Patent ID: 8011908

Claim:
A variable capacity vane pump having a pump control ring which is moveable to alter output capacity of the pump, the pump being operable at least two selected equilibrium pressures, comprising: a pump casing having a pump chamber therein, the pump casing having an inlet port and an outlet port; a vane pump rotor rotatably mounted in the pump chamber; a control ring enclosing the vane pump rotor within said pump chamber, a plurality of vanes operatively engaging said rotor and frictionally engaging said control ring, defining a series of pumping chambers whereby driven rotation of said rotor effects drawing fluid into the pumping chambers through said inlet port and exhausting fluid out of the pumping chambers through said outlet port; said control ring being moveable within the pump chamber to alter volumetric capacity of the series of pumping chambers; a control chamber between the pump casing and the control ring, the control chamber operable to receive pressurized fluid to create a force to bias the control ring towards a position of minimum volumetric capacity of the pumping chambers; a primary return spring acting between the control ring and the casing to bias the control ring towards a position of maximum volumetric capacity of the pumping chambers, the primary return spring acting against the force of the control chamber to establish a first equilibrium pressure; and a secondary return spring mounted in said casing and configured to engage said control ring after said control ring has moved a predetermined amount towards a position of minimum volumetric capacity, said secondary return spring biasing the control ring towards a position of maximum volumetric capacity, the secondary return spring acting against the force of the control chamber to establish a second equilibrium pressure.