GB2204540A - Hydraulic power assisted vehicle steering system - Google Patents

Abstract

The system has a control valve which controls assistance pressure to a servo motor, the control valve having two valve elements which are rotationally displaceable relative to each other in response to a steering input and from a neutral condition to determine the power assistance which is provided. The two valve elements are biased to the neutral condition by a torque rod or spring. A vehicle speed responsive pump 26, the output from which increases in pressure as vehicle speed increases communicates by way of a non-return valve 30 with a ball clutch that reacts in response to an increase in hydraulic pressure to increase restraint to relative rotation between the valve elements. The non-return valve 30 communicates downstream thereof with an hydraulic accumulator 31, the accumulator 31 communicates with the ball clutch and maintains a source of hydraulic pressure for temporary actuation of the clutch in the event of the pump 26 failing. <IMAGE>

Description

"Hydraulic power assisted vehicle steering system" TECHNICAL FIELD & BACKGROUND ART The present invention relates to an hydraulic power assisted vehicle steering system. More particularly the invention concerns a system in which a control valve having two valve elements which are displaceable relative to each other in response to a steering input and from a neutral condition determines power assistance which is to be provided by hydraulically actuated servo motor means and in which the two valve elements are biased to the neutral condition by elastic restraining means (for example a torque rod or a C-spring component in the case of rotary displaceable valve elements).In such a system it is known to provide a clutch means which is responsive to variations in fluid pressure applied thereto and reacts -to increasingly restrain relative displacement between the valve elements as vehicle speed increases and vice versa so that maximum power assistance becomes available at low vehicle speed (as may be required for parking manoeuvres) while zero or minimum power assistance is available (to provide maximum steering feel) at high vehicle speed.

Examples of power assisted steering systems of the kind generally discussed above are to be found in our British Patent Specifications Nos. 1,465,901; 1,470,975; 1,535,360; 1,591,309 and 2,088,795.

The fluid pressure which is applied to the clutch means and which varies in accordance with vehicle speed may be derived from a speed responsive hydraulic pump or from a speed responsive governor by way of which hydraulic fluid pressure is supplied from a constant pressure pump or other source. It will be apparent that if there is a failure in the vehicle speed responsive pressure which is applied to the clutch means while the vehicle is at high speed, the clutch means may immediately revert the control valve to a condition in which maximum power assistance is available and thereby minimum feel. This reversion could occur while a steering manoeuvre is being effected and the sudden change in steering characteristics that would be sensed by the driver could have obvious undesirable consequences.

It is an object of the present invention to provide an hydraulic power assisted vehicle steering system of the kind generally discussed above and by which the aforementioned disadvantage may be alleviated.

STATEMENT OF INVENTION & ADVANTAGES According to the present invention there is provided an hydraulic power assisted vehicle steering system comprising servo motor means; a control valve controlling assistance pressure to the servo motor means, said valve having two valve elements which are displaceable relative to each other in response to a steering input and from a neutral condition to determine the power assistance which is provided by the servo motor means; elastic restraining means which biases the valve elements to said neutral condition; a vehicle speed responsive hydraulic pressure source in which hydraulic pressure therefrom increases as vehicle speed increases, the hydraulic pressure from said speed responsive source communicating by way of a non-return valve with clutch means which reacts in response thereto to increase restraint to relative displacement between the valve elements as vehicle speed increases; fluid restrictor means by which pressure fluid applied to the clutch means can bleed to low pressure, and an hydraulic accumulator located downstream of said non-return valve and in communication therewith and with the clutch means, said accumulator being charged by pressurised fluid derived from the non-return valve and serving to maintain a source of hydraulic pressure for temporary actuation of the clutch means in the event of the vehicle speed responsive pressure failing.

The present invention was developed to alleviate the possibility of a spontaneous and possibly drastic change in steering feel characteristics being sensed by a driver while a vehicle is being manoevured at high speed in the event that the speed responsive pressure supply for the clutch means fails. Under these conditions the hydraulic accumulator may be fully charged and if the pressure source fails, hydraulic pressure from the accumulator closes the non-return valve and is %available to maintain pressure on the clutch means and thereby provide the driver with a relatively heavy steering feel, albeit for a temporary period but which may be adequate for completion of a steering manoevure which is being undertaken at the instant of failure.The hydraulic fluid under pressure which is applied to the clutch means communicates by way of a restrictor with low pressure (such as a return to reservoir conduit) and as such the hydraulic accumulator will discharge at a rate determined, in part, in accordance with the characteristics of the fluid restrictor. When the accumulator is exhausted full power assistance will become available irrespective of vehicle speed, however, the dissipation of fluid pressure from the accumulator may reduce progressively so that the driver of the vehicle may sense a progressive decrease in steering feel as more power assistance becomes available and thereby adapt to the changing steering characteristics whilst realising that there is a failure in the system.

Control valves for power assisted vehicle steering systems are usually of the rotary type whereby the two valve elements are rotationally adjustable relative to each other to control power assistance, however it will be realised that the present invention can be applied to control valves in which the two valve elements are axially adjustable relative to each other for controlling power assistance.

DRAWINGS One embodiment of an hydraulic power assisted vehicle steering system of the rack and pinion type will now be described, by way of example only, with reference to the accompanying illustrative drawings, in which: Figure 1 is a schematic illustration of the system; Figure 2 is a section through a rotary control valve of the system which incorporates clutch means reacting between two elements of the valve, and Figure 3 is a sectional illustration of an hydraulic accumulator and non-return valve unit incorporated in the system.

DETAILED DESCRIPTION OF DRAWINGS The power assisted vehicle steering system illustrated is generally of a well known rack and pinion type having a rack bar with tie rods 1 longitudinally displaceable in a housing 2 in response to rotation of a pinion 3 which engages with a rack on the rack bar. Coaxial with the pinion 3 is an input shaft 4 to which is connected a steering column 5. The input shaft 4 has an integral valve rotor 5 which is received in a valve sleeve 6 within a cylindrical chamber of the housing 2. The sleeve 6 is coupled to rotate in unison with a shaft 7 of the pinion 3.

An elastic torque rod 8 located in a bore 102 of the rotor 5 and coaxial with the shafts 4 and 7 connects between those shafts and has its opposite ends pinned thereto at 9 and 10 respectively. The rotor 5 and sleeve 6 together comprise a rotary valve for the control of hydraulic fluid between an inlet port 11 and an exhaust port 12 to ports 13 and 14 which serve a servo motor (not shown) such as double acting ram on the rack bar within the housing 2. The valve elements 5 and 6 have ports and recesses to provide the necessary control of hydraulic fluid in conventional manner as is well known in the art. The torque rod 8 rotationally biases the rotor 5 relative to the sleeve 6 to a neutral condition of the control valve (which may conveniently be regarded as of the open centre/open return type).Upon a steering torque being applied to the input shaft 4 to rotate the pinion 3, the resistance to rotation of the pinion causes the. rotor 5 to be displaced rotationally relative to the sleeve 6 and thereby the control valve to be adjusted to control pressure fluid from the input port 11 to the ports 13 and 14 and thereby to the servo motor to provide power assistance to displacement of the rack bar in accordance with the steering manoeuvre which is intended.

Reacting between the valve rotor and sleeve elements 5 and 6 is a clutch device 15 comprising two diametrically opposed ball members 16 radially displaceable while being retained in bores 17 in the valve sleeve 6. The balls 16 are displaceable radially inwardly of the sleeve 6 to engage in recesses 18 in the rotor 5. Hydraulic fluid under pressure is applied by way of a port 19 in the housing 2 to a chamber 20 which communicates with the bores 17 and thereby the pressure from such fluid reacts on the balls 16 to urge them into engagement with the recesses 18.

It will be apparent thatas the pressure of hydraulic fluid at the port 19 and in chamber 20 increases, the balls 16 are urged more firmly into engagement with the recesses 18 and thereby increasingly restrain relative rotation between the valve rotor and sleeve elements from the neutral condition of the control valve. The arrangement and operation of the assembly shown in Figure 2 is well known in the art, being disclosed in Figure 1 of our British Patent Specification 2,088,795.

Referring more particularly to the system shown in Figure 1, an engine driven and constant flow power assistance pump 21 draws fluid from a reservoir 22 and provides fluid under pressure by way of conduit 23 to the inlet port 11 for power assistance purposes while a low pressure return conduit 24 communicates between the port 12 and reservoir 22.

Hydraulic fluid pressure for the port 19 is provided by way of a conduit 25 which includes a pump 26 that is driven by an electric motor 27 and draws fluid from the reservoir 22. The motor 27 has a controller 28 associated with which is a vehicle speed sensor 29. The motor 27 is controlled by the vehicle speed sensor 29 through the controller 28 so that the hydraulic pressure derived from the pump 26 is proportional to the speed of the vehicle in which the system is incorporated in the sense that as the vehicle speed increases the pump pressure increases to a maximum and conversely as vehicle speed decreases the pump pressure decreases to a minimum or zero. The output from the pump 26 communicates through the conduit 25 with the port 19 by way of a non-return valve 30.Communicating with the conduit 25 downstream of the non-return valve 30 and thereby in constant communication with the port 9 is an hydraulic accumulator 31.

The non-return valve 30 and accumulator 31 are conveniently constructed as a unit as shown in Figure 3 which may readily be incorporated in an hydraulic fluid pressure supply line for a clutch device of an existing power assistance vehicle steering system of the kind to which the present invention generally relates.

As shown in Figure 3 the non-return valve/accumulator unit comprises a ball 32 which is spring loaded at 33 within a valve housing 42 (which forms part of the conduit 25) to normally close a port 34 of the conduit 25. When hydraulic pressure from the pump 26 is greater on the upstream side of the ball valve 32 than on the downstream side of that valve the pressure differential moves the ball 32 from its seating to open the port 34 and provide fluid pressure supply whereas if the pressure differential is reversed, for example due to a failure of the pump 26, a greater pressure on the downstream side of the ball valve 32 will close the port 34. Communicating with the conduit 25 in the valve housing 42 and downstream of the port 34 is a branch passage 35 which is in constant communication with an accumulator chamber 36 formed within a piston housing 37.The chamber 36 is variable in volume by displacement of a piston 38 in the housing 37. The piston 38 is biased to contract the chamber 36 by a helical spring 39 which reacts between the piston and an end cap 40 of its housing 37. Annular fluid seals 41 are provided between the piston and its cylinder in the housing 37 and between the piston housing 37 and the valve housing 42 to which the piston housing 37 is connected, for example by screw thread.

In use of the system and during low speed manoeuvring of the vehicle, hydraulic fluid from the pump 26 will have very little pressure and the accumulator 31 may be regarded as discharged so that little pressure is applied to urge the balls 16 into the recesses 18; negligible restraint is thereby provided by the balls to relative rotation between the valve rotor and sleeve elements and maximum power assistance is available from the servo motor.As vehicle speed increases and the hydraulic pressure from the pump 26 increases, the fluid pressure at the port 19 urges the balls 16 into engagement with the recesses 18 to restrain relative rotation between the valve elements and thereby reduce the power assistance which is available while the accumulator 31 is charged (by fluid under pressure expanding the accumulator chamber 36 against the biasing spring 39 - possibly to the extent that when the pressure output from pump 26 is at a maximum, the chamber 36 is fully expanded). In the event that there is a sudden loss of hydraulic pressure upstream of the nonreturn valve 30 (say due to a failure of the pump 26), a pressure differential develops over the non-return valve by the greater pressure of fluid in the accumulator 31 under the spring loading 39.This causes the ball valve 32 to close and hydraulic fluid pressure is supplied from the accumulator to the ball members 16 to urge those members into engagement with the recesses 18 and provide restraint against relative rotation between the valve rotor and sleeve elements.

It will be realised that the hydraulic pressure provided from the accumulator 31 will exist for a relatively short period. The reason for this is that fluid under pressure which is applied to the ball members 16 of the clutch device bleeds through a restrictor in communication with the clutch device to low pressure, normally to return to the reservoir 22. In the absence of such a restricted return or bleed, once the non-return valve 30 had closed in response to fluid pressure in the accumulator 31, that pressure would be maintained on the ball members 16 and the system could not return to a condition in which full power assistance is available.In the present example the restricted bleed is provided by an appropriate tolerance between the balls 16 and their respective bores 17 whereby fluid under pressure in the chamber 20 can bleed through the aforementioned restriction in the bores 17 to enter the recesses 18. The recesses 18 communicate with a chamber 100 in the valve sleeve from which fluid can flow by way of a cross bore 101 in the valve rotor to the central bore 102 of that rotor and therefrom flow into the reservoir return conduit 24 by way of a passage indicated by the broken line 103.

Although hydraulic pressure from the accumulator 31 may dissipate rapidly, the temporary period for which that pressure can react on the ball members of the clutch device may be adequate for a steering manoeuvre to be completed at high speed of the vehicle without the driver sensing a material change in the steering characteristics (for example should the system fail while a manoeuvre is being effected). Furthermore, the accumulator 31 will discharge so that the fluid pressure derived therefrom progressively decreases; consequently in the event of a system failure there will be a progressive increase in the power assistance which is available and this progressive increase should be discernible to the driver who can accommodate for it even though the steering characteristics will change while the vehicle may be moving at constant speed.

Claims (2)

1. An hydraulic power assisted vehicle steering system comprising servo motor means; a control valve controlling assistance pressure to the servo motor means, said valve having two valve elements which are displaceable relative to each other in response to a steering input and from a neutral condition to determine the power assistance which is provided by the servo motor means; elastic restraining means which biases the valve elements to said neutral condition; a vehicle speed responsive hydraulic pressure source in which hydraulic pressure therefrom increases as vehicle speed increases, the hydraulic pressure from said speed responsive source communicating by way of a nonreturn valve with clutch means which reacts in response thereto to increase restraint to relative displacement between the valve elemets as vehicle speed increases; fluid restrictor means by which pressure fluid applied to the clutch means can bleed to low pressure, and an hydraulic accumulator located downstream of said non-return valve and in communication therewith and with the clutch means, said accumulator being charged by pressurised fluid derived from the non-return valve and serving to maintain a source of hydraulic pressure for temporary actuation of the clutch means in the event of the vehicle speed responsive pressure failing.

2. A system as claimed in claim 1 and substantially as herein described with reference to the accompanying illustrative drawings.