Abstract:
A high performance intermediate servo assembly wherein the servo piston has been modified to provide at least a 17% increase in surface apply area and a corresponding increase in hydraulic pressure delivered to the 2 nd  gear clutch band which it actuates. The present servo piston is reconfigured to an overall length equivalent to the assembled stack-up dimensions of the original equipment servo piston, accumulator, accumulator spring, and spring retainer components and integrates the critical working surfaces thereof to retain proper function. Such original equipment components have been eliminated to simplify the intermediate servo assembly operation since shift feel is not a concern in this application. O-ring seals on the original equipment servo piston are replaced by high performance seals such as quad-lobed seals to ensure hydraulic integrity under the increased fluid pressure generated. A method of replacing the original equipment components with the present servo assembly is disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a divisional application of U.S. patent application Ser. No. 10/408,054 filed Apr. 4, 2003, now U.S. Pat. No. 6,907,974, entitled High Performance Intermediate Servo Assembly and claims the benefits thereof under 35 U.S.C. § 121. 

   BACKGROUND OF INVENTION 
   The present invention relates to the field of automatic transmissions and, more particularly, to a high performance intermediate servo assembly for use in General Motors 200 4R transmissions (hereinafter “GM transmissions”). 
   Automatic transmission systems of the prior art have a hydraulic circuit sub-system 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 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”) to hydraulically actuate various 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 the intermediate servo assembly is applied in 2 nd  gear and is used as an accumulator in 3 rd  gear. An accumulator is a spring-loaded device that absorbs a certain amount of apply fluid pressure to cushion the application of a clutch band against fluid shock to control shift feel and to prevent damage to these components. 
   In high performance applications it is desirable to increase the hydraulic fluid capacity and fluid pressure generated by the intermediate servo assembly to increase the holding capacity of the clutch band under high load conditions. 
   There are known prior art patents in the field and their discussion follows. U.S. Pat. No. 4,601,233 to Sugano discloses a hydraulic servo device with a built-in accumulator comprising a servo piston fit in a servo cylinder and connected via a stem to a band brake, an accumulator piston, and an accumulator piston spring biases the accumulator piston toward the servo piston. A servo release pressure acts on a pressure acting area, a servo apply pressure acts on a pressure acting area, and an accumulator pressure acts on a pressure acting area in the operation thereof. 
   Another example of this type of servo is shown in U.S. Pat. No. 4,787,494 to Ogasawara et al., which discloses a hydraulic operating unit for a friction-engaging band of a transmission that permits the introduction of a low-pressure working fluid to allow forcible disengagement of the hydraulic operation unit to work against the fluid, which would normally urge the unit into engagement. 
   U.S. Pat. No. 4,388,986 to Umezawa discloses a speed change control for an automatic transmission having a planetary gear mechanism and a brake band for altering the transmission ratio. The system comprises a servo-piston, a piston rod secured to the servo-piston and to the brake band, and an accumulator piston axially slidably disposed in the bore of the servo-piston. 
   U.S. Pat. No. 5,944,627 to Darling-Owen and also owned by the applicant discloses a replacement servo mechanism for applying a band in an automatic transmission wherein the servo assembly includes two servo piston members, a major piston member and a minor piston member, to which an apply pin is connected. 
   It is also known in the prior art to adapt the original equipment manufacture (hereinafter “OEM”) intermediate servo for high performance applications by providing an aftermarket intermediate servo kit. A commercially available kit of the type marketed by Shift Technology Products, a division of Superior Transmission Parts, Inc., Tallahassee, Fla., has been designed for this purpose ( FIG. 3 ) and is described hereinafter in further detail. For the high performance application, the user of this prior art kit is instructed by the manufacturer to disable the function of the integrated accumulator. All of the remaining OEM components are effectively used as spacers in reassembly. However, reassembly of the intermediate servo in such a manner creates mismatched part surfaces causing inaccurate operation and the potential for noise and abnormal wear. 
   While these devices fulfill their respective, particular objectives and requirements, the aforementioned patents do not disclose the high performance intermediate servo assembly of the present invention, which substantially departs from the conventional concepts and designs of the prior art. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention is a high performance intermediate servo assembly wherein the servo piston has been modified to provide at least a 17% increase in surface apply area and a corresponding increase in hydraulic fluid delivered to the 2 nd  gear clutch band, which it actuates to obtain 2 nd  gear. This is accomplished by increasing the diameter of the servo piston and its mating cover while maintaining its overall dimensions to fit the OEM transmission case. 
   In addition, the present servo piston is reconfigured to an overall length equivalent to the assembled stack-up dimensions of the original equipment servo piston, accumulator, accumulator spring, and spring retainer components and integrates the critical working surfaces (i.e. spring seat, axial travel stop) thereof to retain proper function. Such original equipment components have been eliminated to simplify the intermediate servo piston&#39;s operation since shift feel is not a particular concern in this high performance application. Further, the O-ring seals on the OEM servo piston are replaced by high performance seals such as quad-lobed seals to ensure the hydraulic integrity of the intermediate servo under the increased fluid pressure generated by the present high performance servo piston. 
   There has thus been outlined, rather broadly, the important features of the present invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are additional features of the invention that will be described hereinafter and which will form the subject matter of the claims appended hereto. 

   
     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 an exploded view of an intermediate servo for the GM transmissions labeled Prior Art; 
       FIG. 2  is a perspective view of an intermediate servo assembly shown in its functional position engaging a clutch band labeled Prior Art; 
       FIG. 3  is an exploded elevational view of the intermediate servo including components of an aftermarket kit labeled Prior Art; 
       FIG. 4  is a cross-sectional view of the intermediate servo of the present invention in the release position; and 
       FIG. 5  is a cross-sectional view of the intermediate servo of the present invention in the apply position. 
   

   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 an intermediate servo assembly of the GM transmissions. With reference to  FIG. 1  there is shown an exploded view of the OEM intermediate servo assembly, indicated generally at  100 . The OEM servo assembly  100  is comprised of a servo cover  105 , O-ring  106 , retaining ring  107 , piston  110 , inner lip seal  111 , outer lip seal  112 , accumulator piston  115 , Teflon seal  117 , accumulator spring  118 , retaining clip  119 , spring retainer  120 , band apply pin  122 , cushion spring  124 , and O-rings  125  arranged coaxially for installation in the transmission case  135  ( FIG. 2 ). 
   The OEM intermediate servo assembly  100  functions to apply the intermediate clutch band  130  in 2 nd  gear as shown in  FIG. 2  and is used as an accumulator in 3 rd  gear. An accumulator is a spring-loaded device that absorbs a certain amount of apply fluid pressure to cushion the application of a clutch band against fluid shock and also improves shift feel. 
   In 2 nd  gear the intermediate band  130  is applied by servo  100 . The intermediate band  130  is compressed around the direct clutch drum (not shown) with its distal end  130   a  mechanically anchored to the transmission case  135 . The other end of the intermediate band  130  is hydraulically actuated by the intermediate servo  100 . When the intermediate band  130  is applied, the rotation of the direct clutch drum (not shown) is prevented, which places the transaxle in 2 nd  gear. 
   In 2 nd  gear the servo piston  110  is actuated by 2 nd  gear clutch fluid pressure at  132  which is delivered via an oil passage  27  ( FIG. 4 ) in the apply pin  122  and acts on the apply side of piston  110 . The apply rate of the intermediate band  130  is controlled by the 2 nd  gear fluid compressing the cushion spring  124 . By applying the servo piston  110  the band apply pin  122  is forced into the band  130 . Thus, the band  130  is compressed around the direct clutch housing or drum (not shown) until the drum is held stationary to obtain 2 nd  gear. 
   The OEM intermediate servo  100  releases in 3 rd  gear by channeling 3 rd  clutch fluid to the 3 rd  accumulator exhaust valve (not shown). The force of the cushion spring  124  and the 3 rd  gear clutch fluid will overcome the pressure of the 2 nd  gear clutch fluid and move the piston  110  to release the band  130 . The return stroke of the servo piston  110  to release the band  130  acts as an accumulator for 3 rd  gear by absorbing some 3 rd  gear clutch fluid. This allows the band  130  to release and the direct clutch to apply at a controlled rate for a smooth 2–3 shift. 
   It is known in the prior art to adapt the OEM intermediate servo  100  for high performance applications by providing an aftermarket intermediate servo kit. A commercially available kit of the type marketed by Shift Technology Products, a division of Superior Transmission Parts, Inc., Tallahassee, Fla., has been designed for this purpose. As shown in  FIG. 3  this kit produces a modified intermediate servo  100 ′ including a servo cover  105 ′ and a servo piston  110 ′, which are machined oversize to provide additional fluid volume. New lip seals  111 ,  112  of the same type are also provided. 
   For the high performance application, the user of this kit is instructed by the manufacturer to remove the Teflon seal  117  from the accumulator piston  115  to disable its function. All of the remaining OEM components except for spring  118  (i.e. accumulator piston  115 , clip  119 , spring retainer  120 , apply pin  122 , and cushion spring  124 ) are effectively used as spacers during reassembly. However, reassembly of the modified intermediate servo  100 ′ in such manner with the spring  118  omitted creates mismatched part surfaces and the potential for noise, abnormal wear, and inaccurate operation. 
   Accordingly, the present high performance, intermediate servo assembly has been developed to resolve these problems and will now be described. With reference to  FIG. 4  there is shown therein an intermediate servo assembly in accordance with the present invention, indicated generally at  10 . The present intermediate servo assembly  10  is comprised of a modified servo cover  15 , a modified servo piston  20 , and a modified pin  29 , which are configured as shown. The present servo assembly  10  utilizes a heavier gauge return spring  24  having a spring rate in the range of 150 to 155 pounds, which differs significantly from the OEM cushion spring  124 . The return spring  24  is seated on at least one washer or spacer  25  as shown. 
   The modified pin  29  is provided with seals  125  and seal  126  at locations designed to prevent leakage of the increased hydraulic pressure generated by the servo piston  20  from the piston chamber  50 . The modified pin  29  is also provided with structures comprising length-adjusting means including, but not limited to, the following structures. In the present design the thickness and/or number of spacers  25  may be varied to adjust the effective overall length “L” of the pin  29  and the operating characteristics of the band  130 . Thus, the band  130  can be brought into closer proximity with the clutch drum (not shown) by increasing length “L” thereby reducing the actuating stroke and time required to apply the clutch. 
   In addition, the length-adjusting means further include a plurality of tapered segments  28  delineated by circumferential undercut markings  29   a ′ formed in the tip  29   a  of the pin  29 . Utilizing a method of the present invention the distal segment  28  of pin  29  is removed by machining (e.g. grinding) to shorten the overall length of pin  29 . Thus, the application of the clutch can also be adjusted by shortening of the tip  29   a  to increase the actuating stroke and/or delay the time required to apply the clutch. 
   Of course, such length adjustments and the resultant clutch performance can be fine tuned by varying the thickness and/or number of spacers  25  as described hereinabove in combination with grinding the tip  29   a  of the pin  29  to provide optimal shift performance. 
   In the preferred embodiment the modified servo cover  15  and servo piston  20  are fabricated from aluminum bar stock, castings, or extruded stock. The inside diameter (I.D.) of the piston bore  15   b  has been substantially increased over the OEM and aftermarket designs to generate an increase in fluid capacity and, thus, an increase in holding power of the band  130  and the direct clutch. 
   As shown in  FIG. 4  the servo cover  15  is provided with a tool groove  15   a , which facilitates removal of the cover  15  from the transmission case  135  for service/overhaul purposes. The servo cover  15  is provided with an O-ring seal  116  substantially the same as the OEM version for sealing within the mating bore  150  ( FIG. 2 ) of the transmission case  135 . 
   Still referring to  FIG. 4  the axial length “AL” of the servo piston  20  has been increased to correspond to the sum total of the assembled stack-up dimension of the OEM piston  110 , the accumulator  115 , accumulator spring  118 , and spring retainer  120  while integrating the critical working surfaces thereof. More particularly, the spring retainer  120 , which functions as a seat for spring  124  in the OEM design is effectively built into the present piston  20  as at tapered counterbore  21 . Thus, it will be understood that all of the aforementioned OEM components are unnecessary in the present design and may be discarded when the intermediate servo assembly  10  is retrofitted to the GM transmissions. 
   Further, it can be seen in  FIG. 4  that lip seals  111 ,  112  have been replaced by a seal having a multi-lobed configuration known as a quad-lobed seal  30 . Quad lobed seals are very reliable because they have twice the sealing surfaces as standard round O-rings. In the present invention the quad-lobed seals  30  are preferably made from fluorocarbon material (i.e. PTFE), which provides improved heat resistance. Thus, the present intermediate servo  10  is capable of generating and maintaining increased hydraulic fluid pressure and holding power to the direct clutch. 
   In order to install the present intermediate servo assembly  10 , the OEM intermediate servo is removed from the transmission case and discarded. Next, a modified apply pin  29  having a shortened axial length is fitted with new seals  125 , at least one washer  25  is mounted in position, and the apply pin  29  is pressed into the servo piston  20  as shown in  FIG. 4 . Next, the quad-lobed seals  30  are installed in the appropriate grooves  32  formed in the outside diameters (O.D.) of the servo piston  20  with a suitable lubricant and the piston  20  is engaged in the piston bore  15   b  of the piston cover  15  as shown. Thereafter, the return spring  24  is placed in position and the servo assembly  10  is installed in the mating bore  150  ( FIG. 2 ) of the transmission case  135  such that the tapered tip  29   a  of the apply pin  29  engages a receptacle  123  on the moveable end  130   b  of band  130 . 
   In operation 2 nd  gear clutch fluid pressure is delivered to the piston chamber  50  via oil passage  27  in the apply pin  29  and acts on the apply side of piston  20  as shown in  FIG. 5 . As the piston chamber  50  is filled with fluid under pressure, the piston  20  is stroked (i.e. to the right in  FIG. 5 ) against the force of spring  24 . Actuation of the servo piston  10  forces the apply pin  29  into the band  130 , which compresses around the direct clutch drum to prevent it from turning to obtain 2 nd  gear. When clutch fluid pressure is released, the servo piston  20  returns to the position shown in  FIG. 4  and the clutch is released. 
   It will be noted that the return stroke of the present servo piston  20  is stopped by contact with the end face of cover  15   c  against a shoulder  20   a  formed about the circumference of piston  20  ( FIG. 5 ). This represents an improvement in operating control over the prior art design wherein the proximal end  122   a  of pin  122  bottoms out on the inside of cover  105 ′ to halt the return stroke of the piston  110 ′. The present design also permits the combined axial length of the servo piston  20  and cover  15  to be decreased significantly, which uses less material and reduces manufacturing costs. 
   Thus, it can be seen that the present invention provides a high performance intermediate servo assembly for use with GM transmissions that produces at least a 17% increase in surface apply area of the servo piston and a corresponding increase in hydraulic fluid delivered to the 2 nd  gear clutch band, which it actuates to obtain 2 nd  gear. The present intermediate servo assembly reduces the number of component parts required and improves the performance of the intermediate servo in comparison to aftermarket replacement kits. 
   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 high performance intermediate servo 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.