Ceramic clutch separator plates

Abstract: An improvement in a torsional coupling in which clutch separator plates (24) are interleaved between clutch friction plates (16) having friction linings (18) on the opposite surfaces thereof to provide an increased coefficient of friction without substantially increasing the wear of the friction lining of the clutch plates during operation of the clutch or brake. The separator plates (24) are formed of a solid ceramic material (24a) or of a steel plate (46) which is coated with a thin layer of the ceramic material (48). Suitable ceramics include silicon nitride, aluminum oxide and zirconium oxide.

Description: This invention relates to clutch separator plates, such as utilized in clutch or brake disc packs for vehicle automatic transmissions and the like. Multiple disc clutches and brakes employed in a drive train to transmit torque in an automatic transmission for an automotive vehicle or the like generally include a clutch disc pack of the kind comprising clutch plates having friction linings and interleaved with clutch separator plates. Such a clutch disc pack is is known from FR-A-2056321. Prior efforts to improve the frictional characteristics of a clutch employing a clutch disc pack of the kind described resided in the improvement of the friction lining for the clutch plates, and substantially no attention has been paid to the frictional characteristics of the separator plates themselves. One such "prior effort" is disclosed in the aforementioned FR-A-2056321 wherein the friction lining of the clutch plate is an abrasion-resistant layer of molybdenum nickel aluminide, ceramic or a non-ferrous metal which is sprayed on to the friction surface of the clutch plate, the surface of such abrasion-resistant layer then being subjected to pressure treatment by means of stamping or rolling to form pockets in the otherwise flat surface of the layer for the collection of a lubricant or coolant. One problem of the separator plates is in wear of the separator plate surfaces due to the frictional characteristics of the clutch plates. Less wear of the separator plates would provide more of a "steady state" rotating member and therefore consistent performance. There also is a need for higher friction to support current higher torque applications. More recently where the friction linings for the clutch plates have been formed of a paper-based friction material, the separator plates should not be formed of a material or have a surface texture which would cause undue wear upon the friction lining. As the clutch plates have friction linings secured to the opposite faces thereof, the separator plates are conventionally finished by a tumbling process to have relatively smooth surfaces engaging with the friction lining material. To enhance the frictional characteristics of the steel plates when used in combination with the friction lined clutch plates, it has been suggested that the surface be altered to change the characteristics of the steel plates. Various surface preparation procedures where tried, such as sand blasting, shot peening and knurling; however, the resulting surface either did not enhance the coefficient of friction or caused undue wear and/or damage to the friction lining of the mating clutch disc faces. This, in turn, would shorten the useful life of the clutch plates in the clutch pack for the automatic transmission. Also, coating of the separator plates was attempted using a wear-resistant material, such as sputter coating of the steel plates with titanium nitride or similar metallic nitrides; materials known for their wear resistant properties for tool bits, saw blades, etc. Although a slight increase in the frictional characteristics may have been detected, this improvement was clearly offset by a noticeable increase in the wear on the friction linings. In accordance with the present invention as claimed, a clutch disc pack of the aforementioned kind is characterised in that a separator plate has a surface texture on at least one face thereof which provides an increased coefficient of friction between the separator plate and the cooperating clutch plate without substantially increasing the frictional wear of the friction lining of that clutch plate, the surface texture resulting from the use of a ceramic material at least as a ceramic coating on the at least one opposite face of the separator plate and having a roughness in the arithmetic average (Ra) range of 0.0254 to 1.016 micrometres (1 to 40 microinches). Such a ceramic material could be silicon nitride, aluminium oxide as zirconium oxide, which materials have been found to increase friction by 5 to 10% over a standard 1035 steel separator plate without appreciable wear of the ceramic material. In one form of the invention, the separator plate has a steel core covered with the ceramic coating. As alternative, the entire separator plate is formed of a ceramic material. In one practical embodiment of the invention, one set of the clutch and separator plates has interior splines to engage a rotatable member, and the other set of plates has exterior peripheral splines to engage either a second rotatable member or a stationary member to form a torsional coupling or brake, respectively. Thus, one set of plates is splined to an encompassing clutch or brake drum while the other set of plates is splined on to a central shaft or ring. Attention is also drawn as an example of the prior art to FR-A-2631044. In order that the invention may be well understood, there will now be described some embodiments thereof, given by way of example, reference being made to the accompanying drawings, in which: FIGURE 1 is a partial cross sectional view through a clutch pack showing the friction and separator plates in a torsional coupling. FIGURE 2 is an enlarged partial cross sectional view of a ceramic coated separator plate. FIGURE 3 is a view similar to FIGURE 2 but showing a solid ceramic separator plate. FIGURE 4 is a graph showing the frictional characteristics at a 140°C (300° F) testing sequence of a separator plate formed of either silicon nitride or aluminium oxide with a 6 micron lap compared with a 1035 tumbled steel plate. FIGURE 5 is a graph similar to FIGURE 4 but showing the frictional characteristics of a zirconium oxide plate with an "as received" surface and a zirconium oxide ceramic plate with a 6 micron lap compared with a 1035 tumbled steel plate. FIGURE 6 is a graph similar to FIGURE 4 but showing the frictional characteristics of a D.C. composition and 30 composition tumbling media as moulded surfaces, a D.C. composition tumbling compound with a 6 micron lapped surface, and a 30 composition tumbling compound with a 6 micron lapped surface compared with a 1035 tumbled steel plate. FIGURE 7 is a graph showing the frictional characteristics of a steel separator plate in an automatic transmission fluid with and without additives. FIGURE 8 is graph similar to FIGURE 4 but showing a ceramic coated separator plate in the automatic transmission fluid with and without additives. FIGURE 9 is a graph similar to FIGURES 7 and 8 but showing the frictional characteristics of ceramic and steel plates in an automatic transmission fluid. Referring first to Figure 1, there is shown a portion of the upper half of a typical torsional coupling utilizing a clutch pack 16 in a clutch or brake for the automatic transmission in an automotive vehicle, the coupling comprising a drum or brake member 12 formed of cast iron, steel or powdered metal mounted for rotation on suitable bearings (not shown) and an internal metal hub or shaft 14 formed of one of the same materials also suitably mounted for rotation. The drum 12 is adapted to be- driven by a torque apply member and the hub or shaft 14 is adapted to be drivingly connected to a load to be driven, although the driving and driven portions could be reversed. Between the drum and hub are located a plurality of annular metal friction clutch plates 16 having suitable friction linings 18 secured to the opposite faces of each plate. Each friction plate 16 has internal teeth 20 thereon meshing with external splines or ribs 22 on the external surface of the hub 14 for rotation therewith. Alternating with the clutch plates are separator plates 24 having external teeth 28 meshing with internal splines or ribs 30 on the interior surface of the drum 12; the clutch plates and separator plates being interleaved together. Although shown as operatively connected to two rotating members, the separator plates 24 could be connected to a suitable stationary member as where the coupling is applied to a brake in a transmission rather than a clutch. An annular backing plate 32 has teeth 34 on its periphery meshing with the drum splines 30 and is retained axially through a retaining ring 36 mounted in an internal annular groove 38 in the drum. An annular apply piston 40 is mounted to axially slide within the drum and is normally urged leftward (as seen in FIGURE 1) to a disengaged position by suitable clutch release means, such as release springs (not shown). Fluid under pressure is admitted into the chamber 42 at the left-hand end of the piston 40 to urge the piston towards the clutch plates 16 to squeeze the clutch plates and separator plates 24 together so that the friction linings 18 engage the opposite surfaces 26 of the separator plates 24 between the piston and backing plate. With the plates engaged, a power path is established for torque transmittal between the drum 12 and the hub 14. An annular seal 44 is positioned in a groove in the periphery of the piston to prevent leakage from the apply chamber 42 past the piston 40. When the fluid in the chamber 42 is allowed to exhaust or exit, the return means moves the piston to its disengaged position to disengage the clutch. FIGURE 2 discloses a portion of a single separator plate utilized in the clutch pack separating a pair of adjacent clutch plates 16 having suitable linings formed of a resin-based paper friction material. The separator plate 24 is formed of a steel plate 46 coated with a layer 48 of a ceramic material, such as silicon nitride, aluminum oxide or zirconium oxide; although other ceramic materials may be suitable for the separator plate. The separator plate has a surface finish of from 0.0254 micro-metre (1.0 micro-inch) to 0.9398 micro-metre (37 micro-inches) to provide optimum surface texture, which in turn allows fluid suspension/support in the contact area. It appears that the ceramic coating preserves its initial state surface texture. The insulating properties of the ceramic material forces more heat to the contact area of the mating plate, rather than being allowed to dissipate in the separator plate. This increase of surface temperature does not damage the lining integrity or degrade it. The use of the ceramic material increases friction by 5 to 10% over 1035 steel plate, and it appears that surface hardness and surface finish contributes more to increased friction than does the material composition. Increasing ceramic surface roughness increased the wear of the clutch material; however no ceramic wear occurred in any of the tests involving the ceramic materials. A good working range for ceramic surfaces is from an arithmetic average (Ra) of 0.0254 to 1.016 micrometres (1.0 to 40 microinches). FIGURE 3 discloses a similar separator plate 24a which is formed of a solid ceramic material, such as previously set forth FIGURE 4 discloses a graph showing the frictional characteristics of two ceramic coatings having their surfaces treated with a 6 micron lapping operation versus a standard 1035 tumbled steel plate plotted as the coefficient of friction versus speed in meters per second in a Texaco TL8570 lubricant at a temperature level of 149°C (300° F)- (the reservoir temperature of the lubricant), under a load of 8.3 bar (120 psi), a mean radius of 0.0127m (0.5 inches) and a contact area of 2.02 cm² (0.393 square inches). This graph illustrates the curves for either a solid ceramic separator plate which has had a 6 micron lapping operation or a steel separator plate that has been lapped with a 6 micron lapping operation and then coated with a silicon nitride or aluminum oxide coating against a SD1777 friction lining. The graph shown in FIGURE 5 is similar to that of FIGURE 4 with the same parameters, except the comparison is between a zirconium oxide plate in an "as received" condition and a zirconium oxide plate having a 6 micron lapping operation on the surface when compared with the standard 1035 tumbled steel plate. In FIGURE 6, the graph illustrates a D.C. composition and 30 composition tumbling media as moulded surfaces, the D.C. composition with a 6 micron lapped surface and a 30 composition with a 6 micron lapped surface, all compared with the standard 1035 tumbled steel plate under the same parameters as in the tests of FIGURE 4. Both compositions are aluminium oxide tumbling compounds, the D. C. composition having a greater bulk density than the 30 composition. For the "as received" D. C. composition and 30 composition, the data was compiled at 38°C (100° F) due to the high rate of wear of the friction lining. However, for the two compositions after lapping and the 1035 steel plate, the tests were conducted at 149°C (300° F). FIGURE 7 is a graph which plots the coefficient of friction versus speed of a 1035 tumbled steel separator plate in an automatic transmission fluid (ATF) without any additives and with a full additive package. Similarly, FIGURE 8 shows the same graph for a ceramic separator plate or ceramic coated plate in the same ATF with and without the full additive package. It can be seen from FIGURE 7 that the steel plate in the ATF with full additive package has a relative smooth clutch engagement, while the plate in the ATF without the additive package has a very harsh engagement. FIGURE 8 discloses that the ceramic separator plate or ceramic coated plate has similar curves for the ATF with and without the additive package. Thus, in an ATF, the frictional characteristics will not change very much as the additive package deteriorates or ages over time, and at no time will the clutch engagement be as rough as that shown in the non-additive package shown in FIGURE 7. For the purposes of comparison, deteriorated ATF is believed to be comparable to an ATF without an additive package. A deteriorated or degraded ATF would be where the additive package has become insoluble or burnt off from the transmission fluid. Under these conditions, the ceramic separator plate becomes an attractive alternative. Finally, FIGURE 9 discloses a graph illustrating the frictional characteristics of a ceramic separator plate compared with a 1035 steel separator plate. It is clearly seen that the ceramic material has a higher coefficient of friction compared to the steel plate but retains a smooth engagement characteristic in a clutch operation. This phenomena is experienced using three different lubricating fluids; namely, 1) Texaco TL8570, 2) Ford Motor Company Type F fluid, and 3) Mobil 220 fluid. It has been found that a silicon nitride ceramic resulted in the lowest static/dynamic ratio and is therefore expected to provide the smoothest clutch engagement. These results are shown in the following table. The average frictional values were calculated from friction tests at 38°C (100° F), 93°C (200° F) and 149°C (300° F); the dynamic values being taken at 30.5m (100 feet) per minute and the static values being taken at 0.305m (1.0 foot) per minute using the same parameters as the previous examples. Thus, the use of a ceramic material either as a coating on a steel separator plate or as a solid ceramic plate provides a heightened coefficient of friction in automatic transmission fluid either with or without the additive package, and the ceramic separator plates having a lapped or similar surface treatment will result in an increased coefficient of friction without any substantial increase in the wear of the friction linings on the clutch plates during operation of the clutch pack in an automatic transmission.

Claims: A clutch disc pack comprising clutch plates (16) having friction linings (18) and interleaved with clutch separator plates (24;24a), characterised in that a separator plate (24;24a) has a surface texture on at least one face (26) thereof which provides an increased coefficient of friction between the separator plate (24;24a) and the cooperating clutch plate (16) without substantially increasing the frictional wear of the friction lining (18) of that clutch plate (16), the surface texture resulting from the use of a ceramic material at least as a ceramic coating (48) on the at least one face (26) of the separator plate (24) and having a roughness in the arithmetic average (Ra) range of 0.0254 to 1.016 micrometres (1 to 40 microinches). A clutch disc pack as claimed in claim 1, wherein the separator plate (24) has a steel core (46) coated with the ceramic material on the at least one face (26) thereof. A clutch disc pack as claimed in claim 2, wherein the at least one face (26) of the separator plate (24) is treated by lapping with a 6 micron lapping compound prior to the ceramic coating (48) being applied to the steel core (46). A clutch disc pack as claimed in claim 1, wherein the entire separator plate (24a) is formed of a ceramic material. A clutch disc pack as claimed in claim 4, wherein the at least one face of the solid ceramic plate (24a) is treated by lapping with a 6 micron lapping compound. A clutch disc pack as claimed in claim 2 or claim 3, wherein the ceramic material is coated on the at least one face (26) of the steel core (46) by a sputter-coating process. A clutch disc pack as claimed in any of the preceding claims, wherein the friction linings (18) are of a resin-based paper friction material. A clutch disc pack as claimed in any of the preceding claims, wherein the ceramic material is a silicon nitride, aluminium oxide or zirconium oxide. A clutch disc pack as claimed in any of the preceding claims, wherein one set (16) of the clutch (16) and separator (24;24a) plates are splined to one rotatable member (14), and the other set (24;24a) of the clutch (16) and separator (24;24a) plates are splined to a second rotatable (12) or stationary member to form a torsional coupling or brake, respectively.

Inventors: YESNIK MARC A, YESNIK, MARC A.
Applicants: BORG WARNER AUTOMOTIVE, BORG-WARNER AUTOMOTIVE TRANSMISSION AND ENGINE COMPONENTS CORPORATION
Application year: 1991
Main classification: F16D13
IPC codes: F16D13, F16D69, F16D25, F16D65