The cam ring inside a fixed displacement vane pump is stationary and the rotor is offset inside the cam ring. The slotted rotor is turned by the input shaft and the vane tips move along the inside of the cam ring. The vane tips seal against the cam ring as they move in and out of the rotor slots. Volume between the rotor vanes changes because the rotor is offset in the cam ring. As the vanes pass the inlet passage in the pump, the volume between the vanes increases, therefore, providing space for oil to enter the pump