| 摘要 |
A hydraulic system includes a pilot controlled, pressure operated, spool valve (2) which acts substantially as a by-pass valve when regulating supply of fluid to an actuator (8), and as an in-line valve when regulating the return flow of fluid from the actuator (8) to tank. Both these regulating functions are provided by means of a single main valve spool (31). A flow sensor (20) provides negative feedback to the pilot valve in both by-pass and in-line modes of operation of the main valve, which makes the operation of the system substantially independent of load, temperature, and pump flow. If it is essential that the actuator (8), once it has reached a desired level, remains at that level, the system may additionally incorporate a releasable non-return valve (7) in the service line (71) from the main valve (2) to the actuator (8). This non-return valve is inserted into the service line such that it opens under the pressure of fluid flowing from the main valve (2) to the actuator (8), closes under the back pressure of the load, and is opened by a suitable mechanism when fluid is returned from the actuator to tank under the back pressure of the load. The control system is primarily intended for use with hydraulically operated lifts, but may also be used in other hydraulic systems in which the actuator is to be moved in one direction under pressure supplied to it, and returns to its original position under the action of a return force, e.g. gravity. |
