CONTROL ARRANGEMENT FOR A HYDRAULIC MOTOR

Abstract: A control arrangement for a hydraulic motor, which is, at least in one working direction, loadable by an external force, with a control valve (3) having in this working direction an inflow control throttle (5) in an inflow path (6), and a return control throttle (7) in a return path (8). Further, a load retaining valve (10) is arranged in the return path (8), which is loaded in the closed position by its outlet pressure (PF) and a spring (11) and in the opening direction by the motor inflow pressure (PA). Besides, means are provided for increasing the pressure drop between load retaining valve (10) and tank connection (T) to a value higher than the pressure drop between pump connection (P) and inflow motor connection (A). This gives a better stabilisation towards oscillations.

Description: Control arrangement for a hydraulic motor The invention concerns a control arrangement for a hydraulic motor, which is, at least in one working direction, loadable by an external force, with a control valve having in this working direction an inflow control throttle in an inflow path extending between a pump connection and an inflow motor connection, and a return control throttle in a return path extending between a return motor connection and a tank connection, and with a load retaining valve also arranged in the return path. When hoisting or lowering a load, for example in connection 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. The invention is based on the task of providing a control arrangement of the kind described in the introduction, which is substantially less endangered by oscillations. According to the invention, this task is solved in that in the closing direction the load retaining valve is loadable by its outlet pressure and a spring and in the opening direction by the motor inflow pressure, and that means are provided for increasing the pressure drop between load retaining valve and tank connection to a value higher than the pressure drop between pump connection and inflow motor connection. The means for increasing the pressure drop produce a counter-pressure, which improves a stabilisation against oscillations. At the same time the load retaining valve meets the function of braking the return flow, thus limiting the lowering speed. In a preferred embodiment it is provided that a spring- loaded non-return valve opening in the return direction is arranged in the return path between load retaining valve and return control throttle. In a simple way the non- return valve increases the counter-pressure. In this connection it is recommended that the non-return valve has a flow characteristic according to which the pressure drop increases with increasing flow. This adapta- tion to the flow quantity gives even better results. Another preferred alternative is that the motor is a piston motor whose inflow pressure surface is smaller than the return flow pressure surface and that the return control throttle has an opening, which is maximum as large as the inflow control throttle opening at the same valve position. As the return quantity is larger than the inflow quantity, this gives a simple way of increasing the counter-pressure utilising already available components. If a larger pressure drop is desired, or if the inflow side and the return side of the motor have equally large pressure surfaces, it is advantageous that the return control throttle has an opening, which is smaller than the inflow control throttle opening at the same valve position. In each case the opening dimensions can be chosen so that the object of stabilisation with regard to oscillations is reached. Advantageously, the load retaining valve is additionally made as pressure-relief valve, as in the opening direction it is loaded by its inlet pressure on a pressure surface, which is smaller than the pressure surface loaded by the inflow motor pressure. In this way a separate pressure- relief valve can be saved, which is advantageous with regard to space and costs. The influence on the load pressure during normal operation is negligible. In the following the invention is described on the basis of preferred embodiments on the basis of the drawings, showing: Fig. 1 a circuit diagram of a control arrangement according to the invention Fig. 2 a modified embodiment Fig. 3 a diagram referring to Fig. 2 Fig. 1 shows a motor 1 made as a piston-cylinder unit and loaded with an external force 2. The figures may, for example, relate to a crane, whose boom is loaded by an external force. Fig. 1 only shows the circuit during the lowering operation. For lifting an ordinary flow control is used. A control valve 3 adjusted by means of an operating arrangement 4, manually or via a remote control, has an inflow control throttle 5 in an inflow path extending between a pump connection P and an inflow motor connection A, and a return control throttle 7 in a return path 8 extending between a return motor connection Be and a tank connection T. An auxiliary valve 9 arranged in series with the inflow control throttle 5 maintains a constant pres- sure drop at this control throttle. Each position of the control valve 3 thus corresponds to a certain inflow quantity. A load retaining valve 10 is loaded in the closing direc- tion by its outlet pressure P R and a spring 11 and in the opening direction by the motor inflow pressure P A , and with the purpose of forming a pressure relief valve by its inlet pressure P E . The pressure surface 12 loaded by the motor inflow pressure P A is substantially larger than the pressure surface 13 loaded by the inlet pressure P E , for example with a relation of 4:1, whereas the pressure surface 14, which is exposed to the outlet pressure P R , corresponds to the sum of the two pressure surfaces 12 and 13. The spring 11 can be arranged in a chamber, which is pressure-relieved to the atmosphere or to the tank. However, it can also be active in the chamber acted upon by the reference pressure. Further, the load retaining valve 10 is bypassed by a non-return valve 15 opening during the lifting operation. Between the load retaining valve 10 and the return control throttle 7 there is a non-return valve 16 loaded by a spring 17 and bypassed by non-return valve 18 opening during the lifting operation. The non-return valve 16 provides that the outlet pressure P R exceeds the pressure P B at the inlet B of the return control throttle 7 by a certain amount, for example 20 bar. The preload by the spring 17 must be adapted to the actual application and the actual oscillation problem. Further, it is recommended to make the pressure drop over the non-return valve 16 depending on the flow. Such a flow characteristic may be linear, and, for example, increase from 20 bar at flow 0 to 30 bar at maximum flow. The loading of the load retaining valve 10 by the outlet pressure P R acting as counter-pressure has an oscillation reducing effect, particularly when acting in the spring chamber. If the lowering speed is too high, the motor inflow pressure P A is reduced, and the load retaining valve 10 reduces its opening. If, on the other hand, the lowering speed is too low, the motor inflow pressure P A is increased, and the load retaining valve 10 opens more. When assuming that the load retaining valve 10 opens at an inlet pressure P E of 300 bar, a relation between the pres- sure surfaces 12 and 13 of 4:1 will cause this opening to occur at the sole effect of the motor inflow pressure P A of 300/4 = 75 bar. .An outlet pressure P R of 20 bar will have the effect of 100 bar in the load retaining valve due to the relation of the pressure surfaces 13 and 14 of 1:5. This causes that the spring 11 must correspond to a pressure of 200 bar to create balance. However, the result of this is that each reduction of the pressure by the non return valve 16 by 1 bar corresponds to a reduction of 5 bar at the load retaining valve 10. Imagining, for example, a load acting with 275 bar, which is not sufficient to operate the load retaining valve 10, as it is acted upon in the opposite direction by the 300 bar (spring + pressure) , in principle only 5 bar must leak over the nonreturn valve 16 (corresponding to 25 bar at the load retaining valve 10) to make 275 bar rule on both sides, which might eventually lead to self-opening. This can be prevented in that the "counter-pressure" is integrated in the slide function of the control valve 3, in a way as described below. In the embodiment according to Fig. 2 the non-return valve 16 is left out. In each position of the control valve 3 the openings of the inflow control opening 5 and the return control opening 7 are equal to each other. As, however, the piston 19 of the motor 1 has a small inflow pressure surface 20 and a large return pressure surface 21, the fact that a balancing system is not provided between the inflow path 6 and the return path 8 causes that the return quantity is larger than the inflow quantity, and thus also that the pressure drop, which must produce the desired counter-pressure, is larger at the return control throttle 7 than at the inflow control throttle 5. When, for example, a pressure drop of 8 bar at the inflow control throttle 5 is assumed, and a relation between the pressure surfaces 20 and 21 of 1:1.5 is anticipated, the pressure drop at the return control throttle 7 is equal to 1.5 2 x 8 = 18 bar. A further adaptation to the conditions given, be it that a larger pressure drop is desired or that the piston has two equally large pressure surfaces, is obtained in that the opening of the inflow control throttle 5 is in each valve position enlarged by a certain relation in relation to the opening of the return control throttle 7. This is shown schematically in Fig. 3, in which the opening surface F of both control throttles are shown over the distance X of the valve slide. The dotted curve K5 corresponds to the inflow control throttle 5 and the drawn out curve K7 to the return control throttle 7.

Claims: Patent claims 1. Control arrangement for a hydraulic motor, which is, at least in one working direction, loadable by an external force, with a control valve having in this working direction an inflow control throttle in an inflow path extending between a pump connection and an inflow motor connection, and a return control throttle in a return path extending between a return motor connection and a tank connection, and with a load retaining valve also arranged in the return path, characterised in that in the closing direction the load retaining valve (10) is loadable by its outlet pressure (P R ) and a spring (11) and in the opening direction by the motor inflow pressure (P A ) , and that means are provided for increasing the pressure drop between load retaining valve (10) and tank connection (T) to a value higher than the pressure drop between pump con- nection (P) and inflow motor connection (A) . 2. Control arrangement according to claim 1, characterised in that a spring-loaded non-return valve (16) opening in the return direction is arranged in the re- turn path (8) between load retaining valve (10) and return control throttle (7) . 3. Control arrangement according to claim 1 or 2, characterised in that the non-return valve (16) has a flow characteristic according to which the pressure drop increases with increasing flow. 4. Control arrangement according to one of the claims 1 to 3, characterised in that the motor (1) is a piston motor whose inflow pressure surface (20) is smaller than the return pressure surface (21) and that the return control throttle (7) has an opening, which is maximum as large as the opening of the inflow control throttle (5) at the same valve position. 5. Control arrangement according to claim 1, characterised in that the return control throttle (7) has an opening, which is smaller than the opening of the inflow control throttle (5) at the same valve position. 6. Control arrangement according to one of the claims 1 to 5, characterised in that the load retaining valve (10) is additionally also made as pressure-relief valve, as in the opening direction it is loaded by its inlet pressure (P E ) on a pressure surface (13), which is smaller than the pressure surface (12) loaded by the inflow motor pressure (P A ) .

Inventors: BUHL JAN MAIBOELL, BUHL, JAN, MAIBOELL
Applicants: BUHL JAN MAIBOELL, DANFOSS AS, BUHL, JAN, MAIBOELL, DANFOSS A/S
Application year: 1999
Main classification: F15B11
IPC codes: B66C13, B66F9, E02F9, F15B13