Volumetric hydraulic pumps rely on the volume of the sealed working oil chamber to work continuously. It must have one or more sealed working oil chamber, when the hydraulic pump is running, the volume of the oil chamber must continue to gradually increase from small to small, the formation of a vacuum, the tank of oil can be inhaled, when the oil chamber volume from large As the oil decreases, the oil is squeezed in the sealed working oil chamber to increase the pressure. The pressure depends on the resistance (such as the spring force of the check valve) when the oil is output from the pump. The pump capacity of the oil (or output flow) depends on the size of the sealed working oil chamber volume and the size and frequency changes, it is called positive displacement pump.