Abstract:
A subassembly for a transmission including a seal having a protrusion, a piston including a tab, and a lip about said piston, wherein an axial gap is formed between said tab and said lip, said piston also including a hole corresponding to said protrusion and operatively arranged to receive said protrusion for securing seal to said piston, a spring element disposed in said axial gap between said tab and said lip, wherein said spring element includes a finger corresponding to said tab, wherein said finger is aligned with said tab for axially locking said spring element between said tab and said lip, and wherein said tab includes a recess operatively arranged for receiving said finger for rotationally locking said spring element to said piston.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application No. 61/381,252 filed Sep. 9, 2010 which application is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention broadly relates to automobile transmissions, more specifically to piston assemblies for automobile transmissions, and even more particularly to a piston subassembly for an automobile transmission that includes a return spring. 
     BACKGROUND OF THE INVENTION 
     Automobile transmissions are well known in the art. For example, some known systems include three-piece assemblies having two pistons and an uncontained spring that must be preloaded during assembly. U.S. Pat. No. 5,893,445 (Dover) is incorporated herein by reference, and provides another example of a piston assembly for engaging the clutch of a transmission. The design in Dover includes a single piston with an overmolded seal for enabling pressurized actuation of a piston in order to engage a clutch and/or clutch pack and a split release spring retained in a slot for urging the piston away from the clutch pack. In these conventional systems, the multiple components require increased inventory management and results in an increased chance of misassembly. Overmolded seals and dual pistons can be costly to produce. Furthermore, these systems typically use the seal as a positive stop for the piston, which can lead to inefficiencies in engagement of the clutch due to the compressibility and less accurate tolerances of rubber seals. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention broadly comprises a subassembly for a transmission including a piston including at least one tab and a lip about the piston, wherein an axial gap is formed between the at least one tab and the lip, a spring element disposed in the axial gap between the at least one tab and the lip, wherein the spring element is preloaded between the at least one tab and the lip, and wherein the subassembly is formed as a single contained unit with the spring element axially and rotationally locked to the piston. 
     In one embodiment, the spring element includes at least one finger corresponding to the at least one tab. In one embodiment, the spring element is installable on the piston by pressing the spring element against the lip while the at least one finger is misaligned with the at least one tab, then rotating the spring element with respect to the piston for aligning the at least one finger with the at least one tab. In one embodiment, the at least one tab includes a recess for engaging with the at least one finger of spring element for locking the spring element rotationally to the piston. In one embodiment, the invention further comprises a seal having at least one protrusion and wherein the piston includes at least one hole, wherein the at least one hole is operatively arranged for receiving the at least one protrusion therein. In one embodiment, the at least one tab extends in a substantially radial direction from the piston and includes a substantially axial surface for acting as a positive stop for the piston. 
     The current invention also broadly comprises an assembly for a transmission comprising the current invention subassembly and a housing, wherein the subassembly is installed in the housing for engaging a clutch of the transmission. In one embodiment, the seal is operatively arranged to seal a pressure chamber of the transmission for enabling a pressure differential between axially opposite sides of the piston for actuating the piston. 
     The current invention also broadly comprises a subassembly for a transmission including a seal having at least one protrusion, a piston including at least one tab, and a lip about the piston, wherein an axial gap is formed between the at least one tab and the lip, the piston also including at least one hole corresponding to at least one protrusion and operatively arranged to receive the at least one protrusion for securing the seal to the piston, a spring element disposed in the axial gap between the at least one tab and the lip, wherein the spring element includes at least one finger corresponding to the at least one tab, wherein the at least one finger is aligned with the at least one tab and preloaded between the at least one tab and the lip for axially locking the spring element between the at least one tab and the lip, and wherein the at least one tab includes a recess operatively arranged for receiving the at least one finger for rotationally locking the spring element to the piston. 
     The current invention also broadly comprises an assembly for a transmission including the current invention subassembly and a housing, wherein the subassembly is installed in the housing for engaging a clutch of a transmission. In one embodiment, the seal is operatively arranged to seal a pressure chamber of the transmission for enabling a pressure differential between axially opposite sides of the piston for actuating the piston. 
     It is a general object of the present invention to provide a subassembly that is a single contained unit that can be preassembled prior to assembly of a transmission. 
     It is another general object of the present invention to provide a piston assembly that does not use an overmolded seal or a dual piston design. 
     It is yet another object of the present invention to provide a piston assembly having an improved positive stop. 
     These and other objects and advantages of the present invention will be readily appreciable from the following description of preferred embodiments of the invention and from the accompanying drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The nature and mode of operation of the present invention will now be more fully described in the following detailed description of the invention taken with the accompanying drawing figures, in which: 
         FIG. 1  is an exploded perspective view of a transmission subassembly according to the current invention; 
         FIG. 2  is a cross-sectional view of the subassembly shown in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view of the subassembly of  FIG. 2  installed into a transmission housing; and 
         FIG. 4  is a schematic block diagram showing two clutches and a transmission. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     At the outset, it should be appreciated that like drawing numbers on different drawing views identify identical, or functionally similar, structural elements of the invention. While the present invention is described with respect to what is presently considered to be the preferred aspects, it is to be understood that the invention as claimed is not limited to the disclosed aspects. 
     Furthermore, it is understood that this invention is not limited to the particular methodology, materials and modifications described and as such may, of course, vary. It is also understood that the terminology used herein is for the purpose of describing particular aspects only, and is not intended to limit the scope of the present invention, which is limited only by the appended claims. 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this invention belongs. Although any methods, devices or materials similar or equivalent to those described herein can be used in the practice or testing of the invention, the preferred methods, devices, and materials are now described. 
     Referring now to the figures,  FIG. 1  shows subassembly  10  for an automobile transmission. Subassembly  10  includes seal  12 , piston  14 , and spring element  16 . Seal  12  includes protrusions  18  which engage with holes  20  in piston  14  in order to secure the seal to the piston. For example, protrusions  18  could be slightly thicker than holes  20  so that the protrusions are force fit into the holes, or the protrusions could include a widened tip for retaining the seal against the piston once the widened tip is passed through hole  20 . 
     Spring element  16  is held in axial gap  22  between tabs  24  and lip  26  of the piston. By axial, it is meant in the direction along the axis of seal  12 , piston  14 , and/or spring  16 . Specifically, for engagement of the spring with the piston, fingers  28  of spring  16  are first misaligned with tabs  24  of the piston (that is, the fingers are aligned with the gaps between the tabs of the piston) such that spring  16  can be axially moved against lip  26 . Next, the ring is compressed as needed, and rotated such that fingers  28  of the spring become aligned with tabs  24  of the piston for locking the fingers between the tabs and lip in order to prevent the spring from becoming disengaged from axial gap  22 . After the spring is compressed so that it can be rotated into position with fingers  28  between tabs  24  and lip  26 , axial gap  22  is not wide enough to allow the spring to return to its equilibrium position, thereby preloading the spring. 
     Recesses  30  are included to restrict rotation of spring element  16  with respect to piston  14 . Specifically, the recesses are arranged having posts  31  included on opposite sides of each recess, with a distance between posts  31  being approximately equal to the width of the fingers of the spring element. Thus, when the spring element is rotated such that fingers  28  become aligned with tabs  24 , the preloading of the spring element between the tabs and lip  26  causes designated fingers to become locked between posts  31  on either side of recess  30 , thereby locking the piston and spring rotationally. That is, the spring element will uncompress into the at least one recess because of the preload, and the preload force will continue to hold the spring in the recess after assembly. By locked rotationally, it is meant that one component can not move in rotation independently from the other. It should be appreciated that recesses  30  also act to center the spring on the piston so that, for example, forces are distributed properly and so that the subassembly installs smoothly into a transmission. 
     As shown in  FIG. 1 , nine tabs  24  extend radially from the piston and included about the circumference of piston  14 , while lip  26  extends radially from the piston and is provided continuously about the piston, and nine fingers  28  correspond to the nine tabs. It should, of course, be appreciated that a different number of tabs  24  or fingers  28  could be included in various embodiments of the invention, and that lip  26  need not be continuous, but could instead be included as discrete portions which extend from the piston, similar to tabs  24 . In the shown embodiment, three recesses  30  can be seen (every third tab includes a recess), however, it should be appreciated that any number of tabs could be include a recess. It should also be appreciated that lip  26  extends out radially and also curves back axially in the shown embodiment, but any other arrangement suitable for preloading the spring in order to lock it in place could be substituted. 
     Subassembly  10  is intended to be installed into a transmission. For example, as shown in  FIG. 3 , subassembly  10  is installed in housing  32  of a transmission system. Housing  32  could be, for example, a hydraulic cylinder or clutch carrier component. In the shown embodiment, housing  32  includes chamber  34  which is in fluid communication with a hydraulic pressure source for enabling chamber  34  to be pressurized for actuating piston  14  axially in order to engage a clutch or clutch pack of the transmission. Seal  12  is included to prevent pressurized fluid from exiting chamber  34  in order to provide a pressure differential on axially opposite sides of piston  24 . The seal is made, for example, from molded rubber. Retaining ring  36  is included to hold subassembly  10  in the transmission. Spring  16  is compressed between tabs  24  and the retaining ring when the piston is axially actuated. It should be appreciated that instead of ring  36 , spring  16  could instead be arranged such that it clips into a groove in housing  32 , similar to the arrangement shown in the incorporated Dover patent. 
     In the shown embodiment, each tab  24  extends radially from the piston and has axial surface  38 . The axial surface can be used, for example, as a positive stop for determining the position of the piston when it is fully retracted and disengaged from the clutch, as shown in  FIG. 3 . Thus, the axial surface of the tab can be used to matingly engage against the housing in order to determine a reference point for the piston at a high degree of accuracy for improving the tolerance stack-up between the piston and the clutch packs.  FIG. 4  is a schematic block diagram showing transmission  40  with sub-assembly  10 , housing  32 , chamber  34  and clutch  42 . 
     Advantageously, subassembly  10  is arranged such that it can be pre-assembled before being installed into a transmission. By pre-assembled it is meant that the components of the subassembly can be secured together into a single contained unit before being placed in a transmission, as opposed to typical piston assemblies that require a transmission housing, cylinder, carrier, or other component in order to secure all the components together. Thus, the pre-assembled nature of subassembly  10  results from seal  12  being secured to the piston via protrusions  18  which are each engaged, such as by a friction fit, in holes  20 , from spring element  16  being secured axially to the piston by pressing the spring against the lip of the piston and rotating the spring element to align the tabs of the piston with the fingers of the spring element, and from the spring element being locked rotationally to the piston by preloading the spring between the tabs and lip of the piston and including a recess in at least one of the tabs. 
     Advantageously, the current invention results in less components having to be installed into a transmission, since subassembly  10  is a single unit. That is, the spring element is axially and rotationally locked to the piston and the seal is also secured to the piston such that subassembly  10  can be pre-assembled before installation in a transmission. For example, after subassembly  10  is created (as shown in  FIG. 2 ), the subassembly can be installed as a single unit into a transmission. Since the subassembly is installable as a single unit, the subassembly can be checked for faults before being installed in a housing, which blocks visual inspection. Also by first assembling the return spring and seal to the piston, installation is simplified because only one component must be placed in the transmission instead of multiple individual pieces. 
     Thus, it is seen that the objects of the present invention are efficiently obtained, although modifications and changes to the invention should be readily apparent to those having ordinary skill in the art, which modifications are intended to be within the spirit and scope of the invention as claimed. It also is understood that the foregoing description is illustrative of the present invention and should not be considered as limiting. Therefore, other embodiments of the present invention are possible without departing from the spirit and scope of the present invention.