Patent Abstract:
A friction plate for a clutch assembly includes a core plate and a plurality of friction segments made of a friction material. A plurality of grooves are disposed between said plurality of friction segments. Each segment has tapered sides so that the grooves have a first width adjacent the outer diameter of the core plate and a smaller second width smaller adjacent the inner diameter of the core plate. This tapered groove structure directs oil from the outside the clutch assembly to the inside to improve cooling efficiency.

Full Description:
TECHNICAL FIELD 
     The present teachings relate to friction material for clutches, and more particularly to a friction material having a configuration suitable for wet clutch applications. 
     BACKGROUND 
     The present teachings generally relate to a friction material that can be used in clutch applications, such as a multi-plate wet clutch pack in a limited slip differential system. Friction plates and the friction material on the plates affect the reliability and quality of clutch engagement. 
     When wet clutches engage and slide against each other, the contacting friction surfaces generate heat. Oil can be applied to the friction plates to cool the contacting components, either by center-fed forced cooling (where oil is pumped through channels in the center of an input shaft and exits through holes in the center of a clutch hub to flow into the clutch pack) or splash cooling (where a differential housing is filled with oil that splashes on the clutch plates). Forced cooling systems allow adjustment of the oil flow rate based on the cooling demand, but incorporating the pump and fluid channels adds complexity to the differential system. 
     By contrast, splash cooling is a passive cooling method and does not require any special modifications to the differential system. However, the clutch packs in LSD systems are always engaged (e.g., adjacent friction plates and separator plates are always in contact with each other). This makes it difficult for sufficient oil to reach the clutch interface and cool the clutch. In other words, splash-cooled wet clutches tend to have low cooling efficiency. 
     SUMMARY 
     One aspect of the present teachings is directed to a friction plate for a clutch assembly. The friction plate comprises a core plate having an inner diameter and a first outer diameter and a plurality of friction segments made of a friction material. A plurality of grooves are disposed between said plurality of friction segments, wherein each segment has tapered sides so that the grooves have a first width adjacent the first outer diameter and a second width, which is smaller than the first width, adjacent the inner diameter. 
     Another aspect of the present teachings is directed to a clutch assembly having a plurality of the friction plates described above disposed in an alternating fashion with a plurality of separator plates. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded view of a wet clutch pack according to one aspect of the teachings; 
         FIG. 2A  is a plan view of a friction plate that can be used in the system of  FIG. 1 ; 
         FIG. 2B  is a close-up view of the friction plate in  FIG. 2A ; 
         FIG. 2C  is a close-up perspective view of the friction plate in  FIG. 2A ; 
         FIG. 3  is a plan view of friction material that can be used in the friction plate of  FIG. 2 ; 
         FIG. 4  is a plan view of another arrangement of friction material that can be used in the friction plate of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates components in a wet clutch  10  according to one aspect of the teachings. The wet clutch  10  can include friction plates  12  and separator plates  14  arranged in an alternating manner. 
     As shown in  FIGS. 2A ,  2 B, and  2 C, the friction plate  12  can include a core plate  16  and a friction lining  18  bonded to the core plate  16  via any known process. The separator plate  14  and the core plate  16  may both be made of steel. The core plate  16  may have ears  17  to act as clutch guides. 
     The friction lining  18  can be formed as a plurality of segments  20  with side surfaces  21  forming grooves  22  in between the segments  20 . The friction lining  18  may be formed of any appropriate material, such as a paper-based friction material containing aramid or other high-strength fibers and fillers and saturated with a thermosetting resin, such as a phenolic resin. Options for the friction material structure include woven, pultruded or compression-molded structures. Options for the friction material itself may include polyamide, carbon, ceramic, or any combination thereof. These options are only examples: the specific material and material structure is not critical to the teachings, and those of ordinary skill in the art will recognize that other options are within the scope of the teachings. 
     Grooves  22  act as fluid paths to allow oil to flow between the friction plates  12  and separator plates  14  for cooling. Deeper grooves  22  improve cooling efficiency and reduces thermal degradation of the friction lining  18  because deeper grooves  22  allow more cooling oil to circulate between the plates  12 ,  14 . The grooves  22  can be made in various ways, such as molding grooves into the friction lining  18  or cutting the friction lining  18  to form the grooves  22 . Cutting or machining allows formation of deeper grooves  22  (i.e., thicker side surfaces  21 ) than molding and therefore can potentially improve the cooling efficiency of the wet clutch  10 . However, cutting the friction lining  18  material can create fuzziness and loose fibers at the cut edges due to fibers in the friction lining  18  material. The fuzzy edges can restrict oil flow, and the loose fibers can contaminate the oil. 
     The friction lining  18  shown in  FIGS. 1 through 3  solves the above problems by dividing the friction lining  18  into discrete segments  20 . Creating separate segments  20  maximizes the depth of the grooves  22  and provides smooth side surfaces  21  that do not impede oil flow. 
     The number and shape of the segments  20  as well as the direction (e.g., radial vs. angled), width, and shape of the grooves  22  all affect the cooling efficiency of the wet clutch  10  and control the amount of thermal degradation in the friction lining  18 . For example, if the segments  20  are too large, there will be fewer grooves  22 , thereby reducing the amount of oil circulating through the wet clutch  10 , diminishing cooling efficiency, and increasing thermal degradation of the friction lining  18 . However, if there are too many segments  20  and grooves  22 , the increased number of grooves  22  reduces the load capacity of the wet clutch  10  by reducing the land area of the friction lining  18 , thereby increasing the amount of friction load each segment  20  must bear. This can potentially increase the wear rate of the friction lining  18  and reduce its compression fatigue life and reliability. 
     The number of friction segments  20  forming the friction lining  18  can be optimized to provide the best compromise between cooling efficiency and load capacity.  FIGS. 2 and 3  illustrate two possible aspects of the teachings for purposes of illustration and not limitation.  FIG. 3  illustrates a friction lining  18  arrangement having twelve friction segments  20  and twelve grooves  22 , while  FIG. 4  illustrates a friction lining  18  arrangement having sixteen friction segments  20  and sixteen grooves  22 . 
     In both of these aspects, adjacent side surfaces  21  of the grooves  22  may taper slightly inward from an outer diameter of the core plate  16  to an inner diameter of the core plate  16 . The side surfaces  21  of the segments  20  can be straight, flared outward, or tapered slightly inward so that the sides surfaces of adjacent segments  20  form the inwardly tapering grooves  22 . Outer edges of the friction segments  20  form an outer diameter of the friction lining  18 , while inner edges of the friction segments  20  form an inner diameter of the friction lining  18 . In one aspect, the width of the tapered grooves  22  may be about 5 mm at the friction lining&#39;s  18  outer diameter and about 3 mm at the friction lining&#39;s  18  inner diameter to allow oil to flow into the wet clutch  10  easily. 
     Also, each friction segment  20  may have rounded corners  38  at the outer edges. In one aspect of the teachings, the radii of the rounded corners are generous, on the order of millimeters. The rounded corners  38 , together with the tapered side surfaces  21  forming the grooves  22 , create a funnel shape  23 , directing oil from the outer diameter of the core plate  16  toward the inner diameter  30  of the core plate  16  via random splashing and gravitational forces. In other words, the funnel shape formed by the friction segments  20  funnels oil into the clutch interface (i.e., between the friction plates  12  and the separator plates  14 ). As centrifugal forces push oil out of the wet clutch  10  during differential rotation, particularly during high-speed rotation without slippage, the rounded corners  38  guide the oil back into the clutch interface to maintain cooling action during low speed rotation with high slip speed. 
     As shown in  FIG. 2B  the friction lining  18  may also be shaped so the outer edges  33  of the friction segments  20  collectively form an outer diameter that is slightly less than the outer diameter of the core plate  16 , creating a margin  44  of bare metal around the perimeter of the friction lining  18 . In one aspect of the teachings, the margin  44  is approximately 1-2 mm wide. This margin  44  can improve cooling efficiency, as will be described in greater detail below. 
     In one aspect of the teachings, the number, shape, size, and arrangement of the friction segments  20  are selected to provide a reaction torque sufficient to provide traction if a driven wheel encounters a slip condition. In such slip conditions, the wet clutch  10  can experience high sliding speeds (e.g., greater than 500 rpm), high pressures (e.g., greater than 3 MPa), and long slip durations (e.g., greater than 5 seconds). Therefore, the friction segments  20  can be designed so the overall friction lining  18  can withstand high density power inputs (e.g., greater than 2 W/mm 2 ). 
     During clutch operation, heat generated by the friction lining  18  is absorbed by the core plate  16  and the separator plate  14 . Thus, the interface between the friction plates  12  and the separator plates  14  can reach temperatures on the order of 400 C, which is hot enough to potentially degrade the friction lining  18 . To optimize cooling, contact between the oil and both the core plate  16  and separator plate  14  should be maximized so the oil can quickly absorb retained heat in the plates  14 ,  16 . Since splashed oil initially tends to sit on the outside of the wet clutch  10 , the margin  44  increases the amount of metal contacting the oil before it is eased into the grooves  22 , thereby improving cooling efficiency. Also, the margin  44  helps create surface tension that holds in the oil between the core plates  16  and the separator plates  14 , prolonging the contract between the metal of the plates  14 ,  16  and thereby increasing the heat transfer between the hot metal of the plates  14 ,  16  and the oil. 
     It will be appreciated that the above teachings are merely exemplary in nature and is not intended to limit the present teachings, their application or uses. While specific examples have been described in the specification and illustrated in the drawings, it will be understood by those of ordinary skill in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the present teachings as defined in the claims. Furthermore, the mixing and matching of features, elements and/or functions between various examples is expressly contemplated herein so that one of ordinary skill in the art would appreciate from this disclosure that features, elements and/or functions of one example may be incorporated into another example as appropriate, unless described otherwise, above. Moreover, many modifications may be made to adapt a particular situation or material to the teachings of the present disclosure without departing from the essential scope thereof. Therefore, it is intended that the present teachings not be limited to the particular examples illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out the teachings of the present disclosure, but that the scope of the present disclosure will include any embodiments falling within the foregoing description and the appended claims.

Technology Classification (CPC): 5