Apparatus for Precision Tensioning of Threaded Fasteners

The present invention is directed to precision tensioning or preloading of threaded fasteners in critical assembly operations, and more particularly to an apparatus for monitoring and controlling the fastener tensioning operation as a function of torque applied to the fastener and angle of rotation of the fastener into the assembly structure.

The importance of accurately and consistently controlling tension or preload applied to threaded fasteners increases with precision or criticality of parameters and tolerances of the assembly as a whole. This is particularly true in mass production of precision-designed equipment which may later be subjected to maintenance or repair, following which load applied to the assembly fasteners must be substantially the same as that applied during original manufacture. For example, in manufacture of internal combustion engines designed for high performance and fuel economy, the head is fastened to the engine block with a plurality of bolts prior to final machining of various block/cylinder critical surfaces. In the event that the head is later removed for repair or replacement, it is important that the same be precisely reassembled to the block so as to restore relationships of critical surfaces obtained during the original manufacturing machining operations.

Conventionally, preloading of threaded fasteners in engine and other assembly applications is controlled by monitoring torque applied to the assembly tool, such as with a mechanical or electrical torque wrench. Fastener preload control through monitoring of fastener torque alone, however, yields unpredictable and inconsistent results due in part to varying friction between the mating threads and beneath the fastener head. Where it has been attempted to obtain greater uniformity through use of lubricants or the like, results have continued to be unsatisfactory. Another approach has been to monitor torque as a function of angle of rotation, determine rate of change of torque, and compare the resulting data during the manufacturing operation to empirically determine data prestored in a computer memory. Such arrangements still do not directly measure fastener tension, and in addition require expensive assembly and control hardware. A third approach has been to tighten the fastener to a point at which the fastener material yields and the fastener head separates from the threaded body. Arrangements of this type suffer from the same inherent drawbacks as the torque wrench technique described above due to varying friction between the fastener and the assembly, and also increases the cost of both manufacture and repair due to requirement for special double-headed fasteners. For a general discussion of conventional techniques for monitoring and controlling pretensioning of threaded fasteners, see Kelly, "Electronic Controls Zero-In On Reliable Fastening," Assembly Engineering, November 1986, pages 34-38.

A further technique for controlling fastener preload has been found to yield particularly consistent results. This technique, termed "torque-turn" or "torque-angle," involves initially tightening the fastener to a specified torque, and thereafter tightening the fastener through an additional prespecified angle. The initial tightening torque is empirically predetermined to be one at which the fastener is tightened in assembly but has not yet been substantially elastically stretched. By thereafter tightening the fastener through an additional angle or fraction of a turn, advantage is taken of the precision machining of the fastener threads so as to obtain a predetermined elastic stretching of the fastener within the assembly. For example, a torque-turn or torque-angle fastening specification ma call for initial tightening to a torque of 25 Newton-meters, followed by an additional one-half turn or 180.degree. rotation in three equal steps. Computer-based equipment has been proposed for implementing such fastener preloading technique in mass production operations. However, as previously noted, control during maintenance and repair is as important as control during original assembly, and there remains a need in the art for inexpensive equipment which may be employed by maintenance and repair technicians in the field for obtaining the same precision control of fastener preloading as is done during the original manufacturing operation. A general object of the present invention is to provide apparatus of such a character.

Apparatus for precision tensioning of threaded fasteners in accordance with a presently preferred embodiment of the invention comprises a fastener drive head coupled by a multiconductor cable to an apparatus electronic control and display enclosure. The drive head includes a shaft with hex drive and socket mounting facility for applying turning torque to a threaded fastener. A strain gage is mounted on the fastener drive shaft for providing a torque signal as a function of torque applied through the shaft to a fastener. An angle transducer, specifically an optical encoder, is coupled to the fastener drive shaft for measuring angle of rotation of the shaft and fastener, and for providing an angle signal as a function thereof. In the preferred embodiment of the invention, the drive head comprises a two-section housing, one section being affixed to the fastener drive shaft and the other being rotatable with respect thereto. The encoder comprises an annular band carried by one of the housing sections, specifically the housing section which is rotatable with respect to the drive shaft, and having alternating translucent and opaque zones at predetermined angular increments surrounding the fastener drive shaft axis of rotation. Two optical couplers are carried by the other housing section and are spaced from each other circumferentially of the shaft axis by the angle Ni/2, where N is an odd integer and i is the incremental angle of the encoder band zones.

The electronic enclosure package includes circuitry, preferably microprocessor-based circuitry, for receiving and integrating the angle signal fed thereto from the sensor head through the multiconductor cable to accumulate total angle of rotation of the fastener. The control circuitry also monitors the torque signal fed to the electronics from the sensor head. The microprocessor-based control circuitry preferably includes facility for establishing selected torque and/or angle thresholds, comparing each of the torque and angle signals to the corresponding threshold, and indicating when one of the signals equals the corresponding threshold. The torque and angle thresholds are programmable by an operator.

The electronics enclosure includes an operator control panel having an alphanumeric display and facility for selecting either a torque-monitoring mode of operation or an angle-monitoring mode of operation. The operator panel also includes facility for selecting either a measurement mode of operation or a setup mode of operation. During a torque-monitoring mode of operation, either the torque signal received from the drive head or the torque threshold set by the operator is shown at the alphanumeric display depending upon whether a measurement or setup mode of operation is selected. Likewise, during an angle-monitoring mode of operation, accumulated angle of rotation at the sensor head or the angle accumulation threshold is shown at the display depending upon whether a measurement or setup mode of operation is selected. During the setup mode of operation, either of the torque and angle thresholds may be selectively incremented or decremented through operation of switches on the enclosure operation panel. During the angle-monitoring mode of operation, the accumulated angle of rotation may be reset to zero through operation of a switch on the enclosure panel or on the sensor head. The various panel switches preferably comprise membrane switches bearing suitable indicia for indicating function thereof. The angle-monitoring circuitry within the control electronics also includes facility responsive to the two optical couplers in the sensor head encoder for determining not only angle of rotation but also direction of rotation of the fastener drive shaft.

The invention, together with additional objects, features and advantages thereof, will be best understood from the following description, the appended claims and the accompanying drawings in which:

FIG. 1 is a fragmentary plan view of apparatus for precision tensioning of threaded fasteners in accordance with a presently preferred embodiment of the invention;

FIG. 2 is an enlarged sectional view of the drive head assembly illustrated in FIG. 1;

FIG. 3 is an enlarged view of the apparatus alphanumeric display on the electronic enclosure package of FIG. 1;

FIGS. 4A-4B together comprise an electrical schematic diagram of control electronics contained within the electronic enclosure package of FIG. 1, FIGS. 4A and 4B being interconnected along the line A-B in each figure; and

FIG. 5 is a schematic drawing illustrating operation of the angle encoder optics and electronics.

FIG. 1 illustrates apparatus 10 for precision tensioning of threaded fasteners in accordance with a presently preferred embodiment of the invention as comprising a fastener drive head assembly 12 coupled by a multiconductor cable 14 to an electronics enclosure 16. Drive head 12 is illustrated in greater detail in FIG. 2 and includes a fastener drive shaft 17 having at one end a hex head 19 for engagement with a suitable wrench or the like, and a detent assembly 18 of suitable conventional construction at the opposing shaft end for removably receiving and mounting a fastener drive socket 20 of desired size and contour. A two-section housing 22 is carried by shaft 17. Housing 22 comprises a generally flat base 24 having a stepped central aperture 26 received over shaft 17 and engaging head 19, so that base 24 is effectively held by aperture 26 and head 19 against rotation with respect to shaft 17. Base 24 is fastened in position by a spring clip 28. A printed circuit-board assembly 30 is mounted within housing 22 on a ledge 32 which is upstanding from base 24.

Housing 22 also includes a cup-shaped shell 34 having a base 36 with a central aperture 38 received over shaft 17 and captured thereon between a shoulder 40 and a grip ring 42. Shell 34 is thus free to rotate about shaft 17 while being constrained by shoulder 40 and grip ring 42 from axial motion with respect thereto. Shell 34 has a cylindrical sidewall 44 which extends as a flange from the periphery of base 36 to a position adjacent to the periphery of base 24, and overlies a cylindrical flange 46 which projects inwardly from base 24 toward shell 34. Wall 44 and flange 46 are positioned closely adjacent to each other in assembly so as to effectively enclose the hollow interior of housing 22 while permitting rotation of shell 34 about the axis of shaft 17 with respect to base 24. An eyelet 48 having a central aperture 50 is affixed to shell base 36 adjacent to the periphery thereof. A flexible cable 52 is slidably received within aperture 50 and has a "hippo clip" or the like 54 fastened to the opposing end thereof. A setscrew 56 having a knurled head 58 is received within eyelet 48 for selectively engaging cable 52 within aperture 50 and thereby clamping the cable in position with respect to housing shell 34.

An optical encoder 60 is mounted within housing 22 and comprises an encoder wheel 62 and a pair of optical detectors 64,66 (FIGS. 2, 4A and 5). Wheel 62 in the preferred embodiment of the invention takes the form of a band or strip of suitable plastic construction coiled on edge in a hoop and fastened to a lip 68 on shell base 36 circumferentially surrounding and coaxial with fastener drive shaft 17. As best seen in FIG. 5, strip 62 has translucent and opaque zones 70, 72 which alternate in a uniformly spaced circumferential array around shaft 17 at predetermined angular increments--e.g., one degree increments in the preferred embodiment of the invention. Optical couplers 64, 66, which are of suitable conventional construction, are mounted on circuit board 30 and receive band 62 for detecting zones 70, 72 as band 62 rotates therethrough. Couplers 64, 66 are spaced from each other circumferentially of shaft 17 by an odd multiple of angular increments plus or minus 1/2increment--i.e., by the angle Ni/2 where N is an odd integer and i is angular increment or length of zones 70, 72 on strip 62. The significance of such spacing will become apparent hereinafter. A pushbutton 74 is carried by base 24 and has conductors coupled to circuitboard 30. A strain gage 76 is mounted on shaft 17 within housing 22 and has conductors extending therefrom to circuitboard 30. Couplers 64, 66, pushbutton 74 and strain gage 76 are connected by circuitboard 30 to cable 14 which extends through a strain relief grommet 78 on base 24 to electronics enclosure 16 (FIG. 1). Strain gage 76 may be of any suitable conventional type for providing an electrical output signal as a function of strain applied thereto, which in turn is a function of the torque applied to shaft 17 against a fastener whose head is received within socket 20. In a working embodiment of the invention, strain gage 76 comprises a type 6/350XY21 strain gage marketed by H.B. Electronics modified by the manufacturer to have a nominal series resistance of 700 ohms.

Electronics package 16 (FIG. 1) includes an enclosure 80 having a front or operator panel 82. An on/off pushbutton toggle switch 84 is carried on panel 82 above a multiple-character alphanumeric display 86 (FIGS. 1, 3 and 4B). Display 86 includes three centrally positioned 7-segment alphanumeric characters 88 (FIGS. 3 and 4B) for displaying angle, torque and other information. In addition, display 86 includes indicators 90 along the left edge of numeric display 88 for indicating a TORQUE-monitoring mode of operation, an ANGLE-monitoring mode of operation, a LO BATTery condition, and a TORQUE LIMIT setup mode of operation. Likewise, indicators 92 are arrayed along the right-hand edge of numeric display 88 for indicating display of torque in N.multidot.M or FT.multidot.LB, display of angle of rotation in DEGREEs, threshold SETup mode of operation, and an ALARM condition.

Returning to FIG. 1, six rectangular membrane switches 94-104 are arrayed in sequence beneath display 86 on panel 82. Each switch 94-104 bears indicia for indicating function thereof. ANGLE ZERO switch 94 selectively resets to zero the angle of rotation accumulated within the control electronics. TORQUE/ANGLE switch 96 is for selecting between torque-monitoring and angle-monitoring modes of operation. N.multidot.M/FT.multidot.LBS switch 98 is for selecting between numeric display of torque in Newton-meters and foot-pounds. OPERATE/ALARM SET switch 100 selects between the normal or measurement operating mode of operation, and the alarm or threshold setup mode of operation. ALARM UP and ALARM DOWN switches 102-104 are for selectively incrementing and decrementing the angle or torque thresholds indicated at display 86 during a setup mode of operation.

FIGS. 4A and 4B together comprise an electrical schematic diagram of apparatus 10, including both electronics carried within drive head 12 (FIG. 4A) and electronics carried within enclosure 80 (FIGS. 4A and 4B). Apparatus electronics are powered by a battery 106 (FIG. 4B) which is carried within enclosure 16. Battery 106 is connected through operator switch 84 (FIGS. 1 and 4B) and through an electronic power switch 108 to a battery voltage bus 110. A regulated voltage bus 114 is connected to bus 110 by a voltage regulator 112. A microprocessor 116 (FIG. 4A), such as an 8049 microprocessor having on-board memory, has data inputs DB0-DB5 connected in reverse order to switches 94-104. Switch 74 at drive head 12 is connected by cable 14 to input DB5 in parallel with switch 94. Optical coupler 64 on head 12 is connected through cable 14 to the non-inverting input of a differential amplifier 118. Likewise, optical coupler 66 is connected by cable 14 to the non-inverting input of a differential amplifier 120. The biassing resistors for couplers 64, 66, illustrated schematically in FIG. 4A, are carried by a circuitboard 30 (FIG. 2) within head 12. The inverting reference inputs of amplifiers 118, 120 are connected to a voltage divider 122, and the amplifier outputs are respectively connected to the DB6 and DB7 data inputs of microprocessor 116. Strain gage 76 on head 12 is connected by cable 14 to the signal input of a A/D converter 124, which provides a digital output to ports P20-P23 of microprocessor 116 as a function of analog input thereto from strain gage 76. The reference input to converter 124 is connected to a factory-adjustable voltage divider 125 across bus 110.

Microprocessor ports P10-P17 and P24-P25 are connected to display 86 through a series of decoders 126. A battery voltage monitor, including a factory-adjustable voltage divider 127 connected to bus 110, and an inverter 129, drives the LO BATTery indicator of display segment 90 through a decoder 126. Display 86 preferably comprises a custom LCD to display information as illustrated in FIG. 3 and described hereinabove. Microprocessor port P27 is connected through an oscillator 128 to an audible alarm 130 for "beeping" preprogrammed alarm conditions to an operator through a suitable aperture or vent (not shown) in enclosure 80 (FIG. 1). Microprocessor port P26 is connected to power switch 108 for removing battery power from the operating and display electronics in the event of non-use for an extended time, and thereby conserving battery power during standby. The T0 port of microprocessor 116 is connected to switch 84, and the T1 port is connected to the control inputs of A/D convertor 124. Power is supplied to sensor head 12 from battery power bus 110 and cable 14.

In operation, with switch 84 initially closed and battery power supply to the control electronics, an operator may first set or program the torque and angle thresholds within microprocessor 116. This is accomplished by activating switch 110 until SETup mode of operation is indicated at display segment 92, and then selecting either TORQUE or ANGLE monitoring mode of operation at display segment 90 through depression of switch 96. In a torque setup mode of operation, for example, switches 102, 104 are depressed as required to increment or decrement the threshold stored during previous use of the instrument in one Newton-meter or one foot-pound increments to the desired torque limit, such as 50 Newton-meters for example. Operation is then switched to angle mode by depressing switch 96, and switches 102, 104 are again depressed as required to increment or decrement the previously-stored angle threshold in one degree increments to the desired level, such as 180.degree. for example. With the instrument then ready for operation, switch 100 is depressed to change from the setup mode of operation to the measurement mode of operation.

Head 12 and electronics package 116 are then positioned as desired in order to tighten the selected fastener. A suitable socket 20 is fastened to head 12 and the head is positioned over the fastener. Clip 54 is then affixed at suitable position on the assembly into which the fastener is to be tightened so as to prevent rotation of head segment 34 as the fastener is tightened, and setscrew 56 is turned to clamp cable 52 in position. With head 12 so positioned, and in accordance with a preferred technique for utilizing the invention, the operating electronics are placed in a torque-monitoring mode of operation by depression of switch 96 and observation of display segment 90, a suitable drive mechanism is positioned over drive head 19, and the fastener is driven into the assembly. When the fastener is driven into the assembly to the previously-set torque threshold--e.g., 50 Newton-meters in the example discussed above--the operator is so advised by energization of audible alarm 130. Thus, the operator need not continuously observe display 86 in order to be advised that the fastener torque has reached the desired threshold level. In order to complete the torque/angle fastener preload procedure for which the present invention is particularly useful, the operator then depresses switch 96 to switch to an angle mode of operation, and then resets the angle accumulation to zero by depression of either switch 94 on enclosure 80 or switch 74 on drive head 12. The operator then continues to turn the fastener into the assembly for the specified number of degrees or fraction of rotation previously stored in the control electronics--e.g., 180.degree. in the example discussed above--at which point alarm 130 is again activated by microprocessor 116 to advise the operator that the fastener angle threshold has been reached.

In normal operation, it is anticipated that a number of fasteners will be tightened into an assembly in a predefined sequence. For example, eight bolts affixing a head to an engine block may be tightened in a predetermined sequence to the torque threshold, and thereafter further tightened in the same or differing sequence to the angle threshold. In this situation, the apparatus of the invention would be operated in the torque-monitoring mode during the initial tightening sequence, and thereafter switched to the angle-monitoring mode during the latter portion of the tightening sequence. Most preferably, a maximum torque limit such as 266 Newton-meters (200 foot-pounds) is preprogrammed into the control electronics and audible alarm 130 is continuously activated if such maximum torque limit is exceeded in any mode of operation.

During the angle-monitoring mode of operation, optical pickups 64, 66 cooperate with zones 70, 72 on strip 62 for indicating to control microprocessor 116 not only degrees of rotation but also direction of rotation of drive shaft 14 and the driven fastener. Thus, if the fastener is turned in the reverse direction during the angle-monitoring mode, the total accumulated angle will be decremented rather than incremented. Referring to FIG. 5, placement of optical couplers 64, 66 as described above ensures that the outputs thereof are, in effect, 90.degree. out of phase with each other. Assuming that band 62 is travelling in the "forward" direction indicated in FIG. 5, each time the output of coupler 64 switches from a logical zero to logical one, the output of coupler 66 is at a low or logical zero state. Likewise, on each transition of the output of coupler 66 from a logical zero to a logical one, the output of coupler 64 is at a high level. However, when band 62 is travelling in the opposite or "reverse" direction, the output of coupler 66 is at a high or logical one state upon each positive transition of coupler 64, and the output of coupler 64 is at a high or logical one state at each positive transition of coupler 66. By examining the state of one coupler output upon positive (or negative) transition at the other, microprocessor 116 can thus readily determine direction of rotation of band 62 within head 12.

A complete program listing for operating an 8049 type microprocessor in the manner described above accompanies this application as an Appendix. ##SPC1##