Abstract:
A micro-torque limiting production tool comprises a hollow cylindrically shaped handle in which a combination bit retainer clutch mechanism is supported. The combination bit retainer-clutch mechanism includes a pair of cylindrical clutch plates of a plastic composite composed of teflon fluorocarbon and acetal resin, having bulbous regions of diameter D1 in axially broad contact with each other within the central cavity of the handle to define a primary braking surface of area π(D1/2) 2 . Reduced regions of the clutch plates face in opposite directions, from a shoulder region to a remote end segment, the remote end segment of one of the reduced regions, extending through a secondary braking plate also of a plastic composite as described above, to define a secondary annular braking surface equal to π[(D1-D2)/2] 2  (where D1 and D2 are exterior and interior diameters of such plate). Such primary and secondary braking surfaces are provided with a highly accurate, longitudinally acting composite friction force that establishes a set point value for such braking surfaces, above which such surfaces do not act as cohesive unit but separate into individual elements that are not permitted to cohesively rotate. Result: only a torque value at or below the set point level can be applied to an exterior fastener by the bit retainer-clutch mechanism of the invention.

Description:
SCOPE OF THE INVENTION 
     The invention relates to torque limiting production tools and more specifically, but not by way of limitation, to a torque limiting production tool having an improved clutch mechanism that provides a smooth linearly loading function to a fastener during torquing thereof in a manner whereby shock wave generation is substantially reduced after a preset micro-torque value has been attained. 
     BACKGROUND OF THE INVENTION 
     Generally, in the majority of assembly applications (especially those related to the manufacture of disc driver assemblies for computers), the reliability of the joint is dependent upon how accurately and smoothly, the particular fastener or series of fasteners, can be made to clamp the assembly elements together. The clamping force is regulated by the amount of torque applied to the fasteners by production personnel (called torque control). 
     Engineers establish the theoretical torque value for a particular fastener using formulas that relate tensile stress of the fastener to equivalent stress of the fastener thread that takes into account the material used, thread pitch and minor diameters. 
     Then, the production personnel take the engineered torque value and after presetting their production tools to provide such torques during the particular mechanical jointing operations, manufacture the assemblies in a rapid manner in which the assembly elements pass from station to station using different preset production tools. Most production lines use production tools in which the torque value for a particular application has been preset for use at the particular station. 
     In disc drive assemblies, designers require greater storage capacity within a given physical space. Result: the longitudinal spacing between the magnetic discs along the central shaft, becomes less and less. Hence, as torque tolerances for assembly of the disc drives increase, say to tolerances of -+1.5 percent accuracy, conventional mechanical preset production tools have been found to be lacking in certain regards. 
     For example, where such tools rely upon ratchet type or other means of stepped loading of the fastener, they have been found to create shock waves at the fastener. Such waves are believed to originate at the fastener and are transmitted to the elements to be joined as stepped loading occurs. Where the elements to be assembled are stacks of magnetic discs of a disc drive assembly of a computer, such waves can cause the discs to change position, touch and otherwise inappropriately affect their operations. 
     One such step leading mechanical tool is shown in U.S. Pat. No. 4,063,474 wherein the described tool loading of the fastener uses a first annular clutch plate that is spring biased (i) longitudinally into contact with a second clutch plate integrally formed within a coextensive cylindrical handle (through a series of longitudinally restrained balls sitting in sets of pairs of longitudinally offset pockets in the clutch plates), and (ii) radially through a second series of bails locked in a series of radial slots in the bit cylinder. The handle, clutch plates (and the bit cylinder) rotate together with the rotation of the handle until the torque at the fastener (through the bit cylinder) is greater than friction response (as provided by the spring) between the second series of balls and the outer surface of the bit cylinder. Then, the bit cylinder remains stationary, with both (or one) of the clutch plates and handle then rotating relative to the axis of symmetry of the bit cylinder. While such tools may be initially accurate, that fact that the clutch plates contact at separate lands associated with the restrained balls about the circumferential extending faces, has been found to contribute to the rise in inaccuracy of the tools with time. It is believed that each radially separated ball and associated pair of pockets as well as the longitudinal end loading of the series of balls against the bit cylinder, non-uniformly contributes to the total force preset into the tool. Moreover, the machining requirements to create the balls and restraining slots and pockets in the bit cylinder and clutch plates, respectively, are usually beyond the capability of production personnel to repair. As a result, repair cannot occur at the work site. 
     I am also aware of a digitally programmable mechanical electrical production tool in which electrical current is used to accurately drive a servo motor whereby the load can be linearly varied with time, which results in the achievement of accurate torque settings on a linear loading basis. However, due to the high initial cost, experience shows there is still a need for a completely mechanical torque limiting production tool, that is low cost, and that uses an easily serviceable clutch mechanism that (especially for use in micro-torque applications) limits shock wave generation and has high repeatability over many cycles of operations. 
     DEFINITION 
     DISC DRIVE ASSEMBLY includes a motor drive, a disk storage system and a head assembly in which a series of magnetic disks are stacked on a central shaft within a central housing rotated by a motor attached to a threaded end of the central shaft. 
     The head assembly consists of READ-WRITE elements cantilevered from disc edges and attached to the housing (or subhousing) by fasteners. The central housing includes a cover attached by cover screws. 
     SHOCK WAVE GENERATION of an assembly during attachment of fasteners such as nuts and screws results from a longitudinal stepping of load by the nut driver against the fastener head causing a shock wave to translate through the fastener and thence to the assembly. 
     TENSIONING of a fastener relates to the force converted by a thread angle against a transverse support. 
     OPTIMUM TENSIONING of a fastener relates to a optimum torque/tension value applied to the fastener for each fastening application. 
     TORQUE relates to amount of force multiplied by the length of application described inch-pounds or inch-ounces. 
     ACCURACY means deviation within acceptable limits of a specified standard. FULL-SCALE ACCURACY relates to multiplying the stated full scale value by the full-scale deviation. READING/SETTING ACCURACY relates multiplying any value by the stated deviation. 
     PRESET VALUE relates the set tool value and lock same to prevent alteration of setting. 
     PRODUCTION TOOL is a tool that is preset to a torque limiting value for production line use. 
     REPEATABILITY is the extent to which repeated cycles of a tool produce identical values. 
     TENSION is the straight line force producing stretching of a bolt or screw fastener during torquing thereof. 
     MICRO-TORQUE values are in a range between about 8 inch-ounces to 8 inch-pounds for the production tool of the invention. 
     SUMMARY OF THE INVENTION 
     The present invention relates to micro-torque limiting production tool including a hollow cylindrically shaped handle having a side wall, an open end, a more closed end defined by a shoulder and a central cavity in which a combination bit retainer-clutch mechanism is supported. The combination bit retainer-clutch mechanism includes a pair of cylindrical clutch plates of a plastic composite such as a blend of teflon fluorocarbon and acetal resin identified with and by Federal Regulations and/or Specifications LP 392 A-type 2, ASP D-2133-8 and ASP D-4181-88, having bulbous regions of a common diameter D1 in axially broad contact with each other within the central cavity of the handle to define a primary braking surface therebetween of area π(D1/2) 2 . The reduced regions of diameter D2 of the clutch plates face in opposite directions each extending longitudinally of the handle from a shoulder region to a remote end segment. The remote end segment of one of the reduced regions, extends from the shoulder region through a secondary braking plate of annular cross section also of a plastic composite as identified above, to define a secondary braking surface equal to π[(D1-D2)/2] 2  (where D1 and D2 are the exterior and interior diameters of the secondary braking plate) and thence through an opening in the closed end of the handle. Such remote end segment is also placed in operable contact with a bit for micro-torquing purposes of a fastener. However, the remote end segment of the other of the reduced regions, extends oppositely from the bit, from the shoulder region through an annular shaped retaining member within the more central region of the handle and thence connects to a compression spring longitudinally spanning a longitudinal space between a transverse pin extending through the remote end segment of the other of the reduced regions and a cap member also rotatably attached to the handle. Rotation of the cap member provides for both micro- changes in the friction force at the primary and secondary braking surfaces to provide for highly accurate preset micro-torquing values for the tool of the invention whereby a smooth linearly loading function is provided to any fastener in operative contact with the bit and shock wave generation thereat is substantially reduced. 
    
    
     DESCRIPTION OF DRAWINGS 
     FIG. 1 is a perspective view of a torque limiting production tool in use in disc drive assembly operations; 
     FIG. 2 is a side view of the torque, and shock limiting tool of FIG. 1; 
     FIG. 3 is a section taken along line 3--3 of FIG. 2; 
     FIG. 4 is a fragmentary side view of clutch mechanism of the invention of FIG. 3 enlarged to better indicate relevant elements positioned near a remote end thereof; 
     FIG. 5 is a detail side view of the handle of the production tool of the invention; 
     FIG. 6 is a detail side view of the driving clutch plate of the clutch mechanism of FIG. 4; 
     FIGS. 7 and 8 are detail side and top views, respectively, of the primary braking clutch plate of the clutch mechanism of FIG. 4; 
     FIG. 9 is a front detail view of the retaining ring of the clutch mechanism of FIG. 4; 
     FIG. 10 is a front detail view of the secondary braking clutch plate of the clutch mechanism of FIG. 4; 
     FIG. 11 is a fragmentary side view of the primary braking clutch plate clutch mechanism of the invention of FIG. 3 enlarged to better indicate relevant elements positioned near the mid-portion thereof. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows torque limiting production tool 10 in use in association with a disc drive assembly 11. The disc drive assembly 11 includes a disk storage system generally indicated at 13, and a READ-WRITE head assembly 14. The disk storage system 13 includes a series of magnetic disks 15 stacked horizontally on a central shaft 16 within a central housing 17. Rotation of the series of magnetic disks 15 and central shaft 16 is via a drive motor (not shown) rotationally attached to the central shaft 16 opposite to fastener assembly 18 that includes assembly nut 19. The head assembly 14 is attached to the housing 17 by fasteners 20 and includes READ-WRITE magnetic sensors 21 operationally linked to mechanical drivers 22 via cantilevered arms 23. The cantilevered arms 23 also includes a transverse shaft 24 parallel the central shaft 16, the transverse shaft 24 being attached to the arms 23 via fastener assembly 25 including nut 25a. After the disk storage system 13 and READ-WRITE head assembly 14 have been assembled within the central housing 17, a cover 26 is releasably attached by cover screws 7. Separate stabilizing screws 8 are attached to the shafts 16, 24 through the broad surface 9 of the cover 26 to stabilize shaft operations. Critical to performance of the disc drive assembly 11 is the use of preset micro-torque values on the production line, such values to be used as standards in tightening down stabilizing screws 8, cover screws 7, fastener assemblies 18, 25 including nuts 19, 25a and fasteners 20. Note that after such preset micro-torque values are calculated by engineers, the production personnel preset same into the production tool 10 of the invention that will be used on the production line for such operation. Result: highly accurate and repeatable achievement of the preset micro-torque values while substantially reducing and limiting the generation of shock waves through the hardware during torquing operations. 
     FIGS. 2-5 show the production tool 10 in more detail. 
     As shown, the tool 10 includes a longitudinal axis of symmetry A and a hollow cylindrical handle 30 that is metallic and annular shaped. The cylindrical handle 30 comprises a side wall 31 concentric of the axis of symmetry A, such side wall 31 being more open at end wall surface 32 than at more closed end wall surface 33. The side wall 31 (see FIG. 5) also includes central main cavity 34 defined by a diameter D1 and inner surface 34a constant from the more open end surface 32 until shoulder 35a of far end wall 35 is encountered wherein the diameter D1 of cavity 34 is reduced to a diameter D2. I.e., far end wall 35 is provided a central opening 36 in communication with the main central cavity 34, such central opening 36 being of diameter D2 also being concentric of axis of symmetry A. As shown, D1 is greater than D2. 
     Adjacently positioned to the more closed end wall surface 33 but within the main central cavity 34 of the handle 30, see FIG. 3, is a clutch mechanism generally indicated at 27 concentric of the axis of symmetry A. 
     The clutch mechanism 27 is seen to include a driving clutch plate 37 of circular cross section having a bulbous region 37a in broad surface contact with a bulbous region 38a of a primary brake clutch plate 38 to create a primary braking surface generally indicated at 28a that is normal to and is bisected by the axis of symmetry A. The driving clutch plate 37 also includes a reduced diametered remote end region 37b opposite to the bulbous region 37a where shoulder region 37c marks the separation of the bulbous region 37a from the reduced remote end region 37b. Such shoulder region 37c is longitudinally positioned adjacent to the shoulder 35a of the end wall 35 of the handle 30, and as shown is axially separated therefrom by the presence of a secondary brake plate 29 of annular shape. The purpose of the secondary brake plate 29: to create a secondary braking surface generally indicated at 28b parallel to the primary braking surface 28a wherein a composite friction force for these braking surfaces 28a, 28b, is created by compression spring 40, as explained in more detail below. 
     The bulbous region 37a of the driving clutch plate 37 has a circumferentially extending outer surface 37d, see FIG. 4, in radial broad contact with interior surface 34a of the side wall 31 of the handle 30 so that the compression force of the spring 40, FIG. 3, is longitudinally confined. The reduced diametered end region 37b of the clutch plate 37, see FIG. 6, includes a mid-segment 41 that includes an outer surface 42 and an end segment 43. Note the outer surface 42 of the segment 41 is placed in radial contact with wall surface 36a, see FIG. 5, of the opening 36 in the end wall 35 of the handle 30. The diameter of the mid segment 41 and of the end segment 43 is constant and equal to diameter D2. That is, such diameter D2 is constant--longitudinally speaking--from the shoulder region 37c to the end segment 43. 
     Note that the end segment 43 of the reduced end region 37b of the clutch plate 37 is defined by a distance L1 and terminates in an end surface 44. Such end surface 44 is seen to be penetrated by a central opening 45. Note in FIG. 3 that central opening 45 in the reduced end region 37b provides a press fit with bit 50. 
     Bit 50 is conventional and includes a hexagon shaped base and can be fitted at an opposite end with any appropriate driver such a Phillips, regular blade for screws and the like or sockets for nuts and the like. 
     As previously mentioned, the shoulder region 37c of the driving clutch plate 37 is longitudinally positioned adjacent to the shoulder 35a of the end wall 35 of the handle 30, and as shown is axially separated therefrom by the presence of the secondary brake plate 29 of annular shape. That is, the shoulder region 37c does not contact shoulder 35a of the end wall 35. Instead, annular shaped secondary braking plate 29 is positioned in contact with both the shoulder region 37c and the shoulder 35a. Its purpose: to provide the secondary braking surface 28b previously mentioned equal to 
     
         π[(D1-D2)/2].sup.2, see FIG. 10 
    
     Note that toward the mid region of the handle 30, the bulbous region 37a of the driving clutch plate 37 is provided with a diameter D1. As shown in FIG. 6, the bulbous region 37a terminates in a continuously extending end surface 55 transverse to the axis of symmetry A. Such diameter D1 of the end surface 55 is designed to match that of similarly sized bulbous region 38a of the brake clutch plate 38, see FIG. 3, to create the primary braking surface 28a previously mentioned. In that way broad surface contact is provided across the entire extent of transverse end surface 55 of the drive clutch plate 37 as well as along adjacent transverse end surface 57 (see FIGS. 7 and 8) of the bulbous region 38a of the brake clutch plate 38 wherein the friction force acting between the end surfaces 55, 57 (a function of the compression force of the spring 40) can be accurately related to a preset micro-torque value. Likewise, the area of the primary braking surface 28a is related to the diameter D1 of the end surfaces 55, 57 in accordance with π(D1/2) 2 . 
     Bulbous region 38a of the primary brake clutch plate 38 has a circumferentially extending outer surface 58, see FIGS. 3, 7 and 8 defined by diameter D1 in radial broad contact with interior surface 34a of the side wall 31 of the handle 30 so as to be longitudinally confined. The reduced diametered end region 38b of the clutch plate 38 extends from shoulder region 38c that marks the separation of the bulbous and reduced diametered regions 38a and 38b and includes a mid segment 59 (see FIGS. 7 and 8) that includes an outer surface 60 and an end segment 61 that includes an end surface 62. The diameter D2 of the end region 38b is constant--longitudinally speaking--from the shoulder region 38c to terminating end surface 62. 
     Returning to FIG. 3, in broad contact with the shoulder region 38c of the braking plate 38, is an annular shaped retaining ring 66. As shown best in FIG. 9, note that the retaining ring 66 includes a side wall 67 radially terminating in circumferentially extending surface 68 that includes threads (not shown), such retaining ring 66 being capable of rotation in operative contact with associated threads (not shown) at the inner surface 34a of the side wall 31 of the handle 30 of FIG. 5 and rectilinear movement along the axis of symmetry A. Such action is provided by inserting a tip of a tool (not shown) into a pair of openings 69 in the side wall 67 of the retaining ring 66. 
     Returning to FIG. 3, note that the amount of rectilinear advancement of the retaining ring 66 along to the side wall 31 of the handle 30 toward the shoulder 35a of the far end wall 35 of the handle 30, establishes a minimum retaining set point for the primary brake plate 38, the drive plate 37 and the secondary brake plate 29 relative to the aforementioned shoulder 35a of the end wall 35 of the handle 30. 
     Also provided within remote end region 61 of the reduced region 38b of the braking (see FIGS. 7, 8 and especially 11) is a transverse opening 70 having an axis of symmetry A1 normal to the axis of symmetry A of the tool 10. Such transverse opening 70 is constructed to slidably accept a transverse pin 71 having a transverse length of L2 greater than D1, see FIG. 11. The transverse length L2 of the pin 71 is designed to provide a central region 73 that resides within opening 70, a pair of extension regions 74 exterior of the opening 70 and end regions 75 that extend through diametrically opposed slots 78 in the side wall 31 of longitudinal length L1, each having a width matched to that of the diameter of the pin 71 to establish rotational integrity between the pin 71 and the handle 30. 
     Returning to FIG. 3, note that extension region 74 of pin 71 accepts into contact therewith, a first interior end coil 40a of the compression spring 40. While, at the opposite end, a second exterior end coil 40b of the spring 40 is seen to be in broad contact with planar end cap 78. The end cap 78 is seen to comprise radial side surface 79 that is fully threaded to releasably attach to the interior surface 34a of the side wall 31 of the handle 30. The end cap 78 is longitudinally positioned adjacent to the more open end wall surface 32 of the handle 30 and is permitted to undergo rectilinear travel toward the shoulder 35a of the end wall 35 of the handle 30 to establish a highly accurate micro-torque value based on the compression force engendered by the compression spring 40 trapped between the end cap 78 and the pin 71. 
     Engineeringwise, length L3 (see FIG. 11) of the slots 76 permit such longitudinal rectilinear movement of the end regions 75 of the pin 71 (and hence the primary braking plate 38). As previously stated with regard to FIG. 3, such movement is relative to the shoulder 35a of the far end wall 35 of the handle 30, and results, of course, from the controlled rotation of the end cap 78 via insertion of tool tip (not shown) in a slot 80 in broad surface 81 of the end cap 78 to provide micro-torque adjustment in establishing preset micro-torque values for the tool of the invention. Such adjustment results in part, by the fact that both primary and secondary braking surfaces 28a, 28b substantially increase the maximum friction force available for adjustment while substantially reducing friction among the primary tool elements, viz., primary brake plate 38, disc drive plate 37 and secondary braking plate 29. 
     It is apparent that the materials used in the manufacture of the tool 10 of the invention, is of importance. In this regard, the primary brake plate 38, drive plate 37 and secondary braking plate 29 are formed of a high impact plastic having a low coefficient of friction such as a composite or blend of teflon fluorocarbon and acetal resin identified with Federal Regulations and/or Specifications LP 392 A-type 2, ASP D-2133-8 and ASP D-4181-88, an example thereof being Delrin 100 AF, a trademark of E.I. DUPONT DE NEMOUR &amp; COMPANY, Wilmington, Del. having the following physical characteristics: 
     Coefficient of friction: 0.14 using thrust washer test at 10 FPM at 300 psi versus carbon steel finished to 16 um, although a range of 0.08 to 0.20 is adequate for the invention; 
     Coefficient of linear thermal expansion: 3.8×10** in/in using ASTM test 696 for a temperature range of -40 to 85 degrees F.; 
     Specific gravity: 1.54 using ASTM test 792; 
     Rockwell hardness: M78, R110 using ASTM test 785; 
     Tensile Elongation using ASTM test 638: 22 at 73 degrees F. and at rate of 0.2 in/min; 
     Tensile Strength using ASTM test 838: 7.6 Kpsi at 73 degrees F. and rate of 0.2 in/min; 
     Modulus of Elasticity using ASTM test 638: 420 Kpsi at 73 degrees F. and a rate of 0.2 in/min; 
     Shear Strength using ASTM test 732: 8 Kpsi at 73 degrees F.; 
     Flexural modulus using ASTM test 790; 340 Kpsi at 73 degrees F. and a rate of 0.05 in/min; 
     Flexural Yield Strength using ASTM test 790: 10.5 Kpsi at 73 degrees F. and a rate of 0.05 in/min; 
     Compressive Stress using ASTM test 695: 4.5 Kpsi at 73 degrees F., 1 percent deflection and a rate of 0.05 in/min as well as 13 Kpsi at 73 degrees F., 10 percent deflection and a rate of 0.05 in/min; 
     Deformation using ASTM test 621: 0.6 percent under load of 2000 psi at 122 degrees F.; 
     Impact using ASTM test 256: 1.2 ft. lb/in at 73 degrees F.; and 
     Tensile impact Resistance using ASTM test 1822: 50 ft. lb/in at 73 degrees F. 
     Likewise the handle 30 is preferably metallic such as polished aluminum; the end cap 76 is also metallic such as stainless steel; and the retaining ring 66 is also metallic also stainless steel. 
     In this regard, the tool 10 has been built as described above and been successfully tested in the following manner. The tool 10 of the invention was preset with a micro-torque value and then used to torque down a nut on a threaded shaft similar to that depicted in FIG. 1, such preset micro-torque value being to an accuracy within -+1.5 per cent deviation. During torquing operations, the loading on such nut was linearly increased with time (without steps) and without generating shock therealong. With the tool 10 of the invention attached to the nut, the tool 10 was chucked to a milling machine while the shaft was fixedly attached to the milling machine table. Then the machine was rotated at 100 rpm for less than 45 minutes whereby the bit 50 and drive clutch plate 37 remained stationary as the handle 30, primary braking clutch plate 38 and secondary braking plate 27 rotated at the aforementioned 100 rpm. Such test is equivalent, it is believed to years of service of the tool in production operation. Then the tool 10 and shaft was removed from the milling machine, and the tool 10 re-tested as to preset micro-torque value. Such value still had an accuracy within -+1.5 per cent deviation. 
     From the foregoing, it will be appreciated that one skilled can make various modifications and changes to the invention within the spirit and scope of the invention.