The difference between relief valve and pressure reducing valve is as follows:

The principle is different:  the relief valve is a pressure control valve, and the pressure reducing valve is a pressure regulating valve.

The structure is different:  the relief valve is generally closed, and the pressure reducing valve is often open.

The role is different:   the relief valve mainly stabilizes the pressure, and the pressure reducing valve reduces the pressure.

Direct action relief valve

When the system pressure reaches the pressure set by the relief valve spring, the spool moves to open the oil discharge port, and the relief valve will unload excess hydraulic oil.

As shown in the figure below, when the pressure reaches 800pis, the hydraulic cylinder is moving, and when the hydraulic cylinder stroke is completed, the pressure rises to 1000pis. At this time, the unloading oil port of the relief valve is opened to return the excess hydraulic oil to the tank.

Pilot operated relief valve

The pilot relief valve is equivalent to a 2-stage control system. When the pilot pressure reaches the pilot spool, the pilot oil enters the pilot spool and then enters the main spool chamber. When the pressure rises to push the main spool, the main oil circuit of the relief valve begins to unload back to the tank.

As shown below

Direct acting pressure reducing valve

The principle of pressure reducing valve is to use the relative movement between the spool and the seat to control the fluid on and off to achieve automatic pressure regulation

Hydraulic pressure reducing valve is mainly composed of valve body, spring, spool and pressure regulating rod. When the system pressure exceeds the set pressure value, the hydraulic pressure reducing valve will automatically open, and the excess pressure will be discharged through the pressure relief port to keep the system pressure within the safe range.

Pilot hydraulic pressure reducing valve

The difference between the pilot pressure reducing valve and the direct acting pressure reducing valve lies in the control mode of outlet pressure, the size of the adjusting spring, the precision of outlet pressure control and the complexity of structure. The pilot type controls the main valve through the pilot valve, and the outlet pressure control precision is high, but the structure is complex and the cost is high. The direct-acting type directly uses the outlet pressure feedback, the adjusting spring is large, and the reliability is high.