Clutch exhaust assembly and method

An assembly and method is provided for reducing clutch wear due to centrifugal apply pressure within a clutch-apply cavity. The assembly comprises a rotatable clutch housing, a clutch-apply piston disposed within a clutch-apply cavity, and a balance piston. The balance and apply pistons define a balance cavity including a liquid fill channel and a dedicated air exhaust channel having a control radius configured to at least partially offset or compensate for the centrifugal apply force generated within the clutch-apply cavity when the clutch assembly rotates. The method includes configuring the balance cavity with an exhaust channel for exhausting fluid, and providing the exhaust channel with a control radius determined by the innermost radius of rotating fluid within the balance cavity as measured with respect to the centerline of rotation of the clutch assembly, the control radius being suitable for exhausting any excess of centrifugal balance force over centrifugal apply force.

TECHNICAL FIELD

The present invention relates to an improved rotatable clutch assembly having at least one dedicated balance cavity exhaust port configured to optimize clutch performance during rapid acceleration and periods of high clutch rotational speed.

BACKGROUND OF THE INVENTION

In a vehicle having an automatic transmission, a clutch assembly engages or couples a rotating shaft such as an engine crankshaft with a stationary shaft such as a driveshaft in order to transmit power from the engine to the drive wheels. Likewise, the clutch assembly disengages the respective shafts to interrupt the power transfer and permit smooth shifting between, for example, the various gears of a planetary gear set. Hydraulic clutch assemblies or hydraulic clutches in particular are hydraulically-actuated torque-transmitting devices typically having a series of friction elements arranged in a clutch pack within a clutch housing. The clutch pack is actuated by a clutch-apply piston disposed within a clutch-apply cavity portion of the housing, the piston being powered or energized by a supply of non-compressible hydraulic fluid. When hydraulic clutch pressure is reduced, the clutch is released or disengaged, and when clutch pressure is increased, the clutch is actuated or engaged.

In a rotating clutch assembly, a clutch housing having a clutch-apply cavity rotates along with one of the two rotating members or shafts, while a clutch return spring helps bias the clutch-apply piston. When the clutch assembly is not engaged, clutch-apply response time may be improved by maintaining a relatively low fluid pressure within the apply cavity. However, when the clutch reaches a relatively high rotational speed, the centrifugal force imparted by the rotation of the clutch housing may induce a substantial pressure head or centrifugal apply force within the clutch-apply cavity. If the centrifugal apply force exceeds the return spring force provided by the biasing spring, inadvertent or premature application of the clutch may result.

The centrifugal apply force may be compensated for by positioning a separate balance cavity opposite the clutch-apply cavity within the clutch housing. The rotation of the clutch assembly fills the balance cavity with fluid, which can then partially drain from the balance cavity when the rotation ceases. When the balance cavity is full of fluid and the clutch assembly is rotating, the rotating fluid within the balance cavity will act to produce a centrifugal balance force or pressure that opposes or counteracts the centrifugal apply force, helping to at least partially offset or negate the apply force created by the pressure head within the clutch-apply cavity. However, when the clutch-apply cavity is attached to a rotating transmission component capable of relatively rapid acceleration and high rotational speed, such as a driveshaft connected to an electric motor within a hybrid vehicle transmission, relatively rapid fluid fill of the balance cavity is essential in order to provide a balancing opposing pressure to the rapidly increasing apply cavity pressure, and avoid premature application and therefore wear or burnout of the hydraulic clutch.

SUMMARY OF THE INVENTION

Accordingly, an improved clutch assembly is provided for use within an automotive transmission, the assembly comprising a main cavity and a balance cavity having at least one air exhaust channel and at least one fluid fill channel, wherein the air exhaust channel is configured to rapidly exhaust air from the balance cavity when fluid enters the balance cavity through the fluid fill channel.

In one aspect of the invention, the air exhaust channel has a control radius configured to substantially offset the centrifugal apply force imparted by the rotation of the clutch apparatus.

In another aspect of the invention, a vehicle transmission has a rotatable hydraulic clutch, a balance piston, and a clutch-apply piston, wherein the balance piston and clutch-apply pistons at least partially define a balance cavity having a pair of fluid channels configured to rapidly fill the balance cavity with non-pressurized fluid during rotation of the clutch assembly.

In another aspect of the invention, a method is provided for reducing clutch wear in a rotatable clutch assembly having a clutch-apply cavity and balance cavity, the method including configuring the balance cavity with an exhaust channel for exhausting fluid from the balance cavity and providing the exhaust channel with a control radius determined by the innermost radius of rotating fluid in the balance cavity with respect to the centerline of rotation of the assembly, the control radius being suitable for exhausting any excess of centrifugal balance force over centrifugal apply force.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings wherein like reference numbers correspond to like or similar components throughout the several figures, there is shown inFIG. 1a cross-sectional view of a transmission8having a rotatable clutch assembly10mounted with respect to a pair of rotatable components such as co-axial inner and outer shafts12,13, respectively, which are rotatable around or with respect to a centerline11. Centerline11dividesFIG. 1into upper and lower halves showing staggered cross-sectional views taken along different planes to more clearly depict the internal detail of clutch assembly10. Rotatable shafts12,13have respective internal diameters or surfaces15,14, and respective outer diameters or surfaces16,18, with a cylindrical channel or volume20defined between shaft surfaces14,16. Volume20is in fluid communication with a non-pressurized source of fluid22, such as a transmission reservoir or sump (not shown). The fluid22, such as transmission fluid, oil, or other fluid suitable for use as a coolant and/or lubricant with a rotating hydraulic clutch, fills volume20, which is also in fluid communication with a preferably cylindrical fluid fill port24that is drilled, tapped, or otherwise formed in outer shaft13. While a representative co-axial two-shaft configuration is shown inFIG. 1, those skilled in the art will recognize that the number and style of rotating shafts used may be varied within the terms of this invention.

Clutch assembly10may be cast or formed in one piece or welded to form a generally bell-shaped configuration or clutch housing25at least partially defining a primary housing cavity26and a secondary cavity27. Primary cavity26is suitably sized and configured for containing a clutch-apply piston28, a clutch piston stop31, a return spring38, and a balance piston32. Secondary cavity27encompasses the remainder of the clutch housing including the clutch pack40, as shown inFIG. 1. Within the primary cavity26, clutch-apply piston28preferably includes an extension or arm portion39, with arm portion39being configured in a bell or cup-shape to engage a clutch pack40when clutch-apply piston28is actuated or otherwise engaged. Balance piston32has a main surface55configured to oppose return spring38. Clutch-apply piston28and balance piston32at least partially define a balance cavity50therebetween within primary cavity26. Likewise, clutch-apply piston28and clutch housing25at least partially define a clutch-apply cavity44therebetween within primary cavity26. Clutch-apply cavity44is in fluid communication with a fluid channel46containing a supply of pressurized fluid23at a line pressure, for example from a controllable source of pressure such as a pump (not shown), with a plurality of dynamic fluid seals64flanking fluid port47.

Within balance cavity50, the balance piston32is held substantially fixed or stationary relative to the sliding or moveable clutch-apply piston28on one side by clutch return spring38disposed between the pistons32,28within balance cavity50, and on the other side by a balance piston stop52. Clutch return spring38is configured to bias the clutch-apply piston28upon disengagement of clutch assembly10. Within clutch-apply cavity44, clutch-apply piston28is operatively attached to clutch piston stop31, with clutch piston stop31containing a plurality of channels or thru-ports30configured to allow pressurized fluid23to freely flow through the clutch piston stop31within the clutch-apply cavity44. Pressurized fluid23enters the clutch-apply cavity44through a fluid port47, which is in fluid communication through fluid channel46with a source of pressurized hydraulic fluid, for example a pump, operable to provide fluid at a low line pressure PLof preferably approximately 2-3 psi (gauge).

Because clutch-apply cavity44is preferably retained at a substantially full level of fluid23due to the low line pressure PL, when clutch assembly10is connected to or coupled with a rotating shaft such as shaft13, the rotation of the clutch assembly10likewise rotates the fluid23within the clutch-apply cavity44to generate a centrifugal force, represented by arrow70inFIG. 2, that operates on the clutch-apply piston28. Absent an opposing centrifugal balance force within balance cavity50, clutch-apply piston28may prematurely actuate and engage the clutch pack40if the centrifugal apply force exceeds the return force (PS) provided by return spring38(seeFIG. 2),

To counter this centrifugal apply effect, upon rotation of clutch assembly10the balance cavity50fills with non-pressurized fluid22through fill port24, and the rotation thereby rotates the fluid22within the balance cavity50to simultaneously generate an opposing centrifugal balance force within the balance cavity50. The centrifugal balance force (arrow72) is ideally equal to the centrifugal apply force (arrow70). However, because the volume of fluid22within balance cavity50can be at a low or empty level when clutch assembly10is stationary, connection or coupling of clutch assembly10to a device capable of rapid acceleration, for example a hybrid transmission motor, requires a rapid influx of fluid22to fill balance cavity50and provide a sustained centrifugal balance force (arrow72) capable of opposing the rapidly increasing centrifugal apply force (arrow70) within the clutch-apply cavity44. Thus, rapid influx of fluid22may thereby help avoid premature clutch engagement and/or clutch wear and burnout.

To optimize the fill rate of balance cavity50, non-pressurized fluid22enters balance cavity50upon rotation of the clutch assembly10through a dedicated balance cavity feed channel56and fluid fill port24, as shown inFIG. 1. Turning toFIG. 2, as fluid22fills the balance cavity50, air contained therein is displaced or purged through a separate, dedicated air purge channel first portion58a, from which it enters a preferably angled second portion58bwhich is in fluid communication with secondary cavity27, and which intersects the first portion58aat point A. After all of the air is purged from balance cavity50, excess fluid22can escape the balance cavity50through the air purge channel portions58a,58b. While an angled second portion58bis preferred, those skilled in the art will recognize that other configurations of the air purge channel may be suitable for establishing fluid communication between the balance cavity50and the main cavity27within the terms of the invention. Using the dedicated air purge channel portions58a,58bas explained hereinabove, air may exhaust from balance cavity50in an unimpeded or unrestricted manner, thus facilitating rapid fill of the balance cavity50with fluid22.

As the clutch assembly10rotates, the rotating fluid23within the clutch-apply cavity44develops a pressure head or centrifugal apply force (arrow70), as previously discussed herewithin. This centrifugal apply force (arrow70) operates on the clutch-apply piston28along with the low line pressure imparted by pressurized fluid23, and represented inFIG. 2as arrow PL. Assuming a negligible centrifugal balance force (arrow72), only return spring force PSwould oppose clutch-apply piston28. Thus, if the combined force (arrow70+PL) were to exceed the return spring force PS, the clutch-apply piston28may move toward and prematurely engage or actuate clutch pack40. Accordingly, air purge channel first portion58aand second portion58bare formed or provided with a control radius RCmeasured in a direction radially from the centerline of rotation11of shafts12,13, as shown inFIG. 2, to intersection point A. In other words, the control radius RCis the most radially inward extent of only the air purge channel first portion58awith respect to the centerline11, with fluid22entering the first portion58aultimately spilling over into second portion58bat intersection point A. The control radius RCis determined by the innermost radius of any rotating fluid22confined within balance cavity50and first portion58a, with the position of point A being selected based on the amount of centrifugal apply force (arrow70) to be offset. For example, by moving point A radially inward toward the centerline of rotation11, the balance force (arrow72) in the balance cavity50increases, and by moving point A radially outward away from the centerline of rotation11, the balance force decreases. In this manner, the control radius RCis selected such that, when applied to balance cavity50for a given centrifugal apply force (arrow70), the resulting centrifugal balance force (arrow72) is substantially equal to the centrifugal apply force (arrow70), thereby compensating for or offsetting the centrifugal apply effect within the clutch-apply cavity44.

Accordingly, a method of compensating for or offsetting this apply effect and reduce clutch wear includes calculating the centrifugal force (arrow70) within the clutch-apply cavity44for a given clutch-apply piston28. Next, the centrifugal balance force (arrow72) within an opposing balance cavity50is calculated. As previously disclosed hereinabove, the centrifugal apply force (arrow70) and the centrifugal balance force (arrow72) are ideally equivalent. Because of the diameter and therefore surface area differences, this is accomplished by designing the balance cavity50to achieve a higher pressure over its smaller respective surface area relative to the clutch apply-cavity44. The pressure head for the balance cavity50is designed to provide the appropriate pressure by the placement of control radius RC, i.e. the placement of point A with respect to the centerline of rotation11. To achieve this force balance within clutch-apply cavity44and balance cavity50having disparate surface areas, control radius RCis preferably determined by the innermost radius of any rotating fluid22confined within balance cavity50, as described previously hereinabove. The resulting centrifugal balance force (arrow72) within balance cavity50is substantially equal to the centrifugal apply force (arrow70) plus the low line pressure (PL) of fluid23, preferably over the entire operating speed range of rotating shafts12,13. The cross sectional diameter of the air purge channel first and second portions58a,58bare preferably cylindrical with a diameter of approximately 2-3 mm, or sufficiently large enough to allow free fluid flow through the air purge channel portions58a,58band to not unduly restrict movement of the clutch-apply piston28.

Once air purge channel first portion58aand second portion58bare configured with the control radius RCas previously described hereinabove to provide the appropriate amount of centrifugal balance force (arrow72) within balance cavity50, the balance cavity is filled with non-pressurized fluid22from, for example, an oil reservoir or other channel of non-pressurized oil within the transmission. The fluid22enters the balance cavity50through fill port24as described previously hereinabove, and as shown inFIG. 1, and air trapped or contained within the balance cavity50is rapidly purged or vented through a dedicated air purge channel portions58a,58bas shown in detail inFIG. 2, without interfering with, blocking, or otherwise restricting the fill of fluid22within balance cavity50. After air is purged, excess fluid22within balance cavity50may then escape through air purge channel portions58a,58b.