Tightening System

This invention relates generally to apparatus for tightening fasteners and, more particularly, to apparatus for tightening fasteners to the yield point of the fastener material or some other predetermined axial load on the fastener characterized by a flattening of the torque-rotation curve plotted for the particular fastener being tightened.

In the design of structural joints secured by mechanical fastener systems it is usual to provide for the fasteners to exert a predetermined clamping force or load on the structural members to insure the integrity of the joint. When the joints are assembled, therefore, it is necessary that the fasteners be tightened to exert a predetermined axial load on the structural members. However, prior art techniques for tightening threaded fasteners such as nuts and bolts to exert a predetermined load are not entirely satisfactory. The most accurate technique involves a measurement of the axial strain or stretch of the bolt while it is being tightened which can be related to the stress or load acting on the bolt through the previously calculated stress-strain relationship for the bolt. While most accurate, practical applications of the fastener do not usually permit measurement of the stretch in the bolt and, in those instances where the stretch can be measured, it is a time consuming and relatively expensive technique. Accordingly, this technique is used in relatively few applications outside of laboratory testing.

Another known technique and that most commonly used in most joint assembly operations involves the use of torque controlled tools, that is, tools that determine when the torque applied to the fastener equals or slightly exceeds a predetermined value and act to interrupt the flow of power to the tool. Torque measurement is relatively easy and since torque is related to the tensile stress or axial force experienced by the fastener, and exerted on the structural members, a predetermined torque theoretically corresponds to the predetermined clamp load specified for the joint. However, when tightening threaded fasteners in assembly line operations, wide variations in the actual torquetension relationships are experienced. These variations are caused by a variety of factors including allowable tolerance variations in the dimensions and strength of the fasteners and structural members and lubrication or absence thereof on the mating surfaces of the fasteners and/or the structural members, all of which, in turn, cause large variations in the coefficient of friction between the mating surfaces. In actual practice, variations of up to .+-. 30% in the axial load on the bolts used for a particular application can be experienced at the same torque level. Accordingly, the torque control technique is not very accurate.

In an effort to overcome the problems associated with the prior art tightening techniques, another technique has been attempted. This technique is disclosed in the U.S. Pat. to Pauley, No. 3,643,501, issued Feb. 22, 1972 and utilizes the characteristic shape of a curve plotted for the torque applied to the fastener being tightened vs. its rotation during the tightening cycle. The torque-rotation curve for any particular fastener includes an initial or pretightening region wherein the fastener is being rotated but has not yet fully seated on the adjacent structural member; a tightening region wherein the fastener is being tightened within its elastic limit and which is generally linear; and, a yield region wherein its limit of proportionality has been exceeded and wherein the curve begins to flatten and finally flattens out at the yield point of the fastener. The technique disclosed in the Pauley patent utilizes a wrench including a first gear train for driving a first control arm in accordance with the torque being applied to the fastener and a second gear train for driving a second control arm in accordance with the rotation of the fastener. Each control arm includes a portion of a switch member connected in a circuit controlling the operation of the wrench. During rotation of the fastener in the tightening region, the control arms are driven in unison and the switch members are closed. When the limit of proportionality is exceeded, the second control arm is driven faster than the first control arm so that the switch members open, the control circuit is broken and the wrench shuts off. Theoretically the wrench shuts off at the yield point of the fastener.

For various reasons, the device disclosed in the Pauley patent is not entirely satisfactory. First, if the control arms are to be driven in unison in the tightening region of the torque-rotation curve, the slope of the curve must be known beforehand so that the gear trains can be properly related. For the same reasons set out with respect to the torque-tension relationship, similar variations occur in the torque-rotation relationships generated for a group of theoretically similar fasteners. Thus, the gear trains required to drive the control arms when tightening any particular fastener in a group of similar fasteners, will be different.

Another problem associated with the device disclosed in the Pauley patent results from the temporary increases or decreases in the torque-rotation relationship which can be caused by areas of increased lubricity on the mating surfaces of the joint or temporary seizing or galling of the threads. These phenomena provide spikes in the tightening region of the torque-rotation curve which would cause the control arm to move relative to each other and cause the wrench to shut off prematurely. Obviously, premature shutoff of the wrench reduces the accuracy and reliability of the system.

It is an object of this invention, therefore, to provide a tightening system which can reliably and accurately determine the deviation from linearity on curves which can be plotted during the tightening cycle or from the point of maximum slope of such curves.

It is yet another object of this invention to provide a tightening system for tightening a fastener to its yield point or other point having a significant change in the slope of the torque-rotation curve and corresponding to a predetermined axial load.

It is still another object of this invention to provide a tightening system for tightening a fastener to a predetermined axial load with minimum previous knowledge of the particular joint being assembled.

Finally, it is an object of this invention to provide a tightening system that is versatile, reliable, economical and accurate.

These and other objects of this invention are accomplished by providing a tightening system including a wrench for applying torque and imparting rotation to a fastener member. Associated with the wrench is a first mechanism for developing a signal representative of the torque applied to the fastener and a second mechanism for developing a signal representative of the angular displacement or rotation of the fastener. When the torque signal is greater than the rotation signal, the rotation signal is increased to equal the former, but when the torque signal is less than the rotation signal, a decision-making signal is generated and is processed to develop a control signal at a yield point of the fastener.

More particularly, a one-way amplifier responsive to the difference between the signals is operative to increase the rotation signal, but, of course, cannot decrease the rotation signal when the torque signal decreases. In the latter situation the difference between the signals is fed to a decision-making circuit for detecting the yield point.

For a better understanding of the invention disclosed herein, reference is made to the following description of several preferred embodiments taken in conjunction with the figures of the accompanying drawing, in which:

FIG. 1 is a plot of a curve illustrating the characteristics of the typical torque-rotation relationship experienced by a fastener during a tightening cycle;

FIG. 2 is a plot of a curve illustrating the characteristics of typical torque-time and rotation-time relationships experienced by a fastener during a tightening cycle;

FIG. 3 is a plot of a curve illustrating an underlying principle of operation of a tightening system in accordance with this invention;

FIG. 4 is a schematic drawing of a tightening system in accordance with one embodiment of the invention;

FIGS. 5-7 each illustrate different circuits for processing signals developed by apparatus in accordance with the invention to determine when a yield point has been reached.

Referring to FIG. 1, there is illustrated a typical torque-rotation curve for a threaded fastener being tightened with the torque plotted along the vertical axis and with the rotation plotted along the horizontal axis. The curve includes an initial or pretightening region extending from the intersection of the torque and rotation axes to point A. In the pretightening region, mating threads of the fastener assembly have been engaged and one of the fasteners is being rotated, but the bearing face of the rotating fastener has either not contacted the adjacent face of the structural member included in the joint or has not completely pulled the structural members together. At point A on the curve the structural members have been completely pulled together by the fastener assembly and actual tightening of the joint commences. In the art, the torque at point A is commonly referred to as the "snug" torque. In the tightening region of the curve, extending from point A to point B, clamp load is actually being exerted on the structural members and the male member in the fastener assembly, that is, the bolt is stretching in the axial direction due to the induced tensile load. Throughout the tightening region the curve is approximately linear and in some cases may be exactly linear. As is most usual, however, the curve will include a slight curvature and, thus, a point M wherein the slope of the curve is a maximum. At point B the limit of proportionality of the fastener has been reached. Beyond point B the torque and the rotation are no longer approximately linearly related and as shown, the slope of the curve is significantly reduced relative to the maximum slope. For purposes of this application, point B will be considered as the start of the yield region, but it will be understood that beyond point B, increasing load will still be induced in the joint assembly. Point C corresponds to the yield point of the fastener and while the definition of yield point varies slightly, can be considered to be the point beyond which deformation of the bolt is no longer purely elastic. As will become apparent, a tightening system in accordance with this invention is capable of detecting points M or B on the torque-rotation curve and responding thereto to generate a control signal at about point C. Further, it should be understood that particular joint assemblies could include further fastener assemblies constructed such as to deviate from linearity or from the point of maximum slope at a predetermined load other than at the yield point. Such deviation could be detected and used to generate a control signal. For this reason, as used hereinafter, the term yield point should be construed to include the yield point of the material from which the fastener is made as well as points on a flattened portion of a torque-rotation curve generated by the configuration of the fastener at a predetermined clamp load.

Referring now to FIG. 2, there are illustrated two more curves that will be explained to facilitate an understanding of certain aspects of the invention. The first curve, marked Torque, is a plot of the torque applied to the fastener versus time elapsed during the tightening cycle, torque being plotted along the vertical axis and time being plotted along the horizontal axis. The second curve, marked Rotation, is a plot of the angular displacement or rotation of the fastener versus time elapsed during the tightening cycle, rotation being plotted along the vertical axis and time again being plotted along the horizontal axis. The curves illustrated in FIG. 2 are not exact, but are generally close approximations useful in understanding certain aspects of the invention. Looking first at the Torque curve, there is a pretightening region in which the torque increases rather slowly and which extends from the intersection of the torque-time axes to point D. Thus, point D corresponds to point A on the torque-rotation curve. Point D to point E represents the tightening region of the Torque curve similar to the region between points A and B of the torque-rotation curve. In the tightening region of the Torque curve, torque increases at a generally faster rate than in the pretightening region. Finally, the Torque curve includes a yield region from point E wherein the torque increases at a relatively slow rate and which corresponds to the yield region beyond point C on the torque-rotation curve. Point F represents the yield point of the fastener. Similarly, the Rotation curve includes a pretightening region extending from the intersection of the rotation-time axes to point G, a tightening region extending from points G to H and a yield region from point H. Point J on the Rotation curve represents the yield point. Reference to FIG. 2 will be made hereinafter during the description of the preferred embodiment of the invention.

In FIG. 4, there is illustrated a tightening system 10 in accordance with this invention. Tightening system 10 includes a wrench 12 having a motor 14, a drive shaft 16 and a driver bit 18. Drive shaft 16 is rotated by motor 14 to transmit torque and rotation to a fastener engaged by driver bit 18. Wrench 12 can be of any conventional type and as is most common, motor 14 can be air-powered with the flow of motive fluid being controlled by a suitable control valve 20. It should be understood that motor 14 could also be electric, hydraulic or any combination of pneumatic, hydraulic or electric. The exact details of the wrench are not necessary for a proper understanding of the invention and, accordingly, a more specific description is not provided.

Wrench 12 is mounted on a rigid frame 22 of any suitable type by a torsion bar 24 which twists about the rigid frame when motor 14 is operative with a torque substantially equal to and opposite to the torque being applied to a fastener being tightened. Carried on torsion bar 24 is a first gear member 26 which drives a second gear member 28 fixed to an output shaft 30. One end of output shaft 30 is connected to drive the inner core of a potentiometer 32 and interposed between gear member 28 and potentiometer 32 is a slip clutch 34. When wrench 12 is tightening a fastener, torsion bar 24 twists and rotates gear member 26 which drives gear member 28 and output shaft 30. The rotation of shaft 30 can be considered a signal representative of the torque applied to the fastener being tightened which signal is converted to an electrical signal by potentiometer 32 when the inner core of the potentiometer is driven by shaft 30. Use of slip clutch 34 precludes rotation of the inner core of potentiometer 32 and prevents an electrical output signal until the snug torque (points A and D on FIGS. 1 and 2, respectively) is being applied to the fastener. In practice, slip clutch 34 is arranged to engage at a reasonable approximation of the snug torque, for example, at about 20% of the nominal torque to be applied to the fastener.

Carried on drive shaft 16 is another gear member 35 which, when a fastener is being tightened, drives an output shaft 36 through a gear member 38. The output end of shaft 36 is connected to drive the inner core of a potentiometer 40 and interposed between gear member 38 and potentiometer 40 is a clutch 42 which is also used to preclude rotation of the inner core of the potentiometer and prevent an electrical output signal until the snug torque is being applied to the fastener. The rotation of shaft 36 can be considered a signal representative of the angular rotation of the fastener which signal is converted to an electrical signal by potentiometer 40. The electrical rotation signal is modified by an amplifier 14 as will be fully explained hereinafter.

Before proceeding with a further description of the apparatus utilized in the preferred embodiments of the invention, an underlying principle of the invention will be explained. Referring to FIG. 2 of the drawing, the torque and rotation curves illustrated therein can also be thought of as curves plotting the torque signal provided by shaft 30 or potentiometer 32 and the rotation signal provided by shaft 36 or potentiometer 40. It can be seen that in the approximately linear region or tightening region of the curves, points D to E on the Torque curve and points G and H on the Rotation curve, one of the signals, preferably the torque signal, is increasing faster than the other signal, preferably, the rotation signal. The exact relationship between the signals is, of course, dependent on the flexibility of torsion bar 24 and the gear ratios between shafts 24 and 30 and between shafts 16 and 36. As noted, it is preferable to arrange the gearing so that after the snug torque is applied, the torque signal initially increases faster than the rotation signal. According to an underlying principle of the invention, the total rotation signal is modified by being multiplied by a factor X so that the rotation signal is equal to the torque signal throughout a certain portion of the tightening region of the curves. In effect the Torque and Rotation Curves are made to coincide.

In the preferred embodiment of the invention disclosed in FIG. 4 of the drawing, multiplication of the rotation signal is accomplished by amplifier 44. When a fastener is tightened and when clutches 34 and 42 have engaged, amplifier 44 is set such that its output signal is equal to its input signal, that is, the initial signal from amplifier 44 is a signal representative of the initial rotation of the fastener. Thus, the initial torque and rotation signals are fed to a comparator 46 which can be in the form of a differential amplifier providing an output signal representative of the difference between its input, at this point the torque and rotation signals. The output signal from comparator 46 is fed to a one-way amplifier 48 that is, an amplifier having a gain characteristic in one direction only and the one-way amplifier provides an output signal to amplifier 44 which increases its gain characteristic and multiplies the rotation signal by a factor X until it equals the torque signal. When the output of potentiometer 40 equals that of potentiometer 32, comparator 46 no longer provides a significant output signal to one-way amplifier 48 and the one-way amplifier, in turn, stops increasing the gain characteristic of amplifier 44. At this point the gain characteristic of amplifier 44 is fixed only temporarily as will be explained hereinafter, and it continues to multiply the rotation signal and provide the modified rotation signal which is representative of a factor X, multiplied by the rotation signal measured from the snug point, point A on FIG. 1 or points D and G on FIG. 2. If the torque signal again increases relative to the modified rotation signal provided by amplifier 44 comparator 46 again provides an output signal to one-way amplifier 48 which increases the gain characteristic of amplifier 44 whereby it multiplies the rotation signal measured from the snug point provided by potentiometer 40 by another factor, X.sub.2, until the modified signal provided by amplifier 44 again equals the torque signal provided by potentiometer 36. The process repeats itself at each instant when the torque signal exceeds the modified rotation signal. From the preceding description it should be understood that the modified rotation signal at any instant is equal to the rotation signal multiplied by an instantaneous factor. Mathematically the modified rotation signal can be expressed as XR, where X is the instantaneous factor and R is the total rotation of the fastener from the snug point.

If the tightening region of the Torque-Rotation curve illustrated in FIG. 1, points A to B, is exactly linear, one-way amplifier 48 would provide an initial signal adjusting the gain characteristic of amplifier 44 and no further adjustment of the gain characteristic would be made. When point B on the curve illustrated in FIG. 1 is reached, the modified rotation signal is increasing faster than the torque signal and the output from comparator 46 is a reverse polarity signal, for convenience the reverse polarity signal will be hereinafter considered negative, and this signal cannot be fed through one-way amplifier 48 because of its one-way gain characteristic. The negative signal is fed to a decision-making circuit 50 which processes that signal to determine when the yield point of the fastener has been reached and to then develop a control signal which is fed to control valve 20 to shut off the wrench. As is most usual, however, the Torque-Rotation curve is not exactly linear in the tightening region and, accordingly, one-way amplifier 48 is continuously adjusting the gain characteristic of amplifier 44 until the maximum slope of the curve is experienced, point M on the curve. When point M is experienced, the modified rotation signal is increasing faster than the torque signal and the signal from comparator 46 reverses polarity, again for convenience also considered negative, and has a relatively small value until point B is reached when it increases significantly. The negative signal is fed to decision-making circuit 50 to be processed and determine when the yield point of the fastener has been reached and to then develop a control signal which is fed to control valve 20 to shut off the wrench. Thus, the negative signals are decision-making signals.

An understanding of the invention can be facilitated and certain aspects will be further explained with reference to FIG. 3 of the drawing which is a plot of the signal from potentiometer 32 which is representative of the torque being applied to the fastener versus the signal from amplifier 44 which is representative of the modified rotation signal. The torque signal is plotted on the vertical axis of the curve and the modified rotation signal is plotted on the horizontal axis. Neither potentiometer provides an output until the snug torque is applied to the fastener, that is until point A on the curve illustrated in FIG. 1 is reached. At this point both potentiometers provide output signals and the rotation signal is modified as noted above. Because the modifed rotation signal is maintained equal to the torque signal the curve illustrated in FIG. 3 is a straight line forming an angle of 45.degree. with the axes until points B or M on the curve illustrated in FIG. 1 are reached as explained above. Beyond the appropriate point, the curve starts to flatten indicating that the output of comparator 46 is negative.

By comparing the curves illustrated in FIGS. 1 and 3, it can be seen that a signal representative of the total rotation of the fastener measured from the snug point is developed and is multiplied by factors X which will be explained are representative of the chordal slope measured from the snug point of the Torque-Rotation curve for the fastener being tightened. Various factors X.sub.1, X.sub.2, etc. are shown on the curve illustrated in FIG. 1 of the drawing. Each time the slope of the Torque-Rotation curve increases, factor X increases and multiplies the rotation signal measured from the snug point until the compared signals are again equal. The process repeats until appropriate points B or M on the Torque-Rotation curve are experienced, whichever occurs first.

As noted above factors X are representative of the chordal slope of the Torque-Rotation curve measured from the snug point. This can be mathematically shown and will now be explained with reference to FIG. 3 of the drawing wherein it can be seen that the torque signal in the tightening region is equal to the modified rotation signal so that the following equation can be written:

in the above and in the following equations T represents the torque being applied to the fastener and R represents the rotation of the fastener. Thus, before the negative signal is provided by comparator 46, that is before points B or M are reached, the above equation can be written as follows:

and

since T and R are measured from the snug point, X is representative of the average chordal slope of the Torque-Rotation curve illustrated in FIG. 1 measured from the snug point, A. One other point to note is that if the slope of the Torque-Rotation curve varies, the factor X, which cannot be decreased, eventually represents the maximum average slope of the curve.

Completing the description of the preferred embodiment of the invention, reference is made to FIGS. 5-7 wherein different decision-making circuits 50 usable therewith are illustrated. In FIG. 5, a first embodiment of decision-making circuit 50 is illustrated and includes a comparator 54 to which the negative or decision-making signal from comparator 46 is fed. Also fed to comparator 54 from a signal generator 56 is a predetermined negative signal indicative of the expected signal from comparator 46 at the yield point. When the decision-making signal from comparator 46 is substantially equal to or more negative than the predetermined signal from signal generator 56, the control signal is developed and fed to control valve 20 closing the control valve and shutting off the wrench. It should be noted that comparator 54 and signal generator 56 are conventional apparatus and need not be explained further.

The decision-making circuit described with respect to FIG. 5 of the drawing, is particularly usable in those instances where the specifications of the joint being tightened are known and where average values representative of the yield point can be predetermined and set into signal generator 56. With the circuit disclosed in FIG. 5, a reasonable degree of accuracy can be obtained. However, in those applications where the specifications of the joint are not known beforehand or where a higher degree of accuracy is desired the decision-making circuit can be of the type illustrated in FIGS. 6 and 7 of the drawing.

In FIG. 6 a second embodiment of decision-making circuit 50 is illustrated and includes a gradient meter 58 which receives the negative decision-making signal from comparator 46 and rotation signals from potentiometer 40. Gradient meter 58 may be in the form of a shift register and a subtraction circuit each of which receives a decision-making signal as an input and which are connected in series. The shift register can be clocked by the rotation signal which would be fed to the shift register through an analogue to digital convertor and the output of the shift register is also fed to the subtraction circuit which of course, subtracts its input signals and provides an output signal representative of the rate of change of the decision-making signal with respect to the rotation signal. The output signal from gradient meter 58 is fed to a comparator 60 which receives another negative input signal from a signal generator 62 which has been determined to be representative of the expected rate of change of the decision-making signal with respect to the rotation signal at the yield point. As will be made clear hereinafter, it has been determined that when the rate of change of the decision-making signal with respect to the rotation signal has a predetermined relationship with the maximum average slope of the Torque-Rotation curve, the yield point has been reached and this relationship is set into signal generator 62 which as will be made clear is in the nature of a calculator. Accordingly, when comparator 60 determines that its input signals are substantially equal or that the signal from gradient meter 58 is more negative than the signal from signal generator 62, it provides a control signal to control valve 20 closing the control valve and shutting off the wrench.

The relationship set into signal generator 62 can be understood from the following explanation. The decision-making signal from comparator 46 can be expressed as follows:

In the above equation D represents the decision-making signal, X.sub.m represents the factor at the point M or the maximum average slope of the Torque-Rotation curve; R represents the instantaneous rotation; R.sub.m represents the rotation at point M; T represents the instantaneous torque; and, T.sub.m represents the torque at point M. Continuing the explanation and using the same symbols explained above, the output of gradient meter 58 can be expressed as follows:

and by substituting for D from above, this equation can be rewritten as follows: ##EQU1## and thus, ##EQU2## From the explanation provided in the co-pending application, Ser. No. 507,417, filed Sept. 19, 1974 for Apparatus for and Method of Determining Rotational or Linear Stiffness by John T. Boys, it can be seen that at the yield point, (.DELTA.T/.DELTA. R) has a value of from about 0.25 to about 0.75 and most commonly about 0.50 of the maximum average slope of the Torque-Rotation curve. By utilizing this fact, the yield point can be defined by rewriting the last equation as follows:

or

or

Thus, it can be seen that when the signal representative of the rate of change of the decision-making signal with respect to the rotation signal is equal to about 0.50 X.sub.m or, if desired, is within a range of about 0.25 X.sub.m to about 0.75 X.sub.m, the yield point has been reached. Accordingly, signal generator 62 receives a signal from one-way amplifier 48 which is representative of the maximum average slope of the Torque-Rotation curve, X.sub.m, and includes a calculator circuit for performing the calculation in accordance with the desired predetermined relationship, as explained, and provides an output signal in accordance therewith which is fed to comparator 60 for comparison with the signal from gradient meter 58 as also explained.

The decision-making circuit 50 illustrated in FIG. 7 of the drawing includes a comparator 64 which receives the negative decision-making signal from comparator 46 and which also receives a negative signal from an amplifier 66. The negative signal from amplifier 66 is proportional to the torque being applied to the fastener or, preferably, to the rotation of the fastener and, accordingly, receives an input signal from either potentiometer 32 or 40. In accordance with this embodiment of decision-making circuit 50, the control signal closing control valve 20 and shutting off the wrench is developed when the decision-making signal has a predetermined relationship with either the torque or rotational signals. Thus, amplifier 66 conditions its input signal in accordance with the predetermined relationship and provides a negative output signal which is compared with the decision-making signal. It has been determined that amplifier 66 should provide an output signal of approximately -0.002 to -0.020 of the torque or the rotational signal. When comparator 64 determines that the decision-making signal is more negative than the signal from amplifier 66, it develops a control signal which is fed to control valve 20 closing the valve and shutting off the wrench.

While in the foregoing a preferred embodiment of the invention has been described, various changes can be made. For example, gears 26, 28, 34 and 38 can be eliminated as well as potentiometers 32 and 40. The torque signal could be generated by a torque cell mounted between output shaft 16 and a rigid frame on which the wrench is mounted. The signal representative of rotation could be developed by a suitable encoder mounted for rotation with drive shaft 16 and cooperating with a proximity probe mounted to sense the passage of teeth formed on the encoder. The probe develops signals representative of predetermined increments of angular rotation of the fastener as is conventional in the art and these incremental signals can be added to provide a total rotation signal. This latter arrangement is also known in the art.

Thus, the intended scope of the invention is recited in the appended claims .