Abstract:
An apply piston seal is provided for a clutch assembly utilizing a balance piston. The apply piston seal is bonded to the apply piston and provides a sealing relationship between the apply piston and balance piston. By bonding the seal to the apply piston, the manufacturing of the balance piston can be simplified in order to reduce cost for the overall clutch assembly.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a clutch assembly, and more particularly, to a clutch and balance piston assembly having a seal disposed between the apply piston and balance piston.  
       BACKGROUND AND SUMMARY OF THE INVENTION  
       [0002]     In a typical clutch assembly for an automatic transmission (as illustrated in  FIG. 8 ), a clutch pack  16  is provided for providing a frictional engagement between a first member  12  and a second member  14  which is rotatable relative to the first member  12 . A piston  122  is provided within a fluid chamber  24  such that the application of fluid pressure to the fluid chamber  24  causes the piston  122  to move into engagement with the clutch pack  16  to frictionally engage the second member  14  to the first member  12 . The contacting of the friction plates with increasing pressure eventually causes the rotation of the second rotatable member  14  which the system is designed to engage. A problem that has been recognized in the art for a typical clutch assembly of this type is that the centrifugal force of the fluid can put a positive pressure on the apply piston  122  as illustrated in  FIG. 8 . The pressure generated by the centrifugal force of the fluid can cause unintended engagement of the clutch pack  16 .  
         [0003]     Thus, a balance piston system has been developed as illustrated in  FIG. 9  in which the balance piston  128  is disposed between the apply piston  122  and the clutch pack  16  so that hydraulic fluid is present on opposite sides of the apply piston  122 . When the balance piston system is spinning, the balance piston  128  traps fluid at the outer edge of the cavity  130  that it creates with the apply piston  122  to counteract the centrifugal forces caused by the fluid on the other side of the apply piston  122  as illustrated in  FIG. 9 . The balance piston  128  is provided with a seal member  126  that is bonded to an outer edge  122 B and contacts an inner surface of the axially extending arm  122 B of the apply piston  122 . This seal is critical for the proper functioning of the balance piston system. With the seal  126  applied to the outer surface of the balance piston  128 , the complexity and cost of the manufacture of the balance piston  128  is significantly increased.  
         [0004]     The present invention provides a balance piston/apply piston assembly that differs from typical assemblies in that the sealing elastomer (or other material) which is used to create the seal between the balance piston and the apply piston is bonded to the apply piston, not the balance piston as in typical assemblies. The fact that all of the rubber molding occurs on the apply piston, and none on the balance piston, provides a significant cost advantage over the typical assembly design where rubber must be molded to both components. With this invention, the balance piston&#39;s fabrication does not require adhesive or molding of rubber, and thus, no quality processes are required relating to a post-molded rubber inspection.  
         [0005]     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0007]      FIG. 1  is a cross-sectional view of a clutch and balance piston assembly incorporating a seal according to the principles of the present invention;  
         [0008]      FIG. 2  is a cross-sectional view illustrating the forming of a pre-molded slab portion on the apply piston;  
         [0009]      FIG. 3  is a cross-sectional view illustrating the cutting process for forming the lip profile into the premolded slab portion of the seal provided on the apply piston according to the principles of the present invention;  
         [0010]      FIG. 4  is a cross-sectional view of an apply piston having an alternative seal design including a seal skirt according to the principles of the present invention;  
         [0011]      FIG. 5  illustrates the apply piston and seal shown in  FIG. 4  in combination with a balance piston according to the principles of the present invention;  
         [0012]      FIG. 6  illustrates a cross-sectional view of a second alternative embodiment of an apply piston and seal arrangement bonded to the apply piston according to the principles of the present invention;  
         [0013]      FIG. 7  is a cross-sectional view showing the apply piston and seal as shown in  FIG. 6  in combination with a balance piston according to the principles of the present invention;  
         [0014]      FIG. 8  is a cross-sectional view of a conventional clutch system with an apply piston; and  
         [0015]      FIG. 9  is a cross-sectional view of a prior art clutch system having an apply piston and balance piston. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0017]     With reference to  FIGS. 1-3 , the clutch assembly  10 , according to the principles of the present invention, will now be described. The clutch assembly  10  includes a first member  12  and a second member  14  rotatable relative to the first member  12 . A clutch pack  16  includes a plurality of clutch plates  18  having external splines which engage internal splines of the first member  12  and a plurality of clutch plates  20  having internal splines connected to external splines of rotatable second member  14 . An apply piston  22  is provided in a piston chamber  24  provided in the first member  12 . The apply piston includes a radially extending hub portion  22 A and an axially extending arm portion  22 B which presses against the clutch pack  16 . A seal  26  is provided on an inner surface of the axially extending arm portion  22 B and includes a molded slab body portion  26 A and a lip profile  26 B.  
         [0018]     A balance piston  28  is provided between the clutch pack  16  and the apply piston  22 . The balance piston includes an inner hub portion  28 A with a central aperture  30  which is received on the shaft portion of member  12 . The balance piston  28  is disposed against a stop ring  32  and includes a spring seat portion  34  against which a return spring assembly  36  is disposed. Return spring assembly  36  also presses against the radially extending hub portion  22 A of the apply piston in order to bias the apply piston to a disengaged position. Hydraulic pressure generated in the piston chamber  24  causes the apply piston  22  to move against the biasing force of the spring assembly  36  and into engagement with clutch pack  16  for causing frictional engagement between first member  12  and rotatably member  14 . The balance piston  28  includes an axially extending outer portion  28 B which is disposed against the seal lip profile  26 B of seal  26 . The seal  26 , along with balance piston  28  and apply piston  22  define a balance chamber  29  that offsets the pressure generated by the centrifugal force on the fluid in the piston chamber  24 .  
         [0019]     With reference to  FIGS. 2 and 3 , a method of forming the seal  26 , according to the principles of the present invention, will now be described. As shown in  FIG. 2 , a molded slab of rubber  38  is formed to the interior surface of the axially extending arm portion  22 B of the apply piston  22 . It is noteworthy at this point that the apply piston also includes a molded-on exterior seal  40 , and that the molded slab  38  and seal  40  can be applied simultaneously in a single mold cavity. With reference to  FIG. 3 , a trimming process is illustrated for forming the lip profile  26 B in the seal  26 . A blade  42 , as is know in the art, is used for creating the lip profile  26 B. It is noteworthy at this point that the exterior seal  40  typically requires cutting in a post-molded state to remove material flash typically created at the exterior seal  40 , and that the cutting process for the exterior seal  40  and interior seal  26  can be performed simultaneously without a significant increase in cost. By placing the seal  26  on the apply piston  22  as opposed to the prior art technique of applying a seal to the balance piston  28 , the cost of manufacturing the balance piston  28  is greatly reduced while the cost of manufacturing the apply piston  22  is insignificantly affected since molding processes and trimming processes are already required for the apply piston  22 .  
         [0020]     With reference to  FIGS. 4 and 5 , an alternative seal  50  is shown applied to the interior surface of the axially extending arm portion  22 B of the apply piston  22 . The seal  50  includes a body portion  51  which is bonded to the apply piston  22  and includes an axially extending skirt  52  extending from an end of the body portion  51 . As illustrated in  FIG. 5 , the balance piston  28 ′ includes a generally J-shaped end portion  54  in which the axially extending skirt  52  is received. The post-molded outside diameter of the lip is designed to interfere with the outside diameter of the balance piston. The slight stretching of the rubber “skirt”  52  is meant to be a means by which the contact between the rubber and the balance piston is maintained. Since only positive pressure occurs in the cavity between the apply piston and the balance piston, this pressure tends to maintain the position of the rubber “skirt”  52 .  
         [0021]     With reference to  FIGS. 6 and 7 , a second alternative embodiment of the present invention will now be described. With reference to  FIG. 6 , a seal  60  includes a body portion  62  which is bonded to the interior surface of the axially extending arm portion  22 B of the apply piston  22 . An axially extending lip portion  64  extends from the body portion  62  and includes a recess portion  66  on the radially outer surface thereof. The lip  64  is bent radially inward and mated with an exterior surface  68  of a balance piston  28 ″. The lip portion  64  can be bonded to the balance piston  28 ″ by adhesives or otherwise fastened thereto. The seal configurations of  FIGS. 5 and 7  allow relative axial movement between the balance piston and apply piston while still maintaining a sealed relationship therebetween. The function of each of the seal systems of  FIGS. 5 and 7  operates the same as the seal  26  as discussed above with respect to  FIG. 1 .  
         [0022]     The description of the invention is merely exemplary in nature and, thus, 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.