GB2424041A

GB2424041A - Hydraulic system

Info

Publication number: GB2424041A
Application number: GB0505022A
Authority: GB
Inventors: Robert William Beaven
Original assignee: Automotive Motion Technology Ltd
Current assignee: Automotive Motion Technology Ltd
Priority date: 2005-03-11
Filing date: 2005-03-11
Publication date: 2006-09-13
Also published as: GB0505022D0

Abstract

A hydraulic system (10) including an actuator (12), a pump (14) having an inlet (22) connected to a reservoir of hydraulic fluid (32) and an outlet (18), a first conduit (16) connecting the outlet (18) of the pump (14) to the actuator (12), and a second conduit (20) which connects the outlet (18) of the pump (14) and the reservoir (32), the second conduit (20) including a valve assembly (24), wherein the valve assembly (24) is adapted to move into a closed position in which flow of fluid along the second conduit (20) is substantially prevented when the pressure of hydraulic fluid required to be supplied to the actuator (12) exceeds a predetermined value. Also claims a locking differential and a transmission system for a vehicle comprising a clutch and a hydraulic system as described above.

Description

Title: Hydraulic System Descrjrjtion of Invention The present invention

relates to a hydraulic system, particularly, but not exclusively to a hydraulic system for an automatic transmission or differential for a motor vehicle.

Where it is desired to actuate electrically an element, such as a clutch in an automatic transmission system or differential for a motor vehicle, it is known to use a hydraulic system in which an electrically operated pump, otherwise known as a power pack, supplies the necessary pressurised hydraulic fluid on demand. In order for the automatic transmission to operate as required it is necessary for the power pack to provide pressurised hydraulic fluid over a very wide range of flow rates and pressures, and it is generally not possible to **S.

provide a pump which is sufficiently compact and which operates efficiently *. S. : . 15 over such a range of conditions.

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It is therefore known for the hydraulic system to include two pumps, a low pressure pump which is particularly suitable for supplying low pressure hydraulic fluid, and a booster pump which may be operated to increase the pressure of hydraulic fluid in the system when pressures at the high end of the range are required. The high pressure pump may be supplemented or replaced by an accumulator which may be charged by the low pressure pump or where present, by the high pressure pump. Such a system is described in US66331 651.

The presence of a second pump or an accumulator inevitably increases the size, complexity, and hence cost, of the hydraulic system.

According to a first aspect of the invention we provide a hydraulic system including an actuator, a pump having an inlet connected to a reservoir of hydraulic fluid and an outlet, a first conduit connecting the outlet of the pump to the actuator, and a second conduit which connects the outlet of the pump A11299G6 11/03/05 and the reservoir, the second conduit including a valve assembly, wherein the valve assembly is adapted to move into a closed position in which flow of fluid along the second conduit is substantially prevented when the pressure of hydraulic fluid required to be supplied to the actuator exceeds a predetermined value.

By virtue of the invention, hydraulic fluid may be supplied to the actuator at a wide range of pressures or flow rates using a pump which is adapted to pump fluid at high pressures and flow rates.

Preferably the actuator is a clutch actuator.

In this case, the clutch may be for use an automatic transmission in a motor vehicle. More specifically, the clutch may be for use in a twin clutch *II.

transmission. *, *1

* Alternatively, the clutch may be for use in a locking differential. **.

The pump may be a screw pump.

Preferably the pump is driven by an electric motor.

The valve assembly may include a valve member which engages with a valve seat substantially to block the second conduit when the rate of flow of hydraulic fluid through the second conduit exceeds a predetermined value.

The valve assembly may include an electrically operated valve, in which case the system may include an electric or electronic control unit which is adapted to operate the electrically operated valve to bring the valve assembly into the closed position when the pressure or flow rate at which hydraulic fluid is required to be supplied to the actuator exceeds a predetermined value.

According to a second aspect of the invention we provide a transmission system for a motor vehicle comprising a clutch, and a hydraulic system A11299GB 11/03/05 including a hydraulic clutch actuator, a pump having an inlet connected to a reservoir of hydraulic fluid and an outlet, a first conduit connecting the outlet of the pump to the clutch actuator, and a second conduit which connects the outlet of the pump and the reservoir, the second conduit including a valve assembly, wherein the valve assembly is adapted to move into a closed position in which flow of fluid along the second conduit is substantially prevented when the pressure of hydraulic fluid required to be supplied to the clutch actuator exceeds a predetermined value.

The hydraulic system in the transmission system of the second aspect of the invention may include any of the features of the hydraulic system according to the first aspect of the invention. I.

According to a third aspect of the invention we provide a locking differential comprising a clutch, and a hydraulic system including a hydraulic clutch actuator, a pump having an inlet connected to a reservoir of hydraulic fluid and . 15 an outlet, a first conduit connecting the outlet of the pump to the clutch actuator, and a second conduit which connects the outlet of the pump and the a. ..* reservoir, the second conduit including a valve assembly, wherein the valve assembly is adapted to move into a closed position in which flow of fluid along the second conduit is substantially prevented when the pressure of hydraulic fluid required to be supplied to the clutch actuator exceeds a predetermined value.

The hydraulic system in the locking differential of the second aspect of the invention may include any of the features of the hydraulic system according to the first aspect of the invention.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings of which, FIGURE 1 is a schematic illustration of a hydraulic system according to the first aspect of the invention, and A11299GB 11/03/05 FIGURE 2 is an illustration of the valve assembly of the hydraulic system shown in Figure 1.

Referring now to the figures, there is shown a hydraulic system 10 including an actuator 12, a pump 14 having an inlet 22 connected to a reservoir of hydraulic fluid 32 and an outlet 18, a first conduit 16 connecting the outlet 18 of the pump 14 to the actuator 12, and a second conduit 20 which connects the outlet 18 of the pump 14 and the inlet 22 of the pump 14 so that fluid from the outlet 18 may be returned to the inlet 22 via the second conduit 20.

In this example, the actuator 12 comprises a piston 12a which is moveable in a cylinder 12b, the piston 12a being connected to a clutch (not shown), such : * that movement of the piston 12a within the cylinder 12b causes the clutch to engage or disengage. It will be appreciated that the actuator 12 need not comprise a piston and cylinder arrangement, and may be any other form of hydraulically operated actuator.

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* 15 In this example, the clutch is incorporated into an automatic transmission system for a motor vehicle, and more specifically is one of the clutches in a twin clutch transmission. The clutch may, alternatively, be a used in a front, rear or centre locking differential for a motor vehicle.

The pump 12 in this example is an electrically operated screw pump, i.e. a pump which comprises a housing in which are mounted two or more rotors each of which carry a generally a generally helical screw thread. The screw threads of the rotors mesh, such that rotation of one of the rotors in the housing causes the other rotor to rotate. One of the rotors is connected to an electric motor such that operation of the motor causes the rotor to rotate, and the consequent meshing and unmeshing of the screw threads causes fluid to be pumped from one end of the rotors to the other. The pump may be of the type disclosed in co-pending UK patent applications 0310509.5, 0310512.9, 0310591.3, 0310516.0, O3l0518.6or03105921 A11299GB 11/03/05 The pump 14 is driven by an electric motor, operation of which is controlled by an electric or electronic controller.

The second conduit 20 includes a valve assembly 24. In this example the valve assembly 24 comprises a velocity fuse and includes a valve element 30 which may be moved to engage with a valve seat 28 to bring the valve assembly 24 into a closed position in which flow of fluid along the second conduit 20 is substantially prevented. The valve assembly 24 is configured such that the valve member 30 moves into engagement with the valve seat 28 when the rate of flow of hydraulic fluid through the valve assemblyexceecjs a predetermined value. It should be appreciated, however that an alternative valve assembly could be used in which the valve member 30 moves into engagement with the valve seat 28 when the pressure of fluid in the valve assembly exceeds a predetermined value.

The valve assembly 24 is illustrated in detail in Figure 2 and includes a tubular, generally cylindrical, valve body 34 having a first end 34a and a second end 34b through which hydraulic fluid may flow. The valve assembly S..

is mounted in the second conduit 20 such that the first end 34a of the valve body 34 is at the pump outlet 18 side, and the second end 34b is at the pump inlet 22 side. A radially inwardly extending formation of the valve body 34 provides the valve seat 28, and the valve member 30 is movable within the valve body parallel to a longitudinal axis A of the valve body 34.

The valve member 30 includes a central stem 30a which extends along the longitudinal axis A between an sealing portion 30b, which is adapted to engage with the valve seat 28, and a flow restrictor plate 30c which engages with the inner surface of the valve body 34 and which includes a plurality of apertures. Thus, fluid flowing through the valve assembly 24 must pass through the apertures in the restrictor plate 30c, and the restriction to fluid flow causes fluid pressure build-up up-stream of the restrictor plate 30c.

Al 1299GB 11/03/05 The valve member 30 is biased into a position in which the sealing portion 30b is spaced from the valve seat 28 by means of a helical compression spring 36 as illustrated in Figure 2a. The helical compression spring 36 and the sealing part 3Db are arranged towards the second end 34b of the valve body 34 with respect to the restrictor plate 30c. If fluid is flowing through the valve body 34 from the first end 34a of the valve body 34 to the second end 34b, I e. from the restrictor plate 30c towards the sealing part 3Db, fluid pressure build-up up-stream of the restrictor plate 30c results in a force on the valve member 30 which opposes the biasing force of the spring 36. The apertures in the restrictor plate 30c are configured such that when rate of fluid flow through the valve assembly 24 exceeds a predetermined value, the pressure build-up up- stream of the restrictor plate 30c becomes sufficient to overcome the biasing force of the spring 36, and the valve member 30 moves in the direction of fluid flow until the sealing part 3Db engages with the valve seat 28, as illustrated in S...

* . 15 Figure2b S...

In an alternative embodiment of the invention the valve assembly 34 may include an electrically operated valve. Such electrically operated valves are well known, and may, for example, be operated by means of a solenoid. In S.....

:." this case the system 10 would include a controller which is adapted to operate * 20 the valve to move the valve member 28 into its closed position when the pressure of hydraulic fluid required to be supplied to the actuator 12 exceeds a predetermined value.

When the hydraulic system described above is used to actuate a clutch in a locking differential or twin clutch transmission for a motor vehicle, it is necessary for hydraulic fluid to be supplied to and drawn from the actuator 12 over a wide range of flow rates and pressures. For example, where the clutch is used in a locking differential, the requirements may be as follows: 1) On vehicle ignition, fluid supply to the actuator 12 at a volume of less than 2Oml in less than 1 second is required, in order to charge the cylinder 12b with A11299GB 11/03/05 hydraulic fluid, and bring the clutch into a standby condition i.e. with clutch pads around 0.3-0.4mm apart, 2) In order to maintain the clutch in a standby condition, fluid pressure in the actuator 12 must be maintained at around 1-2 bar with zero fluid displacement, 3) When the differential is required to become active and the clutch pads brought together to lock the differential, it is necessary to supply less than lmI of fluid to the actuator 12 in less than 100 milliseconds, 4) To maintain the differential in its locked condition, it is necessary to maintain the pressure in the actuator 12 at up to around 50 bar with zero fluid displacement, 5) In order to release the clutch in either the recovery phase after locking of the differential or in the case of an emergency such as sudden application of the vehicle brakes, it is necessary to discharge the pressure in the actuator * 15 from any pressure up to around 50 bar in around 50 milliseconds, 6) If it is required for the differential to be locked again after discharge as described in 5 above, up to 20 ml of fluid must be supplied to the actuator 12 in less than 100 milliseconds to bring the fluid pressure in the actuator 12 up to a maximum of around 50 bar.

In order to achieve controlled discharge of fluid from the actuator 12, the pump 14 must be operated in reverse mode to pump fluid from the outlet 18 to the inlet 23 and return the fluid to the reservoir 32. Thus, it will be appreciated that, to achieve the desired operation of the locking differential, the pump 14 must be capable of pumping fluid at a wide range of flow rates and pressures.

Unfortunately, pumps do not generally work efficiently over such a range of conditions, and the screw pump used in the present embodiment of the invention is particularly suitable for pumping fluid at high flow rates. If such a A11299GB 11/03/05 pump 12 were to be operated at the speed required to produce the flow specified in point 1 or 2 above, the speed would be so low that the motor driving the pump 12 is likely to stall. Clearly this is undesirable as it could result in unacceptable delay if locking of the differential happens to be required just after stalling of the motor.

Thus, where the fluid pressure at the actuator 12 is required to be up to 2 or 3 bar, the motor drives the pump 14 at a sufficiently low rate that the resultant fluid flow rate through the valve assembly 24 is not sufficiently high to cause the sealing part 30b to engage with the valve seat 28. Thus flow of fluid through the second conduit 20 from the outlet 18 side of the valve assembly 24 to the inlet 22 side is permitted and there is leakage of pumped fluid from the outlet 18 of the pump 14 back to the fluid reservoir 32 and hence to the * inlet 22 of the pump. This leakage reduces the rate of flow of fluid to the :.: actuator 12 so that the required low pressure level can be achieved whilst **** permitting the pump 14 to be operated at sufficiently high speeds to function effectively. ***

* Where the fluid pressure at the actuator 12 is required to exceed a predetermined value, such as 2 or 3 bar, the motor drives the pump 14 at a sufficiently high speed that the resultant fluid flow rate through the valve assembly 24 is sufficiently high to overcome the biasing force of the spring 36 and to cause the sealing part 30b to engage with the valve seat 28. Thus flow of fluid through the second conduit 20 from the outlet 18 side of the valve assembly 24 to the inlet 22 side is substantially prevented, and leakage of pumped fluid from the outlet 18 of the pump 14 back to the fluid reservoir 32 is prevented, and the required high pressures at the actuator 12 can be achieved.

It may be desired for the valve assembly 24 to be in its closed position during initial charging of the actuator 12, so that a specified volume of hydraulic fluid may be transferred to the actuator 12 before leakage along the second Al 1299GB 11/03/05 conduit 20 occurs. In order to achieve this, the valve assembly may be adapted to move to its closed position when the pressure of hydraulic fluid required to be supplied to the actuator 12 is less than 0.5 bar, to open when the pressure of hydraulic fluid required to be supplied to the actuator 12 is more than 0. 5 bar, and to close again when the pressure of hydraulic fluid required to be supplied to the actuator 12 is more than 2-3 bar When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a :.: means for performing the disclosed function, or a method or process for s attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse : forms thereof.

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A11299GB 11/03/05

Claims

1. A hydraulic system including an actuator, a pump having an inlet connected to a reservoir of hydraulic fluid and an outlet, a first conduit connecting the outlet of the pump to the actuator, and a second conduit which connects the outlet of the pump and the reservoir, the second conduit including a valve assembly, wherein the valve assembly is adapted to move into a closed position in which flow of fluid along the second conduit is substantially prevented when the pressure of hydraulic fluid required to be supplied to the actuator exceeds a predetermined value.

2. A hydraulic system according to claim I wherein the actuator is a clutch actuator.

3. A hydraulic system according to claim 2 wherein the clutch is for use an e.

automatic transmission in a motor vehicle.

4. A hydraulic system according to claim 3 wherein the clutch is for use in * 15 a twin clutch transmission.

5. A hydraulic system according to claim 2 wherein the clutch is for use in a locking differential in a motor vehicle.

6. A hydraulic system according to any preceding claim wherein the pump is a screw pump.

7. A hydraulic system according to any preceding claim wherein the pump is driven by an electric motor.

8. A hydraulic system according to any preceding claim wherein the valve assembly includes a valve member which engages with a valve seat substantially to block the second conduit when the rate of flow of hydraulic fluid through the second conduit exceeds a predetermined value.

A11299G8 11/03/05

9. A hydraulic system according to any one of claims 1 to 7 wherein the valve assembly includes an electrically operated valve.

10. A hydraulic system according to claim 9 wherein the system includes an electric or electronic control unit which is adapted to operate the electrically operated valve to bring the valve assembly into the closed position when the pressure or flow rate at which hydraulic fluid is required to be supplied to the actuator exceeds a predetermined value.

11. A transmission system for a motor vehicle comprising a clutch, and a hydraulic system including a hydraulic clutch actuator, a pump having an inlet connected to a reservoir of hydraulic fluid and an outlet, a first conduit connecting the outlet of the pump to the clutch actuator, and a second conduit which connects the outlet of the pump and the reservoir, the second conduit including a valve assembly, wherein the valve assembly is adapted to move I... . . . . . * * into a closed position in which flow of fluid along the second conduit is substantially prevented when the pressure of hydraulic fluid required to be * supplied to the actuator exceeds a predetermined value. S..

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12. A transmission system according to claim 11 wherein the hydraulic system is in accordance with any one of claims 2 to 10.

13. A locking differential for a motor vehicle comprising a clutch, and a hydraulic system including a hydraulic clutch actuator, a pump having an inlet connected to a reservoir of hydraulic fluid and an outlet, a first conduit connecting the outlet of the pump to the clutch actuator, and a second conduit which connects the outlet of the pump and the reservoir, the second conduit including a valve assembly, wherein the valve assembly is adapted to move into a closed position in which flow of fluid along the second conduit is substantially prevented when the pressure of hydraulic fluid required to be supplied to the actuator exceeds a predetermined value.

A11299GB 11/03/05

14. A locking differential according to claim 13 wherein the hydraulic system is in accordance with any one of claims 2 to 10.

15. A hydraulic system substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

16. A locking differential substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

17. A transmission system substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

18. Any novel feature or novel combination of features described herein and/or in the accompanying drawings. * ** * I S S.. I II.. * I I... I. IS * I S * S

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A11299c33 11/03/05