A novel variable displacement vane pump is provided wherein pressurized
working fluid is provided to a portion of the pump chamber to act on the
outside of the capacity varying ring to substantially balance the force
created by the high pressure working fluid inside the ring. As the
resultant high pressure force acting on the pivoting pin is reduced, the
movement of the displacement control ring is smoother, reducing
undesirable hysteresis, the wear on the pin is reduced and the additional
force required to move the ring to vary the volumetric displacement of
the pump is less than would otherwise be needed, allowing the related
control mechanisms to be smaller.