A PRIORITY FLOW CONTROL VALVE FOR HYDRAULIC POWER CIRCUITS

Abstract: The invention falls into the art field of components for hydraulic power circuits, and relates to a valve for controlling the flow of oil directed from a main system into a priority circuit. The valve utilizes an additional piston (19) to impinge on the movable seat (17) of a spring (16) housed in a cylindrical chamber (14) that connects with the constant flow passage serving the priority outlet; the additional piston (19) operates internally of a chamber (20) having cross-sectional area greater than that of the chamber (14) housing the spring (16), and is exposed to high pressure flow, which can be routed into the chamber (20) either from the priority outlet or from the main valve inlet, which will be connected to the high pressure line from the pump.

Description: The invention relates to a priority flow control valve for hydraulic power circuits. The existing prior art embraces flow control valves consisting substantially in a housing with an inlet, connected to the high pressure outlet of a hydraulic pump, and a main cylindrical bore; the bore slidably accommodates a piston by which it is split into two chambers, and receives oil direct from the inlet. The first cylindrical chamber is connected with the second cylindrical chamber by way of a passage that incorporates a flow control element, so that the second chamber is in receipt of oil under pressure at a steady rate of flow for operation of a given service. Accordingly, the second chamber connects with a fixed flow outlet, supplying the priority circuit, whilst the first connects with an outlet that may either unload the oil to the tank or direct it toward another service. The piston is of length such as to block either one or the other outlet at any given moment, and associates by way of a rod, occupying the second chamber, with a second piston that is biased in one direction by a spring housed in a further chamber connected hydraulically with the second chamber. The valve is solenoid-operated, and will be wired in such a way that energization of the coil activates the constant flow outlet, i .e. the priority circuit. Flow control valves of the type outlined above are widely utilized to operate the wrecking actuators with which excavators are equipped. It has emerged, however, that this conventional valve design betrays a number of drawbacks, one of which being that the priority outlet is not always faultlessly blocked when not in use; thus it can happen that there is a continual leakage of oil, sufficient to occasion movement of the actuator, the consequences of which can be extremely hazardous when such movement is not desirable. Indeed, manufacturers stipulate a minimum pressure rise for such valves. In another version of this same type of flow control valve, the leakage of oil is prevented by installing a preloaded backpressure valve to balance the force of the spring. Leakage is certainly prevented by the adoption of this expedient, though one has a further drawback inasmuch as there is a permanent pressure drop during operation of any one of the excavator's different services. The resultant power loss will automatically dictate additional heath which must be dissipated; practical experience shows that power losses of between 1.5 and 5 kW occur, depending on pump flow, and it is absolutely essential that this surplus heat be dispersed. Furthermore, in FR-A-1 585 839 is known a hydraulic device, which permits of establishing a variable pressure in a coupling or a brake, which device serves to vary the velocity in a change gear. Said device comprises a slide valve and a piston being coaxial to but having a greater diameter than said slide valve, between which piston and slide valve a compression spring is arranged. A passage places the chamber in which the piston is situated in communication with an unloading passage. The object of the present invention is to overcome the problems aforementioned, and in particular, to embody a priority flow control valve in which the priority outlet is guaranteed to close efficiently, so that power losses will not occur during normal operation of the machine in which it is installed. The stated object is achieved, comprehensively, with a priority flow control valve for hydraulic power circuits having the characterizing features according to claim 1. Two preferred embodiments of the invention will now be described in detail, by way of example, with the aid of the accompanying drawings, in which: fig 1 is the longitudinal section through a first embodiment of the valve; fig 2 is the longitudinal section through a second embodiment of the valve. Referring to fig 1 of the drawings, 1 denotes the housing of a valve affording an inlet 2 into which oil is directed at high pressure through a line denoted 3. The inlet 2 connects with a cylindrical bore 4 internally of which a piston 5 is slidably accommodated, and positioned in such a way as to create two chambers 6 and 7. The first cylindrical chamber 6 connects with the second 7 by way of a passage 8 incorporating a flow control element 9 of conventional embodiment; oil thus enters the second chamber 7 at a constant rate of flow. The first chamber 6 connects, by way of a passage denoted 10, with a port that either unloads the oil to tank or directs it to another service, whereas the second chamber 7 connects, by way of a passage denoted 11, with a port from which oil is supplied at a constant rate of flow to a service, such as the actuator of a wrecking implement. The piston 5 can be positioned along the bore 4 so as to block the one passage 10 and open the other passage 11, and vice versa. 12 denotes a rod by which the piston 5 is connected with a second piston 13, slidably accommodated in a further chamber 14 disposed coaxial with the bore 4. This chamber 14 and the bore 4 are interconnected hydraulically by way of a drilling 15 formed through the rod 12 and the second piston 13. 16 denotes a coil spring lodged between the second piston 13 and a movable plate 17; spring and plate alike are accommodated by the chamber denoted 14. The plate 17 is positioned with one side offered to the spring 16, and its opposite side engaged by the rod 18 of a further piston 19 slidably accommodated in a cylindrical chamber 20 of cross-sectional area greater than that of the chamber 14 occupied by the spring 16. This larger chamber 20 is connected with the high pressure oil line 3 by way of a line 21 incorporating a restriction 23, and if considered desirable, a check valve 22; needless to say, the line 21 in question might be routed directly through the valve housing 1. 24 and 25 denote a drain line and a conventional relief valve, respectively, both of which connect with a chamber 26 that is also open, via a relative passage 27, to the solenoid valve 29 by which the entire flow control valve is operated. 28 denotes a check valve installed in a passage 30 that connects the spring chamber 14 with the large diameter chamber 20; accordingly, these two chambers can be isolated from one another. The advantages of the valve will be evident from the foregoing description; the most obvious is that of having obtained a considerable reduction in pressure fluctuations with the valve in the de-energized configuration, obtained by pilot operation utilizing flow from the high pressure inlet and exploiting two construction expedients, namely, the difference in pressure between the chambers denoted 14 and 20, and the difference in cross-sectional area between these same two chambers. A further advantage is that the priority outlet 11 remains securely blocked with the valve de-energized. Fig 2 illustrates an alternative embodiment of the valve, in which operation is piloted by directing oil from the spring chamber 14 through a passage 40 formed in the movable plate 17, the rod 18 and the piston 19; the passage 40 slidably accommodates a stem 41 provided with a poppet 42 that registers to an exact fit in a corresponding seat 43. With this arrangement, pressure fluctuation is reduced solely by virtue of the difference in cross-sectional area between the two chambers 14 and 20, since their pressures are substantially the same.

Claims: A priority flow control valve for hydraulic power circuits, comprising: an inlet (2), connecting with a cylindrical bore (4); a first piston (5), slidably accommodated by the bore (4) and dividing the bore (4) into chambers (6, 7) communicating through a passage (8), that opens and closes an unloading passage (10) and a passage (11) through which oil is directed at a constant rate of flow; and a second piston (13), slidably accommodated together with a relative bias spring (16) internally of a chamber (14) disposed coaxial with the bore (4); characterized in that the second piston (13) is connected through a rod (12) with the first piston (5) and the relative bias spring (16) impinges on the surface of said second piston, and in that it comprises: a further chamber (20), in receipt of oil under pressure, the cross-sectional area of which is greater than that of the chamber (14) occupied by the bias spring (16); a piston (19), slidably accomodated in the chamber (20) of greater cross-sectional area and associated with a rod (18) that impinges on a movable plate (17) against which the bias spring (16) is seated, the pressure acting on the piston (19) within chamber (20) being directed opposite to the biasing force of spring (16). A valve as in claim 1, wherein the chamber (20) exhibiting cross-sectional area greater than that of the chamber (14) occupied by the spring (16) is connected hydraulically with the inlet (2) of the valve and with the cylindrical bore (4). A valve as in claim 1, wherein the chamber (14) occupied by the spring (16) and the chamber (20) of greater cross-sectional area occupied by the piston (19) are interconnected hydraulically. A valve as in claims 1 and 2, wherein the chamber (20) of greater cross-sectional area and the chamber (14) occupied by the spring (16) are interconnected by way of a passage (30) incorporating a check valve (28). A valve as in claims 1 and 2, wherein the chamber (20) of greater cross-sectional area connects with the inlet (2) by way of a passage (21) incorporating a restriction (23). A valve as in claim 1, wherein the chamber (20) of greater cross-sectional area connects with the inlet (2) by way of a passage (21) incorporating a check valve (22).

Inventors: BEDOGNI, GIORGIO
Applicants: OLEOSTAR S.R.L.
Application year: 1988
Main classification: F16K31
IPC codes: F15B13