Piston pumps, in which the flow rate is pressure-controlled by suction and pressure valves. Here the suction valve opens when facing a weak spring force as soon as the returning piston creates negative pressure. At the reversal point, the spring closes the suction valve and as the compression stroke starts, the pressure valve is opened when facing the system pressure acting on it (Figure V 7).
Valve-controlled pumps are used in only small oil hydraulic systems but are standard in water hydraulic systems.
As the effective direction of these valves cannot be reversed, they cannot be used as hydraulic motors.
Figure V 7: Principle of valve control of a pump