VARIABLE CAPACITY VANE PUMP WITH DUAL CONTROL CHAMBERS

A variable capacity vane includes a pump control ring which is moveable to alter the capacity of the pump and the pump can be operated at either of at least two selected equilibrium pressures. The pump ring is moved by at least first and second control chambers, the control chambers abutting the control ring such that pressurized fluid supplied to them acts on the pump control ring to move the pump control ring to reduce the volumetric capacity of the pump. When pressurized fluid is supplied to only one control chamber, the pump operates at a first equilibrium pressure and when pressurized fluid is also supplied to the second chamber, the pump operates at a second equilibrium pressure. Pressurized fluid can also be supplied only to the second control chamber to operate the pump at a third equilibrium pressure and/or additional control chambers can be provided if required.