Clutch piston assembly

The clutch piston assembly includes a one piece piston body which extends about an axis and has at least one radially inwardly facing surface and at least one radially outwardly facing surface. At least one seal is engaged with the piston body and extends either radially outwardly from the radially outwardly facing surface or radially inwardly from the radially inwardly facing surface. The seal is made of polyetheretherketone (PEEK) or polyaryletherketone (PAEK) for establishing a low friction and fluid tight seal between the piston body and a wall in the automatic transmission.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to clutch pistons for automatic transmissions of automobiles.

2. Related Art

Automatic transmissions in automobiles typically include one or more planetary gear sets which may be manipulated in various ways to produce a plurality of different gear ratios which may be selected for different operating conditions as programmed by a computer. The planetary gear sets are manipulated by selectively disengaging and engaging a plurality of clutch packs contained within an outer casing of the transmission. In general, each clutch pack is selectively engaged and disengaged by hydraulically actuating an associated clutch piston to energize the clutch piston against the clutch pack or withdraw the clutch piston from engagement with the clutch pack.

Typically, the clutch piston is formed of stamped steel or cast/machined aluminum with one or more rubber adjoining lips either retained within a groove or bonded directly. The rubber lip provides a fluid resistant seal between the piston assembly and the clutch housing creating a constrained cavity where fluid pressure will actuate the clutch piston in an axial direction inside the clutch housing. The rubber lip seals are typically made of a high modulus elastomer with a base polymer of FKM, AEM or ACM. The elastic nature of the rubber material allows for some distortion of the lip where the sealing contact patch is over a broader area then just the lip radius creating some resistance to movement.

SUMMARY OF THE INVENTION

One aspect of the invention provides for a clutch piston assembly for an automatic transmission of a vehicle. The clutch piston assembly includes at least one piston body component which extends about an axis and has at least one radial outwardly facing surface and may also have one radially inwardly facing surface. At least one sealing lip is attached with the piston body either retained inside a groove or bonded to the body surface. The sealing lip extends radially outwardly or inwardly. The seal is made of polyetheretherketone (PEEK) or polyaryletherketone (PAEK) for establishing a low friction and fluid tight seal between the piston body and the clutch housing wall in an automatic transmission.

The subject clutch piston sealing lip may be molded more quickly than traditional elastomer materials resulting in lower cost as cooling times are much lower. Specifically, the cycle time for injection molding PEEK or PAEK seals has been found to be significantly shorter than the cycle times for manufacturing rubber seals. Most importantly, PEEK or PAEK materials offer lower coefficients of friction giving the ability to reduce the frictional losses between the sealing lip and clutch housing wall resulting in better shifting feel and performance

Another aspect of the present invention provides for a method of making a piston assembly for use in an automatic transmission. The method includes the step of making a piston body as a single piece of material and having at least one radially inwardly facing surface and at least one radially outwardly facing surface. The method continues with the step of engaging a seal of PEEK or PAEK with either the radially outwardly facing surface or the radially inwardly facing surface of the piston body for establishing a fluid tight seal between the piston body and a wall in the automatic transmission.

DESCRIPTION OF THE ENABLING EMBODIMENT

Referring to the Figures, wherein like numerals indicate corresponding parts throughout the several views, an exemplary embodiment of a hydraulically actuated clutch piston assembly20as installed in an automatic transmission22is generally shown inFIG. 1. As is common in the art, the automatic transmission22includes a planetary gear mechanism (not shown) and a clutch pack24for selectively adjusting the operation of the planetary gear mechanism to produce a desired gear ratio from an input to an output of the automatic transmission22. Although only one is shown inFIG. 1, it should be appreciated that the automatic transmission22could include a plurality of such clutch piston assembly20and clutch pack24combinations for allowing the automatic transmission22to produce many different gear ratios. The automatic transmission22further includes a clutch housing26with an inner outer wall30and in some instances an inner wall28that form a confined cavity, and the piston body32is moveable in an axial direction within the cavity. The piston body32is engageable with the clutch pack24to selectively establish contact between or separate a plurality of clutch plates34within the clutch pack24from one another, thereby activating the clutch pack24. Herein the clutch housing will be referred to as housing26.

Referring now toFIG. 2, the exemplary embodiment of the piston assembly20includes a piston body32which is made as one or more components of material, such as aluminum or plastic. The piston body32is generally annular in shape when viewed from above and is generally symmetrical in shape around an axis A. The piston body32further has a radially outwardly facing surface36and a radially inwardly facing surface38, each of which extends continuously around the axis A. Each of the radially outwardly and radially inwardly facing surfaces36,38also may present a groove40,41with each groove40,41extending circumferentially around the axis A. Specifically, the radially outwardly facing surface36is provided with a first groove40, and the radially inwardly facing surface38is provided with a second groove41. It should be appreciated that the piston body32may take a wide range of shapes depending upon space constraints within the automatic transmission22. The piston body32may also be made through any suitable process, e.g., casting, injection molding, stamping, turning, etc.

Referring back toFIG. 1, the piston assembly20includes a pair of lip seals42,44which are partially disposed in the grooves40and extend radially therefrom to establish fluid-tight seals between the piston body32and the walls28,30of the housing26. Specifically, the piston assembly20includes a first seal42which is received within the first groove40on the radially outwardly facing surface36of the piston body32and a second seal44which is received within the second groove41on the radially inwardly facing surface38of the piston body32. When the clutch piston assembly20is installed in the automatic transmission22, the seals42,44, piston body32and housing26together define an expandable cavity46with an annular shape. Each of the exemplary seals42,44is provided with a lip45(best shown inFIG. 3) which extends both radially and also axially in the direction of the expandable cavity46for improving the fluid tight seals established with the housing26.

The expandable cavity46is in communication with a hydraulic fluid48port50within the automatic transmission22. During operation, a hydraulic fluid48(shown inFIG. 3) is pumped into the expandable cavity46. Because the seals42,44are in a fluid tight engagement with the walls28,30and the fact that hydraulic fluid48is, for all practical purposes uncompressible, pumping the hydraulic fluid48into the expandable cavity46has the effect of urging the piston body32in a first axial direction to contact and activate the clutch pack24as discussed above. Although not shown, the piston body32may be biased towards the resting position shown inFIG. 1by, for example, a spring.

Each of the seals42,44is made as one integral piece of polyetheretherketone (PEEK) or polyaryletherketone (PAEK). These materials have a relatively high modulus. The high modulus materials allows for the sealing lips45to be designed with less contact area and coupled with the lower coefficient of friction properties, which allows for very low parasitic losses compared to traditional seals made of elastomer materials where the sealing lip contacts one or both of the associated walls28,30of the clutch housing26. As such, the PEEK or PAEK seals42,44offer improved performance and durability as compared to other known seals by reducing the friction between the seal42,44and the housing26. Additionally, the PEEK or PAEK seals42,44may also be manufactured more quickly and cost effectively than other known seals because the cycle times for injection molding PEEK and PAEK are significantly lower than the cycle times for injection molding parts of rubber or synthetic rubber materials.

Preferably, the PEEK or PAEK seals42,44are injection molded directly onto the piston body32, i.e., the seals42,44are overmolded onto the piston body32. This provides for very strong and durable connections between the seals42,44and the piston body32and may be carried out at a low cost.

The PEEK or PAEK seals42,44may be also be bonded to the piston body32to further improve the connections therebetween. One such bonding connection could be provided by subjecting the outer surface of the piston body32at the groove40to an air plasma discharge process before overmolding the PEEK or PAEK seal onto the piston body32. Adhesive may need to be applied after the plasma treatment.

Another aspect of the present invention is a method of making a clutch piston assembly20. The method includes the step of making a piston body32of a single piece of plastic or aluminum. The piston body32may be made through, for example, injection molding or casting operations. As formed, the piston body32includes a radially outwardly facing surface36and a radially inwardly facing surface38. A first groove40is formed in the radially outwardly facing surface36, and a second groove40is formed in the radially inwardly facing surface38.

Referring now toFIG. 4, the method may continue with the step of surface treating the grooves40on the radially inwardly facing surface38and the radially outwardly facing surface36of the piston body32with an air plasma treatment process using an air plasma emitter52. In operation, the air plasma emitter52blows atmospheric gas past two high voltage electrodes (not shown) which positively charge the air before it is blown onto the piston body32. The method then continues with the steps of injection molding a first seal42of PEEK or PAEK directly onto the groove40of on the radially outwardly facing surface36of the piston body32and injection molding a second seal44directly onto the groove40of the radially inwardly facing surface38of the piston body32. The plasma treatment process has been found to improve the bond established between the housing and the PEEK or PAEK seal42,44following the overmolding process.