A device of this kind is known from DE 36 35 162 C2. In this connection, the individual throttles are usually made up of a cooperation of grooves, bores or other openings on two rotary slides, one rotary slide being displaced by a steering handwheel or another steering angle trigger, so that in dependence of the direction, some throttles open and others close, while the measuring motor causes the other rotary slide to follow the first rotary slide in dependence of the hydraulic fluid passing. Thus, it is possible, in dependence of the steering angle determined by the steering angle trigger, to displace a steering motor arranged at the working connections to such a degree that the steered wheels or rudders get the corresponding deflection.

Another steering device of this kind is known from DE 43 42 933 C2. This steering device has one throttle less in the flow.

Such devices are preferably used in tractors, farming tractors, tractor trucks or mobile equipment, which for reasons of their intended use have relatively coarsely structured tyres, that is, with a relatively coarse pro- file. A profile of this kind is very appropriate offroad, however, when driving on a smooth ground, for example a street, it reduces the driving comfort. For one, the driving comfort is impaired in that the driver is exposed to vibrations caused by the coarse tyre profile. However, these vibrations can to a high degree be reduced by pro- viding a corresponding spring cushioning of the driver's cabin or seat. More critical is that the vibrations have a retroaction on the steering system. This causes substan- tial noise in the driver's cabin.

The invention is based on the task of improving the driv- ing comfort.

With a device as mentioned in the introduction, this task is solved in that the load sensing throttle is separated from the return and that a pressure control valve is arranged in the return.

With this device it is obtained that a back pressure can be produced at the working connection device, so that in a manner of speaking the steering motor is subjected to pressure from both sides. This gives a more rigid system, which tends less to swinging. Until now, this back pres- sure has been equal to the tank pressure, for example 0.8 bar. The pressure control valve now permits an increase of this back pressure, for example to 20 bar, so that it reaches the range of the idle pressure. However, this measure alone would have a negative effect on the load sensing pressure. The device being otherwise unchanged, would cause the load sensing pressure to increase by the

same pressure difference, so that the idle pressure would increase accordingly. A more rigid system would still be ensured, however, the components used for producing and controlling the hydraulic pressure would have to be corre- spondingly stronger. Additionally, the pressure difference across the device would not change during idling. These disadvantages are avoided, when the load sensing throttle is separated from the return. In this case, the load sensing connection of the load sensing throttle is still connected with the pressure sink, so that the pressure drop across the load sensing throttle permits a reliable statement of the pressure required in the system. The normal load sensing connection can be used as control pressure connection. Therefore, the term"load sensing connection"is also used in the following.

Preferably, the pressure control valve is made as a pre- stressed non-return valve. The opening pressure of the non-return valve determines the pressure received by the pressure control valve. Such an embodiment is relatively easily realised. A replacement or modification of a spring used to prestressing the non-return valve permits an easy change of the pressure of the pressure control valve and thus also an adaptation to various conditions.

Preferably, the pressure control valve is arranged between the pressure sink and the return throttle. This arrange- ment ensures that a flow exists through the return throt- tle on each displacement movement of the steering unit, meaning that the return throttle still contributes to the steering behaviour. This is particularly advantageous, because the corresponding vibrations mainly occur in connection with curve driving, when the wheels are running on their outer edges. Normally, the coarsest profiling is found here.

Advantageously, the working connection arrangement is connected with a suck back valve arrangement, which is connected with the pressure sink via a suck back line.

This reduces cavitation appearances in the steering unit or the steering motor, respectively, when forces from the outside act upon the steered wheels. Such a suck back pipe arrangement is known per se.

Preferably, the suck back line is connected with the return between the pressure control valve and the return throttle. The suck back valve arrangement can then ini- tially act upon the"trapped"pressure, by supplying additional hydraulic fluid in case of a movement of the steering motor caused by external forces. This enables a faster reaction, as hydraulic fluid under increased pres- sure is readily available for the refilling.

In this connection, it is preferred that in the suck back line a second non-return valve opening in the opposite direction is arranged in parallel with the pressure con- trol valve. This second non-return valve enables on the one hand the suck back from the pressure sink, for example the tank. On the other hand, however, it blocks reliably, when a pressure difference occurs, which is directed in the opening direction of the pressure control valve. The use of two non-return valves in anti-parallel connection simplifies the design.

Preferably, the load sensing throttle is connected with the flow between a first and a second throttle, and a connecting throttle connects the flow before the first throttle with the return between the return throttle and the pressure control valve. This connecting throttle has two functions. Firstly, the connecting throttle, which is open in the neutral position of the steering angle trig- ger, provides a fluid connection between the flow and the

return, via which heat can be transmitted. Thus, the temperature of the control device can be equalised, which improves the steering behaviour, particularly reactions to changes of the predetermined steering angle. Secondly, the suck back through this connecting throttle is easier, as also here hydraulic fluid with an increased pressure is available. The connecting throttle is also able to supply the required back pressure in the return, the pressure control valve being opened by the exceeding of the set pressure.

Preferably, the pressure control valve is set at a value, which amounts to more than 50% of an idle pressure. De- pending on the size of the set value, the"rigidity"of the system can be influenced. Values of more than 50% of the idle pressure have turned out to be advantageous.

Advantageously, the high pressure connection is connected with a priority valve. This priority valve is at the same time used to provide the load sensing pressure. Thus, the distribution of hydraulic fluid to the steering unit and possible other working units is more easily controlled.

In the following the invention is described on the basis of a preferred embodiment in connection with the drawings, showing: Fig. 1 a schematic view of a device for the control of a pressure for a hydraulic steering unit, and Fig. 2 a diagram with a schematic view of the opening behaviour of individual throttles across a steering angle.

Fig. 1 shows schematically a device 1 for the control of a pressure for a hydraulic steering unit 2, which in the

present case is formed by a double-acting hydraulic cylin- der 3 with pistons 4, a piston rod 5 fixed to the piston 4 deflecting the wheels of a tractor in a manner not shown in detail.

The cylinder 3 has steering motor connections R, L, which are connected with corresponding connections R, L on the device 1. In this connection, a suck back valve arrange- ment 6 is provided between the steering unit 2 and the device.

The device 1 has a high pressure connection P which is connected with a pump 9 via a non-return valve 7 and a CF- connection of a priority valve 8. Thus, the high pressure connection P can also be called"pump connection". The high pressure connection P is connected with a flow of the device 1, in which flow a first throttle Al, a second throttle A2, a measuring motor 10, a third throttle A3 and a fourth throttle A4 are arranged in series. The device 1 also has a return 12, in which a return throttle A5 is arranged. The measuring motor 10 and the five throttles Al to A5 mentioned are acted upon by a steering handwheel 13 on the one side and by the measuring motor 10 on the other side. The throttles Al to A5 are made by two rotary slides, of which one is rotated by the steering handwheel, whereas the other is made to follow by the measuring motor 10. An embodiment of this kind is known per se, for exam- ple from DE 36 35 162 Cl. The throttles Al to A5 can also be made in different ways.

The majority of the throttles is required, as both the flow 11 and the return 12 must be throttled, as the work- ing connections R, L are connected with the high pressure connection P and a low pressure connection T in dependence of the rotation direction of the steering handwheel 13, and as also the flow direction of the fluid flowing to the

steering motor 2 through the measuring motor 2 must occur in dependence of the rotation direction.

As appears from Fig. 2, the throttles Al, A4 and A5 are closed in the neutral position. Also the throttles A2, A3 are closed in the neutral position, however opening ear- lier than the throttles Al, A4, A5. The opening character- istic, that is, the opened surface F in dependence of the rotation angle a is shown schematically in Fig. 2.

At a point 14 between the first throttle Al and the second throttle A2 a load sensing line 15 branches off, through which a pressure signal LS is derived, which corresponds to the load pressure of the steering motor during opera- tion. The load pressure LS at the same time controls the priority valve 8 and possibly also the pump 9, which can for this purpose be provided with an adjustment mechanism 16.

The return 12 is connected with the tank 19 or another pressure sink via a pressure control valve 17, which in the present case is made as a non-return valve with a preloading spring 18. However, a connection is missing between the return 12 and the point 14 in the flow, at which the load sensing line 15 branches off. Instead, a load sensing throttle 21 is arranged in a line 20 between the point 14 and the tank 19, the opening characteristic of said throttle being shown in Fig. 2. In the neutral position, the load sensing throttle 21 is completely opened. It closes in step with the increase of the steer- ing angle a. Thus, the load sensing pressure LS occurs as a pressure at an intermediary return of a pressure di- vider, which is formed by the throttles Al and 21. Accord- ingly, the return 12 and the load sensing throttle 21 are disconnected from each other.

However, the flow 11 and the return 12 are connected with each other through a connecting throttle 22, the connect- ing throttle 22 being connected before the first throttle Al with the flow 11 and between the return throttle A5 and the low pressure connection T with the return 12. The opening characteristic of the connecting throttle 22 is shown in Fig. 2 with a line 22. From this it occurs that the connecting throttle 22 is also completely open in the neutral position (a = 0°), and is closed in step with an increasing steering angle.

Further, a suck back line 23 is provided, with which the suck back valve arrangement 6 can draw hydraulic fluid from the tank 19. The suck back line 23 is connected with the return 12 at a point 24. Between the point 24 and the tank 19 an additional non-return valve 25 is provided, which is connected antiparallel to the pressure control valve 17. Accordingly, the suck back valve arrangement 6 can draw hydraulic fluid either via the non-return valve 25 from the tank 19 or via the connecting throttle 22 from the high pressure connection P.

The priority valve 8, the pump 9 and the throttles Al to A5, 21,22 are set so that a pressure of, for example, 22 bar is available at the working connection arrangement RL during idling. At the tank 19 there is a pressure of approximately 0.8 bar. Through the pressure control valve 17 it is now possible to maintain a pressure in the return of, for example, 20 bar, so that it is ensured that the piston 4 of the cylinder 3 is"jammed"between two higher pressures, without requiring a corresponding increase of the load sensing pressure LS. This leads to a more"rigid system", in connection with which the risk of producing swinging caused by coarsely structured tyres is substan- tially reduced. Accordingly, the noise occurring in the device 1 is reduced, which increases the driving comfort.