COMBINED PRESSURE RELIEF AND UNLOADER VALVE

Abstract: A combined unloader and pressure relief valve for a hydraulic system comprises a main valve member (13) the opening of which is controlled by pressure in a pilot valve chamber (23). A lever (28) is normally held against a stop by a spring (31) and rests lightly on a pilot valve member (27) which thus maintains a low pressure in chamber (23) allowing the main valve to open under the influence of pressure at inlet (4). When the valve is to be closed to charge an accumulator (8) a transducer (10) energises a coil (37) to pivot lever (28) about pilot valve member (27). This lifts the lever from the stop and applies a spring load to the pilot valve member thereby allowing pressure to build in chamber (23) to close the main valve member. If the transducer (10) fails and maintains the coil (37) permanently energised, the main valve member will act as a pressure relief valve since the pilot valve member will lift when pressure in chamber (23) is able to overcome the applied spring load.

Description: COMBINED PRESSURE RELIEF AND UNLOADER VALVE This invention relates to a combined pressure relief and unloader valve suitable for use in the high pressure hydraulic system of a motor vehicle It is known to provide in high pressure hydraulic systems of motor vehicles, e.g. in powered hydraulic braking systems1 a pump which charges an accumulator with pressurised hydraulic fluid. In such systems the pump may run to charge the accumulator until the pressure in the accumulator reaches a predetermined cut-out pressure, whereupon an unloader valve is opened to connect the pump outlet to reservoir, The pump then runs against a small pressure head determined by pressure losses in the system until, due to use of powered equipment, the pressure in the accumulator falls to a predetermined cut-in pressure at which the unloader valve is again closed, thereby causing the pump output to flow to the accumulator to recharge the accumulator. In order to prevent damage to the pump or other system components in the event that, due to malfunction, the unloader valve does not open at the cut-out pressure, a relief valve may be provided to connect the pump to reservior if the pump output pressure exceeds a predetermined relief pressure, the relief pressure being higher than the cut-out pressure. According to the present invention there is provided a combined pressure relief and unloader valve comprising: an inlet; an outlet; a main valve member for controlling communication between the inlet and the outlet; a pilot valve for controlling movement of the main valve member; and an electrical actuator for controlling the opening pressure of the pilot valve, the electrical actuator being operative, when actuated, to apply the predetermined load of a spring to the pilot valve to maintain the pilot valve closed until it is subjected to a pressure determined by the load of the spring. If such a valve is incorporated in a high pressure hydraulic system with the inlet of the valve connected to the outlet of the pump and the outlet of the valve connected to reservoir the electrical actuator can be controlled by a pressure transducer which emits a signal when the pressure in an accumulator connected to the pump falls below a predetermined cut-in pressure. The control signal will then cause the valve to close at the cut-in pressure and allow the pump to charge the accumulator. When the accumulator pressure reaches a cut-out pressure the transducer should cease to emit a signal and this will cause the valve to open and re-connect the pump to reservoir If, however, the transducer continues to emit a signal even when the accumulator pressure reaches the cut-out pressure, the valve will remain closed until the pump output pressure reaches a value sufficient to open the valve in the presence of a control signal, whereupon the valve will open to limit the pump output pressure. In such a system the pressure at which the valve will open in the presence of a control signal is preferably about lOOpsi (689 kPa) above the cut-out pressure, and accordingly in the event of transducer failure the valve operates as a relief valve which opens at lOOpsi (689 kPa) above cut-out pressure. The invention will be better understood from the following description of preferred embodiments thereof, given by way of example only, reference being had to the accompanying drawings, wherein: Figure 1 is a view, partially in cross-section, showing a preferred embodiment of the invention; Figure 2 is a cross-section on the line II - II of Figure 1; Figure 3 shows a blank for use in the preferred embodiment of the invention; and Figure 4 shows part of a modified embodiment of the invention. Referring firstly to Figure 1 an embodiment of control valve 1 according to the present invention is mounted in the wall 2 of a hydraulic fluid reservoir 3 such that the valve is emmersed in hydraulic fluid. The inlet 4 of the valve is connected to the outlet 5 of a hydraulic pump 6 which is continuously driven e.g. by a motor vehicle engine. The outlet 5 of the pump is connected via a non-return valve 7 to a hydraulic accumulator 8 of any, suitable design. An outlet 9 from the accumulator supplies hydraulic fluid, as required, to operate vehicle system e.g. a powered braking system or a powered suspension system. A transducer 10 senses the pressure in the accumulator and emits a signal in the form of an electrical voltage when the pressure in the accumulator falls below a cut-in pressure, for example lOOOpsi (6.89 MPa). As a result the valve 1 closes (as hereinafter explained) and the pump 6 begins to re-charge the accumulator. When the pressure sensed by the transducer 10 reaches a cut-out pressure, for example 1500psi (10.34 MPa), the transducer 10 ceases to emit a signal and thereby indicates that the accumulator 8 is fully charged. The valve 1 includes an outlet 11 which is connected to the inlet 4 by a valve chamber 12 in which is located a main valve member 13. The main valve member is engagable with a main valve seat 14 formed on a bush 15 to isolate the inlet 4 from the outlet 11. The bush 15 is screw threaded into the body 16 of the control valve 1 and it will be noted that since the valve 1 is located in the reservoir, the screw threaded connection between the bush 15 and body 16 need not be sealed. The main valve member 13 is mounted as a clearance fit on a stem'17. The stem 17 is free to move relative to the valve body 16, and this fact, combined with the small axial length of the land 18 of the main valve member, means that the main valve member can seat against the main valve seat 14 even if the main valve seat is slightly eccentric to the valve chamber 12. The main valve member 13 is biased towards the seat 14 by a light spring 19 which acts on the main valve member 13 via a dished washer 20 and O-ring 21. Thus, in the rest state of the valve, the main valve member 13 is in engagement with the seat 14 and the clearance passage 22 between the main valve member 13 and stem 17 provides a flow-restricting connection between a pilot chamber 23 formed to the rear of the main valve member 13 and an opening 24 in the end of the main valve member 13. Relative movement which occurs in-use between the main valve member 13 and the stem 17 helps to dislodge any debris which may become lodged in the passage 22 and thus renders the passage 22 self-cleaning. A pilot passage 25 connects the pilot chamber 23 to the reservoir 3 and is formed with a pilot valve seat 26 which is engagable by a pilot valve member 27 to isolate the pilot chamber 23 from the reservoir. The upper end of the pilot valve member 27 stands proud of the adjacent surface of the valve body 16 and provides a fulcrum for a steel lever 28 which is mounted on the valve body. The edge 29 of the valve body 16 provides a line-contact stop 30 for the lever adjacent the valve member 27, and a generally C-shaped spring 31 applies a force F to the lever biasing the lever towards the stop 30. The spring 31 is formed of flat strip material and includes a tongue 32 (Figure 2) which passes through a slot 33 formed in the lever 28 and seats in a groove 34 formed in the valve body. Shoulders 35 formed on each side of the tongue 32 engage the lever 28 so that the line of action 36 of the spring 31 passes through the stop 30. Thus, although the spring 31 is pre-loaded to apply a well defined force F to the lever 28 it does not resist rotation of the lever 28 about the stop 30. Accordingly, the spring 31 does not prevent the pilot valve member 27 lifting in respose to fluid pressure acting on the pilot valve member 27 over the area of the pilot valve seat 26. Finally, a coil 37 is located about an upstanding portion 38 of the valve body 16 to form an electro-magnet 39 which, in response to the voltage emitted by transducer 10, attracts the lever 28. The electro-magnet 39 is sufficiently powerful to rotate the lever 28 about the fulcrum 40 formed by the pilot valve member 27, thereby bringing the edge 41 of the lever 28 into engagement with a stop provided by the upstanding portion 38. This will cause the lever to lift off the stop 30 and the spring 31 will thus apply a vertical force F v to the lever. This force will act on the pilot valve member 27 via the lever 28 to load the pilot valve member 27 with a force KF where K is a constant. v In use, when the vehicle engine is started the pump 6 will start to deliver hydraulic fluid. If the pressure in the accumulator 8 is below the cut-in pressure the transducer 10 will emit a signal which energizes the electro-magnet 39 to attract the lever 28 and thereby apply a load to the pilot valve member as described above. Fluid pressure at the inlet 4 will be supplied via opening 24 and passage 22 to pilot chamber 23 and will there assist the spring 19 to maintain the main valve member 13 in engagement with seat 14, thereby maintaining the inlet 4 isolated from outlet 11. The pump 6 will therefore charge the accumulator 8. If the transducer 10 is working correctly, when the accumulator pressure reaches the cut-out pressure, the transducer will cease to emit a signal and the electro-magnet 39 will thus become de-energized. The lever 28 will return to engagement with the stop 30 and the pilot valve member 27 will lift to allow pressure in the pilot chamber 23 to decay. The main valve member 13 will accordingly move under the influence of inlet pressure away from seat 14 to connect the inlet 4 to outlet 11. The pump will then discharge through the valve 1 direct to reservoir against a small pressure head determined by spring 19 and system pressure losses. If, however, the transducer 10 continues to emit a signal after cut-out pressure is reached, the electro-magnet 39 will remain energised and pump output pressure will continue to rise until the pressure in the pilot chamber 23 reaches a level at which the loading KF V on the pilot valve member is insufficient to maintain the pilot valve member in engagement with the pilot valve seat 26. The pilot valve member 27 will then lift to limit the pressure in the pilot chamber. The limited pressure in the pilot chamber will in turn limit the pressure at the inlet 4 to a value p determined by the equation: max EMI8.1 where: A = effective area of the main valve member over which input pressure acts A2 = effective area of the main valve member over which pressure in the pilot chamber acts A3 = area of the pilot valve seat S = a small constant determined by the spring 19 When the inlet pressure exceeds P the main max valve member 13 will move out of engagement with the main valve seat 14 to reduce the inlet pressure to p max It will thus be seen that the above described valve acts as an electrically controlled unloader valve and as a pressure relief valve. Since the load KF which the spring 31 imposes v on the pilot valve member 27 when the electro-magnet 39 is energized can be accurately set by suitable selection of spring 31, the relief pressure P can be accurately max selected. Preferably P is about lOOpsi (689 kPa) max above the normal cut-out pressure. Referring now to Figure 3, the valve body 16 is advantageously formed by cutting a suitable length of rectangular steel bar-stock 42 and then drilling a hole 43 to form the valve chamber 12. A groove 44 is then milled in the blank to intersect the bore 43, thereby forming the outlet 11 and providing a seat for the spring 31. A second groove 45 is then formed in the blank to define the upstanding portion 38. The blank is then machined as necessary to complete the valve body 16. Finally, referring to Figure 4 a modified embodiment is shown in which the lever 28, spring 31 and electro-magnet 39 are replaced by a solenoid 46 secured to the valve body 16. When de-energized the solenoid applies substantially no load to the pilot valve member 27. How ever, when the solenoid is energized the armature 47 is pulled into engagement with the solenoid coil 48 and thereby applies a load to the pilot valve member 27 via a compression spring 49. The loading of the pilot valve member is thus determined by the spring 49, and in the event of transducer failure, the pilot valve member 27 can lift against the load of the spring 49 when the pressure in the pilot chamber reaches a suitable valve.

Claims: CLAIMS: 1. A combined pressure relief and unloader valve comprising: an inlet; an outlet; a main valve member for controlling communication between the inlet and the outlet; a pilot valve for controlling movement of the main valve member; and an electrical actuator for controlling the opening pressure of the pilot valve, the electrical actuator being operative, when actuated, to apply the predetermined load of a spring to the pilot valve to maintain the pilot valve closed until it is subjected to a pressure determined by the load of the spring. 2. A combined pressure relief and unloader valve according to claim I wherein the main valve member engages a main valve seat to isolate the inlet from the outlet, and including a restricted passage extending from upstream of the main valve seat to a pilot chamber, the pressure in the pilot chamber being controlled by the pilot valve and being effective to control the opening pressure of the main valve member. 3. A combined pressure relief and unloader valve according to claim 2 wherein the restricted passage extends through the main valve member. 4. A combined pressure relief and unloader valve according to claim 3 wherein the restricted passage is formed by two surfaces which move relative to each other as the main valve member moves. 5. A combined pressure relief and unloader valve according to any preceding claim wherein the electrical actuator includes a lever pivotable on a fulcrum defined by the movable member of the pilot valve between a first position in which the lever is biased by the spring into engagement with a stop located to one side of the fulcrum and a second position in which the lever is clear of the stop and the load of the spring is applied to the movable member of the pilot valve. 6. A combined pressure relief and unloader valve according to claim 5 wherein the lever is pivotable from the first position to the second position by the action of an electromagnet formed by a coil mounted on part of the body of the valve and operable to attract a portion of the lever located on the opposite side of the fulcrum from the stop. 7. A combined pressure relief and unloader valve according to any of claims 1 to 4 wherein the pilot valve includes a movable pilot valve member and the actuator selectively applies either substantially no load to the pilot valve member or the predetermined spring load, the actuator comprising: a lever mounted for pivotal movement about a fulcrum defined on the pilot valve member; a first stop for the lever adjacent the pilot valve member; a spring biasing the lever into engagement with the first stop with the line of action of the spring passing through the first stop so that the spring provides substantially no resistance to pivotal movement of the lever about the first stop; and electrical means for pivoting the lever about the fulcrum to bring a portion of the lever remote from the valve member into engagement with a second stop and thereby lift the lever out of engagement with the first stop and cause the spring to load the pilot valve member via the lever. 8. A combined pressure relief and unloader valve according to any of claims 1 to 4 wherein the electrical actuator comprises a solenoid having an armature, the spring being a compression spring located between the armature and a movable member of the pilot valve to apply a predetermined force to the movable member when the solenoid is actuated. 9. A combined pressure relief and unloader valve according to any preceding claim wherein the valve body is a unitary structure machined from a length of steel bar.

Inventors: HARRISON, ANTHONY WILLIAM
Applicants: LUCAS INDUSTRIES PUBLIC LIMITED COMPANY
Application year: 1982
Main classification: B60T17
IPC codes: B60T15, F15B1, F16K17, B60T13