GB2056928A

GB2056928A - Power steering systems

Info

Publication number: GB2056928A
Application number: GB8023572A
Authority: GB
Original assignee: ZF Friedrichshafen AG
Current assignee: ZF Friedrichshafen AG
Priority date: 1979-08-01
Filing date: 1980-07-18
Publication date: 1981-03-25
Also published as: DE2931240A1; GB2056928B; FR2463041B1; SE8005463L; DE2931240C2; FR2463041A1; JPS6349671B2; JPS5625057A

Abstract

In a power steering system including a pump 1, a pressure reservoir 2, a steering valve 5, a working cylinder 10 and a return tank 11 there is provided in the supply from the pump 1 and pressure reservoir 2 to the steering valve 5 a pressure reducing valve 12. The forces acting on the valve 12 are a spring 17 and the pressure to the cylinder 10 against the pressure to the valve 5. The valve 12 acts to maintain a constant pressure gradient at the control edges 26, 27 of the steering valve 5, and the system has small leak oil losses. In a modification, the valve 12 is a seat valve. <IMAGE>

Description

SPECIFICATION Power steering systems The invention concerns power steering systems, in particularfor motor vehicles, comprising a pump, a pressure reservoir, a steering valve which is closed in the neutral position and operable from a steering handwheel, at least one working cylinder with two pressure chambers and a return tank.

Normally the pressure generator consists of a servo-pump and a pressure reservoir which is capable of producing pressures in the range from 140 to 180 bar. A power steering system of this kind, in conjunction with a highly charged pressure reservoir, is capable of supplying the total hydraulic power required by a motor vehicle for steering, brakes, level control etc. With such an arrangement the servo-pump may be small as the reservoir acts as a collector. It is, however, necessary to stop the oil flowing through the power system steering valve, when the valve is in the neutral position.

The problem with this arrangement is high leak oil losses. Since the control edges of the steering valve are under constant high pressure, and only a small overlap of the inlet control edges is desirable to give a good "feel" on the steering handwheel a high amount of oil is lost. In addition, accumulations of dirt particles lead to deterioration of the closeness of the fit, and so increase the amount of oil lost.

A further disadvantage is the different pressure gradients arising between the pressure at the inlet control edges of the steering valve and the pressure present in the chambers of the working cylinder This also has an adverse influence upon the steering behaviour of the vehicle.

This invention is based on the requirement to create a power steering system with a closed centre, with small leak oil losses and a constant pressure gradient. According to the invention a pressure reducing valve provides a pressure balance between the pressure reservoir and the steering valve, the forces acting on one side of the balance being a spring and the pressure supplying the working cylinder, and on the other side the supply pressure to the steering valve.

By making the pressure reducing valve influenced by the pressures and the spring, it is no longer at the high working pressure which is present in the supply line to the steering valve or on the inlet control edges of the steering valve, but only at a pre-selected pressure which is dependent upon the force of the spring. Of particular advantage is the fact that, in the neutral position, there is only low pressure at the steering valve, e.g. 10 bar. Consequently, only a small overlap at the inlet control edges is achieved, giving the driver a better feel on the steering wheel.

A further advantage is a constant pressure gradient between the inlet control edges at the steering valve and the pressures in the pressure chambers of the working cylinder, resulting in a uniform supply current to the pressure chambers and thus a proportional control. The very high working pressure of the pressure reservoir is reduced to the lower operating pressure in the pressure reducing valve. Since smaller diameters are feasible in the pressure reducing valve and the amount of overlapping may be selected according to requirement, the pressure reducing valve suffers from substantially lower leakage losses than the steering valve.

Power system steering valves with open centre and hydraulic feedback may be used as closed valves by employing the pressure reducing valve according to the invention. All that is necessary is to obtain overlapping of the inlet control edges in neutral position by employing an appropriate control piston.

A pressure reducing valve used according to the invention may be equipped with an axially displaceable control piston having a floating mounting. The circumference ofthe piston has a control edge cooperating with the housing. Behind the ends of the piston are arranged, on one side, a spring chamber with a spring and the link line leading to the pressure chambers, and on the other side, a pressure equalising chamber connected with the supply line to the steering valve. Alternatively, the pressure reducing valve may be a seat valve comprising an inlet to an annular chamber and an outlet to the steering valve from a valve chamber behind the valve seat, whereby the spring chamber with the spring and the link line to the pressure chambers is arranged behind one end of the control piston.

Furthermore it is advantageous to provide a short-circuit line between the supply line to the steering valve and the link line, whereby the short-circuit line comprises a pre-tensioned relief valve opening in the direction of the link line with a closing spring which is somewhat stronger than the spring arranged in the pressure reducing valve. In the neutral position the working pressure of the pressure reservoir is present, for a short time, at the inlet control edges of the steering valve. This pressure is slow to reduce. The relief valve succeeds in dispersing this high working pressure immediately.

It is furthermore advantageous to provide for a cross-connection with a reversing valve between the two pressure lines leading from the steering valve to the pressure chambers of the working cylinder, and to let the link line leading to the pressure reducing valve branch off from this cross-connection. Due to this cross-connection and the reversing valve one link line only to the pressure reducing valve is sufficient. The reversing valve operates depending upon which pressure chamber is acted upon by the oil and connects the oil pressure line with the link line.

Figure lisa diagram of a power steering system according to the invention; and Figure 2 is a pressure reducing valve of a different design useable in the system of Figure 1.

In Figure 1, the power for the system is generated by a servo-pump 1, a reservoir 2 and a reservoir charging valve 3. A steering valve 5 (not shown in detail) is supplied with compressed oil via a supply line 4. The steering valve 5 is actuated by a steering handwheel (not shown). The steering valve 5 is direction-controlled, but it is possible to have a power-controlled valve. Pressure chambers 8 and 9 of a working cylinder 10 are connected with the steering valve 5 via pressure lines 6 and 7. A return tank 11 is provided for the return oil.

A pressure reducing valve 12 with an axially displaceable control piston 13 is arranged between the reservoir 2 and the steering valve 5. The valve housing and the control piston 13 together form a control edge 14. Behind one end of the control piston there is a pressure equalising chamber 15, and behind the other there is a spring chamber 16 with a spring 17, The pressure equalising chamber 15 is connected with the supply line 4 by a side line 18, and with the inner chamber ofthe pressure reducing valve 12 behind the control edge 14 buy a bore 19, as illustrated by the broken line. The spring chamber 16 is connected with the two pressure lines 6 and 7 by a link line 20. The connection is effected by a reversing valve 21 arranged in a cross-connection 22 between the two pressure lines 6 and 7.Between the supply line 4 and the link line 20 is a short-circuit line 23 which includes a relief valve 24 opening in the direction of the link line and equipped with a closing spring 25.

A high working pressure P1 of the reservoir 2 (140-180 bar) is reduced to a substantially lower pressure P2 supplying the steering valve 5, and to an operating pressure P3 for the pressure chambers 8 and 9 of the working cylinder 10in the following manner: In the neutral position of the steering valve 5, the inlet control edges 26 and 27 are closed. The pressure equalisation chamber 15 is counterbalanced by the force of the spring 17. As a result only leak oil is exchanged. The pressure P2 at the inlet control edges 26 and 27 corresponds to the com pressiveforce of the spring 17. In general it is sufficient ifthe force of the spring 17 produces a pressure of 10 bar at the piston surface of the associated end 28 of the control piston 13.

The pressure reducing valve 24 reduces the pressure in the supply line 4. For a short time, the pressure at the inlet control edges 26 and 27 is equal to the working pressure Fl. This implies that the pressure equalising chamber 15 is under the same pressure by the side line 18, with the effect that the control piston 13 is immediately pushed to the right, where it takes up a blocking position. The closing spring 25 of the relief valve 24 is slightly stronger than the spring 17 of the pressure reducing valve 12.

This means that in case of high working pressure, the relief valve 24 will open immediately allowing the compressed oil to return to the tank 11 via pressure lines 6 or 7, the return control edges of the steering valve 5 and the return line 29. Once the pressure in the supply line 4 has dropped accordingly, the supply pressure is as desired, i.e. a low P2.

As the steering is operated, which means a movement of the piston of the steering valve 5 to the right for example in the direction of the arrow, supply pressure P2 drops. The spring 17 moves the control piston 13 of the pressure reducing valve 12 to the left, thus enlarging the control edge 14. This, in turn, means a rise in the supply pressure P2 and thus a corresponding rise in the operating pressure P3. The reversing valve 21 leaves its neutral position illustrated and moves to the left. Due to the crossconnection 22 the link line 20 is thus under the same operating pressure P3 as the pressure line 7 leading to the pressure chamber 9. In this way the spring chamber 16 is also under the same operating pressure P3. Thus the control piston is influenced by the operating pressure P3 and the force from the spring on one side, and by the supply pressure P2 on the other side.

As a result the following pressure ratios are obtained: P2 xA=P3 xA+F,where A = piston surface area of control piston 13, and F = force withe spring 17.

It follows that: P2 = P3 + FIA now the pressure difference at the control edges 26 and 27 of the steering valve 5 is P2 - P3 = P3 + F/A - P3 = F/A = constant. This means that the differential pressure at the control edges 26 and 27 is constant and does not depend upon the overall pressure. In this way the pressure chambers 8 and 9 of the working cylinder 10 are always supplied with the same amount of oil via the inlet control edges 26 and 27. The differential pressure depends only on the chosen force ofthe spring 17 and the piston surface area A.

In Figure 2, a valve seat 31 takes over the function of control edge 14 in Figure 1. The inflow to a valve 30 leads into an annular chamber 32, and the outflow into the supply line 4 to the steering valve 5 is from a valve chamber 33 arranged behind the valve seat 31.

This obviates a side line 18 or a bore 19 as illustrated in Figure 1, since the front end of the control piston acts directly upon the supply pressure P2. The pressure reducing valve may be integrated either into the steering gear mechanism or into the reservoir charging valve 3. It all depends upon the given parameters. Integration into the reservoir charging valve 3 has the advantage that the supply line to the pressure reducing valve is not under the constantly high working pressure of the reservoir 2. On the other hand, it has the disadvantage of possibly longer control lines.

Claims

1. A power steering system comprising a pump, a pressure reservoir, a steering valve which is closed in the neutral position and operable from a steering handwheel, at least one working cylinder, a return tank, a pressure reducing valve providing a pressure balance between the pressure reservoir and the steering valve, the forces acting on one side ofthe balance being a spring and the pressure supplying the working cylinder, and on the other side the supply pressure to the steering valve.

2. A power steering system according to claim 1 in which the pressure reducing valve is equipped with an axially displaceable control piston having a floating mounting, the circumference of the piston having a control edge cooperating with the housing of the valve, behind the ends of the piston being arranged, on one side a spring chamber with a spring and a link line leading to pressure chambers of the working cylinder, and on the other side a pressure equalising chamber connected with a supply line to the steering valve.

3. A power steering system according to claim 1 in which the pressure reducing valve is a seat valve comprising an inlet into an annular chamber and an outlet to the steering valve from a valve chamber behind the valve seat, spring chamber with the spring and a link line to the pressure chambers of the working cylinder arranged behind one end of the control piston.

4. A power steering system according to any preceding claim in which a short-circuit line is provided between a supply line to the steering valve and a link, whereby the short-circuit line comprises a pre-tensioned relief valve opening in the direction of the link line with a closing spring which is stronger than the spring in the pressure reducing valve.

5. A power steering system according to any of the preceding claims in which a cross-connection with a reversing valve is provided between pressure lines to the working cylinder, and a link line leads to the pressure reducing valve branching off from this cross-connection.

6. A power steering system as herein described with reference to Figure 1 of the drawings.

7. A power steering system according to claim 6 as modified by Figure 2 of the drawings.