To supply a flow of fluid to the actuator, a pump must be provided. It is important to realize that the pump does not deliver pressure. Hydraulic pressure is created only when an attempt is made to compress a fluid. Fluid pumped through an open-ended pipe will have no pressure, but, if the pipe is connected to an actuator, the resistance to the flow will create pressure. The rate at which a single hand pump can deliver fluid is very slow. In the aircraft hydraulic system an engine driven pump is therefore installed in addition to the hand pump. The purpose of the hand pump would then be for emergency use and for ground testing of the system.
These are usually of double acting type, delivering fluid on each stroke. As the piston moves upward in the cylinder, fluid is drawn in through the inlet non return valve (NRV) into the cylinder. At the same time fluid above the piston is discharged through the outlet NRV. As the piston moves downward, the inlet NRV closes and the transfer NRV opens, allowing fluid to flow through the piston. Since the volume above the piston is smaller than below the piston, part of this fluid is discharged through the outlet NRV.