When hoisting or lowering a load, for example in connec- tion with crane booms, lifting platforms, fork lifts, excavators etc., it is important out of regard for the controllability that the movements are performed without oscillations. It is also desirable for the movements to be performed at a speed which is determined by the operator not by the load size. With hoisting movements there are normally no problems in meeting both wishes. With lowering movements, however, it is very difficult to meet these requirements.

A control arrangement as mentioned in the introduction is known from DE 38 00 188 C2. A normal manually operated three-position valve constructed as a proportional valve serves as control valve. In the inflow path a switching valve has a first throttle assuming such a position that the pressure drop at the throttle is constant. Fixedly connected with the first throttle is a second throttle arranged in the return path, the second throttle serving as load retaining valve and assuming a position determined by the first throttle. A compensation valve is connected

in series with the load retaining valve in the return path, which compensation valve keeps the pressure drop at the load retaining valve constant. Due to the connection between the inflow-side first throttle and the return-side second throttle and the balancing system with suction pipe between the return path and the inflow path required for this reason it is difficult to obtain an accurate, load independent control, which is stable towards oscillations.

From DE 38 02 672 it is known to arrange an auxiliary valve in series with a control valve on the inflow side of a hydraulic motor. When this auxiliary valve is operated so that the pressure drop at the inflow control throttle is kept constant, a constant inflow quantity occurs inde- pendently of the load pressure. If, however, a pilot pipe is provided between inflow path and return path, which pilot pipe has a series connection of a fixed pilot throt- tle and a pilot throttle adjustable by means of the con- trol valve, the auxiliary valve can be activated in de- pendence of the pressure drop at the fixed pilot throttle, which causes that a pressure which is substantially de- pendent on the position of the control valve prevails at the inflow motor connection, which pressure is independent of the load.

The invention is based on the task of providing a control arrangement of the kind described in the introduction, which is to a large extent load independent and stable towards oscillations.

According to the invention this task is solved in that the control valve is part of a pressure control in which the motor inflow pressure is substantially determined by the position of the control valve, and that the load retaining valve opens in dependence of the difference between the motor inflow pressure and a reference pressure, which can

be picked off between the load retaining valve and the return control throttle.

This construction gives an inflow side pressure control and a return side flow control. Seen from a purely control technical point of view the pressure control causes an internal pressure feedback, which is extremely stabilising and enables an approximately oscillation-free lowering.

Due to its control, the load retaining valve ensures that a lowering is only possible when a positive motor inflow pressure is available, that is cavitation cannot occur, and together with the compensation valve the load retain- ing valve provides that during lowering the returning quantity is limited and cannot exceed a maximum quantity specified by the full opening of the control valve. This relation with the motor inflow pressure causes a load independent lowering. In this connection the return quan- tity may certainly have a different value than the inflow quantity, as is the case with a hydraulic cylinder with different piston areas.

It is recommended that the pressure control has a pilot pipe extending between the inflow path and the return path, which pilot pipe comprises the series connection of a fixed pilot throttle and a pilot throttle adjustable by means of the control valve, as well as an auxiliary valve opening in dependence of the pressure drop at the fixed pilot throttle and arranged in series with the inflow control throttle. A pressure control of this kind has a very simple construction and can be accommodated in the housing of the control valve.

Further, it is advantageous that the load retaining valve has a slide, which is loaded in one direction by the reference pressure and a spring and in the other direction

by the motor inflow pressure. This gives a very simple construction of the load retaining valve.

In this connection it is advantageous that the slide is additionally loaded by the inlet pressure of the load retaining valve, the slide having a larger pressure sur- face for the motor inflow pressure and a smaller pressure surface for the inlet pressure. Thus, the load retaining valve can also work as pressure-relief valve. As the influence of the inlet pressure proportional to the pres- sure surface only amounts to a fraction of that of the motor inflow pressure, the load retaining valve does not open until the excess pressure has reached a value which is a multiple of the normal working pressure.

Further, it has turned out to be advantageous that the load retaining valve has a spring chamber, which can be pressure-relieved independently of the reference pressure.

The intended effect particularly occurs when the compensa- tion valve maintains a constant pressure drop at the load retaining valve. However, also the frequently preferred opportunity exists that the compensation valve maintains a constant pressure drop at the return control throttle. In this case the inflow side pressure control and the return side flow control occur in a particularly pronounced way at the control valve.

Expediently, the compensation valve is arranged in the return path. It may be arranged between the return motor connection and the load retaining valve, between the load retaining valve and the return control throttle or between the return control throttle and the tank connection.

Another preferred opportunity is that the compensation valve is arranged in a pilot pipe system and additionally

influences the pilot pressure for an adjustable throttle.

In this connection only a small compensation valve is required, which is interesting for weight and space rea- sons.

In a preferred embodiment it is provided that a pilot pipe extending between the inflow motor connection and the tank connection comprises a series connection of a fixed throt- tle and the compensation valve and therebetween the branching to a pressure surface of the load retaining valve. The fact that the additional control with the compensator overrides the control of the load retaining valves causes that a constant pressure drop can be main- tained at the return control throttle.

Another, also preferred embodiment provides that the compensation valve bypasses the pilot throttle adjustable by means of the control valve. Thus, the overridden con- trol of the auxiliary valve available in the inflow side by means of the compensation valve causes the auxiliary valve to maintain a constant pressure drop at the return control throttle.

An additional advantage in these latter cases is obtained in that the compensation valve is built into the control valve, which is usually possible because of the small dimensions of the compensation valve.

An additional advantageous embodiment involves that the load retaining valve is constructed as a pressure relief valve and the return control throttle is open in the neutral position of the control valve. This gives an additional utilisation of the load retaining valve.

In the following the invention is described on the basis of preferred embodiments on the basis of the drawings, showing: Fig. 1 the connection diagram of a control arrangement according to the invention Fig. 2 an associated working diagram Fig. 3 a second embodiment of a control arrangement according to the invention Fig. 4 a third embodiment Fig. 5 an associated working diagram Fig. 6 a fourth embodiment Fig. 7 a fifth embodiment The control arrangement in Fig. 1 operates a motor 1, here shown as a so-called piston-cylinder unit, which can be loaded in the lowering direction by an external force 2.

The diagram only shows the details required of the lower- ing movement. For the hoisting a usual flow control can be used.

A control valve 3, which is adjustable manually by a regulating arrangement 4 or via a remote control, has an inflow control throttle 5 in an inflow path 6 extending between a pump connection P and a motor connection A, and a return control throttle 7 in a return path 8 extending between a return motor connection Bc and a tank connection T. A pilot pipe 9 connects the inflow path 6 at the inlet of the inflow control throttle 5 with the return path 8 at the outlet of the return control throttle 7. In this pilot

pipe 9 a fixed pilot throttle 10 and a pilot throttle adjustable together with the inflow and return throttles 5 and 7 are arranged. The inflow control throttle 5 is connected in series with an auxiliary valve 12, which maintains a constant pressure drop at the fixed pilot throttle 10. This results in a pressure control according to which the motor inflow pressure PA has a value, which is mainly determined by the position of the control valve 3.

In the return path 8 a compensation valve 13 and a load retaining valve 14 are arranged in series. The load re- taining valve 14 receives the motor inflow pressure PA via a pilot pipe 15 and a reference pressure PR via an addi- tional pilot pipe 16, which reference pressure rules at the outlet B of the load retaining valve 14. Thus the load retaining valve adjusts under the influence of a spring 17 so, that it does not open until the pressure difference PA -PR has overcome the spring force. The compensation valve 13 is controlled so that it maintains a constant pressure drop at the load retaining valve 14. The series connection of compensation valve 13 and load retaining valve 14 is bypassed by an antiparallel connection of a pressure relief valve 18 and a non-return valve 19 opening during the lifting movement.

In the working diagram of Fig. 2 the motor inflow pressure PA is applied on the right half via the adjusting path X of the control valve, that is in particular the slide of a proportional valve. In the left half it is shown how the control path Z of the load retaining valve 14 changes in dependence of the motor inflow pressure PA. The area from 0 to XO corresponds to the dead band of a proportional valve. Here the pressure is built up from a standby pres- sure to PO. This pressure is required to overcome the preload of the load retaining valve 14. From XO to Xl the

motor inflow pressure changes from PO to Pstop, which is the pressure required to move the slide of the load re- taining valve 14 towards the end stop. The area from XO to X1 is the real control area when lowering the load. When the movable part of the motor meets a resistance, for example, is pressed into the earth or has to be packed up, a pressure higher than Pstop may be required. In such cases the slide of the control valve must move beyond X1. This gives a higher motor inflow pressure PA, while the move- ment speed of the motor is still limited by the opening of the load retaining valve 14 in the position Zmax and from the throttling through the compensation valve.

The vertical line starting at S is idealised. It only appears when the control throttles of the control valve 3 work without losses, which is not possible in practice.

The dash-and-dot line L shows that a certain pressure PA on the motor inflow connection A belongs to a certain position X of the slide of the control valve 3, and that with this motor inflow pressure a certain position Z of the load retaining valve is specified, which position Z in connection with the compensation valve 13 results in a predetermined return quantity. Thus, on the inlet side it is ensured that in the inflow path 6 a cavitation prevent- ing pressure is maintained and that in the return path 8 the quantity drained off is limited.

In the embodiment according to Fig. 3 the slide of the load retaining valve 114 is loaded in the closing direc- tion by the spring 17 and the reference pressure PR and in the opposite direction by the motor inflow pressure PA and, via an additional pilot pipe 20, by the inlet pres- sure PE of the load retaining valve 114. However, the motor inflow pressure PA is still prevailing, which ap- pears from the fact that the pressure chamber connected with the pilot pipe 15 has a larger pressure surface 21

than the pressure surface 22 of the pressure chamber connected with the pilot pipe 20. The pressure surface exposed to the reference pressure through PR is preferably equal to the sum of the pressure surfaces 21 and 22.

The embodiment of Fig. 4 differs from the one in Fig. 3 in that the compensation valve 13 is connected to the output side of the load retaining valve 114 in the return path 8, and that the compensation valve maintains a constant pressure drop at the return control throttle 7. Here again the outlet pressure of the load retaining valve 114 is chosen as reference pressure PR. Further, the function of the pressure relief valve 18 is incorporated in the load retaining valve 114, meaning that the spring 17 gets a corresponding preload and the pressure surface relation is chosen accordingly. The chamber for the spring 17 can be relieved separately from the chamber for the reference pressure PR, for example towards the atmosphere or the tank.

For the embodiment in Fig. 4 the working diagram of Fig. 5 applies, which corresponds to the right half of the dia- gram of Fig. 2. Here again the area from 0 to XO forms the dead band of the control valve 3. Here the motor inflow pressure PA is built up from a small stand-by pressure to a pressure PO, which must be high enough to serve as pilot pressure, to open the load retaining valve without avail- able force 2. This is explained by the following example: The load retaining valve 114 has a pilot relation 4: 1. As pressure relief valve it is set at 300 bar. Without the force 2 a load pressure of 20 bar is measured due to the own weight. The required pilot pressure PPmax therefore is (300-20)/4 = 70 bar. As shown in Fig. 5, the start pres- sure PO at the end of the dead band is slightly larger than this maximum pilot pressure. With a force 2 the

required pilot pressure gets lower. With maximum force, for example 240 bar, the pilot pressure Ppmin = 15 bar.

Thus, it is provided that on each operation the motor inflow pressure PA is so high that the load retaining valve 114 is opened. When the pressure drop over the return control throttle 7 is then kept constant by the compensation valve 13, for example at 10 bar, the quantity drained off is so limited that it can do no damage.

In the embodiment according to Fig. 6 the compensation valve 113 is made as a small pilot valve. It is arranged in a pilot pipe 115 in series with a fixed pilot throttle 23. The pick-off 24 arranged between them leads to the pressure chamber with the larger pressure surface 121 of the load retaining valve 114. The compensation valve 113 maintains a constant pressure drop at the return control throttle 7, as deviations are balanced by a superposed adjustment of the slide of the load retaining valve 114.

In the embodiment according to Fig 7 the compensation valve 113 is again made as a pilot valve. It maintains a constant pressure drop at the return control throttle 7 in that the adjustable pilot throttle 11 is bypassed through a pilot branch 25 meaning that the inflow side auxiliary valve 12 is additionally acted upon. The compensation valve 113 can be arranged in the housing or in the slide of the control valve 3. Further, the advantage is obtained that only the required pressure is built up on the outlet side.

Deviations from the embodiments shown are possible in many ways without leaving the basic idea of the invention. For example, the compensation valve can also be connected to the output side of the return control throttle.