Abstract:
A replacement accumulator piston for various General Motors transmissions, which absorbs clutch apply fluid pressure to cushion the application of a clutch band against fluid shock is disclosed. The present pinless accumulator piston design eliminates the accumulator pin whereon the original equipment accumulator piston resides thereby resolving the problem of hydraulic leakage at the accumulator pin/piston interface. The elimination of the accumulator pin is accomplished by increasing the axial length of the piston sidewall or skirt, which permits the use of additional guide rings about circumference of the piston. The additional guide rings facilitate the reciprocating movement of the piston within the piston bore in the absence of the accumulator pin and also serve to maintain the hydraulic integrity of the accumulator. The pinless accumulator piston is a direct replacement for both the forward clutch accumulator, and the 1-2 gear and 3-4 gear accumulators in General Motors transmissions.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Patent Application No. 60/359,476 filed Feb. 26, 2002, entitled Improved Accumulator Piston. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to the field of automatic transmission systems and, more particularly, to a pinless accumulator piston for use in General Motors 4L60, 4T60, 4T60E, 4L60E and 4L80E transmissions (hereinafter “GM transmissions”). 
     Automatic transmission systems of the prior art have a hydraulic circuit subsystem which includes at least a hydraulic pump, a valve body having fluid conducting passages or circuits, input and exhaust ports formed within the fluid circuits, and a plurality of valves comprised of generally cylindrical pistons having control diameters or lands formed thereon, which alternately open and close the ports to the fluid circuits to regulate the flow and pressure of automatic transmission fluid (hereinafter “ATF”). 
     Once released into a specific fluid circuit, the pressurized ATF functions to actuate hydraulic clutches, servo pistons, and other components of the transmission. It will be understood that in describing hydraulic circuits, ATF usually changes names when it passes through an orifice or control valve in a specific fluid circuit. 
     In the GM transmissions accumulators are used to control shift feel during hydraulic clutch application. An accumulator is a spring-loaded device that absorbs a certain amount of apply fluid pressure to cushion the application of a clutch or band against fluid shock. The apply fluid pressure is directed to an accumulator piston that opposes a spring force in the manner of a shock absorber. 
     In the original equipment manufacture (hereinafter “OEM”) of the GM transmissions, an aluminum accumulator piston typically reciprocates against spring pressure on a cylindrical steel accumulator pin, which eventually causes wear at the accumulator pin/piston interface. Such wear causes hydraulic leakage at the accumulator pin/piston interface resulting in poor shift quality and damaged clutches and bands. 
     Although accumulator pistons without such an accumulator pin are known, such prior art accumulator pistons invariably have an axial length, which exceeds the piston diameter or in some cases have a stepped design for engaging multiple bores in the valve body and, thus, are stable in operation and do not require a center pin to support their axial movement. 
     There are several known prior art patents that are available in the field and their discussion follows. One example of a prior art accumulator piston is disclosed in U.S. Pat. No. 4,601,233 to Sugano, which teaches a hydraulic servo device with a built-in accumulator used to dampen a rise in hydraulic fluid pressure. This patent shows an accumulator piston  14 , which is fit into the inner diameter portion  16  of a servo piston  12 . The servo piston  12  is connected to a central stem  18 , which guides the axial travel of the accumulator piston  14 . 
     Another example is shown in U.S. Pat. No. 4,867,294 to de Tuesta, which discloses a stepped accumulator piston  24  that engages two separate cylindrical bores  20 ,  22  to stabilize axial travel of the piston (FIG.  1 ). 
     Another example of an accumulator device is disclosed in U.S. Pat. No. 3,985,063 to Lemon, which teaches an accumulator structure for use in a hydraulic control system for controlling a hydraulic motor. This accumulator piston  50  ( FIG. 1 ) engages multiple bores  56 ,  58  to stabilize axial travel of the piston and also has an axial length, which exceeds its diameter. 
     U.S. Pat. No. 5,025,823 to Stevenson and U.S. Pat. No. 4,046,162 to Rodeghiero also disclose accumulator pistons having axial length to diameter ratios, which are not practical for the present application. 
     Thus, the present invention provides a pinless accumulator piston having an axial length, which is substantially less than the piston diameter as is required in the GM transmission applications and which is stable in operation. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is a replacement pinless accumulator piston for the GM transmissions having an axial length substantially less than the piston diameter and which eliminates the accumulator pin thereby resolving the problem of hydraulic leakage at the accumulator pin/piston interface. This modification is facilitated by providing additional seals and/or guide rings about circumference of the piston. The additional seals and/or guide rings guide the reciprocating movement of the piston within the piston bore in the absence of the accumulator pin and serve to maintain the hydraulic integrity of the accumulator. 
     The present pinless accumulator piston is a direct replacement for both the forward clutch accumulator, and the 1-2 and 3-4 accumulators in the aforementioned GM transmissions. In addition, the pinless accumulator piston may be fabricated from alternative materials such as steel bar stock, cast aluminum and engineering grade resins such as phenolic resin to reduce manufacturing costs. 
     Other features and technical advantages of the present invention will become apparent from a study of the following description and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features of the present invention are set forth in the appended claims. The invention itself, however, as well as other features and advantages thereof will be best understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying figures, wherein: 
         FIG. 1  is a perspective view of an OEM accumulator piston assembly of the Prior Art shown in exploded view and removed from its functional position within the valve body of a GM 4L60E transmission; 
         FIG. 2  is a schematic representation of the forward clutch accumulator and its associated hydraulic circuits labeled Prior Art; 
         FIG. 3  is a schematic representation of 1-2 accumulator, the 3-4 accumulator, and the accumulator valve with its associated hydraulic circuits labeled Prior Art; 
         FIG. 4  is an exploded perspective view showing the components of the 1-2 accumulator and the 3-4 accumulator labeled Prior Art; 
         FIG. 5A  is a cross-sectional view of the pinless accumulator piston of the present invention; 
         FIG. 5B  is an end view of the pinless accumulator piston; 
         FIG. 5C  is a cross-sectional view of an alternative embodiment of the present accumulator piston; and 
         FIG. 6  is a cross-sectional view of the present accumulator piston installed within the cover of the 1-2 accumulator. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Prior to describing the present invention in detail it may be beneficial to briefly review the structure and function of the accumulator assemblies of the prior art GM transmissions. It will be understood that a plurality of such accumulator assemblies are typically utilized within a particular transmission and may be integrated into the transmission valve body, mechanically attached to the valve body, or integrated into the transmission case as described hereinafter. 
     With reference to the drawings there is shown therein a forward clutch accumulator assembly of the GM 4L60E transmission, indicated generally at  100 , in accordance with the prior art and illustrated in FIG.  1 . The forward clutch accumulator assembly  100  (enclosed within broken line) is shown in exploded view and removed from its functional position within the piston bore  105 , which is machined into the valve body, indicated generally at  110 , as shown. 
     In the prior art the forward clutch accumulator assembly  100  comprises an accumulator piston  102  having a central bore  102   a , a piston seal  104 , an accumulator pin  106  pressed into the valve body  110 , and a compression spring  108 , which are arranged coaxially within the piston bore  105  and secured in position by an accumulator cover  112  using machine bolts  114 . 
     In operation the forward clutch accumulator assembly  100  of the GM 4L60E transmission controls shift feel into the forward drive range from Park, Reverse or Neutral. Referring to  FIG. 2  forward clutch feed fluid pressure (FWD CL FD) as at  120  applies the forward clutch whenever the manual valve, indicated generally at  125 , is shifted into a forward gear range using the gear selector, indicated generally at  130 . Simultaneously, forward clutch feed fluid pressure as at  122  is routed to the forward clutch accumulator piston  102 . Forward clutch feed fluid pressure at  122  moves the accumulator piston  102  against the force of spring  108  as the clutch begins to apply. This action absorbs some of the initial increase of clutch apply fluid force to cushion the forward clutch apply. 
     Referring to  FIG. 3  the 1-2 gear accumulator assembly, indicated generally at  135 , and the 3-4 gear accumulator assembly, indicated generally at  140 , help to cushion the application of the 2-4 gear clutch band. These assemblies  135 ,  140  use accumulator fluid pressure to assist spring force. Accumulator fluid pressure is regulated by the accumulator valve  116  in relation to torque signal fluid pressure as at  118 . The Pressure Control Solenoid (PCS) (not shown) regulates torque signal fluid pressure in relation to engine torque, throttle position, and other vehicle operating conditions. 
     The 1-2 gear accumulator assembly  135  is used to control the apply feed of the 24 gear clutch band in 2 nd  gear. The prior art 1-2 gear accumulator assembly  135  is mechanically attached to the valve body  110  and is comprised of an accumulator piston  102 , a piston seal  104 , spring  145 , and an accumulator pin  147 , which is pressed into cover  150 . 
     During a 1-2 gear upshift as illustrated in  FIG. 3 , clutch fluid is routed to both the 2 nd  clutch servo and the 1-2 gear accumulator assembly  135 . The rapid build up of fluid pressure in the 2 nd  gear clutch fluid circuit strokes the accumulator piston  102  against the force of spring  145  and accumulator fluid pressure. This action absorbs some of the initial buildup of 2 nd  clutch fluid pressure and provides a time delay to cushion the 2-4 gear clutch band apply. 
     The prior art 3-4 gear accumulator assembly  140  also comprises an accumulator piston  102  having a central bore  102   a , a piston seal  104 , a spring  145 , and piston pin  147  ( FIG. 4 ) pressed into the transmission case (not shown). The 3-4 gear accumulator assembly  140  is the primary device for controlling the apply feel of the 2-4 gear clutch band in 4 th  gear. The 3-4 gear accumulator assembly  140  functions in a manner similar to the 1-2 gear accumulator assembly  135 . During the 3-4 gear upshift, the 3-4 gear accumulator absorbs the initial increase of the 3-4 gear accumulator fluid pressure to control the apply rate of the 2-4 gear clutch band. 
     An inherent problem in the OEM design of the prior art accumulator assemblies  100 ,  135 , and  140  is that the accumulator pistons  102  reciprocate against spring pressure on steel accumulator pins  106  and  147  extending through the bore  102   a  formed in the piston web, which causes wear at the pin/piston interface. This inherent mechanical wear problem causes leakage of clutch fluid pressure resulting in poor shift quality and damaged clutches and bands. As wear at the pin/piston interface becomes more severe, the pistons  102  can be deflected or cocked out of perpendicular to the pins  106 ,  147  scoring the piston bores and eventually causing seizing of the pistons  102  therein. 
     Referring to  FIG. 5A  there is shown a pinless accumulator piston in accordance with the present invention, indicated generally at  10 . It can be seen that the modified structure of the present piston  10  eliminates the OEM pins  106 ,  147 , bore  102   a , and, consequently, the pin/piston interface, which is the primary source of clutch fluid pressure leakage in the OEM accumulator design. Because the accumulator pin  106  in the OEM forward clutch accumulator  100  and the accumulator pins  147  in the 1-2 gear and 3-4 gear accumulators are deleted from the present accumulator assembly, the central bore  102   a  found in the OEM piston  102  is also omitted from the structure of the present piston  10  providing a solid web structure as at  12   b  in the present design. 
     This design change is accomplished in the present invention by providing an accumulator piston  10  comprised of a body member  12  including a sidewall or piston skirt  12   a , which has a slightly increased axial length as at dimension “L” ( FIG. 5A ) for contact within the bore  105  in comparison to the OEM piston  100 . 
     The piston skirt  12   a  is radially disposed about a central recess  20  including a spring groove  20   a  formed therein at one end thereof, which provides a seating surface for compression spring  145  when assembled. 
     The present accumulator piston  10  also provides structures comprising piston guiding means including, but not limited to, the following structures. The increased axial length of the skirt  12   a  permits the installation of at least one additional seal ring or guide ring  18  on the piston skirt  12   a  to guide the reciprocating, axial movement of the piston  10  within the piston bore  105  in the valve body  110  and/or the transmission case in the absence of the accumulator pins  106  and  147 . 
     As shown in  FIG. 5A , the accumulator piston  10  is provided with at least two seal rings  15  or, in the alternative, guide rings  18 , or a combination thereof, which reside in parallel grooves  17  formed about the periphery of the piston skirt  12   a . Both seal rings  15  and guide rings  18  are fabricated from materials such as Teflon, fluorocarbon rubber, or other materials suitable for this purpose and are provided in various cross-sectional configurations such as lip seals, D-rings, O-rings or combinations thereof for specific applications. In addition to guiding the reciprocating movement of the pistons  12  in lieu of a pin  106 ,  147  to ensure accurate operation during clutch apply, the seal rings  15  and guide rings  18  function to maintain the hydraulic integrity of the pinless accumulator assembly  10  in order to cushion the application of its corresponding clutch. 
     In an alternative construction the accumulator piston  10 ′ is provided with a plurality of raised, circumferential guide ridges  16  integrally formed with the skirt portion  12   a ′ of the body member  12 ′ in lieu of seals  15  or guide rings  18  as shown in FIG.  5 C. In this embodiment ridges  16  function to guide the reciprocating, axial movement of the piston  10 ′ within the piston bore  105  in the valve body  110  and/or the transmission case in the absence of the accumulator pins  106 ,  147 . Ridges  16  may be provided with a coating such as Teflon or other solid lubricant coating suitable for this application. In an alternative embodiment the piston  10 ′ including guide ridges  16  may be provided with a hard anodize coating if aluminum is used to fabricate the piston  10 ′ to reduce surface friction and increase service longevity. 
     In yet another alternative embodiment at least one ridge  16  may be utilized in combination with at least one seal ring  15 . Of course, more than one of either ridges  16  and/or seals  15  and guide rings  18  may be utilized in combination with the other to achieve a desired configuration. 
     To retrofit the present accumulator pistons  10 ,  10 ′ respectively into the OEM valve body  110 , the covers  112 ,  150  are initially removed and the OEM accumulator assemblies  100 ,  135 , or  140  are disassembled. Next, the OEM pistons  102  and seals  104  are discarded. Thereafter, the accumulator pins  106 ,  147  are removed from the valve body  110 , the cover  150 , and/or the transmission case respectively. Next, the resulting hole  30  in the valve body  110 , the cover  150 , or the transmission case wherein the pins  106 ,  147  were previously located are blocked using a ball bearing  25  or other suitable plug, which is pressed into the open hole using a sealant as at  30  (FIG.  6 ). If necessary, the ball bearing is retained in hole  30  ( FIG. 6 ) by a staking process. Thereafter, a piston  10 ,  10 ′ of the present pinless type is inserted into the bore  151  of the cover  150  ( FIG. 6 ) as shown or, in the alternative, into the piston bore  105  within the valve body  110  ( FIG. 1 ) or within the transmission case as described hereinabove. 
     It will be noted that the present accumulator pistons  10 ,  10 ′ are provided with a plurality of integrally formed protuberances or feet  35  ( FIG. 6 ) positioned at the leading end face  12   c  thereof, which function to maximize exposure of the leading end face  12   c  to ATF entering the accumulator assembly  10  and also to limit the travel of the piston  12  within the bore  105  during operation. 
     In the preferred embodiment the pinless accumulator piston  10  is fabricated from ASTM 6061-T6 aluminum, steel bar stock or other material such as engineering grade resins (i.e. phenolic resin) having suitable physical and mechanical properties for this application. 
     Although not specifically illustrated in the drawings, it should be understood that additional equipment and structural components will be provided as necessary and that all of the components described above are arranged and supported in an appropriate fashion to form a complete and operative pinless accumulator piston and method of its use incorporating features of the present invention. 
     Moreover, although illustrative embodiments of the invention have been described, a latitude of modification, change, and substitution is intended in the foregoing disclosure, and in certain instances some features of the invention will be employed without a corresponding use of other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of invention.