Shut-off device for a fluid driven motor

Abstract

The present invention relates to a device for shutting off a supply of pressurized motive fluid to an associated fluid driven motor in response to a predetermined motor back pressure indicating a predetermined load on the motor. The device comprises a housing having a fluid inlet connected to a supply of motive fluid and a fluid outlet connected to supply pressurized fluid to the associated fluid driven motor. A passageway extends through the housing between the fluid inlet and the fluid outlet. A shut-off piston is slidably mounted with the housing and is shiftable to close the housing passageway and shut off the supply of pressurized fluid to the motor in response to a predetermined motor back pressure. An orifice of variable size is provided in the housing passageway and is utilized to reduce the pressure of the fluid supplied to the motor. The size of the orifice is controlled by a pilot piston slidably mounted within the housing and shiftable between a first and second position to vary the size of the orifice from a restricted to an open size. The pilot piston is shiftable in response to a predetermined motor back pressure to enlarge the orifice prior to the closing of the shut-off piston. Enlarging the orifice at this time results in an increase in pressure to the motor just prior to shut off. It has been found that this increase in final pressure to the motors permits the final shut-off torque of the motor to be more accurately controlled, regardless of the type of load encountered by the motor.