GB1597057A

GB1597057A - Power-assisted steering systems for motor vehicles

Info

Publication number: GB1597057A
Application number: GB17452/78A
Authority: GB
Original assignee: ZF Friedrichshafen AG
Current assignee: ZF Friedrichshafen AG
Priority date: 1977-05-04
Filing date: 1978-05-03
Publication date: 1981-09-03
Also published as: DE2719895B2; JPS6216B2; ES469348A1; IT7849172A0; SE436635B; DE2719895A1; FR2389524B1; IT1102057B; FR2389524A1; BR7802808A; JPS549840A; SU833151A3; DE2719895C3; SE7805067L

Description

(54) POWER-ASSISTED STEERING SYSTEMS FOR MOTOR VEHICLES (71) We, ZAHNRADFABRIK FRIEDRI CHSHAFEN AKTIENGESELLSCHAFT, of Friedrichshafen-on-the-Bodensee, Federal Republic of Germany, a Joint-Stock Company organised under the laws of the Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following state menu: The present invention relates to powerassisted steering systems for motor vehicles.

A power-assisted steering system is known from U.S. Patent Specification No.

3,822,759, comprising two hydraulic steering circuits and a mechanical connection from the steering wheel to the steered wheels and also a first steering gear unit incorporating a servomotor and containing a control valve. A second control valve for operating a servomotor acting on a second steering gear unit is flanged on the first steering gear unit.

In this arrangement two servomotors are operated simultaneously, that is to say the power assistance of both servomotors is always applied to the steering linkage.

The invention takes this known powerassisted steering system as its starting point and seeks to provide a power-assisted steering system comprising two hydraulic steering circuits which act separately from one another and which can be adjusted in optimum manner to the operating condition or power assistance required at any given moment in a vehicle, so that the proportion of the auxiliary force acting on the steered wheels is dependent on the steering resistance encountered; for example for parking operations or travelling cross-country the power assistance must be considerably greater than when travelling at high speed on a motorway.

At the same time the entire steering arrangement should be of compact construction and have a high degree of operational reliability.

To this end, the present invention consists in a power-assisted steering system for a motor vehicle provided with two hydraulic steering circuits independent of one another, comprising a servomotor combined with a steering gear unit and an appertaining control device for a first steering circuit, and a second control device, assembled with the steering gear unit, of a second steering circuit, the second control device being hydraulically connected to a second servomotor acting on the steering linkage, characterised in that both steering circuits have pumps which are coupled to the engine of the vehicle and whose power is designed for about half the maximum power required by the power-assisted steering system, the arrangement being such that the first steering circuit is used for low steering resistance and responds immediately on operation of the steering wheel, while the second steering circuit is used for increased steering resistance and can be brought into action automatically when the increased steering resistance is encountered.

A power-assisted steering system having two control pistons disposed eccentrically and transversely to the steering-wheel shaft for controlling the pressure oil is already known from German Patent Specification No. 1,133,642. Pins formed on the end of the steering-wheel shaft nearer the gear unit here engage in the control pistons. The steeringwheel shaft is connected by a torsion bar to a threaded spindle which engages by means of a chain of balls in a working piston and in the head of which the control pistons are accommodated.

In addition, a power-assisted steering system is known which in the working piston has only one control piston disposed transversely to the steering-wheel shaft, see German Patent Specification No. 1,816,295. A driver which is connected to a steering nut, and which together with the steering nut can perform a limited turning movement, engages in this control piston. Since all the components belonging to the control device are accommodated inside the working piston, this steering device is distinguished by a very short overall length.

The composite construction of the servomotor, steering gear unit and control device in these power-assisted steering systems has in many cases proved satisfactory in respect of operating behaviour, operation reliability and compactness. The dual-circuit steering system of the invention therefore builds on these steering arrangements which are of particularly short length.

The power-assisted steering system according to the invention combines the constructional advantages of the two steering arrangements mentioned above, so that a very short overall length is achieved for dualcircuit steering systems. The additional advantage is also achieved that in normal running operation only half the amount of oil is circulated under pressure during operation of the steering system. The oil for the second circuit is circulated without pressure.

This results in particularly economical operation of the entire steering arrangement.

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 one embodiment thereof, and in which: Figure 1 is a simplified longitudinal section through the power-assisted steering system of the invention, with the control devices for two separate steering circuits; Figure 2 is a section drawn on a larger scale, on the line II--II of Figure 1; and Figure 3 is a section, drawn on a larger scale, on the line IIIIII of Figure 1.

As shown in Figure 1, a first steering circuit consists of an oil tank 1, a pump 2, a control device 3 and a servomotor 4 incorporated in a steering gear unit casing. A second steering circuit acting separately from the first consists of an oil tank 5, a pump 6, a control device 7 and a servomotor 8 connected to the steering linkage. A steeringwheel shaft 10, which is operated by a steering wheel (not shown), is formed in its lower portion as a steering worm 11. A working piston 12 can be loaded by means of pressure chambers 13 to 14, while a toothed segment 15 disposed in the pressure chamber 13 engages in the teeth of the working piston 12. The toothed segment 15 is connected by a steering-arm shaft 19 to the steering linkage (not shown).

The control device 3 of the first steering circuit is incorporated in the working piston 12. In addition, a steering nut 18 capable of- limited rotation is disposed together with a driver 20 in the working piston 12, between axial bearings 16 and 17. This driver 20 engages through a stop bore 21 in the working piston 12 into a control piston 22 of the steering device 3. Two springs 23. 23' supported in the working piston 12 and acting on both sides on the driver 20 act as centring system for the steering nut 18 or for adjusting the neutral position of the control piston 22. The control piston 22 guided in a bush 24 is adapted to be displaced from the neutral position to the extent of a predetermined control clearance limited by the stop bore 21.

As can be seen from Figure 2, the pump 2 is connected to supply chambers 25 and 26 of the control piston 22. Seals 27, 28 and 29, 30 respectively protect the supply chambers 25 and 26 against oil losses. In the neutral position shown the supply chambers 25 and 26 are in communication through respective open supply control edges 31 and 32, respective control grooves 33 and 34, respective return control edges 41 and 42, and respective holes 35 and 36 with a central return groove 37 connected to the tank 1. Consequently the working pressure can build up in the pressure chambers 13 or 14 when the pump 2 is driven. The control grooves 33 and 34 are in communication through throttle holes 38 and 39 respectively with retroaction chambers 45 and 46 respectively, which are closed by piston inserts 43 and 44 respectively, these retroaction chambers 45 and 46 being likewise relieved of load in the neutral position shown.

On deflection of the control piston 22, for example to the left, the supply chamber 25 is connected to the pressure chamber 14 through the wide open supply control edge 31 and the control groove 33. The other pressure chamber 13 is in communication with the central return groove 37 by way of the control groove 34, the wide open control edge 42, and the hole 36, so that the working piston 12 moves to the right. Since the working pressure prevailing in the pressure chamber 14 acts by way of the throttle hole 38 in the retroaction chamber 45 also, a retroactive force directed oppositely to the deflection builds up on the control piston 22.

Since the control groove 34 is relieved of load, the other retroaction chamber 46 remains neutral in respect of pressure. This hydraulic force directed in the retroaction chamber 45 in the direction of the driver 20 can be felt on the steering wheel and produces the so-called "road feeling".

The control device 7 (Figures 1 and 3) of the second steering circuit consists of two control pistons 50 and 50A which in the head 51 of the steering worm 11 are mounted perpendicularly and eccentrically to the axis of- the steering-wheel shaft 10. The steeringwheel shaft 10 is connected by a torsion bar 52 to the steering worm 11. Two pins 53 and 53A formed at the bottom end of the steering-wheel shaft 10 project through bores 54 and 54A in the head 51 and engage in the control pistons 50 and 50A, so that on initiation of a turning movement they are displaced axially oppositely to one another, turning with the head 51. Pressure oil thus flows in each case to one pressure chamber of the servomotor 8 by way of control grooves which are formed on the control pistons 50 and 50A and which are opened and closed respectively, while the pressure oil is displaced out of the other pressure chamber into the tank 5.

As shown in Figure 3, the pump 6 pumps pressure oil from the tank 5 into a supply chamber 55. When the steering wheel is turned, the control pistons 50 and 50A guided in bushes 56 and 56A are moved out of the neutral position shown oppositely to one another by the pins 53 and 53A, depending on the direction of rotation, within the control clearance determined by the holes 54 and 54A (Figure 1) in the head 51. From the supply chamber 55 the pressure oil passes through a control groove 57 or 57A to a pressure chamber 58 or 59. The volume of oil displaced from the pressure chamber 59 or 58 flows back through a return control groove 60 or 60A and an annular groove 61 or 61A to the tank 5. In the neutral position of the control pistons 50 and 50A which is shown in the drawing all the control grooves are open, so that oil can flow from the pump 6 to the tank 5. In one ehd face of each of the control pistons 50 and 50A there is inserted a respective centring device 62 and 62A, which on the one hand serves for the accurate adjustment of the control pistons in relation to the control grooves and on the other hand supports the torsion bar 52 when the control pistons are returned to the neutral position.

Between the supply chamber 55 and the annular groove 61 and 61A which are in communication with the tank 5 there is inserted a non-return valve 63, which in the event of failure of the pump 6 or when the vehicle engine is stationary permits the further drawing-in of pressure oil. A non-return valve acting in the same way is also contained in the first steering circuit 1, 2, 3, 4, this valve not being visible in the sectional planes shown but being shown at 42 in Figure 8 of German Patent Specification 1,816,295.

The springs 23 and 23' used in the control device 3 of the first steering circuit and the torsion bar 52 incorporated in the control device 7 of the second steering circuit act as restoring elements for the neutral position of the control pistons 22 and 50, 50A respectively. If restoring elements having different spring forces are used, different response of the control devices 3 and 7 is achieved. In the present case a relatively stiff torsion bar spring 52 and springs 23, 23' having smaller spring force are used.

When the steering-wheel shaft 10 is turned, the control piston 22 is first deflected by means of the steering nut 18 and the driver 20, so that the working pressure is built up in one of the pressure chambers 13 or 14. The control device 7 retains its neutral position as shown in Figure 3, so that the volume of oil of the simultaneously driven servomotor 8 can circulate. Only when the manual force on the steering-wheel shaft 10 is increased with increasing steering resistance, for example during parking or crosscountry travel, will the torsion bar 52 be subjected to resilient twisting, so that the control pistons 50 and 50A will then also be displaced oppositely to one another. Thus the pressure chamber 58 or 59 of the servomotor 8 corresponding to the direction of rotation is brought into action. As soon as the turning force on the steering-wheel shaft 10 is reduced again, the control pistons 50 and 50A are moved back to the neutral position by the restoring forces of the torsion bar 52 and centring device 62, so that only the first steering circuit 1, 2, 3, 4, is in action.

The power-assisted steering system of the invention is so designed that both pumps 2 and 6 are driven by the engine of the vehicle, so that in the event of a breakdown in the first or second steering circuit resulting from a fractured pipe or pump failure the power assistance of one steering circuit is maintained. In the event of the failure of the first steering circuit 1, 2, 3, 4, for example, the vehicle can continue to be safely steered by the application of a slightly increased proportion of manual power, but without the otherwise customary proportional retroaction force, by means of the second steering circuit 5, 6, 7, 8.

Since, as previously fully explained above, a retroaction device consisting of the retroaction chambers 45 and 46 is provided in the control device 3 of the first steering circuit, the second steering circuit can be made without a retroaction force. The control device 7 is thus less expensive. After the second steering circuit has been brought into operation the manual force is thus not further increased, and this is also desirable in order to facilitate steering operations during parking.

Because the total power required is split up into two steering circuits, there is a saving of power. Because during normal running only half the amount of oil circulates under pressure when the steering system is oper ated, only half power is taken from the power-assisted steering system of the inven tion. Moreover, during normal running there is an improvement of road feeling, since the retroaction force can be made greater, thus improving straight-ahead running properties.

In addition, it is possible for a reserve pump driven by the axle drive or by the vehicle wheels to be brought into action in conjunction with the first or second steering circuit with the aid of a sequence valve, see German Patent Specification No. 1,157,945.

A high degree of reliability in respect of steerability is thus ensured even in the event of the failure of the engine, particularly in heavy vehicles.

WHAT WE CLAIM IS: 1. A power-assisted steering system for a motor vehicle provided with two hydraulic steering circuits independent of one another, comprising a servomotor combined with a steering gear unit and an appertaining control device for a first steering circuit, and a second control device, assembled with the steering gear unit, of a second steering circuit, the second control device being hydraulically connected to a -second servomotor acting on the steering linkage, characterised in that both steering circuits have pumps which are coupled to the engine of the vehicle and whose power is designed for about half the maximum power required by the power-assisted steering system, the arrangement being such that the first steering circuit is used for low steering resistance and responds immediately on operation of the steering wheel, while the second steering circuit is used for increased steering resistance and can be brought into action automatically when the increased steering resistance is encountered.

2. A power-assisted steering system according to claim 1, wherein restoring elements provided for the centring of the control device ofthe first steering circuit have a smaller restoring force than a torsion bar acting in the second steering circuit.

3. A power-assisted steering system according to claim 2, wherein the control device of the first steering circuit is provided with a retroacting device.

4. A power-assisted steering system according to any of claims 1 to 3, characterised in that the control device of the first steering circuit has a control piston which is guided eccentrically in a working piston and transversely to the steering-wheel shaft, that a driver, connected to a steering nut mounted for limited rotation and extending in the working piston parallel to the steering-wheel shut, engages in the control piston, that the control device of the second steering circuit consists of two control pistons installed in the head of a threaded spindle transversely and eccentrically to the axis of the steering-wheel shaft, and that the restoring elements for the respective control devices have different rigidities.

5. A power-assisted steering system according to any of claims 2 to 4, wherein the restoring element associated with the control piston of the first steering circuit has only a low initial stress.

6. A power-assisted steering system according to any of claims 2 to 5, wherein in addition to the torsion bar, a centring device is provided for the second control device.

7. A power-assisted steering system according to any of claims 1 to 6, wherein the respective pumps of the first and second steering circuits are in the form of tandem pumps.

8. A power-assisted steering system for a motor vehicle, substantially as described and with reference to the accompanying draw ings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. improving straight-ahead running properties. In addition, it is possible for a reserve pump driven by the axle drive or by the vehicle wheels to be brought into action in conjunction with the first or second steering circuit with the aid of a sequence valve, see German Patent Specification No. 1,157,945. A high degree of reliability in respect of steerability is thus ensured even in the event of the failure of the engine, particularly in heavy vehicles. WHAT WE CLAIM IS:

1. A power-assisted steering system for a motor vehicle provided with two hydraulic steering circuits independent of one another, comprising a servomotor combined with a steering gear unit and an appertaining control device for a first steering circuit, and a second control device, assembled with the steering gear unit, of a second steering circuit, the second control device being hydraulically connected to a -second servomotor acting on the steering linkage, characterised in that both steering circuits have pumps which are coupled to the engine of the vehicle and whose power is designed for about half the maximum power required by the power-assisted steering system, the arrangement being such that the first steering circuit is used for low steering resistance and responds immediately on operation of the steering wheel, while the second steering circuit is used for increased steering resistance and can be brought into action automatically when the increased steering resistance is encountered.