GB2092080A

GB2092080A - Power steering servo valves

Info

Publication number: GB2092080A
Application number: GB8102927A
Authority: GB
Original assignee: BRITISH LEYLAND CARS Ltd; Rover Co Ltd
Current assignee: BRITISH LEYLAND CARS Ltd; Rover Co Ltd
Priority date: 1981-01-30
Filing date: 1981-01-30
Publication date: 1982-08-11
Also published as: GB2092080B

Abstract

A servo valve for the power assisted steering of a vehicle, has slots 28 provided on the surface of a rotor 16 by hobbing. At each end of each slot 28 there are metering profiles 29 produced by grinding. <IMAGE>

Description

SPECIFICATION Improvements relating to servo valves This invention relates to servo valves, and particularly but not exclusively to valves for power-assisted steering in vehicles.

Such valves commonly comprise a rotor carried coaxially within a sleeve and able to rotate relative thereto in proportion to steering loads. The rotor and sleeve co-operate to define variable flow cross-sectional area orifices which control the flow of fluid under pressure to an hydraulic actuator, typically in the form of a double acting hydraulic ram. In a known type of such valve the rotor is provided with substantially axially extending slots which constitute part of said variable orifice. The slots have generally hitherto been milled into the surface of the rotor which, because the slots are at different circumferential locations on the surface of the rotor, must be indexed accurately between milling operations to ensure the correct relative positioning of the slots.

Viewed from one aspect, the present invention provides a method of producing a servo valve having a rotor carried coaxially within a sleeve, relative rotation of said rotor and said sleeve serving to vary the flow cross-sectional area of an orifice, including thestep of hobbing a slot into the surface of said rotor to define said orifice.

Viewed from another aspect, the invention provides a servo valve having a rotor carried coaxially within a sleeve, relative rotation of said rotor and said sleeve serving to vary the flow cross-sectional area of an orifice, wherein said rotor has a hobbed slot in its surface which serves to define said orifice.

An embodiment of the invention will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a cross-section of a servo valve in accordance with the invention; Figure 2 is a side elevation of a rotor of the valve of Figure 1; Figure 3 is a cross-section on the line Ill-Ill of Figure 2; Figure 4 is a cross-section on the line IV-lV of Figure 3; and Figure 5 is a cross-section of a sleeve of the valve of Figure 1.

Figure 1 shows a servo valve 11 for a motor vehicle power assisted steering system. A pinion 12 meshes with a steering rack (not shown) such that rotation of the pinion 1 2 moves the rack to steer the car. The pinion is carried at one end by roller bearings 13 set in a valve housing 14 and at the other end by a bearing in the steering unit (not shown). The pinion has a splined connected to one end of a torsion bar 1 5. The other end of torsion bar 1 5 is pinned for rotation with a valve rotor 16 which is carried by roller bearings 17 in the valve housing 14 at one end and is a sliding fit in the pinion at the other end.A shaft connected to the vehicle's steering wheel is connected to the valve rotor 1 6 so that steering effort by the vehicle's driver is transmitted from the rotor to the torsion bar 1 5 and thence to the pinion. Splines 18 and 1 9 on the rotor and the pinion respectively, engage to limit relative rotation of the rotor and pinion only after a predetermined torsional deflection of the bar 1 5.

Pinned for rotation with the pinion is a valve sleeve 20, which is free to move with the pinion within the housing and relative to the rotor when the torsion bar twists. The sleeve is provided with six equi-spaced axially extending slots, comprising two groups 21, 22 of three slots. The slots in group 21 and group 22 are connected by way of radial bores through the sleeve with a respective annular space 24 or 25 between the sleeve and the housing. Bores 23 for slots 21 are shown in Figure 5. Spaces 24 and 25 communicate, by piping (not shown), with a cylinder on respective sides of a double acting piston connected to the steering rack.Three further equi-spaced radial bores 26 extend between an annular space 27 and the innner face of the sleeve, each bore 26 being interposed between bores communicatiny respectively with space 24 and space 25, and emerging at the inner face of the sleeve through a land between a. slot of group 21 and a slot of group 22.

Valve rotor 1 6 is prcvided with six equi-spaced slots 28 extending parallel to the axisof the rotor.

The slots are produced by hobbing, which although an operation generally reserved for cutting gear teeth, has been found by the inventors to provide more consistent spacing of the slots which is vital for the proper operation of the valve, while at the same time enabling the slots to be produced in the rotor more quickly. At or adjacent each end of the slots 28 the edge of each slot is reduced by grinding to produce metering profiles 29. Three of the slots 28, each disposed alternately with the remaining slots, are connected by a radial bore 30 with the hollow interior of the rotor, and thence to an annular space 31 between rotor and sleeve by way of three radial bores 32 in the rotor. Annular space 31 communicates via three radial bores 33 through the sleeve and a further passageway (not shown) through the housing 14, with a return pipe to a hydraulic pump (not shown).

In use, fluid is supplied by the pump to annular space 27. When there is no steering load, bores 26 are aligned with those slots 28 not associated with a radial bore 30. Fluid then flows from the bores 26 back to the pump by way of a constricted passageway comprising, in sequence, an orifice defined by the edges of the slot 28 and of the land from which the fluid emerges, slots 21 or 22 at each side of the land, and an orifice defined by the edge of the slot 21 or 22 respectively and the edge of a slot 28 associated with a bore 30. Upon flowing into the last mentioned slots 28, fluid flows through the bores 32 and 33 back to the pump. Thus fluid in bores 26 is maintained at high pressure when there is no steering load.

As steering load is applied, rotor 1 6 is rotationally displaced relative to the sleeve causing certain of the above mentioned orifices to open and others to close, resulting in a pressure difference between slots 21 and slots 22 which is transmitted to corresponding sides of the piston in the actuator to assist steering. The profiles 29 are arranged to provide that the opening of the effective orifices between the annular space 27 and either space 24 or 25 is gradual to enhance the flow characteristic of the orifice and thus the operation of the power steering.

It will be apparent that the embodiment is open to considerable modification within the scope of the invention. For example fewer slots could be used and the slots could be helically disposed, whilst still employing hobbing as the means for producing the slots.

Claims

1. A method of producing a servo valve having a rotor carried coaxially within a sleeve, relative rotation of said rotor and said sleeve serving to vary the flow cross-sectional area of an orifice, including the step of hobbing a plurality of slots into the surface of said rotor to define said orifice.

2. A method as claimed in claim 1, wherein said slots are substantially parallel to the axis of said rotor.

3. A method as claimed in claim 1 or 2, including the step of reducing the edge of the slot by grinding to enhance the flow characteristic of said orifice.

4. A method as claimed in claim 1,2 or 3, wherein said plurality of slots is equi-spaced around said rotor.

5. A method of producing a servo valve, substantially as hereinbefore described with reference to the accompanying drawings.

6. A servo valve having a rotor carried coaxially within a sleeve, relative rotation of said rotor and said sleeve serving to vary the flow cross-sectional area of a plurality of orifices, wherein said rotor has a plurality of hobbed slots in its surface which serve to define said orifices.

7. A valve as claimed in claim 6, wherein said slots are substantially parallel to the axis of said rotor.

8. A valve as claimed in claim 6 or 7, wherein an edge of at least one of said slots is reduced to enhance the flow characteristic of a said orifice.

9. A valve as claimed in claim 8, wherein said slot edge is reduced at or adjacent an end of the slot.

10. A valve as claimed in any one of claims 6 to 9, wherein said plurality of slots is equi-spaced around the rotor.

11. A servo valve substantially as hereinbefore described with reference to the accompanying drawings.

12. A power assisted steering system for a vehicle incorporating a servo valve according to any one of claims 6 to 11.