GB2051714A

GB2051714A - Hydraulic power-assisted steering system for motor vehicles

Info

Publication number: GB2051714A
Application number: GB8021809A
Authority: GB
Original assignee: ZF Friedrichshafen AG
Current assignee: ZF Friedrichshafen AG
Priority date: 1979-07-04
Filing date: 1980-07-03
Publication date: 1981-01-21
Also published as: DE2927039A1; ES8102950A1; IT1127504B; FR2460829A1; ES493072A0; JPS5621970A; SE8004859L; IT8049137A0

Abstract

A hydraulic power-assisted steering system for motor vehicles has provision for damping hydraulic oscillations in the steering system while retaining satisfactory suction conditions for the working fluid. The system comprises a pump, a steering valve accommodated in a housing 1 and a power cylinder 4 in which a power piston 7 is slidable and forming two pressure chambers 8 and 9. Ducts 10 and 11 respectively connect annular channels 12, 14 in the steering valve housing 1 to annular channels 13, 15 respectively communicating with the pressure chambers 8 and 9. Hollow non-return valves 16 are screwed in the housing 1 of the steering valve and in parts 3, 5 of the power cylinder housing. The interior bore 17 of the valve 16 receives a valve disc 19 which is urged into a valve closing position against a seat 23 by a weak compression spring 21 and which is provided with a throttle hole 20. Transverse bores 18 enable flow of the working fluid through the valve. The flow is substantially unimpeded in one direction (arrow), but is throttled in the opposite direction. <IMAGE>

Description

SPECIFICATION Hydraulic power-assisted steering system for motor vehicles This invention relates to a hydraulic power-assisted steering system for motor vehicles, of the kind comprising a pump, a steering valve and a power cylinder having two pressure chambers, throttles being arranged in the connecting bores or connecting ducts leading from the steering valve to the pressure chambers.

In such a steering system, the manual force at the steering wheel is assisted hydraulically. The pressure oil required for a steering operation is supplied by an engine-driven pump. During steering, and with the corresponding counteracting force at the road wheels, the steering valve feeds the pressure oil to one side of the power cylinder into a pressure chamber. The force generated in this way assists the rotary movement at the steering wheel and relieves the driver of most of the necessary steering work.

With a rack-and-pinion hydraulic steering system, for example, the track rods are usually joined to the respective end faces of the rack bar.

In order to actuate the steering system, the respective pressure chamber of the power cylinder is filled with pressure oil while simultaneously the other pressure chamber is emptied by the piston movement. A disadvantage in this arrangement is, however, that this results in "touchy" vehicle be haviourwith vehicles which react very sensitively during straight-ahead driving. With short actuation of the steering wheel, such vehicles also often tend to oscillate about the straight-ahead direction of driving (follow-up oscillation). During this, the piston of the power cylinder effects corresponding axial movements about its middle position. This means that hydraulic oscillations occur in the steering system.

For this reason, it is already known to provide throttles in the hydraulic ducts extending from the steering valve to the pressure chambers. With these measures the disadvantageous oscillations are avoided. It is of disadvantage, however, that in such arrangement suction in a pressure chamber can also take place only via the throttle. This gives rise to the problem that with rapid steering movements the mechanical system "overtakes" the hydraulic servo system. This means that then the sucking in of more oil cannot take place quickly enough so that a hard spot or short-time increase in manual force at the steering wheel occurs. For this reason the throttling for the prevention of oscillations could not be selected to be too great in practice since otherwise the replenishing behaviour would have deteriorated further with increasing damping.

It is an object of the invention to provide a hydraulic power-assisted steering system of the foregoing kind, by means of which hydraulic oscillations in the steering system are avoided while retaining good replenishing characteristics.

Accordingly, the present invention consists in a hydraulic power-assisted steering system for motor vehicles, comprising a pump, a sLeering valve and a power cylinder having two pressure chambers, throttles being arranged in the hydraulic connecting ducts or bores leading from the steering valve to the pressure chambers, characterised in that a nonreturn valve, which is provided with a throttle hole and which opens toward the respective pressure chamber, is arranged in the hydraulic bores or hydraulic ducts.

The non-return valve according to the invention substantially provides a "one-way throttle". Hydraulic oil flowing to the respective pressure chamber can flow substantially unthrottled into the pressure chamber through the open non-return valve. Hydraulic oil flowing out, on the other hand, is throttled since it can flow off only through the throttle passage hole in the non-return valve. This throttling effectively prevents any follow-up oscillation of the vehicle and in some cases also prevents additional rattling virbations in the steering valve since the column of oil provides an appropriate resistance in the direction of displacement.

It is of advantage if the non-return valve has a low opening resistance. Due to this measure the hydraulic oil can flow without great resistance into the pressure chamber to be filled.

A simple constructional development of the non return-valve consists in that the non-return valve is constructed as a hollow screw in the interior bore of which a valve plate with a throttle hole is arranged which co-operates with a valve seat.

Preferably, the hollow screw is screwed into the housing of the working cylinder, the valve seat being formed by a shoulder in the interior bore and a closing spring being supported at its end facing the power cylinder on a clamping ring located with press fit in the interior bore.

By means of this measure the non-return valve is combined in a simple manner with the power cylinder.

Conveniently, the hollow screw is screwed into the housing of the power cylinder and the end of the hollow screw facing the power cylinder has at its periphery an undercut which is engaged by a valve plate which is clamped over the end face of the hollow screw and which has a hook-like annular beading, the valve plate being provided with a spring tongue which opens towards the power cylinder.

This development of the hollow screw represents a very simple and cheap solution. When the hydraulic oil flows out of the power cylinder, throttling is produced in a simple manner since the oil can flow off only via the throttle hole. For the intake, on the other hand, the spring tongue lifts off the end face and the oil can flow unimpeded into the pressure chamberto be filled.

Advantageously, the hollow screw is scrrewed into the housing of the steering valve, the valve seat being formed at the end facing the housing of the steering valve by a valve ring pressed into the interior bore, and a closing spring being supported on a shoulder in the interior bore.

By virtue of such arrangement, the non-return valve is combined with the housing of the steering valve.

Preferably, the hydraulic ducts at the housing of the steering valve and at the housing of the power cylinder (pinion housing, end pieces) respectively open into an annular channel and the non-return valve (hollow screw) is arranged in one of the two annular channels associated with one hydraulic duct.

In such arrangement, the hydraulic oil can reach the interior bore of the hollow screw from the annular channel, known per se, through appropriate transverse bores in the hollow screw or, conversely, flow off into the annular channel from the interior bore via the transverse bores.

In order that the invention may be more readily understood, reference is made to the accompanying drawings which illustrate diagrammaticaily and by way of example embodiments thereof, and in which Figure 1 shows a view of a rack-and-pinion steering system; Figure 2 shows a view in direction of the arrow in Figure 1; Figure 3 shows a hollow screw as a non-return valve which is screwed into the hydraulic duct in the steering valve; Figure 4 shows a hollow screw as a non-return valve which is screwed into the hydraulic duct in the power cylinder; and Figure 5 and Figure 6 show a hollow screw as a non-return valve of different construction which is screwed into the hydraulic duct in the power cylinder.

The illustrated rack-and-pinion steering system is of conventional construction having a steering valve housing 1 in which a steering spindle 2 is supported.

Furthermore, a pinion housing 3, a tubular cylinder 4 and an end piece 5 are provided. A rack 6 is supported in the pinion housing 3 and the end piece 5 and has a power piston 7, not shown in detail, which divides the power cylinder into two pressure chambers 8 and 9. The two pressure chambers 8 and 9 are respectively connected to hydraulic ducts 10 and 11. The hydraulic duct 10 is connected via annular channels 12 and 13 to the valve housing 1 and the end piece 5, the pressure chamber 8 being supplied with hydraulic oil via the annular channel 13. The hyraulic duct 11 is connected via an annular channel 14to the valve housing 1 and via an annular channel 15 to the pinion housing 3, the pressure chamber 9 being supplied with oil also via the annular channel 15.

Non-return valves are now arranged in the annular channels 2 to 15 and it is sufficient if there is only a single non-return valve in each of the two hydraulic ducts 10 and 11. The construction of the respective non-return valve depends here on whether it is located in the annular channel 13 or 15 or in the annular channel 12 or 14.

The non-return valve shown in Figure 3 is arranged in the annular channels 12 and 14 in the valve housing 1, whereas the embodiments of Figures 4 to 6 are arranged in the annular channels 13 and 15 in the power cylinder 4.

As shown, the non-return valve is constructed as a hollow screw 16 having an in'r-iorbore 17. The hydraulic oil can flow from the annular channels 12 to 15 via transverse bores 18 into the interior bore 17 or flow off into the annular channeis.

The arrows in Figures 3,4 and 5 indicate the direction of hydraulic oil flowforfilling a pressure chamber. This direction of flow in each case determines the different arrangement of a valve plate 19 which is in each case provided with a throttle 20.

According to Figure 3, the valve plate 19 is urged buy a closing spring 21,which is supported on a shoulder 22 of the interior bore 17, against a valve seat which is constructed as valve ring 23. In this arrangement, the ring 23 is pressed into the interior bore 17.

As can be seen, the intake behaviour of the pressure chamber 8 or 9 is not interfered with by this kind of construction of the non-return valve. This is because, with a closing spring 21 of appropriate strength, the valve plate 19 very easily lits off the valve seat 23 and the hydraulic oil can flow unimpeded into the pressure chamber to be filled.

Conversely, however, when hydraulic oil is displaced from the power cylinder, this displacement can take place only via the throttle 20.

The non-return valve shown in Figure 4, which is also constructed as hollow screw 16, produces the same effect. Since this hollow screw 16, however, is screwed into the end piece 5 or into the pinion housing 3 of the power cylinder, a shoulder 24 in the interior bore 17 serves as valve seat and the closing spring 21 is supported on a clamping ring 25 which is inserted with press fit into the interior bore 17 of the hollow screw 16.

The non-return valve of Figures 5 and 6, which is also constructed as hollow screw 16, may be screwed into the end piece 5 or the pinion housing 3 in the power cylinder. A valve plate 26 of elastic material is provided with a hook-like annular beading 27. The valve plate 26 is clamped over the front face of the hollow screw 16 and the hook-like annular beading 27 engages an undercut 28 of the hollow screw 16. The valve plate 26 has also a throttle 20.

Furthermore, it is provided with a spring tongue 29 (see Figure 6). The spring tongue 29 is a little larger than the diameter of the interior bore 17. In this way also a non-return valve is formed. This is because if the hydraulic oil flows outward the spring tongue 29 lifts off the bore 17 so that the oil can flow unimpeded into the pressure chamber of the power cylinder to be filled. Conversely, however, this spring tongue 29 comes to rest across the opening of the interior bore 17 so that the hydraulic oil flowing off has only restricted passage through the throttle 20.

Of course, other types of non-return valves are also possible within the scope of the invention. The essential feature is only that the non-return valve is constructed in such a manner that it opens in the direction of oil flow to the pressure chambers and that, moreover, it is provided with a throttle for a reverse direction of flow.

Instead of a valve plate 19, for example, a ball or a conical valve plate may be used.

If the available space provides the possibility, the throttle valves can in each case also be arranged directly in an appropriate bore or groove, communicating with the respective pressure chamber 8 or 9, provided in the steering valve 1.

Claims

1. A hydraulic power-assisted steering system for motor vehicles, comprising a pump, a steering valve and a power cylinder having two pressure chambers, throttles being arranged in the hydraulic connecting ducts or bores leading from the steering valve to the pressure chambers, characterised in that a non-return valve, which is provided with a throttle hole and which opens toward the respective pressure chamber, is arranged in the hydraulic bores or hydraulic ducts.

2. A hydraulic power-assisted steering system according to Claim 1, wherein the non-return valve has a low opening resistance.

3. A hydraulic power-assisted steering system according to Claim 1 or 2, wherein the non-return valve is constructed as a hollow screw in the interior bore of which a valve plate having a throttle hole is arranged which valve plate co-operates with a valve seat.

4. A hydraulic power-assisted steering system according to Claim 3, wherein the hollow screw is screwed into the housing of the power cylinder, the valve seat being formed by a shoulder in the interior bore and a valve closing spring being supported at its end facing the power cylinder on a clamping ring located with press fit in the interior bore.

5. A hydraulic power-assisted steering system according to Claim 3, wherein the hollow screw is screwed into the housing of the power cylinder and wherein the end of the hollow screw facing the power cylinder has at its periphery an undercut which is engaged by a valve plate which is clamped over the end face of the hollow screw and which has a hook-like annular beading, the valve plate being provided with a spring tongue which opens towards the working cylinder.

6. A hydraulic power-assisted steering system according to Claim 3, wherein the hollow screw is screwed into the housing of the steering valve, the valve seat being formed on the side facing the housing of the steering valve by a valve ring pressed into the interior bore, and a closing spring being supported on a shoulder in the interior bore.

7. A hydraulic power-assisted steering system according to any of Claims 1 to 6, wherein the hydraulic ducts at the housing of the steering valve and at the power cylinder (pinion housing end piece) respectively open into an annular channel and the non-return valve is arranged in one of the two annular channels associated with one hydraulic duct.

8. A hydraulic power-assisted steering system for motor vehicles, substantially as herein described with reference to and as shown in the accompanying drawings.