Combined pressure relief and unloader valve.

摘要

A combined unloader and pressure relief valve for a hydraulic system comprises a main valve member (13) the opening of which is controlled by pressure in a pilot valve chamber (23). A lever (28) is normally held against a stop by a spring (31) and rests lightly on a pilot valve member (27) which thus maintains a low pressure in chamber (23) allowing the main valve to open under the influence of pressure at inlet (4). When the valve is to be closed to charge an accumulator (8) a transducer (10) energises a coil (37) to pivot lever (28) about pilot valve member (27). This lifts the lever from the stop and applies a spring load to the pilot valve member thereby allowing pressure to build in chamber (23) to close the main valve member. If the transducer (10) fails and maintains the coil (37) permanently energised, the main valve member will act as a pressure relief valve since the pilot valve member will lift when pressure in chamber (23) is able to overcome the applied spring load.