A steering arrangement of this kind is known from US 5,234,070. In dependence of the direction determined by the steering handwheel, the steering valve releases a connection between the pump and the steering motor, which finally causes the angle movement of the steered wheels of a vehicle. For this purpose, an angle sensor, connected with a microprocessor, is arranged on the steering hand- wheel. This microprocessor controls electromagnetic drives, displacing the slide of the steering valve in one direction or the other. On failure of the electromagnetic drives, the slide of the steering valve can also be dis- placed by a hydraulic pressure, which is produced by a pump connected with the steering handwheel, when the steering handwheel is turned.

An embodiment of this kind meets certain security require- ments, as a vehicle equipped in that manner can still be steered when the electrical components fail. However, the embodiment of the steering valve is relatively compli- cated.

It is the task of the invention to provide a simple em- bodiment of a steering arrangement though securing a safe operation.

In a hydraulic steering arrangement of the kind described in the introduction, this task is solved in that option- ally the steering valve can be operated by a steering

handwheel or by a hydraulic pressure generated by a con- trol pressure generator.

Thus, the embodiment of the steering valve becomes rela- tively simple. Merely a connecting arrangement must be provided, via which the hydraulic pressure for the control of the steering valve can act upon the operating element, for example a slide. A synchronisation between an electro- magnetic additional drive and a hydraulic drive is not required. Accordingly, it must be assumed that the switch- ing from a"servo operation", by which the hydraulic pressure comes from the control pressure generator, to the "manual operation", by which the hydraulic pressure is produced by the steering handwheel, is less felt by the driver.

In a preferred embodiment it is provided that the control pressure generator converts an electrical input signal to a hydraulic control pressure. Thus, all the control oppor- tunities also obtained with a direct electromagnetic drive of the steering valve slide are maintained. For example, the control pressure generator can be controlled by a microprocessor or a remote control. A control pressure generator is manufactured by the applicant, for example under the name PVE.

Preferably, the steering handwheel has a sensor, which is connected with the control pressure generator. Thus, the steering handwheel cannot only be used for building up the hydraulic pressure required for the operation of the steering valve in the case of a failure. By means of the sensor, the signals to be used for the steering of the vehicle, can be obtained directly. Thus, a plurality of control algorithms can be realised. This is particularly advantageous, when the steering arrangement is to be used in driven machines, for example fork lift trucks. In this

connection, the term"steering handwheel"does not only mean the part actually operated by the driver, but also all parts connected with it, for example the steering column.

Preferably, for each movement direction of its slide the steering valve is connected with an outlet of a shuttle valve, whose one inlet is connected with the steering unit and whose other inlet is connected with the control pres- sure generator. With a shuttle valve of this kind, the switching from servo operation to manual operation can be automated. The shuttle valve always passes on the higher pressure of its two inlets to the outlet. On a failure of the control pressure generator, for example caused by a failure in the electrical energy supply, it stops produc- ing hydraulic pressure. Accordingly, the pressure produced by the steering unit is larger on a steering movement of the steering handwheel. This pressure is then passed on to the control inlets of the steering valve in order to operate the slide.

Preferably, the pressure produced by the steering unit is larger than the pressure produced by the control pressure generator. In this case, it will still be possible also to intervene in the steering of the vehicle, when the control pressure generator is working. An intervention of this kind may be required in an emergency situation. As the pressures produced by the steering unit are larger than the pressures produced by the control pressure generator, an operation of the steering unit will cause a switching of the belonging shuttle valve. As the shuttle valve makes a unique choice of the pressure to be passed on to its outlet, an operation of the steering handwheel will cause that the control pressure generator no longer has any influence on the steering behaviour of the vehicle.

Preferably, the pump inlet of the steering valve is con- nected with the outlet of a shuttle valve, whose one inlet has a connection with the pump and whose other inlet has a connection with the steering unit. This adds a third step to the security of the steering unit. A steering operation is namely also provided, when also the pump (or the pres- sure source available in the vehicle) fails. Normally, the pump produces the pressure required for the operation of the pump. As long as the pump works, the shuttle valve passes on the pump pressure to the steering valve. The connection to the steering handwheel or the steering unit, respectively, is, however, interrupted by the shuttle valve, so that there is no risk of negative effects of the pump pressure on the steering unit. When, however, the pump pressure drops below the pressure produced by the steering unit, the pressure produced by the steering unit will be used to operate the steering motor. As such a pressure production only occurs, when the steering valve has been operated by the steering unit, a connection between the steering unit and the steering motor also exists via the shuttle valve and the steering valve. This connection is interrupted, when the steering handwheel is no longer turned, so that there is no risk of a counter- effect of the steering motor on the steering handwheel.

Preferably, the connection between the steering unit and the shuttle valve is interrupted in the neutral position of the steering valve. This is an additional security measure. A connection is only set free, when a pressure actually has to be passed on. In the neutral position of the steering valve, counter-effects on the steering hand- wheel are avoided.

Preferably, the connection between the steering unit and the shuttle valve is led through the steering valve. This

makes it easier to ensure that the connections between the steering unit and the shuttle valve on the one hand and between the shuttle valve and the steering motor on the other hand are always interrupted or released simultane- ously.

In a particularly simple embodiment, this is preferably obtained in that both the connection between the steering unit and the shuttle valve and the connection between the shuttle valve and the steering motor are controlled by the same valve slide. A movement of the valve slide, which is required to produce a connection between the pump and the steering motor, will automatically also produce the con- nection between the steering unit and the shuttle valve.

When, however, the slide returns to its neutral position, not only the first connection, but also the second connec- tion, is interrupted.

Preferably, a pilot pressure valve, controlled by a load sensing pressure, is arranged between the connection of steering unit and shuttle valve and a pressure sink. With this pilot pressure valve it can be obtained that the signal pressure generator can provide relatively small control pressures, also when higher pressures rule in the load sensing pressure line (LS line). This is normally the case, as there the pressure from the steering motor is used to control the belonging pump. By means of the pilot pressure valve, the corresponding excess pressure can be reduced by returning hydraulic fluid to the tank.

In the following, the invention is described in detail by means of a preferred embodiment example in connection with the drawing, showing: Only figure: a schematic view of a hydraulic steering arrangement

A hydraulic steering unit 1 has a steering handwheel 2 connected with a steering unit 3. The steering unit 3 is a conventional steering unit with two mutually rotatable slides. In this connection, the steering handwheel rotates one of the two slides and thus releases a path from a pump connection P to one of the direction connections L, R, while the other direction connection R, L is connected with a tank connection T. The hydraulic fluid flow oper- ates a measuring motor, which accordingly brings the second slide into overlapping the first slide again, to interrupt the hydraulic fluid flow again in dependence of the movement of the steering handwheel 2. The measuring motor can also be used as auxiliary pump, so that on operation of the steering handwheel a hydraulic pressure will be produced at one of the two direction connections L, R. As schematically shown, each of the direction con- nections L, R, is connected in series with a throttle 4, 5.

The steering unit 1 also has a valve block 6 comprising a steering valve 7. At its outlet, the valve block 6 is connected with a steering motor 8 operating the steered wheels 9 of a vehicle (not shown in detail). For this purpose, the valve block has two direction connections CL, CR, which are connected with corresponding working cham- bers of the steering motor 8.

A control pressure generator 10 is flanged onto the valve block 6, which generator, as will be explained later, produces hydraulic pressures, which can be used for the operation of the steering valve 7.

Via a priority valve 11, the valve block 6 is connected with a pump 12. An additional connection exists between the valve block 6 and a tank 13. Instead of the pump 12

shown, another pressure source can be used, and instead of the tank 13 shown, another pressure sink can be used.

The priority valve 11 is known per se. It ensures that the steering arrangement 1 is preferably supplied with pump pressure, also when other consumers are connected to a connection 14.

The steering valve has a slide 15 shown in the neutral position.

The slide 15 has a first section 16, which is responsible for a connection between the steering motor 8 and the pump 12. For this purpose a first connection 18 is provided, which is connected with the outlet of a shuttle valve 19.

An inlet of the shuttle valve 19 is connected with the pump 12 via the priority valve 11. When the slide 15 is moved to the right, the connection 18 is connected with the steering connection CL. The steering connection CR is connected with a connection 20, through which returning hydraulic fluid can reach the tank 13, as will be ex- plained below. At the same time, an LS connection 21 is supplied with the load pressure from the steering motor 8.

The LS connection 21 is thus always submitted to the highest pressure existing in the system.

When the slide 15 is moved to the left, the other outlet CR is accordingly supplied with pressure, and the wheels 9 are steered in the opposite direction.

Further, the steering valve has two connections 22,23, which are connected with the corresponding direction connections L, R of the steering unit 3. When the slide 15 is displaced to the right, the connection 22 gets in connection with the second inlet of the shuttle valve 19.

When the slide 15 is displaced in the other direction, the

connection 23 is connected with the shuttle valve 19 via the connection 24 of the steering valve 7.

The displacement of the slide 15 takes place under the influence of hydraulic pressures. The springs, which are not directly marked, only serve the purpose of keeping the slide 15 in its neutral position.

The hydraulic pressures used to operate the steering valve 7 can be produced in two different ways. For one, the hydraulic pressures are produced by the control pressure generator 10 and led to the front sides of the slide 15 via the shuttle valves 25,26. However, the hydraulic pressures can also be produced by an operation of the steering unit 3. The other inlets of the shuttle valves 25,26 are namely connected with the connections 22,23.

The shuttle valves 25,26 passes on the higher of the two pressures from the control pressure generator 10 or the steering unit 3, respectively, to the front sides of the slide 15 of the steering valve 7. As it is guaranteed that the pressures produced by the control pressure generator are always smaller than the pressures produced by the steering unit 3, it is ensured that it is always possible to intervene in the steering behaviour of the vehicle by means of the steering handwheel 2, independently of the control pressures produced by the control pressure genera- tor 10.

Also when the control pressure generator 10 fails, the slide 15 of the steering valve 7 can be displaced. In this case the control pressure of the control pressure genera- tor is equal to zero, so that the shuttle valves 25,26 pass on the inevitably higher pressure of the steering unit 3 to the front sides of the slide 15, which is thus displaced.

When the slide 15 of the steering valve 7 has been dis- placed to a working position, a connection exists between one of the two connections 22,23 and the connection 24.

When the pump 12 fails, the connection 24 is submitted to the pressure produced by the steering unit 3, for example by means of the measuring motor then serving as pump. This pressure is then passed on to the steering motor 8 via the shuttle valve 19, so that steering of the vehicle is still possible during a pump 12 failure.

As long as the pump produces its pressure, the pressure at the connection 24 is always smaller, as a pressure drop occurs at the throttles 4,5 in the steering unit 3.

Via a pilot pressure valve 27, the connection 20 is con- nected with a tank line T. In the closing direction the pilot pressure valve is acted upon by a spring 28 and in the opening direction via a control pressure line 29.

In the neutral position of the slide 15 shown, the LS connection 21 is connected with the connection 20. Accord- ingly, the control pressure from the valve 11 is passed on to the line 29. When this control pressure is higher than the force of the spring 28, the pilot pressure valve 27 opens, and the excess pressure can flow off to the tank 13. Thus, it is possible to keep the control pressure at a predetermined value, for example 12 bar. In this case, a connection from the connection 24 to the tank 13 is re- leased at the same time, so that the second inlet of the shuttle valve 19 is relieved.

When, however, the slide 15 is displaced to one of its working positions, the connection between the LS-connec- tion 21 and the connection 20 is interrupted. The connec- tion 20 is then submitted to the pressure of the fluid

returning from the steering motor. When this pressure exceeds the force of the spring 28, the pilot pressure valve 27 opens, and the fluid can flow off to the tank 13.

The same also applies, when the pump fails and the steer- ing motor 8 is supplied with pressure from the steering unit 3.

The pump connection P of the steering unit 3 is connected with the connection 20 of the steering valve 7. Accord- ingly, in the neutral position of the slide 15, it re- ceives the pressure in the LS connection 21, so that the steering unit 3 can pass on this pressure as control pressure to one of the two connections 22,23. When the slide 15 is operated, the connection P of the steering unit 3 receives the pressure of the fluid displaced by the steering motor, which pressure always assumes a predeter- mined minimum value due to the pilot pressure valve 27, so that this also contributes to the production of the re- quired control pressures.

On the steering handwheel 2 a sensor 30 is arranged, which is connected (not shown in detail) with the control pres- sure generator 10. By means of the sensor 30, for example, the angle position of the steering handwheel 2 can be detected, the control pressure generator 10 using this information and predetermined algorithms to produce the corresponding control pressures in accordance with size and time, to operate the slide 15.