Screw type vacuum pump

Claim 1

A screw type vacuum pump comprising: a pump casing having a suction port and a discharge port on opposite sides thereof; intermeshed male and female rotors comprising means for pumping a gas from said suction port to said discharge port when said rotors are rotated; power transmission means for rotating said rotors, including a transmission casing having an oil reservoir; oil circulating means including an oil pump and an oil cooler, for circulating lubricating oil to said pump casing for lubricating said rotors; and a check valve fluidically connected to said suction port and comprising: (a) a valve casing defining a gas flow space and a cylinder space, said gas flow space being fluidically isolated from said cylinder space and having a valve seat, (b) a valve body for closing said valve seat and sealing said suction port, said valve body including a piston fitted in said cylinder space to divide said cylinder space into an oil chamber and an air chamber at atmospheric pressure, (c) spring means for normally biasing said valve body in a closed position; and (d) a three way change over valve selectively communicating said oil chamber with said oil reservoir when the vacuum pump is stopped and with said oil circulating means at a position downstream of said oil pump when said vacuum pump is operating, whereby oil pressure is applied to said oil chamber from said oil circulating means via said change over valve when said vacuum pump is operated, so that said valve body moves from said closed position to an open position in opposition to the force of the spring means to open fluid communication to said suction port, and whereby the oil pressure is not applied to said oil chamber so that said check valve is closed to prevent reverse rotation of said rotors when said oil pump is stopped and lubricating oil is not circulated to said pump casing.