GB2055718A

GB2055718A - Hydraulic assistance of a steering handwheel

Info

Publication number: GB2055718A
Application number: GB8024486A
Authority: GB
Original assignee: ZF Friedrichshafen AG
Current assignee: ZF Friedrichshafen AG
Priority date: 1979-08-01
Filing date: 1980-07-25
Publication date: 1981-03-11
Also published as: ES8106674A1; DE2931139A1; IT8046870A1; SE443960B; BR8004815A; FR2463039B1; ES493655A0; FR2463039A1; AR222235A1; SE8005464L; GB2055718B; IT1154070B; DE2931139C2; JPH0134825B2; IT8046870A0; JPS56128270A

Abstract

A control valve 1 supplies an unbalanced working cylinder 12. Control edges 9, 14' and 9', 14 between pistons 4a, 4b and valve body 2 have different sized apertures so that on steering a lower pressure may be applied to a larger area face 110 of a working chamber 11' than of a chamber 11. In this way equal hydraulic assistance is applied for equal operating force. Other embodiments show axial slide and rotary slide control valves, and hydraulic feed back. <IMAGE>

Description

SPECIFICATION Hydraulic assistance of a steering handwheel The invention relates to a device for the hydraulic assistance of the steering movement of a steering handwheel of a vehicle comprising a control valve for supplying hydraulic fluid to an unbalanced working cylinder.

In hydraulically assisted steering systems it is sometimes desirable for constructional and/ or money-saving reasons to employ a single working cylinder comprising only one piston rod extending from one side of the piston. As a result piston areas of varying effectiveness are obtained.

A power steering system with such an unbalanced working cylinder is known from Patent Specification DE-OS 1 9 59 920. Power steering systems of this kind suffer from the disadvantage of producing asymmetric steering forces due to the varying piston areas.

These steering force errors are likely to be heightened if tolerance errors occur resulting in uneven pressures in the two working chambers. For the driver, this means that for equal steering forces, the operating force required to turn the wheel to the left varies from that required to turn the wheel to the right.

According to the invention there is provided means whereby, for an equal operating force on the steering handwheel, unequal working pressures are produced in the control valve for the two piston ends of the working cylinder, in such a way as to obtain equal hydraulic assistance for an equal operating force in both steering directions.

Whilst until now the pressures produced in a control valve were equal in both steering directions, they are now unequal. According to the invention this leads to an asymmetric valve characteristic. Thus, in practice, "less working pressure" is allotted to the working chamber with the larger effective piston area, so that the piston force produced is equal to the piston force from the other working chamber.

The control valve preferably comprises one or several displaceable and/or rotatable control pistons and with control edges between control piston and valve housing. The inflow control edges and the return flow control edges for the working chambers of the working cylinder differ from each other in that the control edges regulating the servo-pressure for the working chamber with the larger piston area produce a lower throttle resistance. This measure provides a simple method of producing the necessary differences in pressure for the two working chambers.

A device according to the invention is suitable for use with various control valve designs, for example rotary piston, axial slide or rotary slide valves.

The difference in the control edges is preferably achieved by equipping the control edges of the control piston or pistons for the working chamber with the larger piston area with larger grooves than the control edges for the other working chamber. Piston inserts may be arranged on one end of the two control pistons on a rotary piston valve with hydraulic feedback have varying effective diameters, to the effect that the piston insert associated with the working chamber with the larger piston area is equipped with the smaller effective diameter. These measures also ensure that the pressures for the two working chambers differ from each other.

DRAWINGS: Figure 1 is a cross-section through a steering device according to the invention including a rotary piston control valve; Figure 2 is a longitudinal section through a device with an axial slide control valve; Figure 3 is a cross-section through a device with a rotary slide control valve; Figure 4 is a cross-section through a device with a rotary piston control valve with hydraulic feedback; and Figure 5 is a pressure/torque diagram of a device according to the invention.

In Fig. 1, a control valve comprises a valve body 2 and two control valve pistons 4a and 4b arranged in an oil supply chamber 3. The pistons are each equipped with a bore 5,6 for engaging a pivot, which serves, in the known manner, as actuating element for the control valve pistons 4a and 4b.

The valve body 2 is equipped with a bore 7 housing a torque rod for transmitting a rotary movement from a steering spindle not shown via a pinion onto a rack. It is thus possible, should the hydraulic assist fail, to use this mechanical link from a steering handwheel to the steered wheels for purely manual steering of the vehicle. A line 8 serves to supply oil to the control valve 1. The oil is supplied to the working chambers 11 and 11' of an unbalanced working cylinder 1 2 via an inflow control edge 9 on the control valve piston 4a and an inflow control edge 9' on the control valve piston 4b and lines 10 and 10'. The working chamber 11' has a larger effective piston area 1 10.The return of the oil into the return line 1 5 is effected through bores 1 spa in the valve body 2 via branch points 1 3 or 13' and a return flow control edge 1 4 on the control piston 4b and a return flow control edge 14' on the control piston 4a.

The control edges 9 and 14' effect a lower throttle resistance owing to the size of the apertures (exaggerated in the drawing for better recognition) than the control edges 9' and 14, and thus produce a lower pressure in the line 10'. By choosing the appropriate setting, an equally high hydraulic assist can thus be obtained in both operating directions despite varying piston areas.

In Fig. 2, the control valve of axial slide design has all parts corresponding to those of Fig. 1 indicated by the same reference numbers. A turn on the steering handwheei is transmitted to the control valve piston 4 which is axially movable. Again the control edges 9 and 14' produce, due to correspondingly larger grooves, a lower throttle resistance than the control edges 9' and 14, thus producing the desired low pressure in the line 10'.

Fig. 3 shows a control valve of rotary slide design. The working medium (oil) is supplied via lines 8, 8a, 8 b, 8e and the subsequent three inflow control edges 9 are to the smaller-area working chamber 11, or via the edges 9' to the larger-area working chamber 11'. The working medium returns to the line 15 via the flow control edges 14 and 14' (three of each) and three radial bores 15a, 15b and 1 sic in the control valve piston 4. As in Figs. 1 and 2, the control edges 9' and 14 are again equipped with smaller grooves for the larger-area working chamber 11'.

In Fig. 4, the rotary piston control valve with hydraulic feedback has equivalent components marked with the same reference numbers as in the preceding Figures.

The bores housing the two axially displacea ble control valve pistons 4a and 4b are closed off at one end by inserts 20 and 21. The two control pistons 4a and 4b are each separated from the piston inserts 20 and 21 by an annular chamber 22,23. The annular chambers are each connected to an annular groove 26 and 27 by a narrow bore 24,25. At the opposite end, the two control pistons 4a,4b are each equipped with an annular groove 28,29. On the control pistons being moved away from the piston inserts 20,21, the annu lar grooves 28,29 are connected to a pressure chamber 30. If the movement is effected in the opposite direction, the annular grooves 28,29 are closed off from the pressure chamber 30.

The operation of the control valve is as usual and will therefore not be described in detail. The annular chambers 22 and 23 are in communication, in the known manner, with the opposite supply annular grooves 29,28 (not shown). The inserts 20,21 have different diameters, and so the insert 21 produces a large resistance to manual force. As it acts upon the working chamber 11' with the larger piston area 11 0, the manual force is converted into "less working pressure" for the larger piston area 11 0.

In Fig. 5, the y-axis shows the pressure in the cylinder and the x-axis the torque on the steering handwheel (to the left and to the right). 30 represents the characteristic for the large cylinder chamber and 31 the characteristic for the small cylinder chamber. The characteristics differ in the decisivs lower range.

For an equally large torque on the steering handwheel, different pressures are obtained in the corresponding cylinder chamber for the right and the left turning direction. The higher pressure is produced for the smali cylinder chamber, and the pressure difference and therefore the path of the characteristic have been chosen so as to obtain an equal hydraulic assist for both steering directions.

The asymmetric characteristic may also be achieved for cross-slide valves. To put the invention into practice it is merely necessary to arrange for unequal feedback areas on the cross-slide valve for example as described in Patent Specification DE-OS 28 25 006.

Claims

CLAiMS

1. A device for the hydraulic assistance of the movement of a steering handwheel comprising a control valve for supplying hydraulic fluid to an unbalanced working cylinder, and means whereby, for an equal operating force on the steering handwheel, unequal working pressures are produced in the control valve for the two piston ends of the working cylinder, in such a way as to obtain equal hydraulic assistance for an equal operating force in both steering directions.

2. A device according to claim 1 in which the control valve comprises one or several displaceable and/or rotatable control pistons and with control edges forming between control piston and valve housing, the inflow con troi edges and the return flow edges for the working chambers of the working cylinder differing from each other in that the control edges regulating the servo-pressure for the working chamber with the larger piston area produce a lower throttle resistance.

3. A device according to claim 2 in which the control edges for the working chamber with the larger piston area are provided with larger grooves than the control edges for the other working chamber.

4. A device according to claim 2 in which piston inserts are arranged on one end of the two control pistons on a rotary piston valve with hydraulic feedback have varying effective diameters such that the piston insert associated with the working chamber with the larger piston area is equipped with the smaller effective diameter.

5. A device for the hydraulic assistance of a steering handwheel as herein described with reference to any of Figs. 1 to 4 of the drawings.