slope of the torque-rotation angle curve has just begun to fall from its maximum value, that is, the point where the fastener is just beginning to yield. Considering X in Figure 1 as a point whose slope is a given ratio of the maximum slope achieved for that fastener, this point gives a clamping load that is essentially independent of the fastener geometry and friction conditions. Determine the actual position where the tightening can be stopped. Another object of the invention is to make it possible to identify point X during the tightening operation as soon as it occurs so that the tightening process can be stopped immediately at point X;
Moreover, the X
The object of the present invention is to provide a device that allows the user to check the points.

According to the invention, a device for measuring rotational stiffness or linear stiffness, i.e. a stiffness meter, comprises an output member for imparting movement to a member, an input member for receiving power, and a device for transmitting power from said input member to said output member. a displaceable coupling member between said output member and said input member; a first sensing device sensitive to said input member for generating a first series of signals representative of displacement of said input shaft; and said coupling member. a second sensing device sensitive to the output member for generating a second series of signals indicative of a displacement of the output member by the coupling member, and a second sensing device for transmitting power from the input member to the output member by the coupling member. the first and second series of signals for measuring the difference in the number of signals of the first series and the second series of signals in a reference number of consecutive signals of the second series of signals; It consists of a comparator device sensitive to the signal, the difference being a function of stiffness.

The stiffness meter may include a memory circuit responsive to the comparator device for sequentially storing information representative of the maximum stiffness or torque gradient obtained up to any given point during operation of the member.

The stiffness meter may be used as a wrench to stop torque input to the input member when the difference reaches a predetermined ratio of the maximum gradient stored by the storage circuit. I can do it.

Each of the first and second sensing devices is an incremental sensor mounted on a respective input or output member and capable of emitting a precise number of equally spaced signals during one revolution of the respective input or output member. - It is convenient to be an encoder.

The present invention also provides a fastener engaging member for applying a tightening action to the fastener, a power input member, and a displacement between the fastener engaging tool and the input member for transmitting power to the fastener engaging member. possible coupling member, a first representing the displacement of the power input member;
a first sensing device responsive to movement of said power input member for generating a series of signals; said fastener engaging member for generating a second series of signals representative of displacement of said fastener engaging member by said coupling member; a second sensing device for sensing the movement of a reference number of consecutive signals of the second series of signals when power is being transmitted from the power input member to the fastener engaging member by the connecting member; The difference in time is a function of instantaneous stiffness. a comparator device sensitive to the signals of the first and second series for measuring the difference in the number of signals in the signals of the first and second series, representing the maximum stiffness obtained during tightening of the fastener; a memory circuit responsive to said comparator for sequentially storing information and generating a control signal when said instantaneous stiffness decreases to a predetermined ratio of the maximum stiffness obtained during tightening of the fastener; and a power input by said input shaft. A device for tightening fasteners is also provided, comprising a control device responsive to the control signal for controlling the fastener.

The device may be used particularly, but not exclusively, for tightening threaded fasteners, and in that case the device includes a fastener engaging member that imparts a rotational movement to the threaded fastener, a rotational input. a shaft, a torsionally displaceable coupling member between the output shaft and the rotary power input member for transmitting rotational power to the fastener engagement member, a first series of signals representing rotational displacement of the rotary power input member; a first sensing device sensitive to the rotational power input member for generating a rotational power input member; a second sensing device sensitive to the fastener engaging member for generating a second series of signals representative of displacement of the fastener engaging member by the coupling member two sensing devices, the first and second series, the difference being a function of the instantaneous torque gradient of the fastener when rotary power is being transmitted from the rotary power input member to the fastener engaging member by the coupling member;
a comparator device sensitive to said first and second series of signals for determining the difference in the number of signals in said series of signals; said comparator device for sequentially storing information representative of the maximum torque gradient obtained during tightening of the fastener; a circuit arrangement responsive to said comparator arrangement for generating a control signal when the instantaneous torque gradient has decreased to a predetermined ratio of the maximum torque gradient obtained during tightening of the fastener, and power provided by said rotary power input member; It comprises a control device responsive to the control signal for controlling the input.

It may also provide a method of measuring stiffness using a displaceable coupling member to transmit power from an input member to an output member that imparts motion to the member; in this case, the measurement method generating a first series of signals representative of displacement of a member; generating a second series of signals representative of displacement of the output member by the coupling member; and transmitting power from the input member to the input member by the coupling member. the difference between the number of signals in the first and second series of signals, the difference between which is a function of stiffness, during a reference number of consecutive signals in the second series of signals as they are being transmitted to the output member; The procedure consists of comparing the signals of the first and second series by counting .

The present invention also provides a method for tightening a fastener by means of a rotational power input member that drives the fastener engagement member via a displaceable coupling member, in which case the tightening method is indicative of the displacement of said rotational power input member. generating a first series of signals representative of displacement of the fastener engaging member by the coupling member; a signal in said first and second series of signals that is a difference between a reference number of consecutive signals in said second series of signals as a function of the instantaneous stiffness of the coupling member when being transmitted to the coupling member; comparing the signals of the first and second series by counting the difference in the number of the fasteners; sequentially storing information representative of the maximum stiffness obtained during tightening of the fastener; and determining the instantaneous stiffness of the fastener. generating a control signal for controlling the power input member when the maximum stiffness obtained during tightening has been reduced to a predetermined ratio.

In particular, but not exclusively, the method is for tightening a threaded fastener by driving a fastener engaging member through a torsionally displaceable coupling member. In this case, the method includes generating a first series of signals representative of rotational displacement of the rotary power input member, and generating a second series of signals representative of displacement of the fastener engagement member by the coupling member. that is, as a function of the instantaneous torque gradient of the fastener during a reference number of consecutive signals in the second series of signals when power is being transmitted from the rotary power input member to the fastener engaging member by the coupling member. Comparing the signals of the first and second series by counting the difference in the number of signals in the signals of the first and second series that is a certain difference, obtained during tightening of the fasteners. sequentially storing information representing a maximum torque gradient, and generating a control signal for controlling the rotational power input member when the instantaneous torque gradient has finished decreasing to a predetermined ratio of the maximum torque gradient obtained during fastener tightening; It consists of the steps involved in generating it.

The control signal is such that the instantaneous torque gradient mentioned above is approximately 50% of the maximum torque gradient obtained during fastener tightening.
It is desirable that this occurs when the temperature has finished decreasing to a certain level.

Control signals are typically used to stop power input members, but in some applications,
It may be used to maintain the power input member to maintain the instantaneous torque gradient at the aforementioned predetermined ratio of the maximum torque gradient.

The theory involved in the device provided by the invention is as follows: The joint between the input member and the output member is torsionally linear in nature with an incremental encoder attached at each end. If the torsional stiffness of this shaft is K and φ ₁ and φ ₂ are the angular displacements of the ends of the shaft, then The applied torque or rotational moment M is M=K(0 ₁ −0 ₂ ).

When a shaft end with an angular displacement of 0 ₂ is connected to a fastener to be tightened and the other end is attached to an electric motor or other drive device, the torque gradient-rotation angle characteristic followed by the fastener due to the joint is as follows: It is given by the following equation: dM/d0 ₂ =K(d0 ₁ /d0 ₂ −1).

This equation can be evaluated in two alternative ways. That is, expand the above equation, and the first method is as follows: However, ω ₁ and ω ₂ are the angular velocities of the two ends of the shaft.

If the time between successive signals, i.e. pulses, from both incremental metal encoders of the axis is t ₁ and t ₂ then t ₁ ∝1/ω ₁ , t ₂ ∝1/ω ₂ and therefore dM/ _d02 = _Kt2 - _t1 / _t1 .

The times t ₁ and t ₂ can be measured electronically by the signals, ie pulses, generated by both incremental metal encoders and, after the necessary calculations by a gradient estimator, are reduced to dM/d0 ₂ . A proportional output signal can be determined.

Alternatively, the equation dM/d0 ₂ =K(d0 ₁ /d0 ₂ -1) is evaluated using small increments as follows: △M/△0 ₂ =K△0 ₁ -△ 0 ₂ / △ 0 ₂ When using a high-resolution incremental metal encoder that provides approximately 4000 to 5000 pulses per revolution, it is possible to directly measure △ 0 ₂ and △ 0 ₁ by pulse counting. can. For example, when calculating △0 ₂ by counting 100 pulses with the encoder at the fastener end of a torque transmission joint, the gradient is generated by the encoder at the other end of the joint while the encoder at the fastener end is generating the above 100 pulses. is determined directly by counting the number of additional pulses that occur. For example, if the 6 residual pulses are counted by the encoder at the torque input end of the joint, then dM/d0 ₂ ∝6 Therefore, the gradient measuring device calculates the time between the pulses generated by the two encoders. One has to find t ₂ - _{t 1} /t ₁ by corresponding to the interval, or by simply counting the number of pulses generated by the two encoders, or by the encoder at the fastener end. △0 ₁ - △0 ₂ /△ by measuring the number of extra pulses generated by the encoder at the torque input while generating a given number of pulses, say 100 pulses.
Either 0 _{or 2} must be found.

Here, as already mentioned, referring to Fig. 1, which shows typical torque or rotational moment versus rotational angle characteristics, the curve traced by the torque transmission joint, and therefore the fast, can be divided into the following three bands. I can do it.

initial pretightening area, tightening area, and Area of surrender and subsequent failure of Huasuna.

Within the range, the effects of curls and irregularities on the threads of the fasteners must be ignored, so that the gradient measuring device of the torque transmission device first of all leaves the range and indicates that the fasteners are being tightened within the range. must be measured or known. Conveniently, the region is entered by a torque measurement derived from the encoder signal. Typically, the gradient in the characteristic is approximately constant within the domain, ie the curve approximates a straight line; however, if the characteristic is curved within the domain, it will reach a typical maximum value A. As the fastener is tightened beyond the area, the area is reached and the gradient of the property begins to decrease as the fastener begins to yield, ultimately leading to the failure of the fastener at point Y. It can be said that the transition point X between the regions is the point whose gradient has been reduced by a certain proportion of the maximum gradient of point A. The fastener should be tightened to point It is desirable that it must be possible to compare with the maximum value A to determine when a point has been reached. The gradient measurement device may be any suitable logic circuit that is sensitive to the signals received from the two encoders. Once the gradient measuring device detects that point X has been reached, it can issue a stop command for the operator to stop tightening the fastener. Alternatively, a stop command can be used to automatically shut down the drive motor or other equipment.

Two types of devices for tightening threaded fasteners according to the invention will now be described with reference to the accompanying drawings.

Referring to Figure 2, the threaded fastener is 1
, the fastener is engaged by a screwdriver or other tool 2 for turning the fastener, which has a shaft mounted for rotation in a bearing 3 . The bearing 3 supports an output shaft 5 together with a mating bearing 4, and the shaft 5 engages the tool 2 in a driving manner.
It supports the first incremental metal encoder 6. The tightening device also comprises a further bearing 7 supporting an input shaft 8 with a second incremental metal encoder 9. The input shaft 8 is a torque applying electric motor 1
It is arranged to be driven at 0. axes 8 and 5
are interconnected by a coil spring 11. When the electric motor 10 is operated, it turns the input shaft 8,
This in turn rotates the coil spring 11 which drives the output shaft 5. The output shaft 5 rotates a tool 2 that tightens the fastener 1.

Incremental encoders 6 and 9 may be of optical, electromagnetic or other type capable of generating signals in cooperation with fixed optical response devices or other fixed receiving devices (not shown). A fixed optical response or receiver generates a series of signals which are provided at precise intervals corresponding to the angular rotation of the two encoders or the angular velocity of their and therefore of the respective axes 5 and 8. The frequency of the signals generated by each encoder 6 and 9 is
Determine their respective angular velocities or their functions. The spacing between the signals generated by encoders 6 and 9 can be measured, for example, by electronic equipment. The slope of the torque-rotation angle curve can be determined from the time interval between the signals generated by the respective encoders as described above or by counting the number of signals as described herein. Therefore, when the slope has decreased to a value indicating that point X in FIG. 1 has been reached, the operator will stop the motor 10. The signals generated by the operation of encoders 6 and 9 are fed into a logic device which generates a "stop" signal informing the operator that the motor 10 should be stopped, or alternatively, automatically generates a "stop" signal. Can be used to stop electric motors. Alternatively, instead of being a "stop" signal, the signal generated by the logic device may be used to hold the signal fastener under a substantially constant load; The signal may be a control signal that prevents additional power input from being provided.

Referring now to FIG. 3, the logic device is illustrated in block diagram form. Starting from the top of the diagram, reference numerals 6 and 9 designate two encoders. The encoder on the left, ie the one closer to the fastener, gives pulses 0 ₂ , and the encoder on the right, ie the one closer to the motor 10, gives pulses 0 ₁ . The two streams of pulses are sent to the circuit shown in block 20. this is pulse
Count the number of 0 ₁ and 0 ₂ , or measure the time interval between pulses of each stream of pulses, or calculate the gradient from the measured angular velocity. The signal from block 20 goes to a logic circuit, block 21, which determines whether the band has been reached. If the answer given by logic circuit 21 is positive, the signal goes to logic circuit block 22. Block 22 calculates the gradient function by determining Δ0 ₁ -Δ0 ₂ /Δ0 ₂ or t ₂ -t ₁ /t ₁ as described above. The output signal of block 22 then goes to block 23. This block is another logic circuit which compares the gradient function with the stored maximum function previously determined in block 22 and previously stored in the circuit indicated by block 24. Block 23 provides a positive or negative signal depending on whether point X in FIG. 1 has been reached. If the output signal from block 23 is positive, this is the stop command described above and tightening is immediately stopped as described above. Block 23
If the output signal of is negative, a signal is sent back to block 22 which continues to update the gradient function continuously and continues to send an output signal to block 23.

FIG. 4 is a schematic block diagram showing the main components of the electronic circuit indicated by block 22 in FIG. As already explained, block 22 determines the gradient Δ0 ₁ -Δ0 ₂ /Δ0 ₂ or ΔM/Δ0 ₂ . _The electronic circuit _shown in _{FIG .} Calculate.
The chosen value of Δ0 ₂ is called the chord length. The reason is that this measurement is based on the torque (M) across a string of approximately constant length that is moved continuously along the M-0 curves shown as a ₁ , b ₁ and a ₂ , b ₂ in Figure 1. This is because it corresponds to taking the difference between the readings. The projection of the string length onto the 0 axis is fixed by the characteristics or settings of the string length feed register 27, which is operated each time an operating pulse is received, as will be explained later.

The pulses (0 ₁ ) received from the first incremental metal encoder 9 at the motor-side end of the coil spring 11 are directly related to the direction of rotation of the rotor.
Gated directly into residual register 25. The function of the residual register 25 is to store pulses 0 ₁ .

The pulse (0 ₂ ) from the second incremental metal encoder 6 at the output or fastener end of the coil spring 11 is detected by the checking device 28 to determine whether the fastener is rotating in the correct direction. is checked and then used to decrease the residual register 25. Assuming that the fastener end encoder 6 is rotated backwards, the amount of backward twist must be counted and restored by an equal forward twist before any pulse ₀₂ is gated into the residual register 25. . As soon as pulse 0 ₂ has decremented the residual register 25, the value indicated in this register is examined. If it is greater than zero, more pulses 0 ₁ than pulses 0 ₂ have occurred, resulting in a signal being sent through gate 29 to both gradient register 26 and chord length shift register 27, and the remaining register 25
decrease. The chord length shift register 27 is operated each time it receives a pulse 0 ₂ passed by the inspection device 28 as shown by arrow 30, and also when the shift register 27 receives a pulse through the gate 29. places “1” in its input field. If no pulse is received through gate 29, register 27 is set to "0". Each time the output of shift register 27 places a "1", it decrements gradient register 26 by one pulse. When the shift register output is set to "0", it has no effect on the gradient register. Therefore, each time the shift register is operated by the generation of a pulse ₀₂ in the correct direction, the gradient register determines whether a positive "reading" is produced by the residual register 25 and the output of the shift register 27. Either the pulses are changed one by one or they are not changed depending on. The chord length shift register 27 has characteristics or settings such that the readings of the gradient register are taken over the entire effective chord length. String length is “noise”
That is, it must be long enough to even out the influence of unwanted signals superimposed on the fundamental signal. For example, 1 corresponds to one pulse 0 ₂
A helix angle of ° is too small for this purpose. It was accepted that 20° was a suitable value for the torsion angle at which torque should be measured, but if readings were only taken every 20°,
The torque-torsion angle curve cannot be followed. Therefore, strings such as a ₁ , b ₁ and a ₂ , _{b 2} in FIG _. That is, this is done every approximately 1° of twist. When gradient register 26 is operating normally, the commanded reading of this register is ΔM/Δ0 ₂ of the fastener being tested.

The signal from gradient register 26 is sent to a maximum gradient storage circuit, block 24, and a comparator in block 23 (see FIG. 3).

The signal provided by comparator 23 to stop the electric motor driving the fastener is generated when the instantaneous torque has fallen to 50% of the maximum torque achieved. The reason for this is M-0 ₂
The derivative of the curve, ie the dM/d0 ₂ -d0 ₂ curve shown in FIG. 5, has an inflection point at or near 50% of the maximum value designated by X in the same figure. This inflection point is also the steepest part of the curve, and therefore the point where the curve most rapidly passes through the superimposed "noise" curve. Therefore, the X point is the point on the curve that provides the highest immunity to noise or spurious signals.

The device of the present invention, in its fastener tightening configuration, is essentially a torque gradient or torsional stiffness meter that allows the instantaneous torque gradient to be compared to the maximum torque gradient, thereby eliminating the need for existing methods of controlling fastener tightening. The restrictions will not go away. These are comparative torque-angular characteristics, either by knowing an approximation of the maximum gradient, by knowing the torque that gives a given clamping load, or by knowing the angular rotation that guarantees the optimum clamping load. requires accurate prior knowledge.

The device described here does not require prior knowledge of special torque-rotation angle characteristics, because the deviation of the instantaneous torque from the maximum torque is automatically determined, and the instantaneous gradient is equal to the maximum gradient. This means that the tightening will be automatically stopped when an arbitrary preset portion (eg 50%) of point X is reached. Accordingly, the present invention provides a preferred device for circumferential fastening schemes.

Using techniques similar to those applied to the fastener tightening type of the present invention, the aforementioned torque gradient or torsional stiffness meter can be used in rotational motion with a rotary-to-linear transducer, such as a screw or rack-and-pinion device. By converting to linear motion, it can be converted to a linear stiffness meter. Alternatively, linear springs or other linearly elastic elements connected between linear encoders and shaft members carrying the encoders can be used. In this way, the yield point in a linear system can be determined in the same way as the yield point in a torsional system described above. Therefore, for example, the yield point of a tensile test specimen in a tensile tester or creep tester can be determined without measuring the gauge length and without having to measure force and displacement separately. .

Instead of using a coil spring 11 as a coupling device between input and output shafts, a spiral spring can be used. Alternatively, the torque transmission coupling may be a torsion rod or tube with ends connected between or forming the input and output shafts. The torsion rod or tube may be metal, rubber, plastic, or may be a composite rod or tube made from any of these materials.

FIG. 6 shows an alternative version of the device that can be used in place of the device shown in FIG. In Figure 2, coil spring 1
In addition to being twisted as the fastener follows the torque-rotation angle curve shown in FIG. 1, it is also rotated about its longitudinal axis. In some cases these may be undesirable and the apparatus shown in FIG. 6 may be used instead. In the apparatus shown in FIG. 6, a non-rotating coil spring 31 transfers torque from an input shaft 32 driven by an electric motor 33 to an output shaft 34 arranged to drive fasteners and other members to be turned.
Communicate to. The input shaft 32 is connected to the output shaft 34 via a gearbox 35 housing a transmission, shown as a differential gear train 36 for diagrammatic purposes. When the input shaft 32 is driven at the same speed as the output shaft 34, that is, when the torque M is constant,
No unitary movement of gear train 36 occurs; however, when the input shaft starts moving faster than the output shaft, i.e. within the zone of FIG. 1, the gear train pivots about the common axis of shafts 32 and 34. and transmits the pivoting motion to the housing of the gear box 35. One end of the coil spring 31 is attached to an end wall 37 of a gear box 35, which is attached to a bearing 42 for pivoting about a common axis of shafts 32 and 34.
The other end of the coil spring is attached to a fixed mounting plate 38, and the output shaft 34 can freely rotate in a bearing 39 of this plate. The input shaft 32 is the second
It carries an incremental metal encoder 40 corresponding to the encoder 9 in the figure, and the output shaft 34 is the second
It carries an incremental metal encoder 41 corresponding to encoder 6 in the figure. When the speed of differential rotation between the input shaft and the output shaft changes as the point approaches X, the coil spring 31 becomes the coil spring 1 in FIG.
1: but the spring 31 does not rotate, only twists, and therefore does not need to be dynamically balanced. The angular deviation between the input and output axes is determined by encoders 40 and 4.
1 and those signals will be passed to and processed by the logic shown in FIGS. 3 and 4 in the manner previously described. Another advantage of the device shown in FIG. 6 is that the gearbox 35 can be shortened and housed within the coil spring 31 as shown, thereby reducing the axial length of the device. Both encoders can also be placed inside a coil spring. Alternatively, the encoder 40 can be positioned at the input end of the gear box as shown;
Therefore, lower resolution encoders can be used to save costs.

In either arrangement of the device (i.e., in FIG. 2 or in FIG. 6), the logic device can calculate the torque without requiring an indication of the deviation from the linear torque-rotational angle relationship and without requiring an automatic stop of the drive motor. A switch may be provided which allows the device to be used as a conventional tightening device or wrench for transmitting power from the motor to the fastener.

In FIG. 2, a coil spring 11 is used as a coupling device between both input and output shafts, i.e., although the tightening device is used as a wrench to which input torque is continuously applied during tightening, the coil spring 11 1
By using a coupling device with a substantially linear inelastic M-0 characteristic instead of 1, the tightening device can be used as an impact wrench, i.e. of the type in which the input torque is applied intermittently and in stages. It can be used as a wrench. In such cases, the coupling device may advantageously be a detent-type spring to prevent the device from returning during each period of application of input torque.

[Brief explanation of the drawing]

Figure 1 is a graph showing torque plotted against angle of rotation for a typically used threaded fastener; Figure 2 is an elevational view of the first type of stiffness meter; Figure 3 is an elevational view of the first type of stiffness meter; A diagram illustrating the logic device used in connection with the stiffness meter shown in FIG. 2; FIG. 4 is a block diagram illustrating in detail certain portions of the logic device of FIG. 3; FIG. is the curve of dM/ _d02 plotted against ₀₂ , the graph of the derivative curve obtained from Figure 1: and
The figure is a partially sectional front view of another form of the stiffness meter. 8, 32... Input shaft, 5, 34... Output shaft, 1
1, 31... Coil spring, 9, 40... First incremental metal encoder, 6, 41... Second incremental metal encoder, 22, 23... Comparator (logic circuit), 24... Memory circuit, 1 ……
Fastener, 2... Fastener engagement tool.

Claims (1)

Claims: 1. In a stiffness meter that determines the yield point or similar meaningful change in the slope of any torque-rotation angle curve, generating a fluctuating signal indicative of the torque slope of a torque applied to a constant member. an apparatus for storing, in response to the gradient signal, an information signal indicative of a maximum torque gradient occurring through a generally linear portion of the torque-rotation angle curve; and comparing the gradient signal and the information signal. and a stiffness meter including a comparator device for generating a control signal when the gradient signal changes by a predetermined percentage of the information signal.