A pressure-reducing valve does not allow pressure downstream of the valve to exceed the set point. Suppose the workpiece must be clamped with two clamps. The second clamp is placed at a point where too much clamping force will damage the workpiece. A pressure-reducing valve is used to limit clamping pressure as shown in Fig. 3.23.
Suppose the valve in Fig. 3.23 is set on 500 psi. If pressure at the outlet of the valve increases above 500 psi, the pressure-reducing valve partially closes to create an orifice. Pressure drop across this orifice reduces the downstream pressure to 500 psi.
A functional diagram of the basic pressure-reducing valve was shown in Fig. 2.19. A second type of pressure-reducing valve, the reducing/relieving valve, is shown in Fig. 3.24. This valve operates like the pressure-reducing valve except that it bypasses fluid to the reservoir when the spool is shifted upward by the hydraulic force. The orifice between the inlet and bypass (to reservoir) opens as pressure increases, thus the valve functions like a relief valve. It combines the functions of the pressure-reducing valve and the relief valve, thus the name reducing/relieving valve. Note that the reducing/relieving valve is internally drained, but the reducing valve (Fig. 2.19) has to be externally drained.
As an additional example of pressure-reducing valve operation, consider Fig. 3.25. In this case, the rotary actuator is turning a screw to tighten a connection. If too much torque is applied, the threads will be damaged. Torque is given by