GB2026965A - Hydraulically operated steering system - Google Patents

Abstract

A hydraulically operated steering system for vehicles comprises an actuating means 38 constituted by the rack 40 of a rack pinion steering gear 40, 42, and an adjusting motor 21 having two working chambers 20 and 20' with which are respectively associated pairs of spring-loaded seat-type valves 22,24 and 22', 24'. Valves 22, 22' are inlet valves each of which is connected to a hydraulic pressure accumulator 4 and is urged against its seat 30, 30' by a compression spring 26, 26'. The two inlet valves 22 and 22' are actuated through the springs 26, 26' by the actuating means 38. Valves 24 and 24' are outlet valves each of which is connected through a common return flow connection 58 to a reservoir 59 and is urged against its seat 36, 36' by a tension spring 32, 32' connected to the actuating means 38. The piston rod 48 and piston 48 of the adjusting motor 21 are arranged fixed on the vehicle, while the cylinder casing 50 is movable in relation to them. The valve seats 30, 30' and 36, 36' are rigidly connected to the cylinder casing 50. In the neutral position of the steering system the inlet valves 22 and 22' are closed. They are adjusted to a pressure which is somewhat higher than the pressure in the accumulator 4. Both outlet valves 24, 24' are open. Both working chambers 20, 20' of the adjusting motor 21 are unpressurised. <IMAGE>

Description

SPECIFICATION Hydraulically operated steering system This invention relates to a hydraulically operated steering system, particularly for vehicles, having a steering valve arrangement, wherein the supply from a pressure source is closed in the neutral position of the valve arrangement. In contrast to a steer ing valve arrangement requiring a continuous neutral flow of the hydraulic medium, a steering valve arrangement of the foregoing kind affords the advantage that less energy is consumed when the steering system is not operated, since there are no circulation losses.

Steering valve arrangements of the foregoing kind are mainly for use in hydraulic steering systems i.e., steering systems in which there is a mechanical connection between the actuating means (steering wheel) and an adjusting motor for the vehicle parts to be steered. Hydraulic boosting is required only when the vehicle travels along curves, but not when it is travelling straight ahead, for example on motorways, so that a considerable amount of energy can be saved when driving straight ahead.

The invention can also be applied to hydrostatic steering systems - i.e., steering systems in which there is no mechanical connection between the actuating means and the adjusting motor. It is true that vehicles equipped in this way are not permitted to reach high speeds. Nevertheless, in this case also energy loss due to the circulation of the hydraulic medium is to be avoided if a particular steering movement remains unchanged, for example, when a working vehicle performs work with a steering movement unchanged, at rest or in movement.

Steering valve arrangements are also known from German Offenlegungsschriften Nos. 21 31 276 and 2446841 (ZF, Jablonsky). In the prior art steering valve arrangement the inlet valves, which form components of the steering valves, are constructed as seated valves which are closed in the neutral position. No circulation losses therefore occur. If the actuating means is operated beyond the response clearance, one seated valve is opened, and a working pressure builds up in one working chamber of the adjusting motor, namely that particular pressure chamber. A particular design of the inlet seated valves ensures that a reaction force proportional to the working pressure is exerted on the seated valves, such reaction force being perceptible at the actuating means (steering wheel) and giving the driverthe necessary "road feel".

With a steering movement, after the response clearance has been used up, the or each of the inlet seated valves is opened by mechanical abutmenti.e., when a connection between the actuating means and the valve suddenly becomes rigid. As a result an excessive amount of pressure fluid suddenly shoots into the particular pressure chamber of an adjusting motor required for the steered vehicle parts. The consequence is unsteadiness in the steering operation, which has hitherto proved impossible to eliminate in the known steering valve arrangements.

A steering valve arrangement of a different kind with inlet seated valves is also known from German Auslegeschrift No. 12 14 554 (Bosch). This steering valve arrangement operates on the pressurereducing principle and calls for an expensive construction.

The present invention aims at providing a steering valve arrangement which has inlet seated valves and is so constructed that no unsteadiness occurs during a steering operation, although the advantage of a proportional reaction from the pressure chamber on the actuating means is maintained.

Accordingly, the present invention consists in a hydraulically operated steering system, particularly for vehicles, comprising an actuating means and an adjusting motor, having two working chambers, for the steered vehicle parts, working fluid supply lines which extend from a pressure source respectively to the working chambers of the adjusting motor and contain two inlet seated valves closed in the neutral position of the steering system, and return flow lines which extend respectively from the working chambers of said motor to a reservoir and contain two outlet valves, the two inlet seated valves being connected via control springs in opposite directions to the actuating means.

The invention uses a different principle from the prior art, namely a pure force control system to control the inlet seated valves. This is achieved by disposing the inlet seated valve actuating springs, which are required in any case, in the line of the linkage. The seated valve starts to open when a given force (so-called limit force) has been reached.

However, after the limit force has been overcome, the pressure in the pressure chamber of the adjusting motor can build up only steadily, so that no unsteadiness can occur in the steering operation. In the adjoining proportionality zone (i.e., in the zone of steering movements, in which a reaction force proportional to the working pressure can be felt at the actuating means), the springs of the inlet seated valves have the effect of control springs for the inlet seated valves operating as pressure-limiting valves.

It is immaterial to the principle of the invention whether the force transmitted to the control springs is picked up directly from a steering spindle, a part of a steering transmission, or a part downstream of the steering transmission.

It is true that in the neutral position of the steering system the steering valve arrangements known from the two above-mentioned German Offen legungsschriften prevent any flow of hydraulic medium. In contrast, however, hydraulic medium must flow constantly if the driver when driving straight ahead must constantly perform counter steering to some extent on a road which falls away to one side, or with a side wind. Then necessarily one of the inlet seated valves is opened, and flow losses occur again.

According to an optional feature of the invention the two outlet valves are also constructed as seated valves and are actuated by the actuating means via their control springs. This has the advantage that even when the limit force is overcome and pressure medium flows via one inlet valve to the adjusting motor, any leakage flow to the reservoir is pre vented. If the steering wheel is merely held fast by force, but not rotated, no pressure medium is required either. This applies to driving with a side wind or on a road sloping to one side.

A further advantage of the foregoing optional fea ture is that at least each pair of valves (inlet and outlet valve associated with one working chamber) can be actuated by the same constructional member of the actuating means, so long as the outlet valve always holds the balance to a pressure greater by a differential pressure than the associated inlet valve.

It is therefore simple to actuate the valves. This advantage is achieved by the fact that pure force control is also used for actuating the outlet valves.

In the simplest case the spring chambers of the inlet and outlet seated valves are connected to the associated working chamber of the adjusting motor.

Consequently the force control of these valves also must come into effect in a chamber at working pressure. According to a further optional feature of the invention this can be avoided by using pressurebalanced inlet seated valves. The power steering for these valves can then be accommodated in a pressure-relieved chamber.

If the inlet and outlet seated valves are equipped with separate control springs, it can be difficu It to make sure that both valves hold the balance over the whole proportionality range to pressures which differ by a predetermined differential pressure. This difficulty can be overcome by a further optional feature of the invention, namely the use of a balance beam for each pair or both pairs of valves. In that case a common control spring acts on the inlet and outlet valves, while an additional spring ensures that the differential pressure is maintained over the proportionality range. There is therefore no need to adapt to one another within close tolerances the rigidities of two springs and the seat cross-section of two valves of each pair.

Beyond a predetermined value of steering movements, such as is exceeded when parking, a proportional reaction on the actuating system is no longer necessary, although it is desirable for the maximum working pressure to be available in the particular pressure chamberofthe adjusting motor. The socalled cutting-off effect must occur- i.e., the proportionality straight line must be cut off. Afterthe cutting-off point has been reached, the working pressure should reach its maximum value without any appreciable increase in the force exerted on the actuating system. To this end a very expensive construction is required for the aforementioned ZF steering valves, since two additional steering limiting valves must be provided. In the case of the aforementioned German Auslegeschrift 12 14554 the cutting-off effect cannot be achieved at all.

By a further optional feature of the invention the cutting off effect, with the use of a balance beam, can be achieved in very simple manner, namely by stops which limit the zone of operation of the control springs.

In order that the invention may be more readily understood, reference is made to the accompanying drawings which illustrate diagrammatically and by way of example embodiments thereof, and in which: Fig. 1 illustrates the principle of the force control system, Fig. 2 shows a part of a steering system having two pairs of inlet and outlet seated valves which are jointly controlled, Fig. 3 shows the various steering zones, represented by graphs, Fig. 4 is a longitudinal section through an embod imentofthe invention having a balance beam by means of which a pair of seated valves is controlled, Fig. 5 shows a similar embodiment of the invention, incorporated in a rack and pinion steering system, Figs. 6 and 6a are a longitudinal section and a cross-section along line a-a in Fig. 6 respectively, of a further embodiment of the invention, in which a common balance beam controls two pairs of inlet and outlet valves on the primary side of a steering transmission, Fig. 7 shows part of a rack and pinion steering system in which valve controls according to another embodiment of the invention having resilient striplike balance beams are provided on the secondary side of the steering transmission, and Fig. 8 shows a further embodiment of the invention, applied to a hydrostatic steering system.

Steering valve arrangement with seated valves and control springs: Fig. 1 shows an inlet seated valve 2 disposed in a supply line 3 extending from a pressure accumulator 4 to one working chamber of an adjusting motor (not shown). The inlet seated valve 2 has a valve ball 6 and a valve seat 8. The closure or control spring 10 is disposed between the valve ball 6 and a member 12 which is mechanically drivably connected to an actuating means, that is a steering wheel (not shown). As a rule the actuating means has a separate centring device, illustrated diagrammatically in Fig. 1 by springs 14. However, the function of the centring device can also be performed by two control springs arranged in opposite directions. The same valve arrangement must also be imagined on the opposite side of the member 12.The spring chamber of each of these inlet seated valves is connected to one of the working chambers of the adjusting motor. The valve performsthefunction of a pressure-limiting valve. In dependence on its actua tion,the member 12 adjusts the suitable working pressure in the pressure chamber of the adjusting motor. The centring springs 14, which are constructed as helical compression springs in this embodiment, are more conveniently constructed in known manner as a bending rod, torsion rod or cup springs.

Function.

The control spring 10, centring springs 14 and valve cross-section are so adapted to one another that in the neutral position, when the actuating means is free from forces, they hold the balance to a pressure which is slightly greater than the accumulator pressure in the supply connection 3. As a result in the neutral position the supply connection is reliably closed. If the member 12 is adjusted to the right (as viewed in Fig. 1), the force acting from the right on the valve ball 6 decreases due to the expansion of the control spring 10, until finally the pressure in the supply connection 3 overcomes the force of the spring 10 and lifts the valve ball 6 off its seat.

Pressure fluid can then flow from the pressure accumulator4 into the pressure chamber of the adjusting motor. The differential pressure between the pressure accumulator and the adjusting motor pressure chamber is held by the control spring 10.

Afterthe actuating means has exceeded the limit force, there is proportionality between the actuating force F and the working pressurep in the respective pressure chamberofthe adjusting motor. Fig. 3 shows these relationships. With a steering movement of 0, the pressure in the adjusting motor first remains 0, until the limit force G is reached. Then the pressure p rises proportionally to the force F exerted at the steering wheel along the straight line 16.

In vehicle steering systems the limit force is very desirable, since hydraulic boosting is preferably avoided in the range of small steering movements and steering forces, to improve road contact.

When the limit force is exceeded, a soft transition is obtained, since as a result of the arrangement of the control spring between the actuating means and the valve member, the latter opens gradually in correspondence with the increase in actuating force.

The absence of any rigid connection between the actuating means and the valve member has the further advantage that a seated valve can also be used as the outlet valve, and this is impossible for manufacturing reasons with a rigid connection (Fig.

4).

Steering valve arrangement having four seated valves.

Fig. 2 diagrammatically illustrates a steering system with pure seated valve control. Associated with each of the working chambers 20,20' of the adjusting motor 21 is a pair of valves, for example, one inlet seated valve 22 and one outlet seated valve 24 being associated with the working chamber 20. The same applies correspondingly to the working chamber 20'. The constructural members in the right-hand half of the drawing have the same references with an index. To the extent that the arrangement issym- metrical, details will be described only as regards the left-hand half of the drawing. They apply similarly to the right-hand half. The inlet seated valve 22 has as the control spring 26 a compression spring which forces its valve member 28 against the valve seat 30.

The outlet seated valve 24 has a control spring 32 constructed as a tension spring which pulls the valve member 34 against its seat 36 when appropriately actuated. The outer ends of the springs 26,32 are attached to a rigid, tongs-like shaped member 38 of the actuating means. The member 38 has a rack 40 which can be adjusted in the direction indicated by the double arrow 44 by a pinion 42 which can be rotated from the steering wheel (not shown).

The piston rod 46 and piston 48 of the adjusting motor 21 are arranged fixed on the vehicle, while the cylinder casing 50 can move in relation to them. The valve seats 30,36 are rigidly connected to the cylinder casing of the adjusting motor. The spring chambers of the control springs 26,32 open into the working chamber 20, this being illustrated diagrammatically in Fig. 2 by lines 52,53.

The supply connections 3, 3' of the inlet seated valves are connected to the pressure accumulator 4.

A common return connection 58 of the two outlet seated valves is connected to a reservoir 59.

In this embodiment no separate centring device is used, since with suitable dimensions the control springs 26, 32 etc. can also perform this function.

Operation:- In the neutral position (Fig. 2) in which the actuating member 38 is free from forces, the inlet valves 22, 22' are closed. They are adjusted to a pressure which is somewhat higher than the pressure in the pressure accumulator 4. Both outlet valves 24,24' are opened. Both pressure chambers 20,20' of the adjusting motor are unpressurised.

When the steering wheel is rotated and the pinion 42 moves the actuating member 38 to the right (as viewed in Fig. 2), the control springs set to such movement a resistance which can be felt at the steering wheel. When after a predetermined displacemenu the limit force G (Fig. 3) has been reached, the control spring 26' is expanded to such an extent that the pressure in the pressure accumulator 4 overcomes the spring force and the pressure fluid can flow into the working chamber 20', which then becomes the pressure chamber. Before this condition is reached the outlet seated valve 24' has closed, and its control spring 32' has been tensioned.The inlet and outlet valves are soadapted to one another that the outlet valves always hold the balance to a pressure which is greater by a differential pressure Ap than the pressure to which simultaneously the inlet valves hold the balance. Consequently at no operating position of the steering system can pressure medium, which is to be fed to the adjusting motor pressure chamber, flow away from the outlet valve connected thereto.

With the movement of the actuating member 38 to the right, the control spring 32 of the outlet seated valve 24, which is connected to the pressure-relieved working chamber 20, remains expanded, so that the working fluid can flow away to the reservoir 59. During this whole operation the inlet seated valve 22 has remained closed.

While a pressure equal to the accumulator pressure minus the pressure gradient at the inlet valve 22' builds up in the pressure chamber 20', the cylindex casing 50 of the adjusting motor moves. The rigid connection of the valve seats to the casing produces a control circuit which makes the cylinder to follow up the movement of the actuating member 38.

In Fig. 3 the straight line 16 shows the relationships at the respective inlet seated valve in operation, namely the pressurep regulated thereby in dependence on the actuating force F exerted on the steering wheel. As already stated, in each case the associated outlet seated valve holds the balance to a pressure higher by pressure differential Ap. For the outlet valve the straight line 60 is obtained which, as illustrated in Fig. 3, ideally extends parallel with the straight line 16. In no case may the two straight lines intersect one another. This would mean that the inlet and outlet valve of the same side are open simultaneously- i.e., that pressure medium could flow directly from the accumulatortothe reservoir, bypassing the adjusting motor.If in contrast both straight lines always have a distance Ap, namely a safety hysteresis freely selectable in principle, the pressure fluid must always follow a course via the adjusting motor. The straight line 60 intersects the F axis at a point S. The distance 0 - S represents the increase in actuating force within the clearance of the spring 24' of the outlet valve.

If the steering system is not actuated from the steering wheel, but from the steered wheels, for example when returning from taking a curve to driving straight ahead, and the actuating means is in or adjacent its neutral position (between 0 and S in Fig.

3), both outlet valves 24,24' are opened, and there is an unimpeded working fluid circuit between the working chambers of the adjusting motor and the reservoir. The cylinder casing 50 and the steered vehicle wheels can therefore move unimpeded in both directions.

If after a steering movement at a curve the driver does not completely release the steering wheel but merely allows it to return slowly with braking, the actuating member 38 again being moved from the end position which it has reached, for example on the right, to the left, first of all again the force of the control spring 26' of the inlet valve increases, and the supply from the pressure accumulator4 is cut off. When the actuating force has been reduced by a value AF corresponding to the differential pressure Ap, the outlet valve 24' opens, and pressure fluid can flow away to the reservoir. Thereafter the control spring 32' is expanded, and as more pressure fluid flows away from the pressure chamber 20', the pressure at the outlet valve drops.The straight line 60 (Fig. 3) is passed through in the direction indicated by the arrow- i.e., downwards- until finally the point S is reached and the working chambers are pressure-relieved. During the return steering operation, therefore, also the pressure in the pressure chamber of the adjusting motor is clearly associated with a force at the steering wheel.

Cutting-off effect.

Beyond a predetermined actuating force at the steering wheel there is no advantage in a further proportional increase in force. More particularly during parking it is undesirable that the steering force should increase appreciably. However, at the same time there is no advantage in any feedback of the forces acting on the vehicle wheels. The proportion alityrange, represented bythe straight line 16 in Fig.

3, should be cut off beyond an actuating force A.

Then the pressurep in the pressure chamber increases steeply along the straight line 62 without any appreciable increase in actuating force. (The position 16a of the straight line has been cut off).

In a steering valve according to the invention the cutting-off effect can very simply be produced by the fact that the effect of the control springs is cancelled out from a predetermined actuating distance and therefore a predetermined actuating force onwards.

This is shown by all the following embodiments.

Pairs of valves with balance beam.

In the embodiments illustrated in Figs. 4 and 5 constructional members already described have the same two-place references, but with a preceding 4 or 5, resulting in reference numerals in the hundreds.

Fig. 4 shows the cylinder casing 450 of the adjusting motor, its piston 448 with a piston seal 477 and piston rod 479. Only the left-hand half (as viewed in the Figure) of the piston is shown, and therefore only one pair of seated valves, namely inlet seated valve 422 and outlet seated valve 424. The same valve arrangement must also be imagined in the righthand piston half. All four valves are therefor integrated in the piston. To avoid difficulties when adapting the valve springs to one another, in this case an inlet valve and an outlet valve are each jointly actuated by a balance beam 464. The inlet valve 422 has no control spring of its own, but both valves are actuated via a common control spring 466 arranged in the centre of the balance beam. The control spring encloses a stop pin 468. The actuating means acts on a tappet 470 which is centred by means of cup springs 472.

The control spring 466 acts via a balance beam 464 with equal forces on inlet valve 422 as it does on the outlet valve 424. To keep the outlet valve 424 free from forces in the neutral position, when the inlet valve 422 is closed, an additional spring 474 is required whose prestressing is approximately half that of the control spring 466. However, since the outlet valve must always stand up to a higher pressure than the inlet valve, the additional spring 474, in this embodiment a helical compression spring, must be adjusted higher by a force corresponding to the pressure differential Ap (Fig. 3) than the force required for closing the inlet valve 422.

Operation:- In the neutral position the inlet valve 422 is closed and the outlet valve 424 opened. If during a steering movementthe tappet 470 moves to the right (as viewed in Fig. 5), the cup springs 472 are stressed, while the control spring 466 is expanded. When the force S (Fig. 3) has been exceeded, the outlet valve 424 closes. When the limit force G is reached, the force exerted on the valve member 428 holds the balance to the supply pressure. When the limit force G has been exceeded, the inlet valve 422 opens.

Pressure fluid can flow from the supply connection 403 into the pressure chamber420 and build up in it a pressure whose value depends on the actuating force exerted at the steering wheel. As a result the piston 448 is moved to the right, so that it follows the movement initiated by the tappet 470.

When the tappet 470, actuated from the steering wheel, has moved far enough to the right for a collar 469 of the stop pin 468 to abut the balance beam 464, the force of control spring 466 is rendered ineffective. The cutting-off point A (Fig. 3) is reached. The inlet seated valve 422 opens, so that the full accumulator pressure can have effect in the working chamber 420. The outlet valve 424 remains closed.

Its additional spring 474 is able to hold the balance to the full accumulator pressure.

After the cutting-off point A has been exceeded, no appreciable further travel of the tappet 470 in relation to the piston 448 is required - i.e., nor any further stressing of the centring springs 472 - for a further adjustment of the steered wheels. The further steering movement therefore is effected without any appreciable further increase in the actuating force at the steering wheel (even though accompanied by further rotation of the steering wheel).

Rack and pinion steering gear.

Fig. 5 shows a rack and pinion steering gear with two pairs of seated valves 522,524 and 522', 524' each having one balance beam 564, 564', in an arrangement similar to the embodiment illustrated in Fig. 4. The tappet 570 is rigidly connected to the rack 540. The rack has two centring springs 572, 572'.

The tappet 570 actuates via two control springs 566, 566' the two balance beams 564, 564'. No separate pins corresponding to the pin 468 in Fig. 4 are provided. End stops for control springs 566, 566' are in this embodiment formed by collars 569, 569' of the tappet 570.

In this embodiment the outlet seated valve 524 has an additional spring 574 in the form of a compression spring, and the balance beam 564 actuates the valve member via a projection 576. The same applies to the right-hand outlet valve 524'.

The supply connection 503 and the return connection 558 are connected via ducts 503', 558' to the seat of the right-hand inlet valve 522' and the outlet chamber of the right-hand outlet valve 524'.

Operation is the same as in the embodiment illustrated in Fig. 4.

Embodiments with balance beam in a pressurerelieved chamber.

In the embodiments illustrated in Figs. 4 and 5 the balance beam and the members 468 and partly 470 used for its actuation are accommodated in the working chambers of the adjusting motor. As a resu It the control mechanism is located in the pressure chambers, and this is preferably to be avoided.

In the embodiment illustrated in Figs. 6 and 6A a balance beam 606 is accommodated in a chamber 610 which is open towards reservoir 608 - i.e., is pressure-relieved - and is arranged between a hollow cylindrical casing 612 to be attached to the vehicle and a cylindrical member 614 which is disposed rotatably, but axially non-displaceable, in casing 612.

The member 614 is in one piece with pinion 616 which engages in a rack (not shown). The member 614 is sealed by means of four annular seals 618 in relation to the casing 612 and has three annular grooves 620, 621A, 621 B, of which the annular groove 620 is connected via supply connection 603 to pressure accumulator 604, while the two annular grooves 620A, 620B are connected to the two working chambers 623A, 623B of the adjusting motor.

The cylindrical member 614 is formed with six longitudinal ducts 625A, 625B, 626A, 626B, and 627A, 627B. The annular groove 620 connected to the pressure source is connected via the two longitudinal ducts 625A, 625B to the supply chambers of the two inlet valves 631 A, 631 B. The working chamber 623A of the adjusting motor is connected via annular groove 621A and longitudinal ducts 626A, 627A to the valve seat of the outlet valve 629A and the spring chamber of the inlet valve 631 A. Correspondingly, the working chamber 623B of the adjusting motor is connected via annular groove 621 B and longitudinal ducts 626B, 627B to the seat of the outlet valve 629B and the spring chamber of the inlet valve 631 B.

The two inlet valves are constructed in the same manner as pressure-relieved valves. Only the lefthand inlet valve as shown in Fig. 6a will be described. A piston 637A on whose piston rod 635A a valve member 633A is arranged is accommodated fluid-tight in a valve bore 632A. The effective crosssectional areas of the piston and the valve seat are equal to one another. There is high pressure inside the valve bore 632A, but it acts in a similar manner on the piston and valve member.

In this case an additional spring 639A in the form of a helical compression spring is arranged in the inlet valve. It maintains the valve closed and provides the necessary differential pressure Ap or hysteresis.

A shaft 641 is driven from the steering wheel (not shown) via a steering spindle. The shaft 641 is connected for co-rotation via a forked member 643 to one end of a torsion rod 645 whose other end is attached for co-rotation in cylindrical member 614.

The forked member 643 bears via two control springs 647A, 647 B, which also act as centring springs, againstthe lower end of the balance beam 606, as viewed in Fig. 63. The pistons 637A, 637B are disposed on both sides in the centre of the balance beam, and the valve members 649A,649B of the outlet valves are disposed at the top end of the balance beam 606.

The cross-sectional areas of the pistons 637A, 637B are twice ar large as the seat cross-sectionai areas of the outlet valves 629A, 6293. The balance beam 606 has identical lengths between the outlet and inlet valves and the places of engagement of the control springs 647A, 647B. Consequently the control springs exert on the inlet valves twice as great a force as on the outlet valves.

Operation:- When the shaft 641 is so rotated from the steering wheel against a resistance of the steered wheels that the forked member 643 (Fig. 6a) is moved to the right, the balance beam 606 under the influence of the control spring 647B and additional spring 639B, closes the outlet valve 629B and relieves the inlet seated valve 631 B. When the limit force has been exceeded, the inlet valve acts as a pressure balance and controls in the proportional range the pressure in the adjusting motor working chamber 623B, which then constitutes the pressure chamber. With a further increase of the force on the steering wheel beyond the cutting-off point A, a mechanical stop is produced between the forked member 643 and the balance beam 606. The inlet valve 631 B is opened by the resulting travel control. The full pressure can then have effect in the adjusting motor pressure chamber. Working fluid can in each case flow away from the pressure-relieved working chamber, in this embodiment 623A, of the adjusting motor through the relieved outlet valve 629A into the reservoir 608.

The embodiment illustrated in Fig. 7 differs from that illustrated in Fig. 5 by the feature that the rack 701 and the two balance beams 703,703' are accommodated in a pressure-relieved chamber 705 and by the fact that the two balance beams are con structed as resilient strips. In accordance with the embodiment illustrated in Figs. 6 and 6s pressure relieved inlet seated valves 707 etc. are provided.

Only the left-hand side of the arrangement (as viewed in Fig. 7) will be described hereinafter. The right-hand side is a mirror-image thereof. A stepped piston 710 is slidably guided in a working cylinder 709. The stepped piston 710 is sealed in relation to the cylinder wall by two annular seals 712 and 713 at both ends of a working chamber715. The piston 710 has a supply line 716 which is at high pressure and extends to the inlet seated valve 707. The inlet seated valve 707 has a valve member717 which is connected via a thrust rod 718 to a fluid tightly guided piston 720.

An additional or hysteresis spring 722 which bears against a radial projection 724 of the stepped piston 710 acts on the valve member7i7.

The balance beam 703 is incorporated in a stop rnc:er726. The central portion of the stop rocker bears via centring springs 728 against the stepped piston 710. The two ends 730 of the stop rocker con statute stops for the ends of the balance beam in the cuaing-ol zone.

The two stepped pistons 710,710' are rigidly interconnected by a push-and-pull rod 731. The rack 701 is slidably guided in the longitudinal direction on the push-and-pull rod 731 and engages a pinion 732 actuatable from the steering wheel.

In the neutral position illustrated both ends of the rack701 bear against the balance beam 703,703' yonder the force of the two centring springs 728,728'. The centring springs and the balance beams co-operate to form the control springs. Otherwise the operation of the valves is substantially identical to that described hereinbefore with reference to Figs. 6 and 63.

edlydrostatic steering system.

Fig. 8 shows the application of the invention to a hydrostatic steering system - i.e., a steering system in which there is a hydraulic connection, but no continuous mechanical connection, between the steering wheel and the steered vehicle parts.

A manual and metering pump 803 - i.e., a pump which in normal operation acts merely as a metering or apportioning pump for the pressure fluid coming from a pressure source, in this embodiment a pressure accumulator 804, and acts as a manual pump only when pressure fails, is driven from steering wheel 801. The pump 803 has a mechanical output in the form of a pivotable arm 806 on which two centring springs 808 act.

Adjusting motor 810 is supplied via grooves 812A, 812B of an axial piston valve 814 whose piston 816 can be adjusted by the arm 806.

Two identical, pressure-balanced inlet seated valves 818A,818B are provided the supply chambers of which are connected to a constant pressure source, in this embodiment the pressure accumulator 804.

Only the left-hand valve will be described. Its piston 819A has a stop pin 821A which is enclosed by a control spring 820A. The control spring is operative in the proportional zone between the inlet valve 81 8A and the piston 816 of the axial piston valve. At the end of the control zone, namely at the cutting-off point A, the inlet valve 818A is entrained by the piston 816 via the stop pin 821A and the valve is opened by travel control. In this embodiment also an additional spring 822A is provided for the purpose described hereinbefore. The outlets of the inlet seated valves are connected via lines 824A, 8243 to the two connections of the pump 803. Branch lines 826A, 826B extend to two separate cylinder chambers of the axial piston valve 814.The central cylinder chamber is connected to reservoir 828.

Operation:- In the neutral position the pressure source 804 is fluid-tightly sealed off by the two inlet seated valves 818A, 8183 so that no flow losses occur.

When the steering wheel 801 is rotated, a differential pressure builds up via the pump 803. Let it be assumed that delivery is in the direction indicated by the arrows. Pressure fluid can flow in through the inlet valve 818A but not out from the line 824B. Consequently the arm 806 is entrained and pushes the valve piston 816 to the left (as viewed in the Figure).

As a result a flow path is opened up via a pair of control edges 830B to the working chamber 832B of the adjusting motor. The working chamber 832A is relieved via a pair of control edges 834A to the reservoir 828.

At the same time a force is exerted on the control spring 820Afrom valve piston 816. After the limit force G has been exceeded, inlet valve 818A oper- ates in the proportionality zone and builds up a pressure in the line 824A. If a higher pressure is required in the working chamber 832B than was produced manually in the line 824B, the inlet valve 818A ensures a pressure increase with an only slightly increasing pressure gradient via the pump 803 (in this phase the pump is driven as a motor).

Suitable adaptation to one another of the additional springs 822A,822B, which act as hysteresis springs, ensures that the system is first of all charged at the start of a steering movement which is exerted on the steering wheel from the neutral position. A pressure is then produced at first on the intake side of the manual and metering pump - i.e., assuming a steering movement of the same direction, at first exclusively in line 824A, before pressure fluid can flow away on the delivery side - i.e., from line 8263, since then the pairs of control edges 830B and/or 834A are still closed.

After a predetermined force on the steering wheel has been exceeded, the steering system operates as described hereinbefore in the cutting-off zone.

The driving force for the control springs can be picked up on the primary side (steering wheel side) of the steering transmission, as shown in Figs. 6 and 6a. Instead of this, it can be picked up on the secondary side of the steering transmission, as shown by Figs. 2, 5 and 7.

Claims (13)

1. A hydraulically operated steering valve par ticularly for vehicles, comprising an actuating means and an adjusting motor, having two working chambers, for the steered vehicle parts, working fluid supply lines which extend from a pressure source respectively to the working chambers of the adjusting motor and contain two inlet seated valves closed in the neutral position of the steering system, and return flow lines which extend respectively from the working chambers of said motorto a reservoir and contain two outlet valves, the two inlet seated valves being connected via control springs in opposite directions to the actuating means.

2. A steering system as claimed in claim 1, wherein the two outlet valves are spring-loaded seated valves which are connected via their control springs in opposite directions to the actuating means, the inlet and outlet valves belonging to the same working chamber of the adjusting motor are connected in opposite directions to the actuating means, and in the pressure control zone of the inlet valve the outlet valve connected to the pressure chamber of the adjusting motor holds the balance to a pressure greater by a differential pressure Ap than the inlet valve associated with the same pressure chamber.

3. A steering system as claimed in claim 1 or 2, wherein the inlet seated valves are so pressurebalanced that forces acting on the valve member are independent of the pressure operative in the supply connection, and the control spring and an additional spring acts in opposite directions on the valve member, the additional spring being the decisive factor in maintaining the differential pressure Ap.

4. A steering system as claimed in claim 3, wherein the inlet valve member is connected to a piston which slides fluid-tightly in an associated valve bore and whose operative cross-sectional area is equal to the valve cross-sectional area, and the supply connection discharges into the valve bore between the valve member and the piston.

5. A steering system as claimed in any of the preceding claims, wherein a balance beam or two balance beams disposed in opposite directions actuate the two pairs of valves associated with the working chambers; the associated control springs are disposed between the actuating means and the or each balance beam; the or each balance beam is nonpositively connected to the valve members of the inlet and outlet valves; and additional springs are provided to produce the differential pressure between the or each balance beam and the associated valve members.

6. A steering system as claimed in claim 5, wherein provided between the actuating means and the balance beam is a mechanical lost motion connection whose lost motion zone so corresponds to the working zone of the control spring that, when the lost motion has been used up, the balance beam opens or releases for opening the inlet valve for the unimpeded flow of pressurised fluid into the pressure chamber of the adjusting motor.

7. A steering system as claimed in claim 6, wherein after the lost motion connection has been used up - i.e., the cutting-off point has been exceeded, the balance beam is inoperative, and the additional spring associated with the outlet valve stands up to the full pressure of the pressure source.

8. A steering system as claimed in claim 6, wherein after the lost motion connection has been used up- i.e., after the cutting-off point has been exceeded, the balance beam or a stop rocker forms a non-positive connection between the actuating means, the inlet and the outlet valve, and the inlet valve opens constrainedly against the force of its additional spring and maintains the outlet valve closed against the full pressure of the pressure source.

9. Asteering system as claimed in claim 8, wherein the balance beam is constructed as a resilient strip which, after the lost motion has been used up, is rendered inoperative by stops or entraining elements.

10. A steering system as claimed in claim 1, wherein in a hydrostatic steering system a manual and metering pump drivable by the actuating means is connected downstream to the inlet seated valve, the said pump having a mechanical output which actuates a piston valve; the piston valve performs interalia the function of outlet valves; and the control springs are non-positively connected to a movable part of the piston valve.

11. A steering system as claimed in claim 10, wherein the piston valve also has a supply valve connected downstream of the pump, to the adjusting motor.

12. A hydraulically operated steering system, substantially as herein described with reference to and as shown in the accompanying drawings.

13. A steering valve arrangement for use in a hydraulically operated steering system, comprising supply lines which extend from a pressure source respectively to the working chambers of an adjusting motor and contain two inlet seated valves closed in the neutral position of the steering system, and return flow lines which extend respectively from the working chambers of said motor to a reservoir and contain two outlet valves, the two inlet seated valves being connected via control springs in opposite directions to an actuating means of the steering system.