ITMI20012264A1 - HYDRAULIC ACCUMULATORS AND, IN PARTICULAR HYDRAULIC ACCUMULATORS FOR USE IN TRANSMISSION SYSTEMS - AUTOMOTIVE GEARBOXES - Google Patents

Description

DESCRIPTION

The present invention relates to hydraulic accumulators and, in particular, hydraulic accumulators for use in automotive transmission-gearbox systems. Hydraulic accumulators according to the present invention can however be used in other applications.

Automatic transmission-gearbox systems, for example of the type described in W097 / 05410 or W097 / 40300, the content of which is expressly incorporated in the descriptive content of the present application, fluidic actuators are used to control the operation of a clutch actuating mechanism and / or a gear engagement mechanism.

Such systems conventionally use gas accumulators, in which a gas-containing chamber is separated from a hydraulic fluid chamber by a flexible membrane, whereby fluid can be pumped into the accumulator, compressing the gas, so as to store the fluid under pressure. Gas accumulators of this type have the advantage that they follow the laws of gas, whereby the pressure is proportional to the volume. Consequently, the gas accumulator can be designed to store a relatively large volume of fluid without large pressure variations. However, gas accumulators have the drawbacks that:

(a) change in temperature causes changes in stored pressure / volumes;

(b) the permeability of the membrane causes loss of gas pressure, requiring periodic replacement of the diaphragm and / or regular recharging of the accumulator;

(c) the gas, membrane and fluid must be selected for chemical compatibility;

(d) the container must be sturdy (and heavy) and must contain both the fluid and the gas under pressure; And

(e) the flexibility of the system is inconsistent and consequently it is difficult to control the pressure in the system.

In more recent hydraulic drive systems for controlling transmission-shift systems of motor vehicles, for example as described in UK patent applications GB0024999.5 and GB0025000.1, the content of which is expressly incorporated into the descriptive content of this application, is It has been proposed to use spring accumulators instead of a gas accumulator.

Spring accumulators which comprise a piston mounted in a cylinder, the piston sealing the walls of the cylinder and being urged towards the end thereof by spring means have the advantage that the elastic constant and consequently the pressure of the fluid stored in the cylinder The accumulator does not vary significantly with temperature. Furthermore, the use of a piston opposite a diaphragm solves the problem of permeability and simplifies the problem of compatibility, allowing many more combinations of seals and fluids to be used. Spring accumulators also provide consistent flexibility, which is likely to simplify pressure control in the system.

These spring accumulators normally employ a helical compression spring. Since the force / flexion relationship in helical compression springs is linear, a long spring with a large number of turns is required to obtain a high load but with a low spring constant. This significantly increases the weight and size of the accumulator. Since spatial weight constraints are critical in the design of hydraulic drive systems for automotive transmission-shift systems, this represents a significant drawback associated with the use of spring accumulators.

The present invention provides an improvement in the spring accumulator structure which can alleviate this problem.

According to one aspect of the present invention, a hydraulic accumulator comprises a cylinder, a piston slidably mounted in the cylinder, the piston being sealed with respect to the cylinder wall, a rod extending from the piston towards one side of the piston, the rod extending coaxially to the cylinder beyond a end of the cylinder and a leaf spring acting on the end of the rod remote from the piston, the leaf spring urging the piston towards the other end of the cylinder, said other end of the cylinder being closed, and a port being associated with the cylinder adjacent to said other end thereof.

The use of a leaf spring in the manner described above avoids the drawbacks of a helical compression spring, providing a low elastic constant without significantly increasing the overall dimensions and weight of the accumulator. The spring force of the leaf or leaf spring can be varied by modifications of the lamellar element structure, for example the width of the lamella, the thickness, the tapering of the lamella or the use of a plurality of lamellae, as well as by means of the material of which it is made of. The leaf spring may also be designed to provide a variation in the spring constant as it is deflected, or it may be provided with means for adjusting the spring constant.

According to a preferred embodiment of the invention, the leaf spring can be formed as an integral part of a mounting bracket for the accumulator. Furthermore, a pump and a motor for the pump can be placed in a common housing with the accumulator.

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

Figure 1 schematically illustrates a semi-automatic transmission-gear system using a hydraulic drive system according to the present invention;

Figure 2 illustrates a gear selector mechanism and associated transmission system selection passage illustrated in Figure 1;

Figure 3 schematically illustrates the hydraulic actuation system of the transmission system illustrated in Figure 1;

Figure 4 illustrates a schematic sectional representation of the main control valve of the hydraulic actuation system illustrated in Figure 3, in a second energized position;

Figure 5 illustrates a view similar to Figure 4 of the main control valve in a third energized position;

Figure 6 illustrates a view similar to Figure 4 of the main control valve in a fourth energized position;

Figure 7 illustrates a schematic sectional representation of the gear shift control valve of the hydraulic drive system depicted in Figure 3, in an energized zero or neutral position;

Figure 8 illustrates a view similar to Figure 7 with the shift control valve in a third energized position;

9 illustrates a view similar to FIG. 7 of the shift control valve in an energized fourth position, and

10 illustrates a sectional side elevation view of a spring accumulator, pump and pump motor employed in the system illustrated in FIGS. 1 to 9.

Figure 1 of the accompanying drawings shows an internal combustion engine 10 with a starter and associated starting circuit 10a which is coupled through the main drive clutch 14 to a gearbox or gearbox 12 of the synchronous geared shaft type. multiple via an input shaft 15 of the gearbox. Fuel is supplied to the engine by a carburetor 16 which includes a throttle valve 18, operated by the accelerator pedal 19. The invention is also applicable to an electronic or mechanical fuel injection gasoline or diesel engine.

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

A gear selector lever 24 operates in a passage 50 having two flaps 51 and 52 joined by a transverse track 53 extending between the end of the flap 52 and in intermediate positions between the ends of the flap 51. The passage 50 defines 5 positions, namely: "R" at the end of the flap 52; "N" in intermediate procession between the ends of the transverse track 53; "S" at the junction of the flap 51 with the transverse track 53; and "+" and - "at the ends of the flap 51. In the flap 51, the lever 24 is urged to the central" S "position. The" N "position of the selection lever 24 corresponds to neutral or neutral;" R "corresponds to reverse gear selections; "S" corresponds to selections of a forward gear mode; momentary movement of the lever to the "+" position provides a command to cause shifting one gear up, and momentarily shifting the lever of the Shift 24 to the "-" position provides a command for determining downshifting of a gear in the transmission.

The positions of the lever 24 are sensed by a series of sensors, for example microswitches or optical sensors, positioned around the passage 50. Signals from the sensors are fed to an electronic control unit 36. An output from the control unit 36 controls a gear engagement mechanism 25, which engages the transmission ratios or gears of the gearbox 12, in accordance with the movement of the gearshift lever 24 by the driver of the vehicle.

In addition to signals from the shift lever 24, the control unit receives signals from:

a sensor 19a indicative of the degree of lowering of the accelerator pedal 19;

a sensor 30 indicative of the degree of opening of the butterfly control valve 18;

a sensor 26 indicative of the engine light;

a sensor 42 indicative of the speed of the clutch driven disc; And

a sensor 34 indicative of the position of the slave cylinder of the clutch.

The control unit 36 uses the signals from these sensors to control the actuation of the clutch 14 during starting from rest state and gear changes, for example as described in the patent descriptions EP0038113, EP0043660, EP0059035, EP0101220 and WO92 / 13208 whose content is expressly incorporated in the descriptive content of this application.

In addition the aforementioned sensors of the control unit 36 also receives signal from a vehicle speed sensor 52, the ignition switch 54 and brake switch 56 is associated with the main brake system, for example the pedal brake. 58 of the vehicle.

A buzzer 50 is connected to the control unit 36 to signal / indicate to the driver of the vehicle when certain conditions occur. In addition to or in place of the buzzer 50, a flashing warning light or other indicator means may be employed. A gear indicator 60 is also provided to indicate the selected transmission-to-gear ratio.

As illustrated in FIG. 2, the gear engagement mechanism 25 includes three gear change rails 111, 112, 113 mounted parallel to each other to move in an axial direction. Each gearshift rail 111, 112, 113 associated with two of the gear ratios of the gearbox 12, through a selector and synchro-meshing fork unit in a conventional manner, whereby movement of the gearshift rails 111, 1112, 113 of one axial direction will cause engagement of one of the associated gear transmission ratios and axial movement of the gearshift rail 111, 112, 113 in the opposite axial direction will cause engagement of the other associated transmission-gear ratio.

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

A selector element 110 is mounted to move in a selection direction X transverse to the axes of the gear change rails 111, 112, 113 and in a gear change direction Y for axial movement of the gear change rails 111, 112 and 113. The selector element 110 may thus be moved in the Y direction 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 Figure 3, the selector element 110 is movable in the selection direction X by means of a selection actuator 114 operated at fluidic pressure along the neutral plane A-B of the passage illustrated in Figure 2 to align the selection element 110 with a gear change rails 111, 112, 113 and thereby select a pair of gears associated with that gear change rail. The selector element 110 may then be moved in the gear shift direction Y by a fluid pressure actuated shift actuator 115 to move the shift rail 111, 112 and 113 axially in either. direction to engage one of the transmission ratios associated with it.

The actuators 114 and 115 each comprise a double-acting sliding element or slide having pistons 116, 117 respectively which divide the actuators 114, 115 into two working chambers 118, 119, the working chambers 118, 119 being arranged on opposite sides of each of the pistons 116, 117. Actuating rods or rods 114a, 115a extend from one side of the pistons 116, 117 respectively and are operatively connected with the selector element 110 for movement thereof in the selection and gear shift directions X and Y, respectively. As a result of the connection of the drive rods 114a, 115a to the pistons 116, 117, the working or active area of the pistons 116, 117 exposed to the working chamber 118 is smaller than the working area of the pistons 116, 117 exposed to the working chamber 119.

A solenoid operated main control valve 120 includes a housing 122 defining a hole 124. A spool 126 is slidably positioned in the hole 124, the spool 126 having three axially spaced circumferential ribs 128, 130, 132 which sealingly engage the hole 124. A solenoid 134 acts on one end of the spool 126 whereby, upon energization of the solenoid 134, the spool 126 is displaced axially to the hole 124 against a load applied by a compression spring 136 acting on the opposite end of the spool 126.

An inlet 138 to hole 124 of valve 120 is connected to a spring accumulator 275. An electrically operated pump 223 is provided to charge accumulator 275 through a non-return valve 276. An outlet 140 from hole 124 is connected to a tank 278. A first port 142 from the hole 124 is connected to work chambers 118 of the gear selection and change actuators 114, 115 and selectively to work chambers 119 by means of selection and gear change valves 144, 146 and a second port 148 is connected to the slave cylinder 22 of the clutch. A pressure relief valve 280 is provided between the outlet of pump 223 and reservoir 278 to ensure that the pressure being supplied by pump 223 does not exceed a predetermined minimum value.

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 hole 151. The spool 152 has three axially spaced circumferential ribs 154, 156, 158, the ribs sealing the hole 151. An axial hole 160 opens towards the end 162 of the spool 152 and connects to a transverse hole 164, the transverse hole 164 opening between ribs 154 and 156 of the spool 152. A solenoid 166 acts on the end 168 of the spool 152 away from the end 162 whereby, upon energization of the solenoid 166, the spool 152 will move axially to the hole 151 against a load applied by a compression spring 170 acting on one end 162 of the spool 152.

An inlet 172 to port 151 is connected to port 142 of main control valve 120. An outlet 174 from port 151 is connected to reservoir 278. Port 178 of selection valve 144 is connected to second working chamber 119 of the actuator gear 114 and the port 178 of the shift valve 146 is connected to the second working chamber 119 of the shift actuator 115.

The construction of operation of the valves 144 and 146 and of the actuators 114 and 115 are identical to those illustrated in Figures 7 to 9.

When the transmission is in gear and the clutch 14 is engaged, the solenoids 134 and 166 will be de-energized and the valves 120, 144 and 146 will be in the rest positions shown in Figure 3. In this position, the slave cylinder 22 of the clutch is connected via port 148 and outlet 140 of the main control valve to tank 278; the working chambers 118 of the gear selection and shifting actuators 114, 115 will be connected to the reservoir 278 through the inlet 172, and passages 164, 160 and the output 174 of the gear selection and shifting valves 144, 146; and the working chambers 119 of the gear selection and shifting actuators 114, 115 will be connected to the reservoir 278 through the port 178 and the output 174 of the gear selection and shifting valves 144, 146. Consequently, there will be no movement of the slave cylinder 22 of the clutch or of the gear selection and gear change actuators 114, 115.

When a gear shift is initiated, for example, by the driver of the vehicle moving the shift lever 24 momentarily to the '+' position, or by automatically starting the energized solenoid 134 to move the spool 126 of the main control valve 120 to a second position, as shown in Figure 4. In this second position, the working chambers 118 of both the gear selector and shift actuators 114, 115 and the inputs 172 of the gear selector and shift valves 144, 146 are connected to the spring accumulator 275 via port 142 and inlet. In this second position, the slave cylinder 22 of the clutch remains connected to the reservoir 278.

Simultaneously, as energizing the solenoid 134 to move the main control valve 120 to the second position illustrated in FIG. 4, the solenoids 166 of the shift and shift control valves 144, 146 are energized to move the spool 152 to an off position. zero or zero as shown in Figure 7. In this position, the rib 158 of the spool 152 closes the opening 178 so as to close the working chamber 119 and create a hydraulic lock preventing movement of the gear selection and gear change actuators 114 and 115 , although the working chambers 118 thereof are connected to the spring accumulator 275 by the valves 144, 146 and the main control valve 120. The connection of the port 172 with the outlet 174 via the holes 160 and 164 of the valves gear selection and shifting 144, 146 is also closed.

Further excitation of the solenoid 134 to the third position illustrated in Figure 5 will then close the connection between the slave cylinder 22 of the clutch and the reservoir 278 and will open the connection between the slave cylinder 22 of the clutch and the spring accumulator 275, operating the release 20 to disengage the clutch 14.

Upon disengagement of the clutch 14, the solenoid 134 of the main control valve 120 can be energized to move the main control valve back to a fourth position, as illustrated in FIG. 6. In this fourth position, port 148 is isolated. from inlet 138 and outlet 140, whereby the clutch 14 will be fixed in the disengaged position. The solenoids 166 of the shift and shift valves 144, 146 can then be selectively energized, by moving the shift and shift valves 144, 146 between the third and fourth positions, in order to disengage the currently engaged gear and engage a new gear.

Energizing the solenoid 166 to move the shift selector valve 144, 146 to the third position illustrated in FIG. 8, in which the working chamber 119 is connected to the reservoir 278 while the working chamber 118 is connected to the accumulator 275, will create a pressure differential across the pistons 116 and 117, causing the drive rod 114a, 115a to extend. Energizing the solenoid 166 to move the shift selector valve 144, 146 to the fourth position illustrated in FIG. 9, in which both the working chambers 118 and 119 are connected to the accumulator 275, will cause the actuation rods 114a , 115a have to retract, due to the differential working areas of the pistons 116 and 117. Consequently, by means of appropriate control solenoids 166 of the shift and shift valves 144, 146, the selector element 110 can be moved to engage 1 desired gear-gear / a.

Potentiometers 226 and 227 are connected to drive rods 114a, 115a, respectively, to provide signals indicative of the position of the associated drive rods. Signals from potentiometers 226, 227 are fed to the control unit 36 to provide an indication of the position of the drive rods 114a, 115a for each of the gear ratios of the gearbox or gearbox 12 and also to indicate the position of the drive rod. drive 115a when the selection element 110 is in the neutral plane A-B of FIG. 2. The transmission system can thus be calibrated such that predetermined position signals from potentiometers 226 and 227 correspond to engagement of each of the gear ratios of the exchange 12.

Measurements from potentiometers 226 and 227 can thus be employed by a closed loop control system to control valves 144 and 146, to move the drive rods 114a and 115a to predetermined positions to engage the desired gear-to-gear ratio.

When the desired gear ratio has been engaged, the solenoids 166 of the shift and shift valves 144, 146 are energized to move the valves 144, 146 back to their zero positions, closing the ports 178 and creating an impeding hydraulic lock. movement of the actuators 114, 115.

The solenoid 134 of the main control valve 120 can then be energized to move the main control valve 120 from its fourth to its second position, thereby allowing fluid from the clutch slave cylinder 22 to be sent to the reservoir 278 allowing re-engagement of the clutch. 14. The main control valve 120 can be switched between the third and second position, whereby the clutch 14 is re-engaged in a controlled manner for example as described in EP0038113; EP0043660; EP0059035; EP0101220 or WO92 / 13208.

When the clutch 14 has been re-engaged, the solenoid 134 of the main control valve 120 can be de-energized, whereby it will return to the rest position shown in FIG. 3. Similarly, the solenoids 166 of the shift and selector valves 144, 146 they can be de-energized. Movement of the gear selector and change valves 144, 146 to the rest position illustrated in FIG. 3 will open the working chamber 119 to the reservoir 278, thereby releasing the pressure therein.

During gear selection with the main control valve 120 in the fourth position, illustrated in Figure 6, the force applied by the gear selector and shift actuators 114, 115 can be controlled by controlling the fluid pressure applied by the pump 223 which in turn time is controlled by the current of the pump motor, employing pulse width modulation, alternatively, the pressure in the system can be controlled by switching the pump 223 to on and off states at predetermined times or by suitable manipulation of the valves 144 and 146 of proportional flow control. Consequently, the pressure can be varied depending on the type of gear change required, for example a fast gear or a slow gear, on the gear involved; and whether it is a shift to a higher gear or a downshift. Furthermore, the pressure, particularly at the gear change actuator 115, can be varied during a gear change, for example to vary the force applied during the phases of commissioning, synchronization, gear change and fixing in gear of the gearbox. In particular, the loads applied during the synchronization phase can be reduced to reduce the stresses of the synchro-gearers.

As illustrated above, the spring accumulator 275 includes a housing 300. The housing 300 defines a closed bore 302 and a piston 304 is slidably positioned in the closed bore-chamber 302, the piston 304 being sealed with respect to the wall. cylindrical hole 302.

A rod 306 mounted relative to the piston 304 extends through one side of the piston 304 coaxially to the bore 302, toward the open end of the bore 302. The rod 306 sized to extend beyond the open end of the bore-chamber 302 when the piston 304 it is located at its extreme limit of movement towards the closed end of the bore-chamber 302. A port 308 opens towards the closed end of the bore 302.

The housing 300 also defines a body 310 of the pump 223, which is positioned in juxtaposed relation to the bore 302 of the accumulator 275. The pump motor 312 is attached to the pump body 310 to drive the pump 223. An inlet 314 is provided for connection of pump 223 to reservoir 278. The inlet into pump 223 which is connected to port 302 adjacent to the closed end thereof is formed by passages (not shown) defined in housing 300, these passages defining a housing for the non-return valve 276. The housing 300 may also define a suitable housing and connections for the pressure relief valve 280.

A bracket formation 320 is attached to the housing 300 by which it can be mounted on a support structure in a suitable manner. An extension 322 of the bracket formation 320 is made of a resilient spring material and rests against the free end 324 of the stem 306, the extension 322 thus acting as a leaf spring urging the piston 304 towards the closed end of the bore-chamber 302. When fluid is pumped by pump 223 into port 302, piston 304 will be displaced towards the open end of port 302, thereby inflecting extension 322 of bracket formation 320. Fluid stored in port 302 will thus be pressurized when extension 322 is inflected.

The pressure of the fluid in the accumulator will depend on the spring constant of the extension 322 of the bracket 320. This can be designed as required, by suitable selection of the material of which it is made and the width and thickness of the extension 322 itself. Furthermore, the extension 322 can be tapered in width or thickness, or made up of a plurality of layers of material, in order to provide the required elastic constant. Further, the extension 322 may be designed to provide a variable spring constant when it is inflected and / or means may be provided for adjusting the spring constant.

Although in the above embodiment the leaf spring is defined by an integral extension 322 of the bracket 320, the leaf spring can alternatively be formed separately and fixed, for example, to the housing 300 or the bracket 320, in a suitable manner.

Furthermore, the accumulator, pump and associated components may alternatively be provided with separate housings interconnected by suitable conduits.

According to a further variant of the invention, the open end of the hole 302 can be closed and vented to the atmosphere to provide the reservoir 278 for the system. With this construction, the vented chamber on one side of the piston 304 will preferably be connected to the inlet side of the pump 223 by passages defined in the housing 300.

According to another embodiment of the invention, the holes 124 and 151 of the main control valve 120 and of the gear selector and shift valves 144, 146 and also of the gear selector and shift actuators 114, 115 can be defined from a common housing, the holes 124, 151 of the various components being suitably interconnected by passages through the common housing. The valve / actuator package thus formed would be mounted on or adjacent to the gearbox or gearbox 12. This valve / actuator package may also include the accumulator / pump compressed package described above.

Various modifications can be implemented without departing from the invention. For example, although in the previous embodiment the hydraulic circuit has been described with reference to a semi-automatic transmission-gearbox system, the invention is also applicable to fully automatic transmission systems or to automatic-manual transmission systems.

Although in the above embodiment the pressure in the system can be controlled by switching the pump 223 in a predetermined manner or by the current of the pump motor, a pressure transducer 282 as illustrated with dashed lines in Figure 3 can optionally be included in the system ( 114, 115, 144, 146) for engaging the gears, said pressure transducer 282 being employed, in a closed loop feedback control system, to provide accurate pressure control.

Furthermore, although in the embodiment described above the slave cylinder 22 of the clutch is connected directly to the main control valve 120, a remote displacement valve with sensor means in position of the type described in EP 0702760 whose contents are expressly incorporated in the contents descriptive of the present application, may be interposed between the main control valve 120 and the slave cylinder 22 of the clutch.

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

References used in the dependent claims refer to further development of the subject matter of the main claim by the features of the respective dependent claim; they are not to be regarded as a waiver with respect to obtaining protection for independent elements for the combination of features in the related dependent claims.

Since the subject of the dependent claims can form separate and independent inventions with reference to the prior art at the priority date, the Applicant reserves the right to make them the subject of independent claims of divisional claims. Furthermore, they may also contain independent inventions demonstrating a structure independent of one of the objects of the foregoing dependent claims.

The embodiments are not to be regarded as a limitation of the invention. Rather, a wide range of amendments and modifications are possible within the scope of the present disclosure and particularly those variants, those elements and those combinations and / or materials which, for example, the skilled person can learn by combining some individual together. with those in the general description and embodiments in addition to features and / or elements or process stages described in the claims and contained in the drawings in order to solve a task thus leading to a new object or to new process stages or sequences of stages of processes with combinable characteristics even when they concern manufacturing, testing and work processes.

Claims (11)

CLAIMS 1. A hydraulic accumulator comprising: a cylinder; a piston slidably mounted in the cylinder, the piston being sealed relative to the cylinder wall, a rod extending from the piston to one side thereof, the rod extending coaxially to the cylinder beyond one end of the cylinder and a leaf spring acting on one end of the rod remote from the piston, the leaf spring urging the piston towards the other end of the cylinder, said other end of the cylinder being closed and a port opening towards the cylinder adjacent to said other end. The hydraulic accumulator according to claim 1, wherein the spring constant of the leaf spring is variable. Hydraulic accumulator according to claim 1 or 2, wherein the spring constant of the leaf spring is adjustable. Hydraulic accumulator according to any one of the preceding claims, wherein the accumulator cylinder is defined by a housing, the housing also defining the body of a positive displacement pump, and an electric motor for driving the positive displacement pump being mounted on the housing. The hydraulic accumulator according to claim 4, wherein a connection between the positive displacement pump and the closed end of the accumulator cylinder is provided by an internal passage formed in the housing. 6. A hydraulic accumulator according to claim 5, wherein the internal passage defines a housing for a non-return valve which prevents fluid from flowing from the accumulator to the pump. Hydraulic accumulator according to any one of the preceding claims, wherein said one end of the cylinder is also closed, the chamber defined between said end of the cylinder and the piston, being vented to the atmosphere, said chamber defining a reservoir for pressurized fluid atmospheric. The hydraulic accumulator according to claim 7 when considered with any one of claims 4 to 6, wherein internal passages in the housing connect the reservoir to the pump. The hydraulic accumulator of claim 8, wherein the housing defines a housing for a pressure release valve between the pump outlet and the reservoir. A hydraulic accumulator according to any preceding claim, wherein a housing defining the accumulator cylinder has a bracket formation for securing the housing to a support structure, an integral extension of the bracket defining the leaf spring. 11. Hydraulic drive system for a motor vehicle transmission system including a hydraulic accumulator according to any one of claims 1 to 10.