GB2360557A

GB2360557A - Transmission actuation system with actuators and valves having a common housing

Info

Publication number: GB2360557A
Application number: GB0105186A
Authority: GB
Inventors: David Anthony Harries
Original assignee: LuK Lamellen und Kupplungsbau GmbH
Current assignee: LuK Lamellen und Kupplungsbau GmbH
Priority date: 2000-03-04
Filing date: 2001-03-02
Publication date: 2001-09-26
Anticipated expiration: 2021-03-02
Also published as: GB0005186D0; FR2805876B1; GB0105186D0; FR2805876A1; DE10102030A1; GB2360557B

Abstract

A hydraulic actuation system, eg for an automatic transmission, comprises a clutch actuator 22, gear engagement actuators 114, 115, a main control valve 120 and gear engagement control valves 144, 146. The actuators 22, 114, 115 and control valves 120, 144, 146 are mounted in a common housing to form a valve/actuator pack. This construction enables the hydraulic connections between the valves/actuators to be as short as possible and made of a non-compliant nature of the connections, any excess pressure in working chamber 118 will be dissipated by infinitesimal movement of the actuators 114, 115. Main control valve 120 selectively connects the actuator 22 to a pump 275 or to the reservoir 278. Engagement control valves 144, 146 selectively connects working chambers 11, 119 of the actuators 114, 115 to the main control valve 120 or to the reservoir 278.

Description

2360557 HYDRAULIC ACTUATION SYSTEMS This invention relates to hydraulic

actuation systems and in particular hydraulic actuation systems for automated transmission systems and such transmission systems.

In automated transmission systems of, for example, the type disclosed in W097/0541 0 or W097/40300, whose content is expressly incorporated in the disclosure content of the present application, fluid pressure actuators are used to control actuation of a clutch actuator mechanism and/or a gear engaging mechanism. In accordance with W097/0541 0, separate control valves are used to control the clutch actuator mechanism and the gear engaging mechanism.

W097140300 discloses a hydraulic actuation system in which a main control valve controls both the clutch actuation mechanism and, together with secondary valves, shift and select actuators of a gear engaging mechanism. The use of a single main control valve in this manner reduces the number of components, providing savings in the overall size and cost of the system. The design of the master control valve is however significantly more complicated, which reduces the cost savings.

Furthermore, with the systems disclosed hitherto, the main control valve is packaged as a powerpack together with a source of hydraulic fluid under pressure and a reservoir. The powerpack is mounted remote from the shift and select actuators and associated valves, which are mounted on or adjacent the gearbox. The powerpack is typically connected to the shift and select actuators and associated valves by elastomeric hoses, the compliance of which presents problems to the operation of the hydraulic system.

1 In operation during a gear change, in order to ensure that the gear remains engaged while the clutch is re-engaged, when the gear selector and shift actuators are in the appropriate positions for the required gear ratio, the shift and select actuators are closed, thereby providing a hydraulic lock.

Once the clutch has been re-engaged it is desirable to release pressure in the actuators. In order to achieve this the pressure on the side of the actuator controlled directly by the main control valve must be released first. The relatively low compliance of the connection between the select and shift actuators and the associated select and shift valves will mean that the pressures across the actuators will equalise with very little movement of the actuators. If, on the other hand, the side of the actuators controlled by the shift and select valves is released first, then because of the high compliance of the main supply line, there will be appreciable movement of the actuators. This is not a problem with the select actuator where movement will be restricted except when in the neutral plane. However, movement of the shift actuator in this manner may cause the transmission to jump out of gear. With the actuating systems proposed hitherto, accurate control of the main control valve and select and shift control valves is required to ensure the correct depressurisation sequence.

The objective of the invention is to create an actuating system which is simple in design and which does not feature the disadvantages of the prior a rt.

According to one aspect of the present invention, a hydraulic actuation system for an automated transmission system comprises:

a hydraulic clutch actuator for controlling engagement of a clutch; a gear engagement actuator for controlling engagement of a gear, said gear engagement actuator comprising a double acting ram including first and second working chambers acting on opposite sides of a piston; 2 a main control valve, said main control valve selectively connecting the clutch actuator to a source of hydraulic fluid under pressure or to a reservoir, said main control valve also controlling connection of a gear engagement control valve to the source of hydraulic fluid under pressure; said gear engagement control valve selectively connecting the first and second working chambers of the gear engagement actuator to the main control valve or to the reservoir; and the connections between the main control valve and the gear engagement control valve and between the gear engagement control valve and the gear engagement actuator being substantially non-compliant.

In accordance with the preferred embodiment of the invention, the main control valve is movable between:

a) a first position in which the clutch actuator is connected to the reservoir and connection of the gear engagement control valve to the source of hydraulic fluid under pressure is closed; b) a second position in which the clutch actuator is connected to the reservoir and the gear engagement control valve actuator is connected to the source of hydraulic fluid under pressure; and c) a third position in which both the clutch actuator and gear engagement control valve are connected to the source of hydraulic fluid under pressure.

Preferably the gear engagement control valve is movable between:

a) a first position in which the first and second working chambers of the gear engagement actuator and the connection to the main control valve are all connected to the reservoir; b) a second position in which the first working chamber of the gear engagement actuator is connected to the main control valve and the second chamber of the gear engagement actuator is connected to the reservoir; 3 c) a third position in which the first working chamber of the gear engagement actuator is connected to the main control valve and the second working chamber is closed; and d) a fourth position in which the first and second working chambers of the gear engagement actuator are connected to the main control valve.

The present invention enables a common main control valve to be used for actuation of a clutch actuator and a gear engagement actuator, while permitting de-pressurisation of the gear engagement actuator without movement of the gear engagement actuator to an extent which may cause disengagement of the engaged gear.

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

Figure 1 shows diagrammatically a semi-automated transmission system utilising a hydraulic actuation system in accordance with the present invention; Figure 2 shows a gear selector mechanism and associated selector gate of the transmission system illustrated in Fig. 1; Figure 3 illustrates diagrammatically the hydraulic actuation system of the transmission system illustrated in Fig. 1; Figure 4 shows a sectional diagrammatic illustration of the main control valve of the hydraulic actuation systern illustrated in Fig. 3, in an energised second position; 4 Figure 5 shows a view similar to Fig. 4 of the main control valve in an energised third position; Figure 6 shows a sectional diagrammatic illustration of the gear shift control valve of the hydraulic actuation system illustrated in Fig. 3, in an energised second position; Figure 7 shows a view similar to Fig. 6 with the gear shift control valve in an energised third position; Figure 8 shows a view similar to Fig. 6 of the gear shift control valve in an energised fourth position; and Figure 9 shows a view similar to Fig. 3 illustrating an alternative embodiment of the hydraulic actuation system according to the present invention.

Figure 1 of the accompanying drawings shows an engine 10 with a starter and associated starter circuit 1 Oa which is coupled through the clutch 14, such as a main drive friction clutch to a multi-speed gearbox 12, such as a synchromesh gearbox, for example, via a gearbox input shaft 15. Fuel is supplied to the engine by a throttle 16 which includes a throttle valve 18, operated by accelerator pedal 19. The invention is equally applicable to electronic or mechanical fuel injection petrol or diesel engine.

The clutch 14 is actuated by a release fork 20 which is operated by a hydraulic slave cylinder 22, under the control of a clutch actuator control means 38.

A gear selector lever 24 operates in a gate 50 having two limbs 51 and 52 joined by a cross track 53 extending between the end of limb 52 and intermediate of the ends of limb 51. The gate 50 defines five positions; "R" at the end of limb 52; "N" intermediate of the ends of the cross track 53; "S" at the junction of limb 51 with the cross track 53; and " + " and - at the extremities of limb 51. In limb 51 the lever 24 is biased to the central "S" position. The "N" position of the selector lever 24 corresponds to neutral; W' corresponds to selection of reverse gear; "S" corresponds to selection of a forward drive mode; momentary movement of the lever to the " + " position provides a command to cause the gearbox to shift up one gear ratio; and momentary movement of the gear lever 24 to the "-" position provides a command to cause the gearbox to shift down one gear ratio.

The positions of the lever 24 are sensed by a series of sensors, for example micro switches or optical sensors, positioned around the gate 50.

Signals from the sensors are fed to an electronic control unit 36. An output from the control unit 36 controls a gear engaging mechanism 25, which engages the gear ratios of the gearbox 12, in accordance with movement of the selector lever 24 by the vehicle operator.

In addition to signals from the gear selector lever 24, the control unit 36 receives signals from:

sensor 1 9a indicative of the degree of depression of the accelerator pedal 19; sensor 30 indicative of the degree of opening of the throttle control valve 18; sensor 26 indicative of the engine speed; sensor 42 indicative of the speed of the clutch driven plate; and sensor 34 indicative of the clutch slave cylinder position.

The control unit 36 utilises the signals from these sensors to control actuation of the clutch 14 during take-up from rest and gear changes, for example as described in patent specifications EP003811 3, EP0043660,

6 EP0059035, EP01 01220 and W092/1 3208 whose content is expressly incorporated in the disclosure content of the present application.

In addition to the above mentioned sensors, control unit 36 also receives signals from a vehicle speed sensor 52, ignition switch 54 and brake switch 56 associated with the main braking system, for example the footbrake 58 of the vehicle.

A buzzer 50 is connected to the control unit 36 to warnlindicate to the vehicle operator as certain operating conditions occur. In addition or in place of the buzzer 50 a flashing warning light or other indicating means may be used. A gear indicator 60 is also provided to indicate the gear ratio selected.

As illustrated in Figure 2, the gear engagement mechanism 25 comprises for example at least two, three or four or more shift rails 111, 112,113 mounted parallel to one another for movement in an axial direction. Each shift rail 111, 112,113 is associated with two of the gear ratios of the gearbox 12, via a selector fork and synchromesh unit in conventional manner, so that movement of the shift rails 111, 112,113 in one axial direction will cause engagement of one of the associated gear ratios and axial movement of the shift rail 111, 112,113 in the opposite axial direction will cause engagement of the other associated gear ratio.

Typically; first and second gear ratios are associated with shift rail 111, so that axial movement of the shift rail 111 in a first direction will engage first gear or axial movement of shift rail 111 in a second direction will engage second gear; third and fourth gear ratios are associated with shift rail 112, so that axial movement of shift rail 112 in the first direction will engage third gear or axial movement of shift 112 in a second direction will engage fourth gear; and fifth and reverse gear ratios are associated with shift rail 113, so that axial movement of shift rail 113 in the first direction 7 will engage fifth gear while axial movement of shift rail 113 in the second direction will engage reverse gear.

A selector member 110 is mounted for movement in a select direction X transverse to the axes of the shift rails 111, 112,113 and in a shift direction Y, for movement axially of the shift rails 111, 112 and 113. The selector member 110 may thus be moved in direction X along a neutral plane A-B, so that it may be indexed with and engaged a selected one of the shift rails 111, 112 and 113. The selector member 110 may then be moved in direction Y to move the engaged shift rail 111, 112,113 axially in either direction to engage one of the gear ratios associated therewith.

As illustrated in Fig. 3, selector member 110 is movable in the select direction X by means of a fluid pressure operated select actuator 114, along the neutral plane AB of the gate illustrated in Fig. 2, to align the selector member 110 with one of the shift rails 111, 112,113, and thereby select a pair of gears associated with that shift rail. The selector member 110 may then be moved in the shift direction Y by means of a fluid pressure operated shift actuator 115, to move the shift rail 111, 112,113 axially in either direction to engage one of the gear ratios associated therewith.

The actuators 114 and 115 each comprise a double-acting ram having pistons 116,117 respectively, which divide the actuators 114,115 into two working chambers 118,119, the working chambers 118,119 being disposed on opposite sides of each of the pistons 116,117. Operating rods 11 4a, 11 5a extend from one side of the pistons 116,117 respectively and are operatively connected with the selector member 110 for movement thereof in the select and shift directions X and Y respectively.

As a consequence of the connection of operating rods 11 4a, 11 5a to the pistons 116,117, the working area of pistons 116,117 exposed to 8 working chamber 118 is smaller than the working area of pistons 116,117 exposed to working chamber 119.

A solenoid operated main control valve 120 comprises a housing 122, defining a bore 124. A spool 126 is slidably located in the bore 124, the spool 126 having three axially spaced circumferential lands 128,130,132 which sealing engage the bore 124. A solenoid 134 acts on one end of the spool 126, so that upon energisation of the solenoid 134, the spool 126 is moved axially of the bore 124 against a load applied by a compression spring 136, acting on the opposite end of the spool 126.

An inlet 138 to the bore 124 is connected to a source of hydraulic fluid under pressure in the form of a high pressure accumulator 275. An electrically-d riven pump 223 is provided to charge the accumulator 275 via a non-return valve 276. A pressure transducer 280 measures the pressure in accumulator 275 and, via control unit 36, controls the electrically-d riven pump 223 to maintain the pressure in the accumulator 275 at an appropriate level. An outlet 140 from the bore 124 is connected to a reservoir 278. A first port 142 from bore 124 is connected to select and shift valves 144,146 and a second port 148 is connected to the clutch slave cylinder 22.

The shift and select valves 144,146 are both solenoid operated valves having a housing 150 defining a bore 151 with a spool 152 slidably mounted in the bore 15 1. The spool 15 2 has three axially spaced circumferential lands 154,156,158, the lands sealingly engaging the bore 1. An axial bore 160 opens to end 162 of the spool 152 and connects to a cross-bore 164, the cross-bore 164 opening between lands 154 and 156 of the spool 152. A solenoid 166 acts on end 168 of spool 152 remote from the end 162, so that upon energisation of the solenoid 166, the spool 152 will move axially of the bore 151 against a load applied by a compression spring 170 acting on end 162 of the spool 152.

9 An inlet 172 to the bore 151 is connected to port 142 of the main control valve 120. An outlet 174 from the bore 151 is connected to the reservoir 278. A port 176 of the select valve 144 is connected to the first working chamber 118 of the select actuator 114. Port 176 of shift valve 146 is connected to the first working chamber 118 of the shift actuator 115.

Port 178 of the select valve 144 is connected to the second working chamber 119 of the select actuator 114 and port 178 of shift valve 146 is connected to the second working chamber 119 of shift actuator 115.

The construction and operation of the valves 144 and 146 and actuators 114 and 115 are identical and consequently in the following description, operation of the shift valve 146 and shift actuator 115 only is described.

It will be appreciated that the description in this respect will apply also to operation of the select valve 144 and select actuator 114.

When the transmission is in gear and the clutch 14 engaged, the solenoids 134 and 166 will be de-energised and valves 120,144 and 146 will be in the first positions illustrated in Fig. 3. In this position, the clutch slave cylinder 22 and both working chambers 118, 119 of the select and shift actuators 114,115 will be connected to the reservoir 278.

There will consequently be no movement of the clutch slave cylinder 22 or select and shift actuators 114,115.

When a gear change is initiated by, for example, the driver of the vehicle moving the gear selector lever 24 momentarily to the '+' position, or by automatic initiation, solenoid 134 is energised to move the spool 126 of main control valve 120 to a third position, as illustrated in Figure 5. In this third position both the select and shift valves 144,146 and the clutch slave cylinder 22 are connected to the accumulator 275, via ports 142 and 138. Connection of the clutch slave cylinder 22 to the accumulator 275 will actuate the release fork 20 to disengage the clutch 14.

Simultaneously, with energisation of solenoid 134, solenoids 166 of the select and shift control valves 144,146 are energised to move the spool 152 to a third position as illustrated in Fig. 7. In this position, the land 158 of spool 152 closes port 178 thereby closing working chamber 119 and creating a hydraulic lock preventing movement of the select and shift actuators 114 and 115, even though working chambers 118 thereof are connected to the accumulator 275 by the select and shift valves 144,146 and the main control valve 120.

Upon disengagement of the clutch 14, the solenoids 166 of the select and shift valves 144,146 may be selectively energised, moving the select and shift valves 144,146 between second and fourth positions, in order to disengage the currently selected gear and engage a new gear.

Energisation of solenoid 166 to move the select or shift valve 144,146 to the second position illustrated in Fig. 6, in which working chamber 119 is connected to reservoir 278, while working chamber 118 is connected to the accumulator 275, will create a pressure differential across the pistons 116 and 117, causing the operating rod 11 4a, 11 5a to extend.

Energisation of solenoid 166 to move the select or shift valve 144,146 to the fourth position illustrated in Fig. 8, in which both working chambers 118 and 119 are connected to the accumulator 275, will cause the operating rods 11 4a, 11 5a to retract, due to the differential working areas of the pistons 116 and 117. Consequently, by appropriate control solenoids 166 of the select and shift valves 144,146, the selector member 110 may be moved to engage the desired gear.

Potentiometers 226 and 227 are connected to the operating rods 1 14a, 11 5a respectively, to provide signals indicative of the position of the associated operating rods. Signals from the potentiometers 226,227 are fed to the control unit 36 to provide an indication of the position of the operating rods 11 4a, 11 5a, for each of the gear ratios of the gear box 12 11 and also to indicate the position of the operating rod 11 5a, when the selector member 110 is in the neutral plane A-B of Fig. 2. The transmission system may thus be calibrated, so that predetermined position signals from the potentiometers 226 and 227 correspond to engagement of each of the gear ratios of the gear box 12.

Measurements from the potentiometers 226 and 227 may thus be used by a closed loop control system to control valves 144 and 146, to move the operating rods 11 4a and 11 5a, to the predetermined positions to engage the desired gear ratio.

When the desired gear ratio has been engaged, the solenoids 166 of the select and shift valves 144,146 are energised to move the valves 144,146 back to their third positions, closing the ports 178 and creating a hydraulic lock preventing movement of the actuators 114,115.

Solenoid 134 of the main control valve 120 may then be energised to move the main control valve 120 from its second to its third position, thereby allowing fluid from the clutch slave cylinder 22 to be returned to the reservoir 278, permitting re-engagement of the clutch 14. The main control valve 120 will be switched between the second and third positions, so that the clutch 14 is re-engaged in controlled manner, for example as disclosed in EP0038113; EP0043660; EP0059035; EPO 10 1220 or W092/13208.

When the clutch 14 has been re-engaged, solenoid 134 of the master control valve 120 may be de-energised, so that it returns to the first position illustrated in Fig. 3. Similarly the solenoids 166 of the shift and select valves 144,146 may be de-energised. Movement of the select and shift valves 144,146 to the first position illustrated in Fig. 3 will open working chamber 119 to reservoir 278, thereby releasing pressure therein.

Due to the non-compliant nature of the connections between the main 12 control valve 120 and select and shift valves 144,146 and between the select and shift valves 144,146 and the select and shift actuators 114,115, the excess pressure in the working chamber 118 will be dissipated by infinitesimal movement of the actuator 114,115, such movement being insufficient to cause disengagement of the gear. Finally, pressure in the line between the main control valve 120 and the select and shift valves 144,146 will be released when the select and shift valves 144,146 reach the first position and the main control valve 120 is connected to the reservoir 278, via bores 164 and 160.

In order to minimise compliance in the connections between the main control valve 120 and the select and shift valves 144,146 and between the select and shift valves 144,146 and the select and shift actuators 114,115, these connections should be as short as possible and made of non-compliant material. According to a preferred embodiment of the invention, the bores 124 and 151 of the main control valve 120 and the select and shift valves 144,146 and also of the select and shift actuators 114,115 may be def ined by a common housing, the bores 124,151 of the various components being appropriately inter-connected by passages through the common housing. The valve/actuator pack so formed would be mounted on or adjacent the gearbox 12.

The electrically driven pump 223, accumulator 275, reservoir 276 and control unit 36 may also be mounted with the valve/actuator pack or may be mounted remotely thereof and inter-connected thereto by, for example, elastomeric pressure hoses.

In the alternative embodiment illustrated in Fig. 9, the select valve 144 is provided with an additional port 190 which is positioned such that when valve 144 is in its de-energised first position, it will be open to reservoir 278, via bores 164 and 160 but will be closed by land 154, upon initial movement of the valve 144 from its first position, before the valve 144 13 reaches its second position. The port 190 is connected to the accumulator charging line, between the electrically driven pump 223 and non-return valve 276.

With this embodiment, if the electrically driven pump 223 is started up when the select valve 144 is de-energised, the output from the pump 223 is dumped straight back to the reservoir 278. The electrically driven pump 223 may thus be started under low load conditions, the select valve 144 being moved to close port 190 when charging of the accumulator 275 is required. This has the advantage that a lower powered electric motor 277 may be used to drive the pump 223. Also, when charging of the accumulator 275 is complete, the charging circuit may be depressurised through the select valve 144, thereby reducing any tendency for that part of the circuit to leak under pressure.

It will be appreciated that while in the above embodiment the select valve 144 has been modified to provide the pump dump facility, the shift valve 146 could alternatively be modified.

Various modifications may be made without departing from the invention.

For example, while in the above embodiment the hydraulic circuit has been described with reference to a semi-automated transmission system, the invention is equally applicable to fully-automated transmission systems or to automated manual transmission systems.

Hydraulic actuation system for an automated transmission system including a hydraulic clutch actuator 22 for controlling engagement of a clutch 14, a gear engagement actuator 114,115 for controlling engagement of a gear, whereby the gear engagement actuator 114,115 features the form of a double acting ram, having first and second working chambers 118,119, acting on opposite sides of a piston 116,117, a main control valve 120, whereby the main control valve 120 selectively 14 connects the clutch actuator 22 to a source 275 of hydraulic fluid under pressure or to a reservoir 278, whereby the main control valve 120 also controls connection of a gear engagement control valve 144, 146 with the source 275 of hydraulic fluid under pressure; whereby the gear engagement control valve 144,146 selectively connects the first and second working chambers 118,119 of the gear engagement actuator 114,115 to the main control valve 120 or to the reservoir 278; and the connections between the main control valve 120 and the gear engagement control valve 144,146 and between the gear engagement control valve 144,146 and the gear engagement actuator 114,116 are substantially non-compliant.

The patent claims submitted with the application are proposed formulations without prejudice to the achievement of further patent protection. The applicant reserves the right to submit claims for further combinations of characteristics, previously only disclosed in the description and/or drawings.

Reference back used in sub-claims refer to the further development of the subject of the main claim by the characteristics of the respective sub claim; they are not to be understood as a waiver with regard to achieving independent item protection for the combination of characteristics in the related sub-claims.

Since the subject of the sub-claims can form separate and independent inventions with reference to the prior art on the priority date, the applicant reserves the right to make them the subject of independent claims or of division declarations. Furthermore, they may also contain independent inventions which demonstrate a design which is independent of one of the objects of the preceding sub-claims.

The embodiments are not to be considered a restriction of the invention.

Rather, a wide range of amendments and modifications is possible within the scope of the current disclosure, especially those variations, elements and combinations and/or materials which, for example, the expert can leamby combining individual ones together with those in the general description and embodiments in addition to characteristics andlor elements or process stages described in the claims and contained in the drawings with the aim of solving a task thus leading to a new object or new process stages or sequences of process stages via combinable characteristics, even where they concern manufacturing, testing and work processes.

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Claims

1. A hydraulic actuation system for an automated transmission system comprising:

a hydraulic clutch actuator for controlling engagement of a clutch; a gear engagement actuator for controlling engagement of a gear, said gear engagement actuator comprising a double acting ram including first and second working chambers acting on opposite sides of a piston; a main control valve, said main control valve selectively connecting the clutch actuator to a source of hydraulic fluid under pressure or to a reservoir, said main control valve also controlling connection of a gear engagement control valve to the source of hydraulic fluid under pressure; said gear engagement control valve selectively connecting the first and second working chambers of the gear engagement actuator to the main control valve or to the reservoir; and the connections between the main control valve and the gear engagement control valve and between the gear engagement control valve and the gear engagement actuator being substantially non-compliant.

2. A hydraulic actuation system according to claim 1 in which the main control valve is movable between:

a) a first position in which the clutch actuator is connected to the reservoir and connection of the gear engagement control valve to the source of hydraulic fluid under pressure is closed; b) a second position in which the clutch actuator is connected to the reservoir and the gear engagement control valve is connected to the source of hydraulic fluid under pressure; and c) a third position in which both the clutch actuator and gear engagement control valve are connected to the source of hydraulic fluid under pressure.

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3. A hydraulic actuation system according to claim 1 or 2 in which the gear engagement control valve is movable between:

a) a first position in which the first and second working chambers of the gear engagement actuator and the connection to the main control valve are all connected to the reservoir; b) a second position in which the first working chamber of the gear engagement actuator is connected to the main control valve and the second chamber of the gear actuator is connected to the reservoir; c) a third position in which the first working chamber of the gear engagement actuator is connected to the main control valve and the second working chamber is closed; and d) a fourth position in which the first and second working chambers of the gear engagement actuator are connected to the main control valve.

4. A hydraulic actuation system according to any one of the preceding claims in which the gear engagement mechanism includes two gear engagement actuators, a select actuator for moving a selector member in a first direction and a shift actuator for moving a selector member in a second direction, the select and shift actuators having independent gear select and gear shift control valves, the gear select and gear shift control valves selectively connecting the gear select actuator and gear shift actuator respectively, to the main control valve or to the reservoir.

5. A hydraulic actuation system according to any one of claims 1 to 4 in which the main control valve comprises a spool slidably mounted in a bore, the spool having three circumferential lands which sealingly engage the bore, an inlet being provided to the bore for connection to the source of hydraulic fluid under pressure, an outlet from the bore being connected to the reservoir; a first port open to the bore, the first port being connected to the gear engagement control valve and a second port 18 opening to the bore, the second port being connected to the clutch actuator: in a first position of the spool, the first port being isolated from the inlet and outlet and the second port being connected to the outlet; in a second position of the spool, the first port being connected to the inlet and the second port being connected to the outlet; and in a third position of the spool, the first and second ports being connected to the inlet.

6. A hydraulic actuation system according to any one of claims 1 to 5, in which the gear engagement control valve comprises a spool slidably mounted in a bore, the spool having three circumferential lands which sealingly engage the bore, an inlet being provided to the bore for connection to the main control valve; an outlet being provided from the bore for connection to the reservoir; a first port opening to the bore, the first port being connected to a first working chamber of the gear engagement actuator; and a second port opening to the bore, the second port being connected to a second working chamber of the gear engagement actuator; an axial bore opening to one end of the spool, the axial bore connecting with a cross-bore opening between first and second lands of the spool:

in a first position of the spool, the inlet and first port being connected to the outlet via the cross-bore and axial bore and the second port being connected to the outlet; in a second position of the spool, the first port being connected to the inlet and the second port being connected to the outlet, the inlet and first port being isolated from the outlet; in a third position of the spool, the first port being connected to the inlet and the second port being closed by a land of the spool; and in a fourth position of the spool, the first and second ports being connected to the inlet.

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7. A hydraulic actuation system according to claim 6 in which the gear engagement actuator comprises a double-acting ram having a piston, the working area on one side of the piston being greater than that of the other side of the piston, the first port of the gear engagement valve being connected to the side of the piston with the smaller working area.

8. A hydraulic actuation system according to any one of claims 5 to 7 in which the bores of the main control valve and the or each gear engagement control valve are formed in a common housing, interconnection between the main control valve and the gear engagement control valves being provided by passageways through the housing.

9. A hydraulic actuation system according to claim 8 in which bores for the or each gear engagement actuator are formed in the common housing, the interconnections between the gear engagement control valves and the gear engagement actuators also being provided by passageways in the common housing.

10. A hydraulic actuation system according to any one of the preceding claims in which the source of hydraulic fluid under pressure comprises an accumulator, the accumulator being charged by an electrical ly-d riven pump; the gear engagement valve, in a first position, connecting the charging line from the pump to the accumulator to the reservoir, the connection between the charging line and the reservoir being closed on movement of the gear engagement valve from its first position.

11. A hydraulic actuation system substantially as described herein with reference to and as shown in Figs. 1 to 9 of the accompanying drawings.

12. An automated transmission system including a hydraulic actuation system as claimed in any one of claims 1 to 11.