Abstract:
A hydraulic safety brake valve arrangement for a motor controlled by a directional control valve. The safety brake valve arrangement has a main valve closing the load lowering conduit in a rest position. Pump pressure opens the main valve to throttle exhausted fluid and at the same time throttle fluid through the supply conduit to the opposed motor chamber. A compensating valve is placed in series with the main valve throttle for the loaded side of the motor. Motor speed is thereby controlled independent of the external loading on the motor.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a hydraulic safety brake valve arrangement for a motor which is actuatable by a control valve and which can be loaded in at least one operating direction by an external force, comprising a main valve which, in the rest position, closes at least that motor conduit which serves as an outlet conduit in this operating direction, opens under the influence of the presssure at the connection on the pump side and thereby forms a first throttling point in the supply motor conduit and a second throttling point in the delivery motor conduit, the openings of both throttling points changing in the same sense and depending on the flow through the first throttling point at least during braking operation. 
     In a known arrangement of this kind (DE-OS 32 25 132), the slide of the main valve comprises two control edges which, in conjunction with two annular grooves, form the first and second throttling points. The slide is loaded at one end by the pressure at the connection on the pump side and at the other end by a spring and by the pressure at the connection of the first throttling point on the motor side. The spring holds the pressure drop at the first throttling point constant. The slide position thereby depends on the quantity. Since the main valve closes both motor conduits in the rest position and can be directly connected to the motor housing, it is ensured that, if there is a fracture in the hydraulic conduits, the liquid volume in the motor is kept shut. Since the opening of the second throttling point depends on the amount of flow through the first throttling point, the quantity delivered by the motor is also limited. This results in a braking effect that reduces the influence of the external force. However, with a given opening of the second throttling point, the quantity delivered depends on the external force. This is often undesirable because, for safety reasons, a particular deceleration must not be exceeded. This applies, for example, to all hydraulically actuated consumers subjected to preloading, such as cranes, excavators, lifts or like equipment, irrespective of whether their motor is a rotating or a linear motor. It is also disadvantageous that the first and second throttles are operative not only during braking operation but also during normal operation and therefore give rise to additional throttling losses. 
     SUMMARY OF THE INVENTION 
     The invention is based on the problem of providing a hydraulic safety brake valve arrangement of the aforementioned kind in which the motor speed, especially the braking speed, is independent of the external force (preloading) with a supply quantity that is predetermined by the control valve. 
     According to the invention, this problem is solved in that the second throttling point is in series with a compensating valve which holds the pressure drop constant at the second throttling point. 
     Since the compensating valve holds the pressure drop at the second throttling point constant, an accurately defined delivery quantity is ensured for each size of opening of the second throttle. This is independent of the size of the external force on the motor. When the control valve has determined a particular supply quantity and thereby the size of the opening of the first throttle, the delivery quantity is also fixed. This results in a very stable operation. Predetermined decelerations are not exceeded. Nor do any marked oscillations arise in the system. 
     In the simplest case, the compensating valve can be loaded in the closing direction by the pressure at the connection of the second throttling point on the motor side and in the opening direction by a spring and by the pressure of the second throttling point at the container side. 
     It is a particular advantage to connect the connection of the second throttling point at the container side by way of a refill check valve to the connection of the first throttling point on the motor side. Whenever refilling becomes necessary as a result of the external force, this can be brought about in the simplest way by way of the refill check valve. 
     Since the refill check valve does not branch off from the container, but rather from a conduit section disposed between the control valve and the second throttling point, there is an increased pressure which is given by the throttling resistance of the conduit and facilitates refilling. 
     In this connection, it is advisable to provide a counter pressure valve in the motor conduit on the delivery side, which holds the pressure at the connection of the second throttling point on the container side substantially constant at a value above the container pressure Refilling is therefore at a stable pressure level. This means that the supply pressure at the motor inlet on the pump side is also constant. This again reduces the oscillating tendency of the system. Further, one ensures that no cavitation will occur even with a high external force. 
     In a preferred embodiment, the compensating valve is disposed between the motor and the second throttling point and is loaded in the opening direction by the pressure at the connection of the first throttling point on the motor side. With this construction, the compensating valve is placed in the open position during normal operation. However, if, as a result of an external force, the pressure on the supply side of the motor is too low, the pressure at the connection of the second throttling point on the container side is applied to the compensating valve by way of the refill check valve so that the compensating valve will then act by way of the second throttling point in the sense of holding the pressure drop constant 
     It is also advisable for the main valve to be loaded in the opening direction by the pressure at the connection of the first throttling point on the pump side and in the closing direction by a spring and by the pressure at the second throttling point on the container side. During normal operation, the main valve is loaded in the closing direction by a comparatively weak pressure. It is therefore fully open. The throttling losses are correspondingly low. However, during braking operation, when the refill check valve is open, the pressure at the connection of the second throttling point on the container side is equal to that at the connection of the first throttling point on the motor side. Consequently, the main valve is operated depending on the pressure drop at the first throttling point. During braking operation, therefore, the main valve assumes the throttling position. 
     It is in this case advisable for the pressure chambers with the control faces for the pressure at the connection of the first throttling point on the pump side and for the pressure at the connection of the second throttling point on the container side to be disposed at the two end sections of the main valve slide. This results in a particularly simple construction because the pressure chambers can be located near the first and second throttling points and therefore short conduit distances are possible. 
     In a further form of the invention, an overpressure valve is connected between the motor connection on the delivery side and a pressure chamber of the main valve that has an overpressure control face acting in the opening direction of the second throttling point. If the overpressure valve responds, the second throttling point is necessarily opened. A small overpressure valve is therefore sufficient rapidly to reduce a large overpressure by way of the second throttling point. 
     It is in this connection advisable for the main valve to have a divided slide and for the pressure chamber with the overpressure control face to be disposed at the division. 
     Another possibility is for the pressure chamber having the overpressure control face to be connected to the connection of the first throttling point on the pump side by way of a throttle. By reason of the twofold use of this pressure chamber, a one-piece slide can be employed for the main valve. 
     Another possibility is for both motor conduits to be each provided with a combination of main valve and compensating valve. In this way, external forces in both operating directions of the motor can be considered. 
     Preferably, the spring of the compensating valve and/or of the main valve is adjustable. In this way, the quantity of the refilling medium can be kept to a minimum. Different inlet and outlet volume ratios of the motor can also be considered, such as are present with stepped pistons. 
     Desirably, the second throttling point is provided with a seating valve. One therefore obtains a leakage-free closure of the motor outlet such as is impossible to achieve with a simple slide valve. 
     Further, the main valve and compensating valve may be accommodated in a valve block on the motor housing. There will therefore be no danger of a conduit for the pressure medium braking between the motor and the valve block. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred examples of the invention will now be described in more detail with reference to the drawing, wherein: 
     FIG. 1 shows one embodiment of a control circuit for a hydraulic motor with a safety brake valve arrangement according to the invention; 
     FIG. 2 shows a modified embodiment; 
     FIG. 3 shows a third embodiment; 
     FIG. 4 shows a further embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a control circuit for a hydraulic motor 1 having a stepped piston 2 in a cylinder 3 and constantly loaded by an external load 4 represented by a force F. The two motor connections ports C1 and C2 are each connected to a motor conduit 5 or 6 which communicate with the two connections ports V1 and V2 of a control valve 8 by way of a valve block 7 fixed to the cylinder 3. This control valve can be moved with the aid of a handle 9 out of the illustrated neutral position into one of two operating positions in which the motor 1 is supplied with pressure fluid from a pump 10 depending on the direction and the discharged fluid is returned to a container reservoir 11. The control valve 8 is designed as a proportional valve which has not been illustrated in more detail. 
     The motor conduit 5 is divided into a delivery return portion 5a and a supply portion 5b and the motor conduit 6 into a supply portion 6a and a delivery return portion 6b. During lifting operation, the supplied pressure fluid is fed through a check valve 12 in the supply portion 6a and the delivered fluid through a check valve 13 in the delivery portion 5a. The overall regulation of the quantity is effected by the control valve 8. 
     During lowering operation, a safety brake valve arrangement 14 becomes effective This consists of a main valve 15 and a compensating valve 16 which is connected in the delivery portion 6b of the motor conduit 6 between the connection C2 of the motor 1 and the operating valve 15. In the rest position, the operating valve 15 is brought by a spring 17 into the illustrated blocking position in which the delivery portion 6b as well as the supply portion 5b are blocked. This ensures that, when the control valve 8 is not actuated, the pressure fluid contained in the motor 1 will not flow off and therefore the load cannot drop in an uncontrolled manner. 
     The main valve 15 forms a first throttling point 18 with a variable aperture in the supply portion 5b and a second throttle point 19 with variable aperture in the delivery portion 6b. The main valve 15 is loaded in the opening direction by the pressure PV1 at the connection 20 of the first throttling point 18 on the pump side and in the closing direction by the pressure PC1 at the connection 21 of this throttling point on the motor side. The main valve 15 therefore assumes a position in which the pressure drop at the first aperture corresponds to the force of the spring 17. The first aperture thus defined corresponds to a second aperture at the throttling point 19. This may have any desired functual relationship to the first aperture and is preferably proportional thereto. 
     The compensating valve 16 is forced into the open position by an adjustable spring 22. A pressure PK at the connection 23 of the second throttling point 19 on the motor side acts in the closing direction and a pressure PM at the connection 24 of the second throttling point 19 on the container side acts in the opening direction. Consequently, during lowering operation, the compensating valve 16 assumes such a position that the pressure drop at the second throttling point 19 is held constant. With a given second aperture, the outflowing quantity is therefore constant independently of the external force F and corresponds to the supply quantity Q. 
     Between the connection 24 of the second throttling point 19 on the container side and the connection 21 of the first throttling point 18 on the motor side there is a refill check valve 25 which opens in a direction towards the motor 1. If, therefore, the external force F creates a pressure in the motor conduit 5 that is too low, refilling takes place immediately by way of the refilling valve 25 so that there is no danger of cavitation. A check valve 35 in the delivery part 6b of the motor conduit 6 prevents a short circuit by way of the refill check valve 25 during lifting operation. 
     FIG. 2 shows a modified circuit in which the same parts are given the same reference numerals and corresponding parts have reference numerals increased by 100. The main valve 115 comprises a slide 26. A control edge 27 together with an annular groove 28 forms the first throttling point 118. A conical closure member 29 together with a seat 30 forms the second throttling point 119. The connection 20 of the first throttling point 118 on the pump side is connected by way of a throttle 39 to a pressure chamber 31 having a control face. The connection 24 of the second throttling point on the container side is connected to a pressure chamber 32 having a control face. 
     In the compensating valve 116, a pressure chamber 33 with an associated control face is, as in FIG. 1, supplied with pressure PK at the connection 23 of the second throttling point 119 on the motor side. On the other hand, the opposite pressure chamber 34 communicates with the pressure chamber PC1 at the connection 21 of the first throttling point 118 on the motor side. 
     In addition, a spring-loaded check valve as a counter-pressure valve 135 is provided in series with the second throttle point 119 between the latter and the control valve 108. This holds the pressure PM at a certain level independently of the quantity of flow. The pressure PM is designed to bring about effective refilling. 
     During normal operation, the operating valve 115 is held open by the pump pressure and the compensating valve 116 by the spring 22 and the pressure PC1. In both valves, throttling losses therefore do not occur. However, if, as a result of external forces F, the supply pressure PC1 of the motor 1 drops below the value PM, a refill quantitity Q N  flows through the refill check valve 25 to the connection C1. The pressures PM and PC1 are therefore substantially equal. Consequently, the slide 126 is under the influence of the pressure drop at the first throttling point 118 and the compensating valve 116 is under the influence of the pressure drop at the second throttling point 119. This results in a braking operation during which the outflowing amount of liquid is held constant. 
     The force of spring 17 is an expression for the amount Q supplied by the control valve 108. The force of spring 22 is a measure of the amount of liquid flowing back from the motor 1. If the return liquid is more than the supply liquid, there is a need for replenishment leading to the refill flow Q N . By reducing the force of spring 22 and/or by increasing the force of spring 17, the need for refilling is reduced. It is therefore readily possible to set a minimal refill quantitity which is nevertheless sufficient for stable operation. As soon as the refill check valve 25 opens, the upper piston chamber of the motor 1 is at a constant pressure PM. Any oscillations that occur are rapidly reduced. 
     An overpressure valve 36 is connected between the connection C2 of the motor 1 and the pressure chamber 31 of the valve 115. In addition, the connection 20 is provided with a check valve 37 which blocks in a direction towards the pump. If, in the neutral position of the control valve 108, i.e. with the throttling point 119 closed, an excessive external force F acts on the motor 1 and overpressure therefore occurs at its connection C2, the overpressure valve 36 will open so that, by reason of the throttle 39, the overpressure is effective in the pressure chamber 31. This opens the main valve 115 for a short time so that the overpressure can be rapidly reduced. A comparatively small overpressure valve 36 is sufficient for this purpose. The pressure face of the pressure chamber 30 therefore not only acts as a normal control pressure face but also as an overpressure control face. 
     FIG. 3 illustrates a very similar circuit in which the same parts have the same reference numerals and similar parts have reference numerals increased by 200. The main difference is that the main valve 215 comprises a slide consisting of two parts 226a and 226b. At the dividing gap there is a pressure chamber 38 which is connected to the connection C2 of the motor 1 by way of the overpressure valve 36. If an overpressure occurs here, the slide portion 226b is pushed to the right so that this overpressure can be rapidly relieved. A throttle passage 40 leading to the pressure chamber 130 permits the slide portion 126b to return when the overpressure goes back. 
     In the FIG. 4 embodiment, there is a set of valves of the kind known from FIG. 3, namely the refill check valve 25, the counter-pressure valve 35, the overpressure valve 36, the compensating valve 116 and the main valve 215 and, in mirror image thereto, the same set of valves 25a, 35a, 36a, 116a and 215a. This utilises the fact that the slides of the main valves 215 and 215a prevent return flow through the first throttling point 218 or 218a because these are closed. Return flow from the connection C1 must therefore take place by way of the left-hand valve group and return flow from the connection C2 by way of the right-hand valve group. In both cases, the desired safety is obtained. 
     Altogether, one therefore achieves a valve arrangement with which the motor subjected to an external force F is secured against movement as a result of hose or tube fracture, wherein the cylinder outlet is sealed against leakage by a seating valve, shock pressure effects can be relieved by an overpressure valve and, above all, the lowering movement occurs uniformly and oscillations in the system are avoided.