Patent ID: 8348635

Claim:
A variable displacement gear pump used for a loading system in an industrial vehicle, the industrial vehicle having an actuator of the loading system, a control valve for hydraulically controlling the actuator, and a discharge conduit of the loading system connecting the control valve to the actuator and in which a pressure is increased when loading operation is performed and a pressure is decreased when no loading operation is performed, the variable displacement gear pump comprising: a main gear pump portion having a main drive gear and a main driven gear engaged with each other, the main gear pump portion having a suction-side space and a discharge-side space formed therein; a sub gear pump portion having a sub drive gear and a sub driven gear engaged with each other, the sub gear pump portion having a suction-side space and a discharge-side space formed therein; a suction passage communicating with the suction-side spaces of the main and sub gear pump portions; a discharge passage communicating with the discharge-side spaces of the main and sub gear pump portions, the discharge passage in communication with the control valve; a bypass passage returning hydraulic fluid in the discharge-side space of the sub gear pump portion to the suction passage; a check valve preventing hydraulic fluid in the discharge-side space of the main gear pump portion from flowing to the discharge-side space of the sub gear pump portion, and an opening valve disposed in the bypass passage so as to open the bypass passage when a first pressure receiving surface of the opening valve receives a pressure in the bypass passage that is greater than a pressure on a second pressure receiving surface of the opening valve and so as to close the bypass passage when the first pressure receiving surface of the opening valve receives a pressure that is less than a pressure on the second pressure receiving surface of the opening valve, the second pressure receiving surface disposed so as to receive a pressure in the discharge conduit when a loading operation is preformed, thus no control signal is required to open or close the opening valve.