Abstract:
Friction clutch plates have a first face exhibiting a surface finish of about Ra 0.5 to 0.9 and a second face having friction material secured thereto. The clutch plates are manufactured by rolling steel to a thickness of about 0.030 inches (0.76 mm) which exhibits a mill finish of about Ra 0.64 to 1.14. The rolled steel is then stamped to form circular clutch plate blanks. One surface is then smoothed, preferably by polishing, so that peaks and sharp points are flattened or removed. The resulting surface finish includes irregular relatively flat and smooth regions interspersed with and separating irregular recesses or pits and exhibits a surface finish of about Ra 0.5 to 0.9. Friction material is then secured to the opposite face of the clutch plates. The clutch plates are used in multiple plate friction clutch packs which are intended for use in dual clutch front and rear axle assemblies and motor vehicle transfer cases.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates generally to clutch plates having a specific surface finish and more specifically to clutch plates adapted for use in multiple plate friction clutch packs wherein a mill finish is polished to produce a surface having irregular flat regions and irregular recessed regions. 
     Amongst motor vehicle manufacturers, two areas of vehicle dynamics are foci of constant study, engineering effort and product improvement. These two areas can broadly be denominated vehicle performance and occupant comfort. Occupant comfort within the context of vehicle dynamics generally addresses noise, vibration and harshness (NVH) issues. These design criteria are nowhere more subjects of concern than in vehicles with adaptive four-wheel drive systems where the complex power train, torque distribution device, control strategy and overall system operation create unique performance and control issues. Such adaptive vehicle drive systems operate under most conditions as two-wheel drive systems and automatically shift or adjust to four-wheel drive when certain operating conditions such as wheel slip or other anomalies are present. 
     In such systems, the commencement of four-wheel drive operation, typically achieved by engagement of a clutch disposed between the full-time or primary drive line and the part-time or secondary drive line, which transfers drive energy to the secondary drive line, must be carefully controlled so that its operation is unnoticeable to the vehicle driver. Competing with this design criteria for smooth, transparent clutch engagement is often the design preference to engage the clutch as quickly as possible in order to optimize vehicle control and slip regulation. Whether the clutch engages slowly or rapidly, such engagement must be smooth, linear and without grabbing, judder or other engagement phenomena which may be detected by the vehicle driver and passengers. The present invention is directed to a multiple plate friction clutch having improved engagement smoothness and torque through put for use in motor vehicle drive lines. 
     SUMMARY OF THE INVENTION 
     Friction clutch plates have a first face exhibiting a surface finish of about Ra 0.5 to 0.9 and a second face having friction material secured thereto. The clutch plates are manufactured by rolling steel to a thickness of about 0.030 inches (0.76 mm) which exhibits a mill finish of about Ra 0.64 to 1.14. The rolled steel is then stamped to form circular clutch plate blanks. One surface is then smoothed, preferably by polishing, so that peaks and sharp points are flattened or removed. The resulting surface finish includes irregular relatively flat and smooth regions interspersed with and separating irregular recesses or pits and exhibits a surface finish of about Ra 0.5 to 0.9. Friction material is then secured to the opposite face of the clutch plates. The clutch plates are used in multiple plate friction clutch packs which are intended for use in dual clutch front and rear axle assemblies and motor vehicle transfer cases. 
     Thus it is an object of the present invention to provide clutch plates or discs having a first friction material surface and a second clutch surface with roughness between Ra 0.5 and 0.9. 
     It is a further object of the present invention to provide a multiple plate clutch pack assembly having improved engagement characteristics. 
     It is a still further object of the present invention to provide a multiple plate friction clutch pack having plates or discs with a defined surface treatment separated by clutch friction material. 
     It is a still further object of the present invention to provide a multiple plate friction clutch pack wherein one surface of each clutch plate is a mill finish which is polished to define irregular flat regions and adjacent irregular recesses. 
     It is a still further object of the present invention to provide a multiple plate friction clutch pack for use in a motor vehicle drive line having clutch plates or discs with a surface finish of Ra 0.5 to 0.9. 
    
    
     Further objects and advantages of the present invention will become apparent by reference to the following description of the preferred embodiment and appended drawings wherein like reference numbers refer to the same component, element or feature. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic view of a vehicle drive system including a twin clutch rear axle incorporating the present invention; 
     FIG. 2 is a fragmentary, full sectional view of a multiple plate clutch pack assembly and an electromagnetic ball ramp operator in a twin clutch axle; 
     FIG. 3 is an elevational view of the front face of a first clutch plate according to the present invention; 
     FIG. 4 is an elevational view of the back or rear face of the first clutch plate according to the present invention illustrating friction material; 
     FIG. 5 is an elevational view of the front face of a second clutch plate according to the present invention; 
     FIG. 6 is an elevational view of the back or rear face of the second clutch plate according to the present invention illustrating friction material, 
     FIG. 7 is an enlarged view of the metal friction surface of a clutch plate according to the present invention; 
     FIG. 8 is a highly enlarged, fragmentary sectional view of a clutch plate according to the present invention taken along line  8 — 8  of FIG. 7; 
     FIG. 9 is a highly enlarged, fragmentary sectional view of the finished clutch plate surface according to the present invention taken along line  8 — 8  of FIG. 7; and 
     FIG. 10 is a diagrammatic view of a manufacturing facility for the production of clutch plates according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, a four-wheel motor vehicle drive system is diagrammatically illustrated and designated by the reference number  10 . The four-wheel vehicle drive system  10  includes a prime mover  12  such as a gasoline or Diesel engine which is coupled to and directly drives a transaxle  14 . The output of the transaxle  14  drives a bevel or spiral bevel gear set  16  which provides motive power to a primary or front drive line  20  comprising a primary or front prop shaft  22 , a primary or front differential  24 , a pair of live primary or front axles  26  and a respective pair of primary or front tire and wheel assemblies  28 . The bevel or spiral bevel gear set  16  also provides motive power to a secondary or rear drive line  30  comprising a secondary or rear prop shaft  32  having appropriate universal joints  34 , a secondary or rear axle assembly  36 , a pair of live secondary or rear axles  38  and a respective pair of secondary or rear tire and wheel assemblies  40 . 
     Referring now to FIGS. 1 and 2, the rear axle assembly  36  includes a housing  44 . The housing  44  receives a stub input shaft  46  which is rotatably supported in the housing  44  by a plurality of anti-friction bearing assemblies such as roller bearing assemblies  48 . The stub input shaft  46  may includes a tone wheel  52  disposed operably adjacent a sensor  54  such as a variable reluctance or Hall effect sensor  54 . The stub input shaft  46  may be integrally formed with and terminate in a bevel gear  56  having gear teeth  58  which mate with complementarily configured gear teeth  62  on a ring gear  64 . The ring gear  64  is integrally formed with or secured to a cylindrical or tubular drive member  66 . The tubular drive member  66  is supported by a pair of anti-friction bearings, such as ball bearing assemblies  68 , one of which is illustrated in FIG.  2 . 
     The tubular drive member  66  includes female splines or gear teeth  72  which engage complementarily configured male splines or gear teeth  74  on a drive collar  76 . The drive collar  76  is freely rotatably received upon a stub output shaft  78  but is restrained from axial movement to the left, as illustrated in FIG. 2, by a shoulder  82  on the end of the stub shaft  78 . The drive collar  76  includes a flange  84  which is disposed between one end of the tubular drive member  66  and a portion of a bell-shaped drive housing  86  which is a component of an electromagnetic friction clutch assembly  90 . It will be appreciated that the rear axle assembly  36  includes a pair, that is, a left and right, electromagnetic friction clutch assemblies which, independently and controllably provide drive torque to the rear axles  38 . The bell-shaped drive housing  86  and the drive collar  76  include an interengaging male and female spline or gear set  92 . Thus, rotation of the tubular drive member  66  is transmitted through the drive collar  76  to the bell housing  86 . The bell housing  86  includes a plurality of axial female splines or gear teeth  94  which drivingly engage a first plurality of friction clutch plates or disks  96  having male splines  98  complementary to the female splines  94  within the bell housing  86 . Thus, the first plurality of friction clutch plates or disks  96  rotate with the bell housing  86 . As illustrated in FIGS. 2,  3  and  4 , the first plurality of clutch plates or disks  96  include friction material  100  on one face and clutch faces or surfaces  102  on an opposite face. 
     Referring now to FIGS. 2,  5  and  6 , interleaved with the first plurality of friction clutch plates or disks  96  are a second plurality of friction clutch plates or disks  104  having female splines or gear teeth  106  which mate with complementarily configured male splines or gear teeth  108  on a clutch hub  110 . The second plurality of friction clutch disks  104  likewise include clutch faces or surfaces  112  and friction clutch material  100  on the opposite face. As illustrated in FIG. 2, the pluralities of friction clutch disks or plates  96  and  104  are arranged such that the plates  96  and  104  alternate with the friction clutch material  100  and form a friction clutch pack. 
     The clutch hub  110  includes female splines or gear teeth  114  which are complementary to and engage male splines or gear teeth  116  on the stub output shaft  78 . The stub output shaft  78  and clutch hub  110  preferably include suitable axial and radial lubrication passageways  118  which facilitate circulation of a suitable cooling and lubricating fluid within the electromagnetic clutch assembly  90 . 
     Splined to the clutch hub  110  and specifically to the male splines  108  by a set of complementarily configured female splines  120  is a circular apply plate  122 . The apply plate  122  is acted upon by a first circular member  124  having female splines or gear teeth  126  which are complementary to the male splines or gear teeth  116  on the stub output shaft  78 . The first circular member  124  includes a plurality of ramp-like first recesses  128  arranged in a circular pattern about the axis of the stub output shaft  78 . The first recesses  128  represent an oblique section of a helical torus. Disposed within each of the first recesses  128  is a load transferring ball  132  or similar load transferring member which rolls along the ramps defined by the oblique surfaces of the first recesses  128 . A second circular member  134  is disposed in opposed relationship with the first circular member  124  and includes a like plurality of complementarily sized, configured and arranged second recesses  136 . The load transferring balls  132  are thus received and trapped within the pairs of opposing recesses  128  and  136 , the ends of the recesses  128  and  136  being curved and much steeper in slope than the interior regions of the recesses  128  and  136  such that the load transferring balls  132  are retained in the regions defined thereby. 
     It will be appreciated that the recesses  128  and  136  as well as the load transferring balls  132  may be replaced with other, analogous mechanical elements which cause axial displacement of the circular members  124  and  134  in response to relative rotation therebetween. For example, tapered rollers disposed in complementarily configured conical helices may be utilized. 
     The second circular member  134  is secured by any suitable means such as welding, interengaging splines or an interference fit to an annular rotor  142  which defines a U-shape in cross-section. The annular rotor  142  is preferably fabricated of soft iron and substantially surrounds a coil housing  144  containing an electromagnetic coil  146 . Electrical current is provided to the electromagnetic coil  146  through a single or double conductor cable  148 . Disposed adjacent the face of the rotor  142  is a flat, circular armature  150 . The armature  150  includes male splines or gear teeth  152  about its periphery which are complementary to and engage the female splines or gear teeth  94  on the interior of the bell housing  86 . Thus, the armature  150  rotates with the bell housing  86 . Both the rotor  142  and the armature  150  preferably include pluralities of arcuate, discontinuous banana slots  154  which direct and enhance the effect of the magnetic flux generated by the electromagnetic coil  146 . A Belleville spring  158  is disposed between the armature  150  and the apply plate  122 . When electrical energy is provided to the electromagnetic coil  146  through the cable  148 , and a speed difference exists between the tubular drive member  66  and the stub output shaft  78 , drag is created between the opposed faces of the rotor  142  and the armature  150 , thereby creating a relative speed difference between the first and second circular members  124  and  134 . Such a speed difference causes the load transferring balls  132  to ride up the ramps or recesses  128  and  136 , separating the first and second circular members  124  and  134  and moving the apply plate  122  to the left, as illustrated in FIG. 2, compressing the pluralities of friction plates or disks  96  and  104  and transferring torque from the tubular drive member  66  to the stub output shaft  78 . 
     Referring now to FIGS. 1 and 2, preferably, a thrust bearing  162  is disposed between the second circular plate  134  and one of the anti-friction bearings  68 . A tone wheel  164  is secured to and rotates with the stub output shaft  78 . A first axle sensor  166 A which may be either a variable reluctance or Hall effect sensor  166  is disposed in sensing proximity with the tone wheel  164  and provides a signal representing the rotational speed of the stub output shaft  78 . This signal, as well as a signal from a second axle sensor  166 B and the sensor  54  are provided to a microprocessor  168 . The microprocessor  168  may include software for conditioning such signals, computing rotational speeds, scaling such speeds, computing speed differences and providing outputs such as PWM (pulse width modulated) drive signals to the left and right electromagnetic friction clutch assemblies  90 . An output flange  170  is preferably secured for rotation to the stub output shaft  78  through a set of interengaging splines  172  and is maintained there by a suitable nut  174  or other fastener. 
     Turning then to FIG. 7,  8 ,  9 , and  10 , the configuration or finish of the faces or surfaces  102  and  1   12  of the clutch disks or plates  96  and  104 , respectively, which engage friction material  100  and the manufacture thereof will now be described. A manufacturing facility  180  for the manufacture of such clutch disk or plates  96  and  104  is diagrammatically illustrated in FIG.  10 . During manufacture, material, typically steel  182 , is roll formed by a plurality of mill rollers  184  to the desired thickness, typically about 0.030 inches (0.76 mm) and more generally between about 0.025 inches (0.63 mm) to about 0.035 inches (0.89 mm). The mill rollers  184  of the forming operation produce a surface finish having a roughness of approximately Ra 0.64 to 1.14. This surface finish is illustrated in FIGS. 7 and 8, wherein FIG. 7 represents an enlarged plan view and FIG. 8 represents a highly enlarged, cross-sectional view. It will be appreciated that this initial surface finish is achieved by and is primarily a result of the surface irregularities or roughness of the mill rollers  184  which is reproduced or “printed” on the steel  182 . The rolled steel  182  is then provided to a conventional reciprocating press  186 . A plurality of clutch plate blanks  188  are formed, preferably by stamping or other analogous forming process, from the roll formed steel  182  in the reciprocating press  186 . The clutch plate blanks  188  are carried from the press  186  by a conveyor  190 . 
     After rolling, which achieves the surface finish described above and illustrated in FIGS. 7 and 8, and stamping which achieves the shapes illustrated in FIGS. 3,  4 ,  5  and  6 , the clutch plate blanks  188  are then polished by a buffer or polisher  192 . The polisher  192  preferably includes a coarse fiber pad  194  or similar relatively soft abrasive media typically including random weave fibers and binder which smoothes, buffs and polishes the surfaces  102  and  112  of the clutch blanks  188 . “Relatively soft” as used herein refers to a comparison with conventional or typical metal working, i.e., grinding or polishing, media such as grinding wheels or other hard, abrasive media. So polished, the roughness of the surfaces  102  and  112  is approximately Ra 0.5 to 0.9. 
     The polished clutch plate blanks  188  then continue along the conveyor  190  to a reversing mechanism  196  which flips the clutch blanks  188  so that the polished faces or surfaces  102  and  112  are face down. Then the clutch blanks  188  pass through a coating station  198  which applies an adhesive to an unpolished surface  202 . The adhesive may be applied by rolling, spraying or other analogous process and to either the clutch blanks  188 , as shown, or to the friction material  100 . Finally, an annulus of friction material  100  is applied to the blanks  188  in an applying station  206 . The clutch plates or disks  96  and  104  are now complete. 
     The clutch plates  96  and  104  manufactured according to the foregoing steps and including the above-described finish on the surfaces  102  and  112  exhibit both improved maximum torque throughput and quieter, smoother clutch engagement relative to clutches having other face treatments. 
     It has been found that surface finishes of less than about Ra 0.5 do not provide either suitable or improved torque transfer through a clutch such as the friction clutch assembly  90  and that surface finishes much above Ra 1.15 may contribute to premature failure of the friction material  100  due to the roughness of the surface. 
     While the complete mechanism of frictional coupling and thus improvement in torque throughput is not understood, it is believed that this roughness in the range of Ra 0.5 to Ra 0.9 represents a compromise or optimization between smoother surfaces or surface finishes which do not provide sufficient frictional coupling and rougher surfaces or surface finishes which result in premature degradation of the friction facing material. 
     Friction clutches such as the electromagnetic friction clutch assemblies  90  described above, incorporating the friction plates or discs  96  and  104  of the present invention, when utilized in motor vehicle drive line components such as dual clutch axles (both front and rear) and transfer cases, exhibit both improved maximum torque throughput and enhanced engagement characteristics, i.e., engagement which is smooth and without judder or other undesirable engagement phenomena. 
     The foregoing disclosure is the best mode devised by the inventor for practicing this invention. It is apparent, however, that devices and processes incorporating modifications and variations will be obvious to one skilled in the art of friction clutch components and their methods of manufacture. Inasmuch as the foregoing disclosure is intended to enable one skilled in the pertinent art to practice the instant invention, it should not be construed to be limited thereby but should be construed to include such aforementioned obvious variations and be limited only by the spirit and scope of the following claims.