Abstract:
A method of operating a friction plate clutch includes activating a clutch control mechanism to engage the friction plate clutch, and engaging a holding clutch. The clutch control mechanism is then deactivated, rendering it unable to maintain engagement of the friction plate clutch. The holding clutch is used to retain engagement of the friction plate clutch. One embodiment of the method uses a wedge clutch as the holding clutch. A latching clutch assembly includes a friction plate clutch movable between an engaged and a disengaged position.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of prior application Ser. No. 12/351,901, filed Jan. 12, 2009, which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to clutch assemblies for selectively transferring rotation between two or more bodies. 
     BACKGROUND OF THE INVENTION 
     Power transmissions, particularly multi-speed power transmissions of the automatic shifting type, utilize torque transmitting members or friction devices to transfer rotational movement and torque between transmission elements. One such torque transmitting member is the friction plate clutch, which enforces frictional engagement between interleaved disc elements formed in a friction pack, and alternately coupled with an input or output member. In some cases, the friction device is a brake and the output member is a stationary housing. 
     Torque transmitting mechanisms include a control mechanism, such as: fluid (hydraulic), mechanical, or electrical control mechanisms. Fluid-operated torque transmitting members have a piston disposed within a housing. The piston travels linearly in a cavity in the housing between an engaged position and a disengaged position, thereby causing selective engagement of the friction elements. 
     SUMMARY 
     A method of operating a friction plate clutch is provided. The method includes activating a clutch control mechanism to engage the friction plate clutch, and engaging a holding clutch, which may be a wedge clutch. The clutch control mechanism is then deactivated, such that the clutch control mechanism is unable to maintain engagement of the friction plate clutch. The holding clutch is used to retain engagement of the friction plate clutch. The method may further include reactivating the clutch control mechanism and disengaging the holding clutch, such that control over engagement of the friction plate clutch is returned to the clutch control mechanism. 
     The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes and other embodiments for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic, cross-sectional view of a latching clutch assembly having a friction plate (primary) clutch and a wedge (holding) clutch, both of which are shown in a disengaged position; 
         FIG. 2  is a schematic, cross-sectional view of the latching clutch assembly shown in  FIG. 1 , showing the friction plate clutch and the wedge clutch in respective engaged positions; 
         FIG. 3  is a schematic, cross-sectional view of a latching clutch assembly having a friction plate (primary) clutch shown in the disengaged position, and a bearing (holding) clutch shown in a released position; and 
         FIG. 4  is a schematic, cross-sectional view of the latching clutch assembly shown in  FIG. 3 , showing the friction plate clutch in the engaged position and the bearing clutch in a locked position. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to the drawings, wherein like reference numbers correspond to like or similar components throughout the several figures, there is shown in  FIG. 1  an embodiment of a clutch assembly  10  for selectively transferring torque between an annular first member  12 , which includes an outer hub portion  14 , and an annular second member  16 , which includes an inner hub portion  18 . The first member  12  may be an input or driving member, and the second member  16  may be an output or driven member. 
     Those having ordinary skill in the art will recognize that the input/output designation of the first and second members  12  and  16  is not limiting, and that the input/output nature of the two members may change during operation of clutch assembly  10 . For example, in some hybrid transmission applications, the direction of rotation, and the input and output direction, may change as the operating modes of the hybrid transmission change. Furthermore, those having ordinary skill in the art will recognize that the specific orientation of the inner hub  18  and outer hub  14  are also not limiting, and that the first member  12  may have either an inner or outer hub attached thereto. 
     While the present invention is described in detail with respect to automotive applications, those skilled in the art will recognize the broader applicability of the invention. Those having ordinary skill in the art will recognize that terms such as “above,” “below,” “upward,” “downward,” et cetera, are used descriptively of the figures, and do not represent limitations on the scope of the invention, as defined by the appended claims. 
     The embodiments shown in the figures utilize fluid (hydraulic) operated clutch control mechanisms, but other clutch control mechanisms may be used within the scope of the appended claims. For example, without limitation, the clutch control mechanism may also be: electrical, mechanical or other control methods recognizable to those having ordinary skill in the art. 
     A clutch pack  20  is disposed within the clutch assembly  10  between the outer and inner hubs  14  and  18 . Clutch pack  20  allows selective torque transfer between the outer hub  14  and the inner hub  18 —and therefore between the first member  12  and the second member  16 . The clutch pack  20  incorporates a plurality of clutch plates or discs  22  and a plurality or friction plates or discs  24 . The clutch plates  22  are drivingly connected with outer hub  14  and the friction plates  24  are drivingly connected with inner hub  18 . The clutch pack  20 , and clutch assembly  10 , is shown in  FIG. 1  in a disengaged position, such that the first member  12  and second member  16  may rotate at substantially different rates. 
     Slidably disposed in a cavity formed by the first and second members  12  and  16  and the outer and inner hubs  14  and  18 , is a friction clutch piston  26 . A friction clutch chamber  28  is defined between the friction clutch piston  26  and either, or both, the first member  12  and outer hub  14 . Friction clutch chamber  28  is filled with a fluid, such as transmission fluid or oil, communicating with a main pressure source. The main pressure source may be, for example, without limitation: a transmission pump, a torque converter, or another pressure source recognizable to those having ordinary skill in the art. Therefore, the main pressure source feeding into the friction clutch chamber  28  acts as a hydraulic actuator for the fluid-operated clutch control mechanism. 
       FIG. 2  shows the clutch pack  20  in the engaged position. As the pressure in the friction clutch chamber  28  is increased, the friction clutch piston  26  is biased rightward, such that it moves to the right and contacts the clutch pack  20 . The force of the friction clutch piston  26  causes the clutch plates  22  to lock to the friction plates  24 , resulting in substantially common rotation between the outer hub  14  and the inner hub  18 , and therefore common rotation between the first member  12  and second member  16 . Furthermore, the main pressure source may be throttled or otherwise controlled to selectively vary the fluid force acting on the friction clutch piston  26 . 
     The clutch assembly  10  further includes a return spring  30 . The return spring  30  acts upon a reaction plate  32 , which is forced into abutment with the friction clutch piston  26 . Return spring  30  also acts upon a stationary return plate  34 ; however, in other embodiments, the return spring  30  could act directly upon the inner hub portion  18 . Thus, the friction clutch piston  26  is urged leftward, as seen in  FIG. 1 , by the force of the return spring  30 , and opposes the rightward fluid force from friction clutch chamber  28 . 
     When there is no longer sufficient pressure from the main pressure source to retain engagement of the clutch plates  22  to the friction plates  24 , the force of the return spring  30  will move the friction clutch piston  26  leftward, causing disengagement of the clutch pack  20 . Thus, the fluid-operated clutch control mechanism provides the clutch pack  20  with two positions relating to the amount of pressure in the friction clutch chamber  28  and the opposing force of the return spring  30 : a pressure set position (engaged, shown in  FIG. 1 ), and a spring set position (disengaged, shown in  FIG. 2 ). 
     The availability of the main pressure source to operate the friction clutch piston  26  may be tied to the operation of the vehicle engine, such that while the engine is off (i.e, deactivated), there may be insufficient pressure to engage, or maintain engagement of, the clutch pack  20 . The pressure is sufficient when the engine is on (i.e., activated). Loss of pressure may occur, for example, and without limitation, in hybrid vehicles or other vehicles which shut off (i.e, deactivate) the combustion engine during traffic stops. However, engagement of the clutch pack  20  may be desired during engine-off periods, or during engine re-starts (i.e, reactivations). 
     Clutch assembly  10  includes a holding clutch mechanism configured to hold or maintain engagement of clutch pack  20  without the force provided by the main pressure source in the friction clutch chamber  28 . The holding clutch mechanism used in clutch assembly  10  is a wedge clutch  40 , which may be selectively engaged during periods of insufficient main pressure to maintain engagement of the clutch pack  20 . 
     The wedge clutch  40  includes a wedge  42  and a wedge actuator  44 . In the embodiment shown in  FIGS. 1 and 2 , the wedge actuator  44  causes the wedge  42  to selectively move downward and upward (as viewed in the figures) between the first member  12  and the friction clutch piston  26 . Wedge actuator  44  may be, without limitation: electrical (such as a solenoid), mechanical (such as a spring or a gear), or may be tied into the fluid-operated clutch control by manipulating fluid pressure tapped from the main pressure source. 
       FIG. 2  shows the wedge clutch  40  in the engaged position, such that the friction between wedge  42 , first member  12 , and friction clutch piston  26  will lock the wedge in place. When the wedge clutch  40  is engaged, the friction clutch piston  26  is prevented from moving leftward, and the clutch pack  20  is prevented from disengaging. 
     Those having ordinary skill in the art will recognize that while  FIG. 2  shows both the clutch pack  20  and wedge clutch  40  in their respective engaged positions, the two need not necessarily coincide. For example, the clutch pack  20  may be engaged (as shown in  FIG. 2 ) by the fluid-operated clutch control mechanism (the main pressure source) while the wedge clutch  40  remains in the disengaged position (as shown in  FIG. 1 ). 
     By preventing leftward movement of the friction clutch piston  26 , engagement of the wedge clutch  40  allows the clutch pack  20  to remain engaged even where there is no pressure in the friction clutch chamber  28 . Engagement of wedge clutch  40  allows for common rotation of the first and second members  12  and  16  even when the vehicle engine is turned off and the main pressure source is not functioning. 
     After the fluid-operated clutch control mechanism is reactivated, and the main pressure source repressurizes the friction clutch chamber  28 , the clutch pack  20  may again be held in engagement by the fluid-operated clutch control mechanism. Once the fluid force is sufficient, the friction load on the wedge  42  is decreased, and the wedge clutch  40  may be disengaged. 
     The wedge actuator  44  may disengage the wedge clutch  40  by moving the wedge  42  upward (as viewed in  FIGS. 1 and 2 ). Alternatively, the wedge  42  may be pushed upward and disengaged by the fluid pressure in the friction clutch chamber  28 . After disengagement of the wedge clutch  40 , the fluid-operated clutch control mechanism is again allowed to completely control engagement and disengagement of the clutch pack  20 . 
     The embodiment of the wedge clutch  40  shown in  FIGS. 1 and 2  holds engagement of the clutch pack  20  by acting upon the friction clutch piston  26 . However, the wedge clutch  40  may be located and configured to maintain engagement by acting on other elements of the clutch assembly  10 . For example, and without limitation, the wedge clutch  40  could act directly on one of the plates  22  or  24  of clutch pack  20 , thereby allowing the friction clutch piston  26  to move to its disengaged position without disengaging the clutch pack  20 . 
     Referring now to  FIGS. 3 and 4 , there is shown another embodiment of a clutch assembly  110  for transferring torque between first member  12  and second member  16 , via the outer and inner hub portions  14  and  18 . Similar to the embodiment shown in  FIGS. 1 and 2 , the clutch assembly  110  includes the clutch pack  20 , which selectively allows torque transfer between the outer hub  14  and the inner hub  18 —and therefore between the first member  12  and the second member  16 . 
     Clutch assembly  110 , and the clutch pack  20 , is shown in  FIG. 3  in a disengaged position, such that the first member  12  and second member  16  may rotate at substantially different rates.  FIG. 4  shows the clutch pack  20  in the engaged position. 
     The friction clutch piston  26  is slidably disposed in a cavity formed by the first and second members  12  and  16  and the outer and inner hubs  14  and  18 . The friction clutch chamber  28  is in fluid communication with the main pressure source. 
     In operation, to engage the clutch pack  20 , as the pressure in the friction clutch chamber  28  is increased, the friction clutch piston  26  is biased rightward, such that it moves to the right and contacts the clutch pack  20 . The fluid force from the friction clutch chamber  28  overcomes the force of the return spring  30  to move the friction clutch piston  26  to the right, locking the clutch plates  22  to the friction plates  24 . Again, the main pressure source may be throttled or otherwise controlled to selectively vary the fluid force acting on the friction clutch piston  26 . 
     Return spring  30  acts upon a reaction plate  132  which is fixedly attached to the friction clutch piston  26 . Return spring  30  also acts upon a stationary return plate  34 . The friction clutch piston  26  is urged leftward (as viewed in  FIG. 3 ) by the force of the return spring  30 , which is sufficient to overcome the opposing fluid force on the friction clutch piston  26  when the main pressure source is insufficient to retain engagement of the clutch plates  22  to the friction plates  24 . Therefore, the fluid-operated clutch control mechanism again provides the clutch pack  20  with two positions: a spring set position (disengaged, shown in  FIG. 3 ), and a pressure set position (engaged, shown in  FIG. 4 ). 
     Clutch assembly  110  further includes a holding clutch mechanism configured to hold or maintain engagement of clutch pack  20  without the force provided by the main pressure source in the friction clutch chamber  28 . The holding clutch mechanism used in clutch assembly  110  is a bearing clutch  150 , which may be selectively locked during periods of insufficient main pressure to hold or maintain engagement of the clutch pack  20 . 
     As explained below, the bearing clutch  150  is configured such that, when locked, bearing clutch  150  will freely move to the right but is unable to move to the left (as viewed in  FIGS. 3 and 4 ). Therefore, the bearing clutch may also be referred to as a selective one-way mechanism. 
     Bearing clutch  150  includes a bearing piston  152  which is slidably disposed between the reaction plate  132  and an outer surface  13  of the input member  12 . A plurality of ball bearings or spheres  154  are disposed between the outer surface  13  and a cam surface  156  formed on an inner surface of the bearing piston  152 . Spheres  154  may be disposed with equiangular spacing about the outer surface  13 , and the number of spheres  154  may vary, but will usually include at least three. The spheres  154  are housed in a cage  158 , and are urged rightward (as viewed in FIGS.  3  and  4 ,) relative to the bearing piston  152 , by at least one tickler spring  160 , which is compressed between the bearing piston  152  and the cage  158 . 
     The cam surface  156  is formed such that rightward movement of the bearing piston  152  allows sufficient space to permit the spheres  154  to roll freely along the outer surface  13 . However, when the tickler spring  160  is able to move the spheres  154  rightward into abutment with the cam surface  156 , leftward movement of the bearing piston  152  is prevented by the locking reaction between the cam surface  156 , the spheres  154  and the outer surface  13 . Therefore, the bearing clutch  150  will move rightward freely but will prevent leftward movement unless the tickler spring  160  is neutralized. This is the locked state of the bearing clutch  150 , and is shown in  FIG. 4 . 
     A shoulder  162  is annular and aligned for contact with an annular locking ring  164  which is secured to the reaction plate  132 . When the bearing clutch  150  is locked, the shoulder  162  and the annular locking ring  164  cooperate to prevent the reaction plate  132  from moving leftward. Therefore, the friction clutch piston  26  is also prevented from leftward movement. If the clutch pack  20  is in the engaged position, preventing leftward movement of the friction clutch piston  26  will prevent disengagement of the clutch pack  20 , even without sufficient main pressure in the friction clutch chamber  28 . 
     In order to disengage the clutch pack  20 , the bearing clutch  150  must be released. To release the bearing clutch  150 , the spheres  154  must be moved leftward, relative to the bearing piston  152 , in order to relieve the spheres  154  of the locking force generated by contact with the cam surface  156 . 
     Leftward movement of the spheres  154 , relative to the bearing piston  152  and cam surface  156 , is provided by a compensator piston  166 , which is slidably disposed between the outer surface  13  and the reaction plate  132 . In the embodiment shown, the compensator piston  166  selectively biases the cage  158 , and therefore the spheres  154 , leftward relative to the cam surface  156 . Compensator piston  166  is in fluid communication with a compensator chamber  168 , which is in fluid communication with a dam or compensator pressure source. 
     The compensator pressure source selectively provides fluid pressure which biases the compensator piston leftward (as viewed in  FIGS. 3 and 4 ). A compensator return spring  170  acts to bias the compensator piston  166  rightward, opposing the fluid force from the compensator chamber  168 . 
     If the fluid force provided by the compensator chamber  168  is sufficient to overcome the force of the compensator return spring  170 , the compensator piston  166  will bias the spheres leftward, relative to the bearing piston  152 , and release (disengage) the bearing clutch  150  (as shown in  FIG. 3 ). Conversely, if the fluid force is insufficient, the compensator return spring  170  will move the compensator piston  166  rightward until it abuts an annular locking ring  172 , and the tickler spring  160  will force the spheres  154  into the cam surface  156 , locking (engaging) the bearing clutch  150  and restricting leftward movement of the bearing clutch  150  (as shown in  FIG. 4 ). 
     When the bearing clutch  150  is released (as shown in  FIG. 3 ), the bearing clutch  150  is free to slide along the outer surface  13  with the movement of the friction clutch piston  26  and reaction plate  132  in either direction. However, when insufficient pressure is supplied to the compensator chamber  168 , the bearing clutch  150  locks (as shown in  FIG. 4 ) and will not allow the bearing piston  152  or the friction clutch piston  26  to move leftward, and can maintain engagement of the clutch pack  20 . 
     In operation of the clutch assembly  110 , the compensator pressure source and the main pressure source may be derived from operation of the vehicle engine. If the clutch pack  20  is engaged and the engine is shut off, the loss of pressure to the friction clutch chamber  28  would cause the reaction spring  30  to attempt to disengage the clutch pack  20 . However, loss of pressure to the compensator chamber  168  will lock the bearing clutch  150 , such that the friction clutch piston  26  is not able to move leftward, and the engagement of the clutch pack  20  is maintained, even with little or no pressure being supplied by the main pressure source (to friction clutch chamber  28 ) or to the compensator pressure source (to the compensator chamber  168 ). 
       FIG. 3  shows the clutch pack  20  in the disengaged position and the bearing clutch  150  in the released position; and  FIG. 4  shows the clutch pack  20  in the engaged position and the bearing clutch  150  in the locked position. However, the states of the two clutches need not always coincide in the manner shown in  FIGS. 3 and 4 . The bearing clutch  150  may be in either the locked or released position while the friction clutch pack  20  is in either the disengaged or engaged positions. For example, it may be desirable to place the transmission in neutral by moving the clutch pack  20  from the engaged to disengaged position while the vehicle engine is running. In such a case, the bearing clutch  150  would be placed in the released position to allow the friction clutch piston  26  to move leftward and disengage the clutch pack  20 . 
     While the best modes and other embodiments for carrying out the claimed invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.