GB2053821A - Steering mechanism - Google Patents

Abstract

Hydraulic power-assisted steering mechanism comprises two steering valves 60, 60' having pressure chambers 21 in permanent communication with a steering motor 3. Pistons 62, 62' have throttle-openings 37, 37' into spring chambers 11, 11'. An actuator 2 comprises pre-control valves 43, 43'. A cut-out device comprises a pressure limiting valve 70, back pressure valves 68, 68' feed-back chambers 66, 66' and throttle-openings 64, 64'. An alternative mechanical cut-out device is also described. The steering pressure is proportional to the force applied to the actuator 2 up to a cut-out point, and then rises steeply, for example for parking. <IMAGE>

Description

SPECIFICATION Steering mechanism The invention concerns a hydraulic power-assisted steering mechanism which is suitable for motor vehicles, i.e. for actuation by a steering handwheel or a steering wheel of a ship.

Power-assisted steering mechanism of this kind provides a working pressure in a steering motor within a proportional band which increases in proportion to the force applied to the actuator. Once pressure and force have exceeded a certain limit (the cut-out point), a slight increase in the force applied greatly increases the working pressure. This leads to high pressure being available e.g. for parking operations with relatively little applied force and no feedback to the actuator.

The invention provides a power-assisted steering mechanism comprising two steering valves, each valve having a pressure chamber in permanent communication with one working chamber of a steering motor, a spring-loaded piston having a throttle-opening from the pressure chamber to an actuator chamber, an actuator for controlling the pressure in the actuator chamber in proportion to the force applied to the actuator up to a cut-out point, and a cut-out device for allowing the pressure to rise independently of the applied force above the cut-out point.

Such a mechanism can be provided with a high hydraulic efficiency, as the steering valves are not required to process all of the working flow passing to the steering motor. It can be a simple design with hydraulic steering device to achieve the cut-out effect. By using as a hydraulic power source a reciprocating piston pump controllable on the suction side, and controlling the suction side via a pair of control edges of the control valves, the hydraulic energy available at the pump is utilised without substantial loss. this can be achieved with a minimum of constructional work at the steering gear unit. Due to the low concentration of energy at the actuator or control component in communication with the steering valves no disturbing flow noises are transmitted via the steering column into the passenger compartment of a motor vehicle. The cut-out device ensures energy-saving.

Drawings Figure lisa schematic illustration of a steering mechanism according to the invention but without a cutout device; Figures 2 and3 illustrate two differentembodi- ments of the invention for rack and pinion steering; Figure 4 illustrates a steering mechanism according to the invention with hydrostatic cut-out means; Figures 5and 6 are an axial and a cross section respectively of a mechanical device for achieving the cut-out effect in the mechanism of Figure 1; and Figure 7 is a graph of the steering characteristics of the mechanism of Figure 1.

As shown in Figure 1, hydraulic power-assisted steering mechanism comprises three main sections: an energy supply unit 1 inside a circle, an actuator 2, and a steering motor 3. The energy supply unit 1 comprises two similar mechanically independent steering valves 5 and 5' and a reciprocating piston pump 7 controllable on the suction side. The steering valves each have a valve piston 9,9' and a working spring 11,11', and act as pressure balances.

The associated valve housings are shown in outline only. As the two are similar, only the left half of the mechanism will now be described in detail. Inside the steering valve 5, the high pressure line 12 is in communication with an annular groove 17 in the valve housing via a control edge 15. This groove 17 is in communication with an inner pressure chamber 21 of the valve piston 9 via through holes 19 in the valve piston wall. The pressure chamber 21 is connected with a working chamber 4 of the steering motor 3 via a working line 23.

From a tank 25 holding working fluid, a tank line 28 leads through branches to the steering valves 5,5'.

The tank line 28 serves as suction and as a return line. Inside the steering valve 5, the tank line 28 is in communication with an annular groove 31 in the valve housing via a pair of control edges 29.

Connected to annular groove 31 is a suction line 34 of the pump 7. A further pair of working edges 35 permits a connection to be set up from the pressure chamber 21 of the steering valve 5 to the tank line 28.

On its side adjacent the working spring 11, the valve piston 9 is equipped with a throttle opening 37. The adjoining spring chamber 39 is in communication with a pre-control valves 43 in the form of a pressure-holding valve via a control line 41. The spring chambers of both pre-control valve 43 and 43' are connected to a tank line 28a.

The actuator 2 is equipped with a steering handwheel 45 which, when turned in opposite directions, acts upon the two springs 47,47' of the pre-control valves 43,43'. In addition, the actuator 2 has a mechanical effect upon a steering element which is schematically through a connection shown as the broken line 49. The pre-control valves 43,43' are open when in the neutral position, i.e. when the handwheel 45 is not acted upon by any force. By a turn on the handwheel 45, one of the springs 47,47' is loaded proportionally to the force acting upon the handwheel 45.

The flow path of the working fluid is from the tank line 28 to the suction side of the pump 7 via the pair of control edges 29 and the suction line 34. The pressure fluid delivered by the pump flows through the higher pressure line 12 and the pair of control edges 15 into the pressure chamber 21, through the throttle opening 37, the control line 41 and the pre-control valve 43 into the tank 25. The pair of control edges 29 and the working spring 11 ensures that the pre-control valve 43 is supplied with a constant control flow. The pump 7 is controlled on the suction side.

If, with a turn of the handwheel 45, the force applied to the spring 47 of the pre-control value 43 is increased, the flow cross-section of the valve 43 is reduced. This leads to an increase in pressure in the spring chamber 39 causing the valve piston 9 in Figure 1 to move downwards. The pair of control edges 29 opens sufficiently wide for the pump 7 to receive the necessary suction flow. The increased pressure in the chamber 21 extends to the left hand working chamber 4 of the steering motor 3, and the pump 7 delivers the amount offluid required for the adjustment. The fluid displaced from the working chamber 4' reaches the pressure chamber 21' of the right hand steering valve 5'. Due to the rising pressure in this chamber, the valve piston 9' moves upwards until the pair of control edges 35' opens allowing working fluid to leave through the tank line 28.

The steering valves 5,5' thus act as limit valves for the suction flow of the pump 7 with a saving in energy. The differential pressure resulting from the working springs 11,11' and the cross-section of the steering valves counterbaiances the currents flowing through the throttle openings 37,37'. The flow currents in the control lines 41,41' are small compared with the working flow currents. Therefore, the energy in the actuator 2 has a low concentration, and no disturbing flow noises are transmitted into the passenger compartment of the vehicle.

The two mutually independent steering valves 5,5' lead to trouble free operation as evidence from the following consideration.

Let us assume that in the neutral position or with a turn at the hand wheel 45 the current flowing through the throttle opening 37 is too small for the steering position. The result is an insufficient pressure differential between the pressure chamber 21 and the spring chamber 39. The working spring 11 will therefore press the valve piston 9 downwards.

The pair of control edges 29 will open further and the delivery rate of the pump 7 will increase until the current flowing through the throttle opening 37 has reached its pre-setvaiue.

Conversely, if the current flowing through throttle opening 37 is too large, for example because the pair of control edges 29' of the steering valve 5' are too far open, it may be that closure of the pair of control edges 29 is not enough (due to the bypass at the pair of control edges 29') to create equilibrium at the steering valve 5 (acting as pressure balance). In this case, the current flowing from the high pressure line 12 to the pressure chamber 21 is limited by the pair of control edges 15.

If even this is not sufficient for limiting the current flowing through the throttle opening 37, for example because due to a movement to the left by the piston of steering motor 3, additional working fluid is delivered via the working line 23 to the pressure chamber 21, the valve piston 9 will be pushed even further against its working spring 11, and the surplus working fluid is allowed to leave via the now-open pair of control edges 35 to the tank 25. Thus it is ensured that the control currents flowing to the actuator are constant in each operational state whatever the position of the valve pistons 9,9' in relation to each other. The pressure balances copy the pressures set at the pre-control valves in the two working chambers of the steering motor 3.

Operational Safety To start with, let us assumethatone of the working chambers of the steering motor 3, e.g. the working chamber 4, is under pressure without the spring 47 having been tensioned by the handwheel 45. This means an increase in the pressure difference between the pressure chamber 21 and the spring chamber 39 resulting in an upward-acting force upon the valve piston 9. Counter-steering also may be used to build up pressure in the working chamber 4' to act upon the piston of the steering motor 3, thus increasing the pressure in the working chamber 4.

The counter-steering opens the pre-control valve 43 and so reduces the pressure in the spring chamber 39. Thus the valve piston 9 is exposed to the full force of the pressure in the working chamber 4. The valve piston 9 is pushed against its spring 11 until the pair of control edges 35 opens and allows working fluid to flow off to the tank 25 and thus relieves the pressure in the working chamber 4.

Now let us assume the opposite: there is a demand for a build-up of pressure in the working chamber 4, but this is not supplied because the pre-control valve 43 does not close owing to the entry of foreign particles or because the pre-control throttle 37 is blocked. The pressure in the pressure chamber 21 ensures that the valve piston 9 con tinges to be pushed upwards until the pressure chamber 21 and working chamber 4 are without any pressure. This is not critical, for it is comparable to a failure of the pump 7, in which case steering is effected by the mechanical connection 49.

Figure 2 shows a mechanical rack and pinion 52, a control head 54, an energy supply unit 1 driven from the vehicle engine and containing the pump 7 and steering valves 5,5' of Figure 1, the tank 25 and the steering motor 3. The pressure lines leading to the control head 54, i.e. the control lines 41,41' and the tank line 28a, are of comparatively small crosssection. The working lines 23,23' carrying the steering energy itself only run between the energy supply unit 1 and the steering motor 3, and between the unit 1 (line 8) and the tank 25. The steering motor 3 may form an integral part of a rack and pinion steering mechanism. However, there are advantages in a separate arrangement, particularly for subsequent installation, as mechanical rack and pinion steering mechanism is usually provided with a steering damper, whose place may be taken by the steering motor.

In Figure 3, the steering valves are integral not with the pump 7, butwiththetanktoform a unit 56.

This is of advantage when for constructional reasons hose line are employed to form the pressure line 12, 34to the pump 7.

Cut-out (Hydraulic) In Figure 4, parts corresponding to those in Figure 1 bear the same reference number and are not further described. Two steering valves 60,60' are integrated not with the pump 7, but with the tank 25.

The pairs of control edges occupy a position different from that in Figure 1, but have the same function as is indicated by their reference numbers.

The valve pistons 62,62' are equipped with throttle openings 64 and 64', i.e. with feedback throttles on the side not facing the springs 11,11'. The associated feedback chambers 66,66' of the steering valves are each connected via a back pressure valve 68,68' with a common adjustable pressure limiting valve 70. The spring chamber of this valve is connected with the tank line 28.

The feedback throttles 64,64', in conjunction with the back pressure valves 68,68' and the pressure limiting valve 70, limit the pressure in the feedback chambers 66,66' and thus control the pressure. For instance, if during a turn on the steering wheel the pre-control valve 43 is set at a higher pressure than the pressure limiting valve 70, no fluid flows to the pre-control throttle 37. This upsets the pressure balance in the steering valves 60,60'. The working fluid flows through the feedback throttle 64, the back pressure valve 68 and the pressure limiting valve 70 to the tank 25. The valve piston 62 is pushed downwards irrespective of any larger forces acting upon the pre-control valve 43. This causes an increase in pressure in the working chamber 4 of the steering motor 3, and the steering motor 3 is adjusted without any feedback.

Cut-out (Mechanical) In Figures 5 and 6, a steering spindle 72, which is rotatable by an actuator (handwheel) is rotatably mounted in an output part 76 by means of needle bearings 74. In a rack and pinion steering mechanism, the output part 76 forms an integral part with a pinion 78. The pre-control valves 43 and 43' are housed in a housing 80, the inside of which is connected with the tank line 28a. The two control lines 41,41' lead to the valve seats via separate ring grooves and channels 82,82'. The steering spindle 72 forms an integral part with a crossarm 88. Attached to the crossarm by means of a screw 90 is a leaf spring 86 fitting into a lateral recess 84 of the steering spindle 72 at the level of the crossarm 88.

The output part 76 is equipped with rigid stops 92 for the ends of the crossarm 88. The output part 76 is equipped with rigid stops 92 for the ends of the crossarm 88 thus limiting the area of free movement of the steering spindle in relation to the output part, and permitting steering in case of failure of the hydraulic power-assistance. The leaf spring 86 forms the two working springs of the pre-control valves 43 and 43' and ensures valve actuation within the proportional band. The cross arm 88 has lateral holes at the ends which loosely hold cups 94 supported against the crossarm by collars 96. Each cup 94 holds a centring spring 98. The centre position is adjusted by means of adjustment screws 100 located in shoulders 102 firmly attached to the output part 76. The centring springs 98 have a far greater stiffness (100-fold for example) than the arms of the leaf spring 86.The centring springs 98 also serve to limit the pressure to a maximum tolerable working figure.

Function In neutral position (as illustrated in Figures 5 and 6) the arms of the leaf spring 86 do not exert any force upon the spheres of the pre-control valves 43,43'. As a result the spring chamber 39 (see also Figure 1) is without any pressure, and the hydraulic system is ineffective.

As a torque is introduced by the steering spindle 72, it is supported against the pinion 78, and both parts rotate against each other. If the crossarm 88 is deflected in the direction of the arrow 104 in relation to the outpart 76, the centring spring 98' is tensioned and the spring 98 is relieved. Figure 7 shows a graph illustrating the dependence of the pressure P operating in the steering motor on the force F acting upon the actuator. At the beginning of a steering turn, starting at point 0, the force F only increases without the hydraulic system being effective. Until point G (the boundary force) has been reached, the left arm of the leaf spring 86 does not exert any force upon the pre-control valve 43. All the same, the force acting upon the actuator increases due to the effect of the two centring springs 98,98'.Once the boundary force G has been reached the leaf spring exerts a force upon the valve 43, and the pressure P in the working chamber of the steering motor rises corresponding to the straight line 106. The device is now within the proportional band. The force noticeable on the actuator emanates almost exclusively from the centring springs 98,98', and to a very small degree from the leaf spring 86, but it is the leaf spring which has the decisive influence upon the pre-control valve 43.

At the end of the proportional band, as cut-out point A is reached; pressure P in the steering motor rises steeply, while the force F on the actuator increases only slightly. In other words, the pressure follows a steeply rising branch 108. This is due to the cup 94 touching the left arm of the leaf spring 86.

Upon a further minute rotation, the valve 43 is closed compietely under the influence of the centring spring 98, and the pressure in the steering motor rises corresponding to the branch 108 to the maximum permissible limit.

From the moment of contact between the cup 94 and the leaf spring arm (the moment which corresponds to the cut-out point) all feedback from the centring spring 98 to the crossarm and the actuator ceases, and only the feedback force from the already highly tensioned centring spring 98' remains. All the same, no sudden increase in force will be felt on the actuating device, as the cut-out point is passed. For the working pressure in the steering motor which rises steeply from the cut-out point onwards, ensures, by means of the mechanical connection from the piston rod of the steering motor via the steering element 110, the rack 112 and the pinion 78 (see also Figure 2), that the pinion and output part 76 (Figure 5) are rotated away from the steering spindle 72.

Thus the hydraulic system comes into action before the sudden increase in force mentioned above can become effective, and no increase in force will be felt at point A.

As point H (Figure 7) is reached, no further increase in pressure in the steering motor is wanted.

This is achieved by one of the centring springs (in the example the left centring spring 98) yielding sufficiently to allow pressure fluid to leave the pre-control valve 43. As a result, the valve piston 9 in Figure 1 will move far enough upwards to ensure that the pressure in working chamber 4 of the steering motor does not exceed maximum value H.

As point H is passed, the cup 94 is lifted from the crossarm 88. The remaining influence is that of the centring spring 98' upon the crossarm, so that a sudden increase in force along the branch 114 (Figure 7) can be felt at the actuator. This increase in force cannot be rendered ineffective by the hydraulic system as its pressure is limited to the maximum value H. In any case, an increase in force is desirable at this point, for the driver should be made aware that greater power assistance is not available. As the force rises further along branch 114,a point is ultimately reached where the crossarm 88 comes to rest against the stop 92, and the output part 76 thus mechanically follows the steering spindle 72. This occurs even in the case of failure of the hydraulic system.

The centring springs 98 and 98' may be adjusted to a centre position by means of screws 100, i.e. in such a way that in the neutral position no force acts upon the actuator. By tightening both adjustment screws equally, the steepness of straight line 106 of the proportional band can be increased. By using centring springs of varying stiffness, the steering device may be set at varying maximum pressures for the right and left working chambers of the steering motor. This is desirable where the piston rod extends from one side of the steering motor only and therefore has piston surfaces of varying size. In this case, it is desirable that the maximum pressure in the working chamber 4' be higher than that in the working chamber 4, so that for each maximum pressure the same maximum force acts upon the piston. The proportional band is not affected by the use of centring springs of varying stiffness. For whilst passing through the straight line 106, the actuator is influenced by the difference in the force of the centring springs, resulting in a linear rise in force for both steering directions along the straight line 106, the inclines are identical.

Claims (9)

1. A power-assisted steering mechanism comprising two steering valves each valve having a pressure chamber in permanent communication with one working chamber of a steering motor, a spring-loaded piston having a throttle-opening from the pressure chamber to an actuator chamber, an actuator for controlling the pressure in the actuator chamber in proportion to the force applied to the actuator up to a cut-out point, and a cut-out device for allowing the pressure to rise independently of the applied force above the cut-out point.

2. A mechanism according to claim 1 in which each steering valve comprises pairs of edges for controlling the pressure between the pressure chamber and a pump and between the pressure chamber and a tank.

3. A mechanism according to claim 1 or claim 2 in which each steering valve comprises a pair of edges for controlling the pressure in a suction line to a reciprocating pump.

4. A mechanism according to any preceding claim in which each valve piston has a further throttle-opening from the pressure chamber to a feed-back chamber, and the feed-back chamber pressure is controlled by a pressure holding valve.

5. A mechanism according to claim 4 in which the feed-back chambers each lead to a back pressure valve and thence to a common pressure holding valve.

6. A mechanism according to any of claims 1 to 3 having a torque-transmitting drive comprising a rotatable part comprising a crossarm and a spring having arms which function as working springs for pre-control valve, the ends of the crossarm having resilient stops contactable with the spring at the cut-out point.

7. A mechanism according to claim 6 in which the stops comprise springs which act as centring springs of the steering mechanism.

8. A mechanism according to claim 7 in which the tension of the centring springs is adjustable.

9. A steering mechanism as herein described with reference to Figure 1 and Figure 2,3,4 or 5 and 6 of the drawings.