Patent ID: 8545200

Claim:
A variable displacement oil pump comprising: a rotor configured to be rotationally driven by an internal combustion engine; a plurality of vanes movable out from and into an outer circumferential portion of the rotor; a cam ring configured to: receive the rotor and the plurality of vanes in an inner circumferential space of the cam ring, cooperate with side walls, the rotor and the plurality of vanes to separately form a plurality of working-oil chambers, wherein the side walls are provided on both axial side surfaces of the cam ring, and swing about a swing fulcrum to vary an eccentricity between a rotation center of the rotor and a center of an inner circumferential surface of the cam ring; a housing configured to receive the cam ring therein, and including: a discharge portion open to at least one of the plurality of working-oil chambers from at least one of the side walls, wherein the at least one of the plurality of working-oil chambers reduces its volume when the center of the inner circumferential surface of the cam ring becomes eccentric relative to the rotation center of the rotor, and a suction portion open to at least one of the plurality of working-oil chambers from at least one of the side walls, wherein the at least one of the plurality of working-oil chambers to which the suction portion is open, increases its volume when the center of the inner circumferential surface of the cam ring becomes eccentric relative to the rotation center of the rotor; a biasing member configured to bias the cam ring in a direction that enlarges the eccentricity between the rotation center of the rotor and the center of the inner circumferential surface of the cam ring; a contact surface configured to become in contact with an outer circumferential surface of the cam ring by means of a biasing force applied to the cam ring by the biasing member, wherein a distance between the swing fulcrum and the contact surface is shorter than a distance between the swing fulcrum and a portion at which the cam ring is biased by the biasing member; a control chamber separately formed by the contact surface and the swing fulcrum of the cam ring on an outer circumference of the cam ring when the contact surface is in contact with the outer circumferential surface of the cam ring, and configured to cause the cam ring to swing against the biasing force of the biasing member by a pressure introduced from the discharge portion to the control chamber; and a choking portion formed on the outer circumferential surface of the cam ring and configured to maintain a pressure of the control chamber even when the cam ring swings.