GB2059638A - Hydraulic pressure supply system - Google Patents

Abstract

A vehicle which includes such a system has a pump 1 supplying fluid pressure by way of a non- return valve 4 and an accumulator 5 to a service station 7 including a closed centre valve by which fluid pressure is to be tapped on demand. The system includes a relief valve arrangement 6 having a normally closed main spool valve 9 and a normally closed pilot spool valve 10. The latter is displaced when the fluid pressure in accumulator 5 reaches a desired value, to open communication between the fluid pressure main spool chamber 17 thereby causing displacement of the main spool valve 9 and unloading of fluid pressure from the pump 1 to the reservoir 2. A different- piston main spool valve can be biased closed hydraulically as well as by the spring shown by communicating chamber 18 with the output from pump 1. The service station 7 can serve hydraulic suspension or braking and fluid pressure supply to the accumulator 5 from pump 1 can be by way of a flow divider valve the other outlet of which serves a power assisted steering gear. <IMAGE>

Description

SPECIFICATION An hydraulic fluid pressure supply system and a vehicle which incorporates such a system This invention relates to an hydraulic fluid pressure supply system and to a vehicle which incorporates such a system.

More particularly, the invention concerns a system in which hydraulic fluid under pressure is supplied from a fluid pressure source such as an engine driven pump of a vehicle in which the system may be fitted, to a service station from which hydraulic fluid under pressure is to be tapped on demand, for example by way of a valve which is normally closed centre to actuate or control auxilliary hydraulic services of the vehicle such as power assistance braking or suspension. In such a system it is usual to include a relief or unloader valve which is responsive to fluid pressure in the supply and reacts when that fluid pressure attains a predetermined value to open communication between the fluid pressure source and an hydraulic fluid exhaust which is usually in the form of a reservoir.In such known system the relief valve is usually in the form of a simple spool valve which is biased to a condition in which it normally closes communication between the fluid pressure source and exhaust and is displaced against its biasing to open said communication when fluid pressure in the supply attains a value which is sufficiently high to displace the spool against its biasing. It frequently occurs in such an arrangement that the relief valve can hunt or oscillate in response to fluid pressure fluctuations in the supply system between its open and closed conditions with the result that there is uncertainty in operation of the relief valve and unacceptable chatter or throttling may be exhibited by that valve; it is an object of the present invention to provide an hydraulic fluid pressure supply system which alleviates the aforementioned disadvantages.

According to the present invention there is provided an hyraulic fluid pressure supply system comprising a fluid pressure source for supplying hydraulic fluid under pressure to a service station from which hydraulic fluid under pressure is to be tapped on demand; relief valve means which is responsive to fluid pressure in said supply and reacts when that fluid pressure attains a predetermined value to open communication between the fluid pressure source and an hydraulic fluid exhaust; said relief valve means comprising a first valve which is biased to a condition in which it normally closes communication between the said source and exhaust and is displaceable against its biasing by hydraulic fluid pressure to open said communication, and a second valve which is biased to a condition in which it normally closes fluid pressure communication between the fluid pressure supply and the first valve and is displaceable against its biasing by hydraulic fluid pressure derived from the fluid pressure supply when that fluid pressure attains a predetermined value to open fluid pressure communication between the fluid pressure supply and the first valve which latter fluid pressure communication serves for displacing the first valve against its biasing to open communication between the said source and hydraulic fluid exhaust.

Still further according to the present invention there is provided an hydraulic system comprising the supply system as specified in the immediately preceding paragraph in which the service station comprises a valve which is normally close centre by which hydraulic fluid under pressure is to be tapped on demand from the supply system; a further system having an open centre valve; flow divider valve means by which fluid flow from said fluid pressure source is directed to two outlets of the flow divider means one of which outlets communicates with said supply system and through which fluid under pressure is supplied to that system and the other of which outlets communicates with the further system and through which fluid under pressure is supplied to that further system.

By the present invention the actuation of the first valve which controls the opening and closing of communication between the fluid pressure source and exhaust is controlled by way of the second valve which is responsive to pressure derived from the fluid pressure supply and this second valve can be arranged so that it permits fluid pressure to react on the first valve only when that fluid pressure is sufficient to cause the second valve to open communication between the fluid pressure source and exhaust clearly and decisively thereby alleviating the uncertainty and chatter in the relief valve operation.

Preferably the second valve, which is conveniently in the form of a spring biased spool, is displaceable in response to hydraulic fluid pressure derived from the fluid pressure supply and controls the pressurising and exhausting of a chamber in the first valve, which chamber, when subjected to fluid under pressure from the fluid pressure supply is expandible to effect displacement of the first valve against its biasing. For example, the first valve may comprise a spool which defines with its spool cylinder an expandible and contractible chamber to effect displacement of the spool and thereby actuation of the valve in response to variations in fluid pressure within the chamber.With such an arrangement the second valve may be arranged so that when it closes the aforementioned chamber to communication with fluid pressure supply that valve is displaceable under its biasing to a condition in which it opens communication between the chamber and exhaust to permit displacement of the first valve under its biasing and when the second valve opens the chamber to communication with the fluid pressure supply that valve is displaced against its biasing to close communication between the chamber and exhaust.

The supply of hydraulic fluid under pressure from the source to the service station may be by way of a non-return valve and it is then preferred that the first valve is located to normally close communication between the source at a position upstream of the nonreturn valve and exhaust and to be displaceable against its biasing to open said communication and that the second valve is displaceable against its biasing by hydraulic fluid pressure which is derived from the fluid pressure supply at a position downstream of the nonreturn valve. The use of a non-return valve is particularly advantageous in a supply system where the hydraulic fluid under pressure is supplied by way of the non-return valve to an accumulator from which the service station is supplied.The use of an accumulator in hydraulic systems is well known particularly where the source is provided from a constantly driven pump and fluid under pressure is to be tapped from the service station by way of a closed centre valve spasmodically.

Still further according to the present invention there is provided a vehicle which includes a system as previously discussed as being in accordance with the invention and in which the service station comprises the or a closed centre valve which serves to actuate or control auxilliary hydraulic services such as power assistance braking or suspension of the vehicle. Furthermore, when the vehicle includes a system as previously mentioned which comprises the further system having the open centre valve, such open centre valve may serve to control actuation of power assistance means for a steering gear of the vehicle. Open and closed centre valves are well known in the art of hydraulic control systems being discussed, for example, in our U.K. Patent Specification No. 1,506,850.

One embodiment of an hydraulic fluid pressure supply system constructed in accordance with the present invention and as applied to a vehicle will now be described, by way of example only, with reference to the accompanying illustrative drawings, in which: Figure 1 diagrammatically illustrates the supply system; Figure 2 diagrammatically illustrates the supply system of Fig. 1 applied to a vehicle and having associated therewith a further hydraulic system which includes an open centre valve for controlling actuation of power assistance means of a steering gear, and Figure 3 illustrates a modified form of valve which can be substituted for the corresponding valve which is incorporated in the systems of Figs. 1 and 2.

The hydraulic fluid pressure supply system has a constant flow engine driven pump 1 which draws hydraulic fluid from a reservoir 2. The output of the pump 1 passes by way of a passage 3 and a non-return valve 4 to an hydraulic accumulator 5 which is thereby charged with the output of pump 1. The accumulator 5 provides a source of fluid under pressure by way of passage 6 to a service station indicated at 7 comprising a normally closed centre valve by which hydraulic fluid under pressure is to be tapped on demand.

Associated with the system thus far described is a relief valve arrangement indicated generally at 8 and comprising a main spool valve 9 and a pilot spool valve 1 0. The main spool valve 9 has an inlet port 11 which communicates by way of passage 1 2 with the output from pump 1 at a position upstream of the non-return valve 4. The valve 9 also has an outlet port 1 3 which communicates by way of passage 14 with the reservoir 2.

Communication between the inlet and outlet ports 11 and 1 3 is controlled by an annular recess 1 5 in the spool 1 6 of the valve. The spool 1 6 is axially slidable in a spool cylinder and forms therewith opposed spool chambers 1 7 and 1 8 in the latter of which is located a biasing spring 1 9 which normally biases the spool 1 6 to a position in which the port 11 is closed by a spool land to communication with the port 1 3 and thereby with the reservoir or exhaust 2.

The spool 20 of the pilot spool valve 10 is axially slidable in a spool cylinder and forms therewith opposed spool chambers 21 and 22. Spool chamber 21 is in constant communication by way of passage 23 with hydraulic fluid under pressure in the accommulator 5 while the spool chamber 22 houses a spring 24 which biases the spool 20 in a sense to contract the chamber 21. The spool 20 has a land 25 which, during displacement of the spool serves to control communication between the passage 23 and a passage 27 which is in constant communication with the chamber 1 7 of the main spool valve 1 6 and also has an annular recess 26 which, together with the spool land 25, serves to control during displacement of the spool communication between the passage 27 and a passage 28 which communicates with the passage 14 and thereby with the reservoir 2. The spring 24 biases the spool 20 in a sense to contract the chamber 21 and into a condition in which passages 27 and 28 communicate with each other by way of recess 26 whereby the spool chamber 1 7 is open to communication with the reservoir 2. In this latter condition the passage 23 and thereby the accummulator 5 is closed by the spool land 25 to communication with the spool chamber 1 7 by way of passage 27.

With the relief valve arrangement 9 inoperative as shown in Fig. 1 the whole output from pump 1 serves to charge the accumulator 5 by way of non-return valve 4 so that fluid pressure is available in the accumulator to be tapped on demand at the service station 7.

Variations in pressure in the accumulator 5 are reflected in the spool chamber 21 of the pilot spool valve and when the accumulator pressure develops sufficiently the spool 20 will be displaced against the biasing of spring 24 until a predetermined pressure is attained in the spool chamber 21 which causes the spool land 25 to be displaced sufficiently to open communication between the passages 23 and 27 and close communication between the passages 27 and 28. Fluid under pressure from the accumulator 5 is thereby admitted by way of passage 23, chamber 21 and passage 27 to the spool chamber 1 7 of the main spool valve 16.The admission of this fluid under pressure to the chamber 1 7 causes the latter to expand and displace the spool 1 6 against the biasing of spring 1 9 and thereby open communication between the inlet port 11 and outlet port 1 3 of the main spool valve. This latter effect opens communication between the output from pump 1 by way of passage 1 2 and the reservoir 2 by way of passage 1 4 thereby relieving the accumulator from further supply of fluid under pressure.

From the aforegoing it will be appreciated that the operation of the main spool valve 9 is determined by actuation of the pilot spool valve 1 0. The biasing spring 24 and the effective pressurised area on the end face of the spool 20 in the chamber 21 of the pilot spool valve can therefore be arranged so that the spool 20 will be displaced rapidly to open communication between the accumulator 5 and the spool chamber 1 7 of the main spool valve when sufficient fluid pressure is available in the accumulator to displace the spool 1 6 of the main spool valve to an extent which opens communication between the inlet and outlet ports 11 and 1 3. By such an arrangement the spool 1 6 in the main valve 9 can have relatively positive opening and closing characteristics between its ports 1 3 so that the spool 1 6 is alleviated from oscillation (hunting) and chatter.

When hydraulic pressure in the accumulator 5 decreases sufficiently (which will occur as the pressure is tapped from the service station) the drop in fluid pressure in the spool chamber 21 permits the spool 20 of the pilot valve to be displaced under its spring biasing to close communication between the passages 23 and 27 and re-open communication between the passages 27 and 28 as shown. The spool chamber 1 7 of the main spool valve 9 is thereby re-opened to communication with the reservoir 2 by way of spool recess 26 permitting the main spool 1 6 to be displaced under its biasing to rapidly close communication between the ports 11 and 1 3 and causing the whole output from pump 1 to be available to charge the accumulator 5.As aforementioned this latter rapid displacement of the spool 1 6 alleviates uncertainty in the opening and closing of relief to the output from pump 1 and chatter which may otherwise be effected by oscillation of the spool 16.

The arrangement shown in Fig. 2 includes the system substantially as above described with reference to Fig. 1 applied to a vehicle in which the closed centre valve in the service station 7 serves to actuate or control auxilliary hydraulic services of the vehicle such a power assistance braking or suspension. Furthermore, the output of pump 1 passes to an input port 29 of a flow divider valve 30 having outlet ports 31 and 32 between which the fluid flow is divided within the valve 30.

The outlet port 31 communicates with the passages 3 and 1 2 of the supply system as above described while the outlet port 32 communicates with a passage 33 of a further system which includes an open centre valve indicated generally at 34 which controls fluid flow to power assistance means of a power assisted steering gear 35. The relationship between the valve 34 and gear 35 may be regarded as conventional whereby in a neutral condition of the valve 34 the power assistance steering gear is inoperative and fluid flow is permitted through the open centre valve from the port 32 by way of passage 33, the valve 34 and passages 36 and 1 4 to return to the reservoir 2.

The flow divider valve 30 comprises a spool 37 which is axially slidable in a spool cylinder and forms with its cylinder axially opposed spool chambers 38 and 39 one with each of which, for practical purposes, the respective outlet ports 31 and 32 may be regarded as being in substantially, constant communication. The spool 37 has an internal passage 40 which is in constant communication with the inlet port 29 and it is this passage which effects the division of fluid flow from the output of the pump 1. The passage 40 includes a restrictor 41 through which it is in constant communication with the spool chamber 38 and also includes a restrictor 42 through which it is in constant communication with the spool chamber 39.It will be apparent that if the spool 37 is subjected to alternating fluid pressure differentials in the chambers 38 and 39 whereby the spool is causes to reciprocate, the chambers 38 and 39 will alternately and repeatedly expand and contract respectively.

If it is assumed that fluid pressure in the spool chambers 38 and 39 is substantially equal, the spool 37 will be centralised as shown. If it is now assumed that the open centre valve 34 is operated to actute the power assistance means for the steering gear 35 a demand for fluid pressure develops in the passage 33. This demand causes fluid pressure to develop in the spool chamber 39 while spool chamber 38 maintains communication with the reservoir 5 so causing the spool 37 to be displaced leftwardly in the drawing to progressively close outlet port 31 until the pressure in chamber 38 equalises with that in the chamber 39 and the ratio of fluid flow is maintained through outlet ports 31 and 32.If however, the power assistance steering gear is inoperative and the accumulator 5 discharges by operation of the close centre valve 7, then there will be a demand for fluid pressure from the outlet port 31 (assuming that port 11 is closed to communication with port 1 3 in the main spool valve 9). This latter demand develops fluid presssure in the spool chamber 38 while port 32 communicates with whe reservoir 2 so causing the spool 37 to be displaced rightwardly in the drawing to progressively close outlet port 32 until the pressure in chamber 39 equalises with that in chamber 38 and the ratio of fluid flow is maintained through the outlet ports 31 and 32.For practical purposes the spool 37 may be regarded as never fully closing either the outlet port 31 or the outlet port 32; the reason for this is that when one or other of these ports is being closed by the spool 37 the hydraulic pressure in the spool chamber 38 or 39 which is associated with the near closed port rises to an extent where it is approximately equal to the hydraulic fluid pressure which is available in the spool chamber 38 or 39 which is associated with the fully opened outlet port and so the spool 37 is no longer subjected to a fluid pressure differential and ceases its axial displacement.It will be apparent that when the demand for fluid pressure by one or other of the supply system which includes the accumulator 5 and the further system which includes the power steering facility is satisfied and both outlet ports 31 and 32 are opened to communication with the reservoir 2, the fluid pressure in the spool chambers 38 and 39 will equalise and the spool 37 centralises.

In the hydraulic system shown in Fig. 2 it is realised that the volume demand for fluid flow to the power assistance means of the steering gear 35 will likely be greater than the volume demand for such flow to actuate the auxilliary services by way of the close centre valve 7.

For this reason the normally required ratio of fluid flow between the respective systems can be determined by appropriate selection of the sizes of the restrictors 41 and 42. Desirably therefore the restrictor 42 provides a far less restriction to fluid flow therethrough than does the restrictor 41. This ratio of fluid flow which is available through the restrictors 41 and 42 is maintained constant as required by the characteristics of the systems and irrespective of operation of the flow divider valve 30.

Fig. 3 illustrates a modified form which is available for the main spool valve and can be substituted for the valve 9 as shown in Fig. 1 and 2. The modified main spool valve in Fig.

3 has a stepped diameter spool 43 with opposed end faces 44 and 45 of different effective pressurised areas of which the end face 44 with the larger effective area communicates with the spool chamber 1 7 and the end face 45 with the smaller effective area communicates with the spool chamber 1 8 (within which is located the spring 19). In addition the spool chamber 1 8 is in constant communication by way of branch passage 46 with the passage 1 2 at a position upstream of the non-return valve 4.Both spool chambers 1 7 and 1 8 are therefore responsive to hydraulic fluid pressure while chamber 1 8 includes additional biasing from the spring 1 9. The modified main spool valve shown in Fig. 3 when substituted for the corresponding valve 9 in the system of either Fig. 1 or Fig. 2 serves the same purpose as the valve 9.In the Fig. 3 arrangement the passage 1 2 communicates by way of annular recess 1 5 in the spool 43 with the passage 14 and thereby with the reservoir 2 and this is consistent with the accumulator 5 being fully charged since the hydraulic pressure in that accumulator reacts on spool 20 and also on the spool end face 44 to retain the spool 43 rightwardly in Fig. 3 against the biasing spring 1 9 while spool chamber 18 communicates with the reservoir 2.When the accumulator 5 discharges to a predetermined extent and passage 27 is opened to communication with the reservoir through passage 28 there is a decrease in pressure in the spool chamber 1 7 which permits the spool 43 to be displaced leftwardly under its spring biasing to close port 11 and thereby effect pressurisation of the accumulator 5 as previously described.

When the stepped diameter spool closes port 11 it will be apparent that the spool chamber 18 is pressurised by way of passage 46 so additionally biasing the spool leftwardly in a sense to maintain the port 11 closed. Upon a predetermined pressure being attained in the accumulator 5, pressure is applied to the spool chamber 1 7 causing the spool 43 to be displaced rightwardly against the spring and hydraulic pressure biasing in the spool chamber 1 8 (by virtue of the differential areas of the spool end faces 44 and 45). During such rightward displacement of the spool 43 the port 11 will again open to the recess 1 5 and thereby to exhaust (reservoir) to open the passage 1 2 and thereby the spool chamber 1 8 to exhaust. The effect of this latter displacement and consequent de-pressurisation of the spool chamber 18 reduces the leftward biasing on the spool 43 causing it to be displaced rightwardly in Fig. 3 at a greater speed than that which would otherwise be achieved. Upon opening of the port 11 to the reservoir 2 it is possible to arrange for the stepped diameter spool 43 to snap to the right in Fig. 3 thereby giving a relatively clean opening of the main spool valve and alleviating fluid throttling; this should also further alleviate possible hunting of the spool 43.

Claims (14)

1. An hydraulic fluid pressure supply system comprising a fluid pressure source for supplying hydraulic fluid under pressure to a service station from which hydraulic fluid under pressure is to be tapped on demand; relief valve means which is responsive to fluid pressure in said supply and reacts when that fluid pressure attains a predetermined value to open communication between the fluid pressure source and an hydraulic fluid exhaust; said relief valve means comprising a first valve which is biased to a condition in which it normally closes communication between the said source and exhaust and is displaceable against its biasing by hydraulic fluid pressure to open said communication, and a second valve which is biased to a condition in which it normally closes fluid pressure communication between the fluid pressure supply and the first valve and is displaceable against its biasing by hydraulic fluid pressure derived from the fluid pressure supply when that fluid pressure attains a predetermined value to open pressure communication between the fluid pressure supply and the first valve which latter fluid pressure communication serves for displacing the first valve against its biasing to open communication between the said source and hydraulic fluid exhaust.

2. A system as claimed in claim 1 in which the second valve is displaceable in response to hydraulic fluid pressure derived from the fluid pressure supply and controls the pressurising and exhausting of a chamber in the first valve which chamber, when subjected to fluid under pressure from the fluid pressure supply is expandible to effect displacement of the first valve against its biasing and wherein the second valve is arranged so that when it closes the chamber to communication with the fluid pressure supply it is displaceable under its biasing to a condition in which it opens communication between the chamber and fluid pressure exhaust to permit displacement of the first valve under its biasing and when the second valve opens the chamber to communication with fluid pressure supply that valve is displaced against its biasing to close communication between the chamber and hydraulic fluid exhaust.

3. A system as claimed in either claim 1 or claim 2 in which the source supplies hydraulic fluid under pressure to the service station by way of a non-return valve; the first valve is located to normally close communication between the source at a position upstream of said non-return valve and exhaust and to be displaceable against its biasing to open said communication, and the second valve is displaceable against its biasing by hydraulic fluid pressure derived from the fluid pressure supply at a position downstream of said non-return valve.

4. A system as claim in claim 3 in which the source supplies hydraulic fluid under pressure by way of the non-return valve to an accumulator from which the service station is supplied.

5. A system as claimed in any one of the preceding claims in which the service station comprises a valve which is closed centre to the flow of fluid therethrough, on demand, from the fluid pressure supply.

6. A system as claimed in any one of the preceding claims in which the first valve is biased by spring means.

7. A system as claimed in any one of the preceding claims in which the first valve is displaceable by fluid pressure differential and has fluid pressure biasing which opposes the fluid pressure which is derived by way of the second valve to react on the first valve, the fluid pressure for said fluid pressure biasing being derived from the fluid pressure supply at a position in that supply which is openable by the first valve to communication with exhaust.

8. A system as claimed in claim 7 when appendant to claim 3 in which the fluid pressure for said fluid pressure biasing is derived from the fluid pressure supply at a position upstream of the non-return valve.

9. A system as claimed in any one of the preceding claims in which the second valve is biased by spring means.

10. An hydraulic fluid pressure supply system substantially as herein described with reference to Fig. 1 or Fig. 3 of the accompanying illustrative drawings.

11. An hydraulic.system comprising the supply system as claimed in any one of the preceding claims in which the service station comprises the or a closed centre valve by which fluid under pressure is tapped from the supply system on demand; a further system having an open centre valve; flow divider means by which fluid flow from said fluid pressure source is directed to two outlets of the flow divider means one of which outlets communicates with said supply system and through which fluid under pressure is supplied to that system and the other of which cutlets communicates with the further system and through which fluid under pressure is supplied to that further system.

1 2. An hydraulic system as claimed in claim 11 and substantially as herein described with reference to Fig. 2 of the accompanying illustrative drawings.

1 3. A vehicle which includes a system as claimed in any one of the preceding claims in which the service station comprises the or a closed centre valve by which fluid under pres sure is tapped on demand from the supply system to actuate or control auxilliary hydraulic services such as power assisted braking or suspension of the vehicle.

14. A vehicle as claimed in claim 1 3 when appendant to claim 11 in which the open centre valve controls actuation of power assistance means for a steering gear of the vehicle.