Abstract:
A hydraulically-operated clutch for an automatic transmission includes a clutch housing assembly with a clutch pack positioned therein. A movable piston is supported in the clutch housing assembly and defines an expandable chamber between the piston and clutch housing assembly. Pressurized fluid is provided to the expandable chamber to effect movement of the piston between engaged and disengaged positions with respect to the clutch pack. The piston and clutch housing assembly are configured such that only a small portion of the expandable chamber is pressurized when the piston is in the disengaged position, thereby providing improved response time for movement of the piston to the engaged position.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to a piston for a hydraulically-operated clutch in which an expandable piston actuating chamber is partitioned so that only a small portion of the expandable chamber is in fluid communication with a source of pressurized fluid when the piston is disengaged, thereby improving response time for movement of the piston away from the disengaged position.  
         BACKGROUND OF THE INVENTION  
         [0002]    Automatic transmission clutches generally include a clutch drum (driving member) having an expandable piston-operating fluid chamber, a piston axially slidably positioned against the piston-operating fluid chamber, a clutch hub (driven member) coaxially disposed in the clutch drum, and a clutch plate pack interposed between the clutch drum and the clutch hub having one end directed to the piston. The clutch plate pack includes first and second groups of plates which are alternately juxtaposed. When the fluid chamber is fed with a pressurized operating fluid, the piston is forced to press the clutch plate pack thereby to engage the first and second groups of clutch plates. Under this engaged condition, the clutch drum and the clutch hub are united and thus can rotate together. When the pressurized operating fluid is drawn from the fluid chamber, the piston releases the clutch plate pack thereby to cancel the engagement between the first and second groups of clutch plates. Under this disengaged condition, the clutch drum and the clutch hub can rotate separately or individually.  
           [0003]    These hydraulically-actuated friction clutches and brakes frequently include balance dams that provide accurate, consistent control of gear ratio changes. The balance dam creates a centrifugal hydraulic pressure that opposes and balances centrifugal hydraulic pressure developed on the apply side of the piston. These centrifugal pressures are caused by rotation of the entire clutch assembly. In this way, pressure effects due to the speed of rotation can be eliminated or reduced to a point where the control system pressure alone creates the force operative to engage the friction element.  
           [0004]    There remains a need in the automatic transmission art to provide improved response time for gear shifting, particularly in those vehicles having push-button shifting available with automatic transmissions. It is desirable that a push-button actuated shift would occur in less than 0.5 second. Existing hydraulically-operated clutch mechanisms are unable to provide such a quick response.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention provides a fast-acting piston for actuating a clutch assembly wherein an expandable chamber on the apply side of the piston is partitioned so that only a small portion of the expandable chamber is initially pressurized to move the piston toward the engaged position against the clutch pack.  
           [0006]    More specifically, the present invention provides a hydraulically-operated clutch for an automatic transmission, including a clutch housing assembly with a clutch pack positioned therein. A movable piston is supported in the clutch housing assembly and defines an expandable chamber between the piston and clutch housing assembly. A source of pressurized fluid is positioned in communication with the expandable chamber to effect movement of the piston between engaged and disengaged positions with respect to the clutch pack. A partition structure is configured to partition the expandable chamber such that only a small portion of the expandable chamber is in fluid communication with the source of pressurized fluid when the piston is in the disengaged position, and the entire expandable chamber is in fluid communication with the source of pressurized fluid when the piston is in the engaged position, thereby providing improved response time for movement of the piston away from the disengaged position.  
           [0007]    The clutch also includes a balance dam member defining a balance dam chamber between the piston and the balance dam member. The piston includes a port connecting the balance dam chamber with the expandable chamber, and a check valve is positioned in communication with the port.  
           [0008]    In a preferred embodiment, the check valve is a movable ring which is juxtaposed against a plurality of openings in the piston to selectively block the openings to discommunicate the expandable chamber from the balance dam chamber.  
           [0009]    Preferably, the clutch housing assembly includes an annular ring which is engageable with the piston to function as a partition structure to divide the expandable chamber to form the small portion of the expandable chamber. The annular ring is positioned on a clutch chamber hub member. Also, the piston includes a substantially cylindrical radially inward portion which cooperates with the ring to define the small portion of the expandable chamber.  
           [0010]    The invention also contemplates a method of actuating a piston in a hydraulically-operated clutch, as described above, wherein the small portion of the expandable chamber is communicated with the source of pressurized fluid when the piston is in the disengaged position, and the entire expandable chamber is in fluid communication with the source of pressurized fluid when the piston is in the engaged position.  
           [0011]    Accordingly, an object of the invention is to provide a hydraulically-operated clutch for an automatic transmission with improved response time for actuating the clutch.  
           [0012]    The above object and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 a  is a partial cross-sectional side view illustrating a hydraulically-operated clutch in a disengaged position in accordance with the invention;  
         [0014]    [0014]FIG. 1 b  is a partial cross-sectional side view of the hydraulically-operated clutch of FIG. 1 a , in the engaged position;  
         [0015]    [0015]FIG. 2 is a partially cut-away perspective view of a slightly modified embodiment of a piston for use with the clutch of FIGS. 1 a  and  1   b;    
         [0016]    [0016]FIG. 3 shows a plan view of an annular ring corresponding with the embodiment of FIGS. 1 a  and  1   b;    
         [0017]    [0017]FIG. 4 shows a cross-sectional side view of the annular ring of FIG. 3;  
         [0018]    [0018]FIG. 5 a  shows a partial cross-sectional side view of the piston of FIG. 2 engaged with the annular ring of FIGS. 3 and 4, with the ring in the closed position;  
         [0019]    [0019]FIG. 5 b  shows a radially offset (i.e., rotated) cross-sectional view of the assembly of FIG. 5 a , illustrating the port between the expandable chamber and the balance dam chamber in the closed position;  
         [0020]    [0020]FIG. 6 a  shows a partial cross-sectional view of the piston and annular ring of FIG. 5 a , with the annular ring in the open position with respect to the port; and  
         [0021]    [0021]FIG. 6 b  shows a radially offset (i.e., rotated) partial cross-sectional view of the piston and annular ring of FIG. 6 a , illustrating the port in the open position.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0022]    [0022]FIGS. 1 a  and  1   b  show partial cross-sectional views of an automatic transmission  10  including a clutch assembly  12  positioned on a driveshaft  14  adjacent a planetary gear set  16 .  
         [0023]    The clutch assembly  12  includes a clutch housing assembly  18 , which includes a clutch drum  20 . A clutch pack  22  is positioned within the drum  20 , and includes a clutch apply plate  24  which is splined to the clutch drum  20  along with the clutch plates  26 ,  28  and  30 . Clutch plates  25 , 27 , 29 , 31  are splined to the driven member  32  and are juxtaposed between the plates  24 , 26 , 28 , 30  for frictional engagement therewith to selectively connect the clutch drum  20  to the driven member  32  for rotation therewith.  
         [0024]    The clutch housing assembly  18  also includes a rotating clutch hub member  34  which is welded to the clutch drum  20 . The rotating clutch hub member  34  includes a centerline supply port  36  formed therein to communicate oil into an expandable chamber  40  from a source of pressurized oil supplied through a channel  38 . The expandable chamber  40  is formed between the clutch drum  20  and the movable piston  42 . The rotating clutch hub member  34  also includes a second centerline supply port  44  which communicates oil from the channel  38  to the balance dam chamber  46 , which is formed between the piston  42  and the balance dam member  48 .  
         [0025]    A return spring  50  biases the piston  42  in a direction to reduce the size of the expandable chamber  40 . The piston  42  includes inner and outer lip seals  62 , 64 , respectively, to seal the expandable chamber  40 .  
         [0026]    When pressurized fluid (oil) is provided to the expandable chamber  40  through the centerline supply port  36 , the pressure of this fluid acts against the piston  42  to force the piston  42  to the right, as viewed in FIGS. 1 a  and  1   b , to force the contact surface  52  of the piston  42  toward the clutch apply plate  24 . Higher magnitudes of oil pressure supplied to the expandable chamber  40  further expands the chamber  40  and provides high forces against the clutch apply plate  24  to forcibly compress the respective clutch plates  24 , 25 , 26 , 27 , 28 , 29 , 30 , 31  together to engage the clutch drum  20  with the driven member  32  for rotation therewith. Accordingly, the piston  42  is movable between the disengaged position shown in FIG. 1 a  and the engaged position shown in FIG. 1 b , wherein the contact surface  52  of the piston  42  contacts the clutch apply plate  24 .  
         [0027]    The clutch housing assembly  18  includes an annular ring  54  (partition structure) which is secured to the rotating clutch hub member  34  and to the clutch drum  20 . The annular ring  54  is operative to partition the expandable chamber  40  so that only a small portion  56  of the expandable chamber  40  is in communication with the centerline supply port  36  when the piston  42  is in the disengaged position, as shown in FIG. 1 a . In this configuration, the piston  42  may be quickly actuated away from the disengaged position because only a small quantity of pressurized oil must be provided through the centerline supply port  36  to quickly fill and pressurize the small portion  56  of the expandable chamber  40  to force the piston  42  toward the engaged position. This configuration substantially improves actuation time for the clutch assembly  12 . The small portion  56  is substantially smaller, radially with respect to the central axis of the piston, than the entire chamber  40 .  
         [0028]    As shown in FIG. 1 a , the piston  42  includes a substantially cylindrical radially inward portion  51  which, in cooperation with the ring  54  and the rotating clutch hub member  34 , defines the small portion  56  of the expandable chamber  40 .  
         [0029]    As shown in FIG. 1 b , as the piston  42  moves toward the clutch pack  22  so that the contact surface  52  of the piston  42  nearly contacts the clutch apply plate  24 , a gap  58  is formed between the annular ring  54  and the seal  60  on the piston  42  so that pressurized oil from the centerline supply port  36  is fed into the entire expandable chamber  40  to provide a desired magnitude of oil pressure induced force against the piston  42  to control the clutch pack  22 .  
         [0030]    As shown in FIGS. 1 a  and  1   b , a port  66  provides fluid communication between the expandable chamber  40  and the balance dam chamber  46 . A check valve  68  closes the port  66  when oil pressure in the expandable chamber  40  is higher than the pressure in the balance dam chamber  46  so that the piston  42  may be forced against the clutch pack  22  to control the clutch pack  22 .  
         [0031]    [0031]FIG. 2 shows a partially cut-away perspective view of a piston  142  in accordance with a slightly revised alternative embodiment of the invention wherein a plurality of openings  144 , 146  are provided in the piston  142  to form the port which connects the expandable chamber with the balance dam chamber.  
         [0032]    [0032]FIGS. 3 and 4 illustrate a molded plastic ring  70  which forms the check valve  66  corresponding with FIGS. 1 a  and  1   b . As shown, the molded plastic ring  70  includes a plurality of integrally molded nylon barbs  72 . The relationship between the check ring  66  and the piston  142  is shown in FIGS. 5 a ,  5   b ,  6   a  and  6   b . As shown in the closed position in FIGS. 5 a  and  5   b , the barbs  72  of the check valve  66  extend through corresponding holes  74  in the piston  142 . As shown in FIG. 5 b , in the closed position, the ring  70  of the check valve  66  blocks the hole  144  of the piston  142  to discommunicate the expandable chamber from the balance dam chamber. The check valve  66  is in this closed position when the pressure in the expandable chamber exceeds the pressure in the balance dam chamber.  
         [0033]    When oil is drained from the expandable chamber (on the apply side  76  of the piston  142 ), the pressure in the balance dam chamber will eventually exceed the pressure in the expandable chamber, and the check valve  66  moves to the open position, as shown in FIGS. 6 a  and  6   b , wherein each of the holes  144  which form the port communicates the balance dam chamber with the expandable chamber to allow flow of oil through the port.  
         [0034]    Accordingly, the invention provides a fast-acting clearance take-up feature achieved by introducing oil into the small portion  56  of the expandable chamber. The small volume of oil in the small portion  56  of the chamber advances the piston, compressing the return spring  50  and rapidly moving the piston  42  into contact with the clutch apply plate  24 . Oil from the centrifugal balance chamber  46  passes through the ports  66  and check valve  68 . Thus, most of the oil volume required to fill the fully stroked apply piston chamber (expandable chamber) does not have to be supplied through the centerline supply port  36  as the piston is initially advanced toward the engaged position.  
         [0035]    When the piston  46  is advanced sufficiently to apply a normal force to the clutch apply plate  24 , the oil pressure applied through the centerline supply port  36  closes the check valve  68  and the clutch capacity may be finely modulated via the centerline apply pressure through the port  36 .  
         [0036]    To release the clutch, the centerline pressure is reduced so as to control the time rate of decay of clutch capacity. When the return spring pack force is greater than the apply normal force, the volume of oil in the apply chamber is displaced to exhaust via the centerline apply port  36 .  
         [0037]    Accordingly, the invention provides improved clutch response time, and the balance dam feature provides precise centerline pressure control of clutch capacity.  
         [0038]    This invention also improves regulator valve performance by avoiding valve overshoot because the valve is not required to handle quick changes from high flow to low flow, thereby easing pressure regulation.  
         [0039]    While the best mode for carrying out the invention has 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.