GB2286807A

GB2286807A - Power-assisted vehicle steering system

Info

Publication number: GB2286807A
Application number: GB9403749A
Authority: GB
Inventors: John Arthur Wise
Original assignee: MG Rover Group Ltd
Current assignee: MG Rover Group Ltd
Priority date: 1994-02-26
Filing date: 1994-02-26
Publication date: 1995-08-30
Also published as: GB9403749D0

Abstract

The system comprises a pump 11 for delivering fluid 12 under pressure, a steering cylinder 16 having a piston 17 for applying power assistance to a steering element 22 of the vehicle, the piston 17 having faces of different areas A1 A2, a control valve 13 operable in response to rotation of a steering wheel 14 for controlling flow of the fluid from the pump 11 to the steering cylinder 16 and a further valve 15 arranged in a fluid path between the control valve 13 and the steering cylinder 16. The further valve 15 causes pressure to be applied primarily to the smaller area A2 of the piston 17 when providing power assistance on one steering direction and to both areas A1, A2 when providing power assistance in the other direction. In that way, the net force applied to the piston 17 will be substantially the same in both directions. <IMAGE>

Description

A POWER ASSISTED STEERING SYSTEM FOR A VEHICLE The invention relates to a power-assisted steering system for a vehicle.

With power-assisted steering systems, a steering cylinder is provided having a piston for transmitting power assistance to a steering element of the vehicle. For certain vehicle installations, the piston in the cylinder has faces of different areas and, therefore, the net force applied to the piston as a result of fluid pressure applied thereto will not be the same in both steering directions. An object of the present invention is to provide system which reduces that particular disadvantage.

According to the invention there is provided a power assisted steering system for a vehicle comprising a pump for delivering fluid under pressure, a steering cylinder having a piston for applying power assistance to a steering eieen of the vehicle, the piston having faces of different areas, a control valve operable in response to rotation of a steering wheel for controiling flow of the fluid from the pump to the steering cylinder and a further valve arranged in a fluid path between the control valve and the steering cylinder which causes pressure to be applied primarily to the smaller area of the piston when providing power assistance in one steering direction and to both areas when providing power assistance in the other direction.

The net force applied to the piston in that way provides a more balanced assistance level in both steering directions preferably from lock-to-lock when steering the vehicle which is desirable from the point of view of vehicle handling.

referably1 the fluid pressure is applied only to the smaler ace wnen providing the power assistance in said one direction.

In the preferred embodiment, the larger area of the piston s osu;e the smaller area of the piston. Where the fluid pressure applied only to tne smaller area 5 the same as the fluid pressure appled t both areas, it is possible to ortain equal net force on the piston n both steer inc directions.

Preferably, the piston divides the steering cylinder into two chambers and the further valve causes fluid fom one chamber to pass s the other chamber during movement of he piston in the steering cylinder when applying pressure to rot areas of the piston. In that case, the fluid preferably passes from the chamber defined in part by the smaller area of the piston to the chamber defined in part by the larger area of the piston. That is useful as the pump needs to deliver less fluid to the chamber defined in part by the piston of larger area as the fluid delivered thereto by the pump is supplemented by fluid from the other chamber. Therefore a pump having a smaller than normal delivery characteristic can be used.

The further valve may also cause fluid from the chamber defined in part by the larger area to be vented when applying pressure to the smaller area of the piston, The further valve may conveniently comprise a shuttle type valve. In such a case, the shuttle is preferably movable by a suitable bias means such as a spring into a first operative position. In that first operative position, the further valve preferably causes pressure s: be applied primarily to the smaller area of the piston.

The pressure of the fluid preferably acts against the shuttle in the bias direction when pressure is applied primarily to the smalier area of the piston.

The shuttle of the shuttle valve may be movable by flu d pressure against the bias means into a second operative position, the second operative position preferably being that in which the further valve causes pressure to be applied to both areas of the piston.

A power-assisted steering system for a vehicle will now be described by way of example with reference to the accompanying drawings in which: Fig 1 is a diagrammatic view of a system in accordance with the invention showing pressure being applied to the smaller area of a piston in a steering cylinder and Fig 2 is a view similar to Fig 1 but showing pressure being applied to the larger area of the piston.

In Fig 1, a reservoir 10 contains hydraulic fluid 12 suitable for a power steering system. X power steering pump 11 delivers the fluid 12 to a controi valve 13 which operates in response to rotation of a steering wheel 14.

The valve 13 has four fluid ports indicated at 13a, 13 13c and 13d. Port 13a is connected directly to the pump 11 and port 13b is connected directly to the reservoir 10.

A further vaive indicated generally at 15 has a port 15a connectea to the port id and a port 15b connected to the port c. The valve ;5 also has ports 15c and 15d connected to a steering cylinder indicated generally at 16.

The cylinder 16 has therein a piston 17 having a larger area face Al and a smaller area face A2. The piston 17 divides the cylinder 16 into first and second chambers 18, 19. The chamber 18 is connected to the port 15d of the further valve 15 and the chamber 19 is ccnnected to the port 15c. The piston 17 has a piston rod 20 drivably connected to a steering rack 22 which meshes with a pinion 23 at the lower end of a steering column 24. The steering rack constitutes the aforesaid "steering element" of the vehicle.

The valve 15 is formed with a bore 25 in which a shuttle 26 is slidably mounted. The shuttle 26 has a peripheral groove 26a therein and is normally biased by a spring 27 against a shoulder 28 at he right hand end of the bore 25 as viewed in the drawing. The spring 27 etenos between the left hand end of the shuttle 26 and a shoulder 21 at the left hand end of the bore 25. The ports 5a and 5e communicate radially with the bore 25, the port 15c having a widened sect ion 29 adjacent the bore 25. A passageway 90 extends between the port 45a and the left hand end of the bore 25. The right hand end of the bore 25 communicates with the port ,5b, 5d.

The operation ot the power-assisted steering system X now be described.

With the system at rest, the shuttle 26 is normally biased against the shoulder 28 by means of the spring 27.

With the engine of the vehicle running and the pump 11 in operation, rotation of the steering wheel 14 so as to move the steering rack 22 in the direction of arrow A in Fig 1 causes fluid under pressure to be directed through port 13d and to pass through port 15a of the valve 15.

The pressure of the fluid acts on the left hand end of the shuttle 26 and maintains the shuttle in the position shown in Fig 1. Fluid entering port 15a passes around the groove 26a and through the widening 29 which is uncovered by the shuttle 26. Fluid then passes out of port 15c and enters the chamber 19 of the steering cylinder 16. Fluid entering the chamber 49 causes the piston to move in the direction of arrow 7. and fluid in the chamber 18 is vented via tne ports 15d, Sb and 43c 13b back to the reservoir 10. The control valve operates in known manner to prevent fluid flowing out of the port 13d once sufficient steering movement has been applied.

With reference to Fig 2, movement of the steering wheel 14 n the opposite direction will call for the rack 22 to be moved in the direction of arrow Y. The control valve 13 causes fluid 0 leave port 3c and enter port 15b of the valve "5. The pressure or the fluid in port 15b s applied to the right hand end of the shuttle 26 and the resulting force causes the shuttle 26 to be moved to the left against the bias of the spring 27 as shown.

Passageway 30 and ports 15a, 13d are connected through valve 13 to port 13b and thus to the reservoir 10. Fluid ieaves port 15d and enters the chamber 18 to cause the piston 17 to move in the direction of arrow Y. However, chamber 19 is connected to port 15c and hence to port 15d via the widening 29 which is uncovered by the right hand end of the shuttle 26. Therefore, the pressure n chambers 18 and 19 will be equal. However, as area Al is greater than the area A2, the net force will result in movement of te piston 17 in the direction of arrow order to ensure that the net force applied to tine piston 17 causing the piston to move in the direction of arrow :'. is the same as the net force causing the piston 17 to move in the direction of arrow Y area A1 is double the area A2. The net resultant forces are as follows: Movement of piston 1 in direction of arrow X (Fic 1) Pressure of fluid supplied to chamber 19 = Pressure of fluid leaving chamber 18 = Fo (zero) Force F1 applied t area 1 = Po X A1 = g Force F2 applied to area A = 1 2 Net force FN = P1 A2 Movement of piston 17 in direction of arrow Y (Fig 2) Pressure of fluid supplied to chamber 18 = P1 Pressure of fluid in chamber 19 = P1 Force F1 applied to area Al = P1 X Al Force F2 applied to area A2 = P1 X A2 Al = 2 X A2 Net force FN = F1-F2 = P1 2A2-P1 An Net force FN = P1 A2 Therefore, the net force FN is the same in each case and will apply from lock-to-lock.

As the piston 17 moves in the direction of arrow Y fluid from chamber 19 passes through the valve 15 to the chamber 18. Therefore, as weii as providing the same net power assistance forces in both directions .7. and Y, the present invention facilitates the use of a smaller delivery pump 11 as the delivery requirement for the system needs to e sufficient only to supply the chamber 19 bearing in mind that fluid pumped to chamber 18 is supplemented by fluid from the chamber 19 during movement of the piston in the direction of arrow Y.

In the unlikely event that the spool 26 should jam in the position shown in Fig 1, the assistance level will be as shown in Fig 1 when steering rack 22 is moving in the direction X. However, when movement of the steering rack 22 in the direction Y is required, pressure P1 will be applied to area Al and fluid from chamber 19 will be vented through ports 15c, 15a, 13d and 13b therefore providing power assistance force on the steering rack 22 which is double that normally provided.

If the spool should jam in the Fig 2 position, there will be no power assistance when calling for movement of the steering rack 22 in the direction of arrow X. However, normai power assistance will be provided when movement of the steering rack 22 is called for in the direction of arrow Y.

It will be noted, however, that the probability of jamming is minimised in the present invention by applying fluid pressure to the left and right hand ends of the spool 26 causing or assisting displacement thereof.

Although specific reference has been made to the use of a shuttle-type control valve 13, it will be appreciated that the control valve 13 could comprise a rotary valve or a multiple ohec:Y vaive.

Claims

1. A power-assisted steering system for a vehicle comprising a pump for delivering fluid under pressure, a steering cylinder having a piston for applying power assistance to a steering element of the vehicle, the piston having faces of different areas, a control valve operable in response to rotation cf a steering wheel for controlling flow of the fluid from the pump to the steering cylinder and a further valve arranged in a fluid path between the control valve and the steering cylinder which causes pressure to be applied primarily to the smaller area of the piston when providing power assistance in one steering direction and to both areas when providing power assistance in the other direction.

2. A power-assisted steering system according to Claim 1 in which fluid pressure is applied only to the smaller face when providing the power assistance in salsa one direction.

3. A power-assisted steering system according to Claim 1 or 2 in which the larger area of the piston is double the smaller area of the piston.

4. A power-assisted steering system according to Claim 1,2 or 3 in which the piston divides the steering cylinder into two chambers and the further valve causes fluid from one chamber to pass to the other chamber during movement of the piston in the steering cylinder when applying pressure to both areas of the piston.

5. A power-assisted steering system according to Claim 4 in which fluid passes from the chamber defined in part by the smaller area of the piston to the chamber defined in part by the larger area of the piston.

6. A power-assisted steering system according to Claim 4 or 5 in which the further valve causes fluid from the chamber defined in part by the larger area to be vented when applying pressure to the smaller area of the piston.

7. A power-assisted steering system according to any preceding Claim in which the further valve is a shuttletype valve in which the shuttle is movable by a bias means into a first operative position.

8. A power-assisted steering system according to Claim 7 in which in the first operative position of the shuttle, the further valve causes pressure to be applied primarily to the smaller area of the piston.

9. A power-assisted steering system according to Claim 8 in which the pressure applied primarily to the smaller area of the piston is also applied to the shuttle to provide a force thereon in the bias direction.

10. A power-assisted steering system according to Claim 7, 8 or 9 in which the shuttle is movable by fluid pressure against the bias means into a second operative position.

11. A power-assisted steering system according to Claim 10 in which in the second operative position, the further valve causes pressure to be applied to both areas of the piston.

12. A power-assisted steering system for a vehicle constructed and arranged substantially as described herein with reference to the accompanying drawings.