VARIABLE CAPACITY PUMP

PURPOSE:To provide a pressure vs rate-of-flow characteristic in linear compliance with the set values over the whole range by releasing part of the discharge pressure of a main pump to a tank line in the predetermined rate of flow being interlocked with motion of a selector valve when the discharge pressure of an aux. pump is led to a proportional solenoid control valve as pilot pressure oil. CONSTITUTION:When the load pressure sinks from the self pressure pilot control condition made by a main pump 11 to cause a selector valve 7 to make changing-over into external pilot control made by an aux. pump 4, a difference in the drain amount from the main pump discharge amount nullifies, and accordingly a rate-of-flow correction valve 8 releases a rate of flow corresponding to the differential drain amount from the main pump discharge amount to the tank line in divergence being interlocked with the selector valve 7. Therefore, the discharge amount to the load does not any more include the portion corresponding to the differential drain amount even though a proportional control solenoid valve 15 is operated with the same control input. As a result the pressure vs rate-of-flow characteristic as external trait complies with the set values linearly over the whole range.