Patent Document

FIELD 
     The present invention relates to a torque-transmitting device and to clutches or brakes for controlling the operation of mechanisms such as a transmission. 
     BACKGROUND 
     Torque-transmitting devices such as clutches or brakes are heavily used throughout the automotive industry. For example, vehicle transmissions employ a multitude of clutches to engage and disengage the gearsets of the transmission to provide forward and reverse gear ratios. The clutch includes a reaction plate for exerting a compression force. The clutch also includes a friction plate disposed adjacent the reaction plate for frictionally engaging the reaction plate to transfer a driving torque between the reaction plate and the friction plate when the compression force is applied. The clutch further includes a friction material layer adhered to the friction plate that opposes the reaction plate. The friction material layer is configured to be compressible by the reaction plate. The clutch further includes a fluid lubricant disposed between the reaction plate and the friction plate for providing a lubrication layer between the plates during clutch engagement. 
     Three dominant lubrication states exist during clutch engagement: hydrodynamic (HD), soft elasto-hydrodynamic (soft-EHL) and boundary. Hydrodynamic lubrication state is characterized by a thick fluid film thickness, low nominal contact pressure of the plates of the clutch, and an extremely low coefficient of traction/friction. Thus, torque transfer during in the hydrodynamic lubrication state is difficult to achieve. 
     During the soft elasto-hydrodynamic lubrication state, the sharp groove edges and/or rough fibers topography of the friction material operates to break through the hydrodynamic film and create islands of film separating the mating surfaces of the clutch plates. The islands are unevenly distributed over the surface area due to the non-homogenous nature of friction material, waviness of the plates and uneven load distribution. Since the nominal load on the friction plate is constant, the islands of fluid experience much greater pressure than exists in the hydrodynamic state. As the lubrication fluid becomes thinner its shear rate rapidly increases, which allows sufficient torque to be transferred through the fluid. 
     During the boundary lubrication state the soft-EHL fluid experiences break down due to a speed decrease and temperature rise. Further, the active additives in the surface of the friction material layer are activated and create a low-shear protective layer of fluid, which acts like a solid lubricant which interacts with the friction material layer to create a solid to solid coefficient of friction which is closer to the EHL value. A tribo-chemical layer is resultant of chemical reaction between the fluid lubricant additives and mating surfaces of the clutch plates. When the fluid lubricant deteriorates, the tribo-chemical layer undergoes shearing. The chemical bonds break-down, additives in the fluid are depleted. The tribo-chemical layer is replenished by using new non-depleted additives from the fluid. As additive concentration in the fluid decrease below a predefined critical level, the local coefficient of friction increases in magnitude and leads to uneven clutch engagement (shudder or stick/slip) or further temperature increase. Provided the temperature increases, the high temperature may result in damage to the friction material (wear, tearing, and glazing). The reaction plate may also be damaged which may include hot spotting. 
     The soft-EHL is the optimum state for clutch operation for the following reasons: (1) The thin fluid film traction provides friction coefficients sufficient to transfer the required torque. (2) The thin film traction does not generate excessive heat, which may damage the friction material (wear, tearing and glazing) and reaction plate (hot spotting) leading to the loss of friction and resulting in shudder. (3) The surface active friction modifiers are not consumed during the soft-EHL, so fluid deterioration is delayed. As long as fluid base stock is chosen from high level grades, the bulk oxidation is not primary failure mode of the clutch up to a fluid sump temperature of 135 degrees Celsius. (4) Fluid film as well as friction material have damping capabilities that reduce the amplitude of friction interface self-induced and/or external vibration. (5) The traction/friction coefficient is proportional to the viscous drag, which decreases with increasing temperature (this is typical for the clutch engagement cycle) positive friction/slip slope. (6) Compared to other lubrication regimes, the clutch can operate in the soft-EHL mode for sufficiently long periods of time without any damage or failure of the mating surface. 
     There is a need in the art to provide a new and improved clutch that prolongs the soft EHL state during clutch engagement. The new and improved clutch should maintain a sufficient coefficient of friction and not create excessive heat that will damage the friction material layer of the clutch. 
     SUMMARY 
     The present invention provides a clutch including a reaction plate configured to exert a compression force. The clutch further includes a friction plate having a surface opposing the reaction plate. The clutch further includes a friction material layer having a first surface attached to the surface of the friction plate, and a second surface opposing the reaction plate. The second surface includes a plurality of dimples formed therein. The clutch further includes a lubrication fluid disposed between the reaction plate and the friction material layer for transferring the compression force and providing a lubrication layer between the reaction plate and the friction plate. 
     The invention prolongs the soft-EHL regime during engagement, increases the actual area of contact during the soft-EHL lubrication regime, and decreases the actual contact pressure of the mating surfaces during the soft-EHL lubrication regime. 
     Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
         FIG. 1  is a side view of a torque transmitting device connected between a drive shaft and a driven shaft, in accordance with an exemplary embodiment of the present invention; 
         FIG. 2  is a partial cutaway view of the torque transmitting device of  FIG. 1 , in accordance with an exemplary embodiment of the present invention; 
         FIG. 3   a  is a magnified partial cross-sectional view of the torque transmitting device of  FIG. 1  having a plurality of dimples formed on a top surface of a friction material layer shown in  FIG. 2 , in accordance with the exemplary embodiment of the present invention; 
         FIG. 3   b  is a magnified view of an dimple shown in  FIG. 3   a ; and 
         FIG. 3   c  is a magnified cross-sectional view of the torque transmitting device of  FIG. 1  having the plurality of dimples shown in  FIG. 3   a  and in a state of engagement, in accordance with an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings, wherein like reference numbers refer to like components, in  FIG. 1  a side view of a torque-transmitting device  10  is shown, in accordance with an exemplary embodiment of the invention. Torque-transmitting device  10  is commonly referred to in automotive applications as a clutch or brake. Torque-transmitting device  10  has a first plate or friction plate  12  and a second plate or reaction plate  16 . Friction plate  12  is separated from the reaction plate  16  by a layer of lubrication fluid  14 . The lubrication fluid  14  is disposed between the friction plate  12  and the reaction plate  16 , and is used to provide a lubrication barrier between the plates  12  and  16 . 
     The torque-transmitting device  10  is connected between a drive shaft  18  and a driven shaft  20 . More particularly, the friction plate  12  is coupled to the drive shaft  18  and the reaction plate  16  is coupled to the driven shaft  20 . The drive shaft  18  is typically connected to a torque-producing device such as an internal combustion engine (not shown). The driven shaft  20  may be connected to a planetary gearset (not shown) for transmitting a driving torque from the engine to the planetary gearset to drive the wheels of a vehicle. However, either plate  12 ,  16  of the torque transmitting device  10  may also be connected to a rotating member or to a non-rotating member. Both the friction plate  12  and the reaction plate  16  are made of steel or a similar material. However, it should be appreciated by one skilled in the art that the present invention may be applied to plates made of different materials, such as metal alloys, composites or the like. 
     Referring now to  FIG. 2 , a partial cutaway view of the torque-transmitting device  10  of  FIG. 1  is shown, in accordance with the exemplary embodiment of the present invention. Portions of reaction plate  16  have been removed to reveal the lubrication layer  14  and a friction material layer  22 . The friction material layer  22  is attached to a surface  23  of the friction plate  12 . The friction material layer  22  may be one of a variety of friction materials commonly used in torque-transmitting mechanisms today. However, the present invention contemplates that the friction material layer  22  shall be made of cellulose, Kevlar, and resin or any combination of these materials in varying percentages by weight that may or may not be in use in present clutch applications. The friction material layer  22  is a compressible resilient material that will return to its initial height and shape prior to being compressed by reaction plate  16 , provided the friction material layer  22  is not compressed beyond its elastic limit. 
     In the preferred embodiment of the present invention, the friction material layer  22  includes a plurality of raised dimples or raised indentations  26  formed on a top surface  24  of the friction material layer  22 . The dimples  26  are arranged within a predefined pattern and equally spaced apart over the top surface  24  of the friction material layer  22 . As shown, the dimples  26  are radially aligned. It should be appreciated that the dimples  26  may be arranged in a multitude of patterns including a random pattern. The density of dimples  26  over the top surface  24  may also be varied depending on the particular application or desired performance criteria. 
     Referring now to  FIGS. 3   a  and  3   b , a partial cross-sectional view of the torque-transmitting device  10  through a plurality of dimples  26  is shown in  FIG. 3   a . Furthermore, a magnified view of a dimple  26  is shown in  FIG. 3   b . Each dimple  26  includes a substantially circular rim or annular flange  28 . The rim  28  has a predefined height relative to the top surface  24  of the friction material layer  22 . Each dimple  26  further includes a cavity  30  defined by the rim  28 . Each cavity  30  has a substantially rounded bottom surface. However, it should be appreciated that the bottom surface of the cavity  30  may be formed with a different shape. Each dimple  26  is formed such that the cavity  30  is deep enough to retain the lubrication fluid  14  during the clutch activation taking into consideration the elasto-plastic deformation and wear of the friction material layer  22 . However, the predefined depth of each cavity  30  allows the lubrication fluid  14  to bleed over the rim  28 . During clutch engagement  10 , the lubrication fluid  14  is squeezed or forced out of the cavity  30  and flows over the rim  28  onto the top surface  24  of the friction material layer  22  to create a thin film of lubrication fluid between layer  22  and plate  16 . Each rim  28  operates to penetrate the thin film of lubrication fluid  14 , which increases the area of contact between the friction material layer  22  and the reaction plate  16 . 
     As shown in  FIG. 3   a , the torque-transmitting device  10  is in a non-engaged state. During the non-engaged state the reaction plate  16  does not contact the friction material layer  22 , and therefore no load is transferred. As shown, the friction material layer  22  is uncompressed. During the non-engaged state the friction material layer  22  has a thickness of T uc . More particularly, when force is applied to the torque-transmitting device  10 , the dimples  26  and the friction material layer  22  are compressed. The rim  28  of each dimple  26  has a predefined height relative to the top surface of the friction material layer  22 . The height of each rim  28  prevents the friction material layer  22  from being compressed beyond a predefined elastic zone. 
     Referring now to  FIG. 3   c , a partial cross-sectional view of the torque transmitting device  10  of  FIG. 3   a  is shown, in accordance with the exemplary embodiment of the present invention. As shown, the torque transmitting device  10  is in an engaged state. During the engaged state the reaction plate  16  and the friction plate  12  are moved towards each other. More particularly, the reaction plate  16  contacts the dimples  26  and compresses the friction material layer  22 . As a result of the compression, the lubrication fluid  14  will be forced out of the cavity  30 . Moreover, each rim  28  of each dimple  26  penetrates the lubrication fluid  14  forced out of the cavity  30 , which results in the load being distributed evenly over the entire top surface  24  of the friction material layer  22 . 
     In conclusion, the present invention has many advantages and benefits over the prior art. The teachings of the present invention may be employed to overcome many problems found in prior art torque-transmitting devices  10 . For example, the dimples  26  break through the film during the initial state of engagement, which increases the area of contact between the reaction plate  16  and the friction material layer  22 , and decreases the contact pressure between the reaction plate  16  and the friction plate  12 . The dimples  26  also distribute the load evenly over the entire portion of the top surface  24  of the friction material layer  22 . The dimples  26  also prolong the soft-EHL state by retaining the lubrication fluid  14  within the cavities  30  and distributing the necessary amount over the friction material layer  22 . Additionally, the dimples  26  prevent the frictional material layer  22  from overheating, because the lubrication fluid  14  is collected within the cavities  30  and evenly dispersed over the friction material layer  22  on an as needed basis during compression of the friction material layer  22 . The dimples  26  minimize the amount of fluid necessary for the torque-transmitting device  10  to operate optimally, which decreases the required oil pump capacity during operation of the torque-transmitting device  10 . The dimples  26  also prolong the slip time without shudder, and increase the torque-transmitting device  10  power density without shudder. 
     The description of the invention is merely exemplary in nature and variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.

Technology Category: 2