GB1598848A

GB1598848A - Power-assisted steering system for motor vehicles

Info

Publication number: GB1598848A
Application number: GB17451/78A
Authority: GB
Original assignee: ZF Friedrichshafen AG
Current assignee: ZF Friedrichshafen AG
Priority date: 1977-05-04
Filing date: 1978-05-03
Publication date: 1981-09-23
Also published as: SE7805066L; DE2719926B2; JPS549839A; ES469347A1; SE436634B; JPS6222B2; DE2719926A1; DE2719926C3

Description

(54) POWER-ASSISTED STEERING SYSTEM FOR MOTOR VEHICLES (71) We, ZAHNRADFABRIK FRIEDRICHSHAFEN 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 statement: This invention relates to power-assisted steering systems.

A power-assisted steering system is already known from U.S. Patent Specification No. 3,822,759, which comprises two hydraulic steering circuits and a mechanical connection from the steering wheel to the steered wheels. This steering device consists of a first steering gear unit enclosing a servomotor and of a control valve contained therein. Another control valve for the operation of a servomotor acting on a second steering gear unit is flanged to the first steeering gear unit. In this arrangement the two steering circuits are operated simultaneously by two pumps driven by the engine of the vehicle, that is to say the assisting power of both servomotors always acts on the steering linkage.

The problem underlying the invention is that of providing optimum prerequisites for the operational reliability of a powerassisted steering system. Another requirement in this arrangement consists in accommodating both steering circuits inside a complete unit with only one steering gear unit, despite their hydraulic separation.

To this end, the present invention consists in a power-assisted steering system for a motor vehicle, having two hydraulic steering.

circuits independent of one another and operable by means of a steering-wheel shaft, one said steering circuit comprising a steering gear unit containing a servomotor and a first control device and actuatable by means of a threaded spindle arranged in prolongation of the steering-wheel shaft, the other steering circuit comprising a second control device which is combined with the steering gear unit and which is arranged co-axial with said first control device and a further servomotor which acts on a steering linkage, a separate pump being provided for the respective steering circuit, characterised in that the two control devices are disposed one behind the other to form a constructional unit enclosed by a common housing, said first control device consisting of two control pistons which are disposed transversely to the axis of the steering-wheel shaft and in spaced relationship therefrom and have a retroacting device, said second control device consisting of a rotary slide valve.

A power-assisted steering system having two control pistons for controlling the pressure oil, which are disposed eccentrically and transversely to the steering-wheel shaft, is already known (German Patent Specification 1,133,642). Pins formed at the end of the steering-wheel shaft nearer the gear unit engage in the control pistons in this arrangement. The steering-wheel shaft is connected by a torsion bar to a threaded spindle which by means of a chain of balls engages in a working piston and in whose head the control pistons are accommodated. The ends of the two control pistons project into an oil supply groove and have differential piston surfaces, so that a determined retroacting force can be felt on the steering wheel. The composite construction of the servomotor, steering gear unit, and control device in this known power-assisted steering system has proved satisfactory in many cases in respect of operating behaviour, operating reliability and compactness. The invention therefore builds on a particularly suitable, short steering arrangement of this kind.

In the steering arrangement of the invention the control device associated with the two servomotors and consisting of piston valves and a rotary slide valve forms a constructional unit enclosed in a common housing, whereby a space saving arrangement results.

A power-assisted steering system of this kind which has two steering circuits acting independently of one another is incorporated mainly in heavy vehicles which could not be steered without power assistance. As is customary in single-circuit power-assisted steering systems, the first steering circuit in this arrangement contains a device for producing a retroacting force which transmits the so-called "steering feel" to the driver.

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 provided with the control device for two steering circuits according to the inven tion Figure 2 is a cross-section on the line II-II of Figure 1; Figure 3 is another cross-section on the line III-III of Figure 1 with the casing omitted; Figure 4 shows a detail of the longitudinal section of Figure 1 drawn on a larger scale; Figure 5 shows on a larger scale a detail of the driver pin engaging with play in the rotary slide valve, as shown in Figure 4; and Figure 6 is a partial section on the line VI-VI of Figure 4, with the centring device.

According to Figures 1 and 4, a working piston 2 disposed in a steering gear casing in the form of a cylinder 1 serves at the same time as a steering nut, which is in engagement on the one hand with a segment shaft 3 and on the other hand through a row of balls 4 with a threaded spindle 5. The threaded spindle 5 is connected by a torsion bar 6 to a steering-wheel shaft 7 which carries a steering wheel (not shown). In the head 8 of the threaded spindle 5 is incorporated a control device 9 comprising control pistons 10 and 10A lying transversely to the axis of the spindle. Two pins 12 and 12A disposed at the bottom end on a flange 11 on the steering-wheel shaft 7, project through holes 14 and 14A in the spindle head 8 and engage in the control pistons 10 and 10A in such a manner that when a turning movement is initiated they are displaced axially oppositely to one another, turning with the spindle head. This results in resilient twisting of the torsion bar 6 until control grooves which areJformed on the control pistons 10 and 10A and which will be explained in greater detail with reference to Figure 2, are opened and closed, so that in each case pressure oil flows to one side of the working piston 2.

The working piston 2 divides the steering gear casing into two pressure chambers 16 and 17. Together with an oil pump 18, a tank 20 and the control device 9, the cylinder 1 forms a first self-contained steering circuit. Pressure oil is supplied to the control device 9 through a supply chamber 21, while an annular groove 22 serves for the return. The pressure chamber 16 can be brought into communication with the pump 18 by way of a channel 23 in the spindle head 8 and of the control device 9. The pressure chamber 17 can be connected to the pump 18 through a channel 24, which is shown in dot-and-dash lines in Figure 1, another channel 25 in the spindle head 8, and the control device 9. The channels 23 and 24/25 leading to the pressure chambers 16 and 17 respectively can likewise be connected through the control device to the annular return groove 22 connected to the tank 20.

According to Figure 2 the pump 18 pumps pressure oil from the tank 20 into the supply chamber 21. When the steering wheel is turned the control pistons 10 and 10A are displaced oppositely to one another, from the neutral position shown in the drawing, by the pins 12 and 12A respectively, depending on the direction of rotation, within the control play determined by the holes 14 and 14A respectively (Figure 4) in the spindle head 8. From the control chamber 21 the pressure oil passes by way of a control slot 26 or 26A to the pressure chamber 16 or 17 respectively. The amount of oil ejected in each particular case is returned by way of a control slot 27 or 27A and an annular groove 29 or 29A respectively to the tank 20. In the neutral position shown (middle position) of the control pistons 10 and 10A, all the control grooves 13, 15 and 13A, 15A are therefore opened, so that oil can flow through from the pump 18 to the tank 20.

A centring device 28 and 28A is resDec- tively inserted in one end face of each of the control pistons 10 and 10A, which serve for the accurate adjustment of the control pistons in relation to the control grooves and support the torsion bar 6 on the return of the control pistons to the neutral position. The other end face of the control pistons 10 and 10A, together with pistons 31 and 31A, respectively forms reaction chambers 32 and 32A respectively. These reaction chambers 32 and 32A are connected to the pressure chambers 16 and 17 through respective channels which are not visible.

On adjustment of the control pistons 10 and 10A the force of the pressure in these reaction chambers 32 and 32A must thus be overcome in known manner, this force transmitting the so-called "road feel" to the steering wheel. If these reaction chambers are eliminated completely hydraulic steering would still be possible and only the force of the torsion bar 6 and the force of the centring device 28 or 28A would have to be overcome. A non-return valve which enables pressure oil to be sucked in when the vehicle engine is stationary, is inserted between the supply chamber 21 and the annular grooves 29 and 29A in communication with the tank 20.

From Figures 1 and 4 it can also be seen that a rotary slide valve 36 in the form of a sleeve is supported on the steering-wheel shaft 7 by means of needle bearings 34 and 35. The upper portion of the spindle head 8 is in the form of a control bush 37. The rotary slide valve 36 and the control bush 37 together form a rotary slide valve. Longitudinal grooves 38 and 39 are formed in the control bush 37 which, in alternating sequence, are in communication by way of respective separate pressure medium ducts 40 and 41 with the respective pressure chambers 42 and 43 of another servomotor 44 (Figure 3). Axially extending longitudinal grooves 45 and 46 are distributed over the outer cylindrical surface of the rotary slide valve 36. These longitudinal grooves 45 are in communication with a pump 50 by way of ducts 47 and an annular groove 48. The longitudinal slots 46, which are cut right through, are in communication with a tank 52 by way of ducts 51. The rotary slide valve 36, 37, the servomotor 44, the pump 50 and the tank 52 here form the components of a second steering circuit separate from the first steering circuit.

In the event of a relative rotation of the rotary slide valve 36 in relation to the control bush 37, the longitudinal grooves 45, 46 and 38. 39, which are connected together in the neutral position (Figure 3), are controlled in such a manner that in dependence on the direction in which the steering wheel is turned either one or the other pressure chamber 42 or 43 is brought into communication with the inlet of the pump 50 and the respective oppositely situated pressure chamber 42 or 43 is brought into communication with the tank.

From Figure 4 it can be seen that axial bearings 53 and 54 take the axial forces of the spindle head 8. while a radial bearing 55 takes the radial forces of the steering-wheel shaft 7. The two steering circuits are hydraulically separated from one another by seals 56, 65 and 57.

A driver pin 58 which engages with clearance "s" in a hole 60 in the rotary slide valve 36 (Figure 5) is inserted into the flange 11 of the steering-wheel shaft 7. In addition, a centring device composed of a spring 62 and two balls 63 and 64 is inserted into a cutout 61 formed in the control bush 37 and rotary slide valve 36 (Figure 6).

When the steering-wheel shaft 7 is turned, only the control device 9 is at first moved, so that the first steering circuit 1, 18, 20 is operated, this circuit being at the same time designed as a running circuit. Since this displacement is effected through the needle bearings 34 and 35, the frictional hysteresis in relation to the rotary slide valve 36 is very low and the latter is consequently not affected by the turning movement of the steering-wheel shaft 7. Only when the force on the steering wheel becomes greater as steering resistance increases, for example in the event of the failure of the first steering circuit resulting from the failure of the engine, will the rotary slide valve 36 be moved by the driver pin 58 of the steeringwheel shaft 7 into the operative position in relation to the control bush 37, so that the second control circuit 44, 50, 52 is brought into play. For this purpose the rotary slide valve 36 performs a relative rotation, for example in the direction of the arrow in Figure 6, in relation to the control bush 37, so that the ball 63 retains its position in the cutout in the control bush 37. The spring 62 is thereby compressed. As soon as the steering wheel is no longer subjected to a turning force, the rotary slide valve 36, 37 is moved back by the centring device 62, 63, 64 and the control device 9 is moved back by the common restoring force of the torsion bar 6 and of the centring device 28, 28A, into the neutral position.

The power-assisted steering system according to the invention is so designed that one pump 18 is driven by the engine of the vehicle and the pump 50 by the vehicle wheels, so that in the event of a failure in the first steering circuit as the result of the failure of the engine, a fractured pipe, or pump failure, the power assistance of the second steering circuit is retained. In the event of a failure of the first steering circuit 1, 9, 18, 20, the vehicle can still be safely steered by the pump 50 driven by the vehicle wheels and the second steering circuit 19, 44 while the vehicle is moving.

Since only the pump 18 is driven by the vehicle engine and the pump 50 is coupled as reserve pump to the vehicle wheels or axle drive, the second steering circuit serves only as an emergency steering system as long as the vehicle is running. The pump 18 of the first steering circuit must be made so large that it is also sufficient for parking purposes. If a small pump 18 were used, additional steering force would have to be applied by hand during parking, because the pump driven by the wheels supplies oil only while the vehicle is moving.

WHAT WE CLAIM IS: 1. A power-assisted steering system for

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Claims

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transmitting the so-called "road feel" to the steering wheel. If these reaction chambers are eliminated completely hydraulic steering would still be possible and only the force of the torsion bar 6 and the force of the centring device 28 or 28A would have to be overcome. A non-return valve which enables pressure oil to be sucked in when the vehicle engine is stationary, is inserted between the supply chamber 21 and the annular grooves 29 and 29A in communication with the tank 20.

From Figures 1 and 4 it can also be seen that a rotary slide valve 36 in the form of a sleeve is supported on the steering-wheel shaft 7 by means of needle bearings 34 and 35. The upper portion of the spindle head 8 is in the form of a control bush 37. The rotary slide valve 36 and the control bush 37 together form a rotary slide valve. Longitudinal grooves 38 and 39 are formed in the control bush 37 which, in alternating sequence, are in communication by way of respective separate pressure medium ducts 40 and 41 with the respective pressure chambers 42 and 43 of another servomotor 44 (Figure 3). Axially extending longitudinal grooves 45 and 46 are distributed over the outer cylindrical surface of the rotary slide valve 36. These longitudinal grooves 45 are in communication with a pump 50 by way of ducts 47 and an annular groove 48. The longitudinal slots 46, which are cut right through, are in communication with a tank 52 by way of ducts 51. The rotary slide valve 36, 37, the servomotor 44, the pump 50 and the tank 52 here form the components of a second steering circuit separate from the first steering circuit.

In the event of a relative rotation of the rotary slide valve 36 in relation to the control bush 37, the longitudinal grooves 45, 46 and 38. 39, which are connected together in the neutral position (Figure 3), are controlled in such a manner that in dependence on the direction in which the steering wheel is turned either one or the other pressure chamber 42 or 43 is brought into communication with the inlet of the pump 50 and the respective oppositely situated pressure chamber 42 or 43 is brought into communication with the tank.

From Figure 4 it can be seen that axial bearings 53 and 54 take the axial forces of the spindle head 8. while a radial bearing 55 takes the radial forces of the steering-wheel shaft 7. The two steering circuits are hydraulically separated from one another by seals 56, 65 and 57.

A driver pin 58 which engages with clearance "s" in a hole 60 in the rotary slide valve 36 (Figure 5) is inserted into the flange

11 of the steering-wheel shaft 7. In addition, a centring device composed of a spring 62 and two balls 63 and 64 is inserted into a cutout 61 formed in the control bush 37 and rotary slide valve 36 (Figure 6).

When the steering-wheel shaft 7 is turned, only the control device 9 is at first moved, so that the first steering circuit 1, 18, 20 is operated, this circuit being at the same time designed as a running circuit. Since this displacement is effected through the needle bearings 34 and 35, the frictional hysteresis in relation to the rotary slide valve 36 is very low and the latter is consequently not affected by the turning movement of the steering-wheel shaft 7. Only when the force on the steering wheel becomes greater as steering resistance increases, for example in the event of the failure of the first steering circuit resulting from the failure of the engine, will the rotary slide valve 36 be moved by the driver pin 58 of the steeringwheel shaft 7 into the operative position in relation to the control bush 37, so that the second control circuit 44, 50, 52 is brought into play. For this purpose the rotary slide valve 36 performs a relative rotation, for example in the direction of the arrow in Figure 6, in relation to the control bush 37, so that the ball 63 retains its position in the cutout in the control bush 37. The spring 62 is thereby compressed. As soon as the steering wheel is no longer subjected to a turning force, the rotary slide valve 36, 37 is moved back by the centring device 62, 63, 64 and the control device 9 is moved back by the common restoring force of the torsion bar 6 and of the centring device 28, 28A, into the neutral position.

The power-assisted steering system according to the invention is so designed that one pump 18 is driven by the engine of the vehicle and the pump 50 by the vehicle wheels, so that in the event of a failure in the first steering circuit as the result of the failure of the engine, a fractured pipe, or pump failure, the power assistance of the second steering circuit is retained. In the event of a failure of the first steering circuit 1, 9, 18, 20, the vehicle can still be safely steered by the pump 50 driven by the vehicle wheels and the second steering circuit 19, 44 while the vehicle is moving.

Since only the pump 18 is driven by the vehicle engine and the pump 50 is coupled as reserve pump to the vehicle wheels or axle drive, the second steering circuit serves only as an emergency steering system as long as the vehicle is running. The pump 18 of the first steering circuit must be made so large that it is also sufficient for parking purposes. If a small pump 18 were used, additional steering force would have to be applied by hand during parking, because the pump driven by the wheels supplies oil only while the vehicle is moving.

WHAT WE CLAIM IS: 1. A power-assisted steering system for

a motor vehicle, having two hydraulic steering circuits independent of one another and operable by means of a steering-wheel shaft, one said steering circuit comprising a steering gear unit containing a servomotor and a first control device and actuatable by means of a threaded spindle arranged in prolongation of the steering-wheel shaft, the other steering circuit comprising a second control device which is combined with the steering gear unit and which is arranged co-axial with said first control device and a further servomotor which acts on a steering linkage, a separate pump being provided for the respective steering circuit, characterised in that the two control devices are disposed one behind the other to form a constructional unit enclosed by a common housing, said first control device consisting of two control pistons which are disposed transversely to the axis of the steering-wheel shaft and in spaced relationship therefrom and have a retroacting device, said second control device consisting of a rotary slide valve.
2. A power-assisted steering system according to claim 1, wherein the two control devices are accommodated in the head of said threaded spindle.
3. A power-assisted steering system according to claim 2, wherein a control bush of the rotary slide valve is made in one piece with the head of the threaded spindle.
4. A power-assisted steering system according to claim 3, wherein between the rotary slide valve and the control bush a centring device is disposed which consists of two balls and a compression spring located therebetween. the centring device being located in a part-circumferential groove formed between the rotary slide valve and control bush.
5. A power-assisted steering system according to anv of claims 4 to 9, wherein the rotary slide valve is supported on the steering-wheel shaft by means of needle bearings.
6. A power-assisted steering system.

substantially as herein described with reference to and as shown in the accompanying drawings.