Field of the Invention

This invention relates to a transmission piston dam and more particularly, though not exclusively, to a piston dam of a transmission clutch for improving control over engagement and disengagement of the clutch.

Background of the Invention

Automatic transmissions use clutch packs to engage and disengage drive trains thereby providing a range of different ratios between an input of the transmission and an output of the transmission. Clutch packs are typically made up of alternate layers of steel and friction discs. When a clutch pack is needed to transmit torque, pressure is applied to a piston which applies force to the clutch pack, pressing together the layers of steel and friction discs into engagement. When the clutch pack is to be released, pressure used to apply the piston is dumped, and a spring is used to return the piston to a disengaged configuration.

However, there exists a problem in that transmission oil on or near an apply surface of the piston will exert pressure on the piston as the clutch accelerates in angular velocity, owing to centrifugal force acting on the oil. Accordingly, changes in forces acting on the piston due to centrifugal forces on the oil can cause difficulties in achieving stable operation of the clutch.

This can lead to lack of control in slip of the clutch, and to erratic operation of the clutch.

Examples of the present invention seek to overcome or at least alleviate one or more of the problems discussed above.

Summary of the Invention

In accordance with the present invention, there is provided a piston dam which is vented to a stable pressure.

Preferably, the dam is sealed. More preferably, the dam is for a clutch piston of a transmission, and the dam is sealed from fluid supply to other components within the transmission.

In another aspect, there is provided a transmission including a dam for receiving a working fluid which is used to counteract movement of a piston within the transmission, the fluid being vented to a stable pressure.

Preferably, the dam is sealed from other working fluids of the transmission.

In accordance with another aspect of the present invention, there is provided a piston dam which is sealed.

In accordance with another aspect of the present invention, there is provided a transmission having a piston, an apply chamber into which fluid is fed for causing movement of the piston to an activated configuration, and a dam as described above, wherein centrifugal force on fluid in the dam results in a force acting on the piston to counteract force exerted on the piston resulting from centrifugal force on fluid in the apply chamber.

Preferably, a housing of the transmission rotates about an axis, and the stable pressure is independent of rotational speed of the housing about the axis. More preferably, the piston is for applying a clutch of the transmission

Preferably, the stable pressure is atmospheric pressure. Alternatively, the stable pressure may be a regulated pressure.

Preferably, the transmission is a sealed unit, and the dam is vented to atmospheric pressure external to the transmission.

Preferably, the dam is vented to a location on the housing of the transmission which location is radially spaced from the axis.

In accordance with another aspect of the invention, there is provided a transmission comprising a transmission housing rotating about an axis, wherein rotation of the transmission housing is transmitted via a damper to a clutch housing containing a clutch operable in response to movement of a piston to selectively engage or disengage a drive train for transmission of drive to an output, an apply chamber into which fluid is fed for causing movement of the piston to engage the clutch, and a dam as described above, wherein, during rotation of the transmission housing about the axis, centrifugal force on fluid in the dam results in a force acting on the piston to counteract force exerted on the piston resulting from centrifugal force on fluid in the apply chamber.

Preferably, the stable pressure is independent of rotational speed of the transmission housing about the axis.

Preferably, the stable pressure is atmospheric pressure. Alternatively, the stable pressure may be a regulated pressure.

Preferably, the transmission is a sealed unit, and the dam is vented to atmospheric pressure external to the transmission.

Preferably, the dam is vented to a location on the transmission housing radially spaced from the axis of rotation.

Preferably, when the clutch is engaged, drive is transmitted from the clutch to a ring gear of a planetary gear set which, in turn, transmits drive to the output of the transmission.

Preferably, the transmission includes a second clutch which is engagable so that drive is transmitted from the damper to a second clutch, then to the output of the transmission so as to provide a 1 : 1 drive ratio between the input of the transmission and the output of the transmission.

Brief Description of the Drawings

The invention is described, by way of non-limiting example only, with reference to the accompanying drawings in which:

Figure 1 is a longitudinal cross-sectional view of an upper half of a transmission having a clutch with a vented piston dam;

Figure 2 is a longitudinal cross-sectional view of an upper half of the transmission shown in Figure 1, in which power flow through the transmission is shown diagrammatically for two gear states; and

Figure 3 is a longitudinal cross-sectional view of an upper half of a transmission having a clutch with a vented piston dam.

Detailed Description

Figure 1 shows a transmission 10 having an input 12 and an output 14, both being rotatable about a common axis of rotation 16. The transmission 10 is encased within a housing 18 compactly shaped so as to enable retro-fitting of the transmission 10 in place of a torque converter in an existing drive train.

A front surface 20 of the transmission 10 is provided with an internally-threaded bore 22 which is configured for connection to an output of an engine, for example by bolting a fly wheel of the engine (not shown) to the internally-threaded bore 22. It will be understood that while Figure 1 shows a longitudinal cross-sectional view of an upper half of the transmission 10, other similar internally-threaded bores 22 would also be provided around the periphery of the frontal surface 20 of the housing 18, and that these other

intemally-threaded bores would similarly be connected to the output of the engine so as to provide a secure driving connection between the engine and the transmission 10.

The output 14 of the transmission may be connected to drive means for driving a vehicle in which the transmission 10 is mounted. In particular, the transmission 10 is intended for mounting in a drive train of a watercraft, in which case the output 14 may be coupled to a drive shaft which, in turn, may be coupled to a propeller. Further drive transmission components may be provided between the output 14 of the transmission 10 and the propeller. For example, a drive transmission selectable between forward and reverse gears may be provided in this way. It should also be understood that the transmission 10 may also be used in other application such as, for example, automotive transmission systems.

The transmission 10 includes a damper 24, a first clutch 26, a planetary gear set 28 and a second clutch 30. In a first gear state, the first clutch 26 is engaged so that drive is transmitted from the housing 18 and damper 24 to a ring gear 32 of the planetary gear set

28 via the first clutch 26. Drive is then transmitted from the ring gear 32 to a carrier 36 of the planetary gear set 28 which, in turn, drives the output 14 of the transmission.

Accordingly, the first gear provides a low gear ratio which is advantageous when low output speeds are desirable, such as, in the case of watercraft, during docking. Power flow in the first gear is illustrated diagrammatically in Figure 2 by a first gear power flow line

37.

A second gear ratio is achieved by engaging the second clutch 30 which effectively bypasses the planetary gear set 28 such that drive is transmitted from the housing 18 via the damper 24 to the clutch and piston housing 38, then via the clutch 30 directly to the carrier 36 of the planetary gear set 28 and from there to the output 14. In this way, the second gear provides a 1:1 gear ratio between the input 18 and output 14 of the transmission, suitable for achieving higher output speeds. The power flow in the second gear is illustrated diagrammatically by second gear power flow line 40.

Advantageously, this construction is extremely compact and allows the transmission 10 to be fitted within the space of a torque converter, or to replace an existing

marine damper. As the damper 24 is internal of the housing 18, it is protected from corrosion and other damage by the housing 18 and also by way of it being immersed in fluid within the housing 18.

Also of benefit in the constructional layout of the transmission 10 is the configuration of the clutch and piston housing 38 which serves several functions. In particular, the clutch and piston housing 38 forms a mounting to which the damper 24 is mounted for supporting the internal components of the transmission 10 from the transmission housing 18. The clutch and piston housing 38 also houses the clutch pack of the first clutch 26, and forms an outside housing for the second clutch pack 30.

Also mounted within the clutch and piston housing 38 is the piston 42 which is operated to engage and disengage the first clutch 26. The piston 42 has an apply surface 44 which, together with an inside surface of the clutch and piston housing 38 forms a piston apply chamber 46 into which fluid is fed at a controlled pressure so as to move the piston 42 relative to the clutch and piston housing 38 to engage the first clutch 26. Actual engagement of the first clutch 26 is achieved by an arm 48 of the piston 42 being brought into abutment with the clutch discs of the first clutch 26 so as to push together the clutch discs into mutual engagement so that drive is able to be transmitted through the clutch 26.

When the first clutch 26 is to be disengaged, pressure is released from within the piston apply chamber 46 by allowing the fluid within the chamber 46 to be expelled, such that a return spring 50 is able to move the piston 42 back to its returned position wherein the discs of the first clutch 26 are disengaged (as shown in the Figures). The return spring 50 is mounted at one end to a wall 52 which is fixed relative to the clutch and piston housing 38. An opposite end of the spring 50 abuts a return side 53 of the piston 42. The wall 52 and the piston 42 are each fitted with O rings 54 so as to provide a fluid tight seal around those components which are subject to relative movement. It will be understood by those skilled in the art that the drawings in Figures 1 to 3 are cross-sectional representations and that, in its actual physical form, the transmission is generally circular about the axis of rotation 16. As such, the space between the piston 42 and the wall 52 is annular, and there may be a series of springs 50 in the annular space so that it is in practice a spring pack which serves to return the piston 42 to its returned position.

The wall 52 is held in place by a snap ring 49 on one side, and by a lip 51 in the clutch and piston housing 38 on the other side.

In practice, there will always be fluid on the apply side 44 of the piston 42 due to leakage and, when the piston and housing 38 are rotating (with drainage at or near the central axis 16 of the transmission 10) the fluid will be flung radially outwardly. Owing to centrifugal pressure exerted by the fluid in the piston apply chamber 46, at some point the. force from the return spring 50 against the return side 53 will be overcome and the piston 42 will start to apply the first clutch 26 inadvertently (ie. the clutch "self-applies"). Accordingly, a dam 56 (or piston return chamber) is provided on the return spring 50 side of the piston 42. Working fluid is present in the dam 56 such that during operation, when the piston and housing 38 are rotating, this working fluid is also subject to centrifugal force. Accordingly, if the surface area of the piston 42 on the return side 53 is equal to the surface area of the piston 42 on the apply side 44, this assists in tuning the centrifugal effects so that the forces due to centrifugal movement of the working fluid on the return side 53 counteract the forces due to centrifugal movement of the fluid in the piston apply chamber 46 to prevent or delay "self-apply" of the clutch 26.

In one previously proposed arrangement, a compensation chamber is provided to prevent unintentional movement of a piston due to centrifugal forces. However, the compensation chamber is subjected to the general supply of working fluid (ie. oil) within the transmission, and thus is also subjected to the changes in pressure of that oil supply. Accordingly, at low speed, angular rotation of the unit may be too low such that when the clutch is being applied the difference in pressure on either side of the piston is too large relative to the magnitudes of the relevant pressures. As such, as pressure in the compensation chamber will be subject to great fluctuations, this can lead to lack of control in operation of the clutch which may result in unpredictable or jerky engagement/disengagement of the clutch.

In accordance with the present examples shown in the drawings, the dam 56 is sealed from other working fluids of the transmission so that the working fluid (eg. oil) in the dam 56 is not subject to the same changes in pressure as the working fluids in the

remainder of the transmission 10. Instead, the dam 56 is fed from either piston apply, lubrication oil or leakage oil. Also, the dam 56 is vented back to atmosphere via a passage 58 which terminates in an opening 60 in an outside surface of the housing 18, so that a stable reference pressure is used.

In the design shown in Figures 1 and 2, the volume of the transmission outside of the piston 42 and dam 56 is filled with oil (indicated generally in Figure 1 by reference numeral 59). The pressure within that volume may possibly be higher than atmosphere, in addition to centrifugal pressure when the transmission is rotating about axis 16. The centrifugal pressure itself in that volume may also be high as the radial column height relevant to pressure internal of the housing 18 potentially begins at location 61 at or near the central axis 16. Accordingly, if not sealed, the pressure in the dam 56 may reach a level higher than that in the piston apply chamber 46. By sealing of the dam 56 from the remainder of the internal volume of the housing 18, the dam 56 is isolated from such pressures. Also, as the radial column height for the dam 56 begins at the opening 60, which is spaced radially outwardly from the axis 16, the effect of centrifugal pressure within the dam 56 will be less than the effect in the remainder of the internal volume of the housing 18. The column of centrifugal oil for the piston 42 begins at location 63.

The remainder of the internal volume of the housing 18 may also be subjected to large fluctuations in pressure owing to other apparatus being operated within that volume which may include, in other examples, a torque converter or other such devices. Due to the sealing of the dam 56 from the remainder of the internal volume of the housing 18, in combination with the venting of the dam 56 to a stable pressure, pressure within the dam 56 is isolated from the change in pressure environment of the general assembly of the transmission 10.

Although the examples shown in Figures 1 and 2 show the dam 56 being vented to the atmosphere external to the housing 18, other stable reference pressures may be used such as, for example, a lube pressure supply or a line pressure supply.

With regard to neutralising the effect of the centrifugal forces on either side of the piston 42, as the surface area on the return spring side 53 of the piston 42 is smaller than

the surface area of the apply side 44 of the piston 42, the piston 42 will in the example shown self-apply at some angular velocity of the housing 18 about the axis 16, however the arrangement is tuned to do this outside of the operating range.

A similar arrangement is shown in Figure 3, in which similar features are identified with similar reference numerals.

Both example transmissions shown also include a one-way clutch 62 which facilitates shifting between gears as the drive path for the first gear is able to be overrun when the transmission is operating in the second gear. Accordingly, the first clutch 26 need not be disengaged for operation in the second gear, ie. with operation of the one-way clutch 62 the transmission can be operated in the second gear with both the first clutch 26 and second clutch 30 engaged.

The above transmissions have been described by way of example only and modifications are possible within the scope of the invention.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that that prior art forms part of the common general knowledge in Australia.

Throughout this specification, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.