GB2096361A

GB2096361A - Method and apparatus for automatically tensioning threaded fasteners

Info

Publication number: GB2096361A
Application number: GB8209344A
Authority: GB
Original assignee: International Harverster Corp
Current assignee: Navistar Inc
Priority date: 1981-03-31
Filing date: 1982-03-30
Publication date: 1982-10-13
Also published as: DE3212038A1

Abstract

Automatic tensioning of threaded fasteners is effected using a microcomputer. The torque and rotation are measured and, when the linear part 52 of the curve is reached, the curve is extrapolated back to the snug angle SA. The fastener is then rotated a predetermined angle from there. In another embodiment, tightening is stopped when the curve begins to deviate more than a certain amount from the linear part (part 58, proof load region). In another embodiment the degree to which previously installed fasteners have been tightened is checked. <IMAGE>

Description

SPECIFICATION Method and apparatus for automatically tensioning threaded fasteners This invention relates generally to fastener tightening methods and devices, and more particularly to a method and apparatus for automatically tightening threaded fasteners such as bolts and nuts to a predetermined tension, and for determining the tension to which previously installed fasteners have been tightened.

Various methods and systems for tightening threaded fasteners are known. One such system is a simple torque wrench that measures the amount of torque applied to a threaded fastener.

However, the amount of torque available for effective tension in threaded fasteners is a small percentage of the overall applied torque. This is because the applied torque is largely dissipated by various frictional forces between the fastener and the device to which it is fastened. Moreover, because the frictional forces vary as a function of fastener material, contact conditions at the thread and at the bolt head and other manufacturing variables, a simple torque measurement may not provide an accurate indication of fastener tension.

Consequently, systems that measure both torque and fastener rotation have been developed. Such systems utilize the parameters of torque and rotation in various ways. For example, systems that tighten a fastener to proof load (the onset of plastic deformation of the fastener) measure the torque versus angular displacement function, and terminate the tightening process when a nonlinearity is detected. Other systems compare successive torque measurements and terminate the tightening process when two successive torque readings are substantially similar. In systems that tighten the fastener to a percentage of the proof load, the fastener is first tightened to proof load, as determined by a nonlinearity in the torque versus angular displacement function, completely backed off and retorqued to a percentage of the proof torque.

While these systems provide a way to tighten a threaded fastener to various preloading conditions, most of the prior art systems are limited in the number of functions they are capable of performing. Moreover, the systems for tightening fasteners to a percent of proof load are not entirely satisfactory, and none of the prior art systems provide a way to determine the load to which a previously installed fastener has been tightened. In addition, none of the prior art systems were capable of satisfactorily tightening fasteners having prevailing torque lock nuts.

According to a first aspect of the present invention an automatic tensioning system for threaded fasteners comprises means for applying rotational force to a threaded fastener; means operatively coupled to said force applying means for determining the amount of torque applied to the fastener; means operatively coupled to said force applying means for determining the angular displacement of said fastener; control means responsive to said torque determining means and to said angular displacement determining means for selectively rendering said force applying means operative to apply rotational force to said fastener; said control means including means for determining the snug angle of said fastener, said control means being operative to terminate the application of force when said angular displacement reaches a predetermined value beyond said snug angle.

In a second aspect of the invention the control means includes means for determining the angle of a linear portion of the torque versus angular displacement characteristic of the fastener after a predetermined torque has been exceeded and for terminating the application of force when the slope of the torque versus angular displacement curve deviates from the slope of the linear portion by a predetermined amount.

The invention also extends to a system for automatically determining the conditions to which a previously installed threaded fastener has been tightened comprising means for rotating the threaded fastener; means for determining the torque applied to the threaded fastener; and control means operatively coupled to said rotating means and to said torque determining means, said control means being operative to render said rotating means operative to rotate the fastener in a reverse direction to loosen the fastener by a predetermined number of degrees and then to rotate the fastener in the forward direction to retighten the fastener by the same number of degrees, said control means being responsive to said torque determining means to determine the torque at the end of the retightening process to thereby determine the condition to which the fastener had originally been tightened.

The automatic tensioning system of the invention is desirably controlled by a microcomputer system that measures the angular deviation (rotation) of the threaded fastener after the fastener has become snug as well as the torque applied to the fastener. The torque and angular deviation measurements may, as aforesaid, be utilized in several ways. For example, in a proof load fastener installation mode of operation, the torque and the angle of rotation may be measured to determine the slope of the torque versus rotation curve, and the fastening process terminated when the slope changes by a predetermined amount. The angular displacement and the torque may be measured at proof load to determine if they fall within predetermined limits.In a percent of proof mode of operation, the torque and angular displacement measurements may also be used to determine the snug angle (the angle of rotational displacement at which the head of the fastener engages the material being fastened) as well as the slope of the torque versus displacement characteristic to calculate the angular displacement at the percent of proof load. An additional check may be made to determine whether the slope thus obtained is within a predetermined range. In such percent of proof tightenings, a friction coefficient which varies with the type of coating on the bolt and the type of bolt material may be employed to alter the multiplication factor between torque and tension.

In addition, a fixed torque factor may be preset into the system to compensate for the absence or presence of a lock nut.

In a quality audit mode, the system can determine the tension to which previously installed fasteners have been tightened. In such mode, a previously installed fastener is backed off a predetermined number of degrees, such as, for example 1 20, and then retightened to the original angular position. The torque at the end of the retightening sequence is then measured to determine the torque to which the fastener had previously been tightened.

In order that the invention may be better understood, a system in accordance therewith will now be described in more detail, by way of example only, with reference to the accompanying drawings in which: Fig. 1 is a block diagram of an automatic tensioning system for threaded fasteners; and Fig. 2 is a graph illustrating the torque versus angular displacement characteristic of a typical fastener.

Referring now to the drawing, with particular attention to Fig. 1 , there is shown a threaded fastener tensioning system generally designated by the reference numeral 10. In accordance with a preferred embodiment of the invention, the tensioning system 10 utilizes a torque wrench, preferably a pneumatic or air wrench, such as the air wrench 12 to drive a threaded fastener (not shown) such as a bolt or a nut. A shaft encoder 14 is coupled to the shaft of the air wrench 12 to provide an indication of the position of the shaft of the air wrench 12.Various shaft encoders such as optical, magnetic or other shaft encoders may be used; however, the shaft encoder 14 must provide an indication of the position of the shaft fairly accurately, and in the embodiment illustrated in Fig. 1, the shaft encoder provides one output pulse for each degree of rotation of the shaft of the air wrench 12. A torque transducer 1 6 is also affixed to the shaft of the air wrench 1 2 and provides an analog signal, typically in millivolts, representative of the torque on the shaft of the air wrench 12. The signal from the torque transducer 1 6 is applied to a signal conditioner 1 8 which serves to amplify the output signal from the torque transducer 1 6 to provide a signal having a range of, for example, 0 through 5 voits.A multiplexer 20 samples the signal from the signal conditioner 1 8 at intervals determined by the system and applies the sampled signal from the signal conditioner 1 8 to an analog-to-digital converter 22, which converts the sampled signal from the multiplexer 20 to a digital signal usable by a microcomputer 24. The microcomputer 24, which may be, for example a Motorola type 6800 microprocessor, controls an air wrench servo valve 26 via an interface 28 to energize and deenergize the air wrench 12 as necessary. In addition, the microcomputer 24 operates a printer/plotter 30 to print the torque and rotation data for each fastener.A data input terminal, which may include, for example, a series of thumb wheels, switches, a keyboard or the like, is used to enter the required torque, angular deviation, slope of the torque-deviation curve, proof load deviation from the slope and mode of operation data into the microcomputer 24.

In operation, the mode of operation is first entered into the microcomputer 24 by the data input terminal 32. In the present embodiment, the system has three basic modes of operation, namely, a proof load tightening mode, a percent of proof load tightening mode and a quality audit mode and the desired mode is selected via the data input terminal 32, for example, by the rotation of a keyswitch. Once the desired mode has been selected, the appropriate data defining rotation angle and/or maximum and minimum torque and angle of rotation limits, the limits of the slope of the torque versus angular displacement curve, the sensitivity to deviation of the slope of torque-displacement curve, the friction coefficient, which varies with the type of coating on the bolt, as well as whether or not a lock nut is employed, are entered.The system is then ready to install or audit the installation of fasteners in accordance with the mode and other parameters selected. However, before giving a more specific description of the operation of the device 10 in the various modes, it is necessary first to discuss the torque versus angular displacement characteristics of a typical threaded fastener.

When a fastener is first installed, the torque required to rotate the fastener is relatively low and remains relatively constant, since the only force that must be overcome is the force of friction between the threads, and that force is relatively low when the fastener is not snug. The torque remains relatively low, as indicated by the area 50 of the graph in Fig. 2, until the fastener becomes snug, at which point the torque required to rotate the fastener increases at a substantially linear rate as illustrated by area 52 of the curve of Fig. 2. The slope of the torque versus angular displacement curve remains relatively constant as the applied torque increases until plastic deformation of the fastener begins to occur. At this point, also known as the proof load, the torque versus angular displacement function deviates from linearity, as illustrated in the area 54 of Fig. 2, as the slope of the curve decreases.

In the fastener installation mode of operation, the system may be operated in two modes. One mode is a proof load mode wherein the fastener is tightened to proof load and the other is a percent of proof mode wherein the fastener is tightened to a specified percentage of the proof load. In the proof load mode, the system gradually increases the torque applied to the fastener and samples several points along the torque versus angular displacement curve. The tightening and sampling continues until a series of points (for example, 6 points) lying along the straight line portion 52 of the torque versus angular displacement curve are obtained. After the predetermined number of points have been sampled, the angle A (Fig. 2) of the linear portion 52 is calculated.After the angle A has been calculated, subsequent points along the torque versus displacement curve are sampled, and the tightening operation is continued as long as the points fall within a predetermined distance or angular deviation from the straight line (dashed line 56 in Fig. 2) defined by the angle A. As soon as the torque versus displacement curve deviates from the straight line by a predetermined amount, the tightening operation is automatically terminated, for example, at point 58 (Fig. 2). The degree of deviation permitted before the tightening operation is terminated is entered via the sensitivity inputs of the data input 32, and may conveniently be entered as an angle A. .In addition, as a further check, maximum and minimum torques Tmax and Tmin, as well as maximum and minimum angular displacements max andO and(6) min may also be entered into the system. The actual torque and angular displacement at thb point of proof load may be compared with the maximum and minimum torque and angular displacements to determine whether the fastener was within specification, since a faulty or cross threaded fastener wouid most probably have a proof load that falls outside of one or both of the maximum and minimum torque and displacement limits. In addition, a time out may be employed to terminate the tightening function after a predetermined amount of time, such as, for example, three seconds, has elapsed.

Such a time out feature is useful, for example, when a fastener has not been properly installed, as in the case of a stripped or crossed thread, or when the thread has been damaged. In addition, a limit on the slope of the linear portion of the torque versus angular displacement curve in the linear region (i.e., angle A) may be entered to assure that the angle A is within the range expected for the particular fastener being utilized.

In the percent of proof mode, the angle A is calculated in the same manner as it was in the case of the proof load mode. However, in addition to calculating the angle A, the snug angle SA (Fig. 2), which is determined by extrapolating the linear portion 52 of the torque versus angular displacement curve until it intersects the abscissa, is calculated. Once the snug angle SA has been determined, the fastener is rotated a predetermined number of degrees which corresponds to the desired percent of proof load from the snug angle. Once the fastener has been rotated the appropriate number of degrees of angular displacement, the torque may be measured to determine if the torque lies within a range of torques expected for the particular fastener being used in order to provide a further check on the condition of the fastener and the installation.

In addition, a friction coefficient which changes the multiplication factor between torque and displacement may be entered. Such a friction coefficient is determined by the type of coating on the bolt, for example, oil or grease, and is used in the percent of proof mode to correct the expected torque limits as a function of the coating on the bolt and may also be used in the proof load mode to appropriately adjust the expected ratio between torque and angular displacement at proofload.

The system described can also be used in a quality audit mode that makes it possible to determine the conditions to which a previously installed fastener has been tightened. In the quality audit mode, the previously installed fastener is backed off a predetermined number of degrees, for example 120, and then retightened the same number of degrees, for example between points 60 and 62 (Fig. 2). At the end of the retightening process, the torque is measured to determine the torque to which the fastener was previously tightened. in addition, the slope of the torque versus angular displacement curve may be measured during the retightening process and checked to see if it is within specifications for the particular fastener and installation conditons being employed. Thus, the system provides a way to audit previously installed fasteners and serves as an important check on quality control.

Another problem that occurs with automatic tensioning devices is that fasteners having a lock nut, lock washer or any prevailing torque locking device cannot readily be installed by prior art systems. The reason for this is that such lock washers or lock nuts place a torque load on the fastener. For example, such a lock washer or lock nut may make it necessary to apply, for example, 20 to 25 pounds of torque to the fastener in order to turn the fastener before any tension is applied.

Thus, in order to solve the problems associated with prevailing torque locking devices, the system may be programmed, via the entry of appropriate data into the data input 32, to avoid taking data until a predetemrined minimum torque greater than the torque provided by the prevailing torque locking device is exceeded. After that torque has been exceeded, the slope of the linear portion of the torque versus angular deviation curve (angle A), as well as the new snug angle SA may be calculated. The fastener may then be tightened either to proof load or to percentage of the proof load based on the angle A and the snug angle SA thus calculated in the manner previously described.

In addition, other correction factors may be entered, for example, factors that compensate for the response time of the air wrench, such as an angular correction factor which adds or subtracts a predetermined number of degrees of rotation to compensate for air wrenches that consistently overtighten or undertighten the fastener may be used.

Obviously, many modifications and variations of the present invention are possible, and it is to be understood that the invention may be practised otherwise than as specifically described above.

Claims

1. An automatic tensioning system for threaded fasteners comprising means for applying rotational force to a threaded fastener; means operatively coupled to said force applying means for determining the amount of torque applied to the fastener; means operatively coupled to said force applying means for determining the angular displacement of said fastener; control means responsive to said torque determining means and to said angular displacement dezermining means for selectively rendering said force applying means operative to apply rotational force to said fastener, said control means for determining the snug angle of said fastener, said control means being operative to terminate the application of force when said angular displacement reaches a predetermined value beyond said snug angle.

2. A system as claimed in claim 1 or wherein said predetermined value beyond said snug angle corresponds to a predetermined percentage of proof load.

3. A system as claimed in claim 1 or claim 2 wherein said snug angle determining means includes means for determining the torque versus angular displacement characteristic of the threaded fastener, determining the slope of a linear portion of the characteristic, and determining the snug angle by extrapolating the linear characteristic to the zero torque point.

4. A system as claimed in claim 3 wherein said snug angle determining means includes means for determining whether the slope of the linear portion of the characteristic is within predetermined limits.

5. A system as claimed in claim 4 wherein said control means includes means for altering the predetermined limits of the slope of the linear portion of the characteristic in accordance with a factor representative of the friction of the fastener.

6. A system as claimed in any one of the preceding claims wherein said control means includes means for detecting whether the torque is within predetermined limits after the fastener has been rotated the predetermined value of angular rotation beyond the snug angle.

7. A system as claimed in claim 6 wherein said control means includes means for altering the predetermined limits of said torque in accordance with a factor representative of the friction of the fastener.

8. A system as claimed in any one of the preceding claims wherein said control means include time out means responsive to said angular displacement determining means for automatically terminating the application of force to the fastener if said fastener has not been rotated a predetermined angular displacement within a predetermined time interval.

9. A system as claimed in any one of the preceding claims wherein said control means includes means for determining the snug angle only after the torque has been determined to exceed a predetermined level.

10. A system as claimed in any one of the preceding claims wherein said control means includes means for altering the termination of the application of rotational force to the fastener by a predetermined time interval.

1 A system for automatically determining the conditions to which a previously installed threaded fastener has been tightened comprising means for rotating the threaded fastener; means for determining the torque applied to the threaded fastener; and control means operatively couped to said rotating means and to said torque determining means, said control means being operative to render said rotating means operative to rotate the fastener in a reverse direction to loosen the fastener by a predetermined number of degrees and then to rotate the fastener in the forward direction to retighten the fastener by the same number of degrees, said control means being responsive to said torque determining means to determine the torque at the end of the retightening process to thereby determine the condition to which the fastener had originally been tightened.

12. A system as claimed in claim 11 wherein said control means is operative to rotate the fastener in the reverse direction by an amount insufficient to loosen the fastener to the snug angle.

13. A system as claimed in claim 12 wherein said predetermined number of degrees is approximately 120.

14. A method for automatically tensioning threaded fasteners comprising the steps of applying rotational force to a threaded fastener; measuring the angular displacement and amount of torque applied to the fastener; determining the snug angle of the fastener based on the amount of angular displacement and torque applied thereto, and rotating the fastener a predetermined number of degrees beyond the snug angle.

1 5. A method as claimed in claim 14 wherein the step of determining the snug angle includes the steps of determining the torque versus angular displacement characteristic of the threaded fastener based on the torque and angular displacement applied thereto, determining the slope of a linear portion of the characteristic and determining the snug angle by extrapolating the linear characteristic to the zero torque point.

1 6. A method as claimed in claim 1 5 wherein the step of determining the predetermined number of degrees beyond the snug angle includes the step of determining the angular displacement corresponding to proof load for the fastener and setting the predetermined angular displacement beyond the snug angle to be a predetermined percentage of the proof load angular displacement.

1 7. A method for automatically determining the conditions to which a previously installed threaded fastener had been tightened comprising the steps of; rotating the fastener in the reverse direction to loosen the fastener by a predetermined number of degrees, rotating the fastener in the forward direction to retighten the fastener by the same number of degrees, and measuring the torque applied to the fastener at the end of the retightening step.

1 8. An automatic tensioning system for threaded fasteners comprising: means for applying rotational force to a threaded fastener; means operatively coupled to said rotational force applying means for determining the amount of torque applied to the fastener; means operatively coupled to said force applying means for determining the angular displacement of said fastener; control means coupled to said torque determining means and to said angular displacement determining means for selectively rendering said force applying means operative to apply rotational force to said fastener, said control means including means for determining the angle of a linear portion of the torque versus angular displacement characteristic of the fastener after a predetermined torque has been exceeded and for terminating the application of force when the slope of the torque versus angular displacement curve deviates from the slope of the linear portion by a predetermined amount.

1 9. An automatic tensioning system as claimed in claim 18 and further including means for altering said predetermined amount.

20. A method for tensioning a threaded fastener comprising the steps of; applying a rotational force to the fastener, determining the amount of torque applied to the fastener, determining the angular displacement of the fastener, determining the angle of a linear portion of the torque versus angular displacement characteristic based on the torque and displacement measurement after the torque has exceeded a predetermined value, and terminating the application of force to the fastener when the slope of the torque versus angular displacement curve deviates from the slope of the linear portion by a predetermined amount.