Hydraulic Applications – Pressure Reducing Valve

In this article, as an example of hydraulic system applications, we’ll cover an application using a pressure reducing valve for clamping a workpiece on a drill press. We’ll identify the hydraulic circuit components for the sample system and build the hydraulic circuit diagram.

Hydraulic Application of a Pressure Reducing Valve

On a drill press, the motion and power transmission for clamping the workpiece and drilling are done hydraulically.

In the hydraulic system, there are two cylinders that provide the motion and power transmission for clamping (A) and drilling (B).

Since different workpieces require different clamping forces, the clamping pressure in cylinder A needs to be adjustable to different pressure values. A pressure reducing valve is used for this.

Pressure valves allow the pressure in a hydraulic system, or in a section of a hydraulic system, to be set and controlled.

Pressure Reducing Valves

These valves reduce the inlet pressure to a lower outlet pressure. The pressure value at the valve’s outlet is used as the control pressure. These valves are open in their rest position. In this application, a 2-way pressure reducing valve is used.

Pressure reducing valve circuit diagram (above)

Cross-section of a pressure reducing valve (above).

Preliminary Circuit Diagram Concept

In this circuit diagram, the 4/2 directional control valve (1.1) is actuated first. The clamping cylinder’s piston rod A moves forward under the motion pressure. Once the piston rod reaches the workpiece, the system pressure, set to a maximum of 50 bar, takes over the system.

Then, for the drilling operation, cylinder B’s piston rod begins its forward motion by actuating the 4/3 directional control valve (2.1). While the piston rod moves forward, the motion pressure in that cylinder takes over the entire system. This means the pressure in the clamping cylinder also drops.

As noted, the maximum pressure set by the pressure relief valve is needed for the drilling operation, while, as stated in the application, different pressures are wanted for the clamping operation. To achieve this, a pressure reducing valve needs to be placed ahead of the cylinder used for clamping.

Circuit Diagram with a Pressure Reducing Valve

Likewise, in the circuit diagram extended with a pressure reducing valve, the clamping cylinder’s piston rod moves forward under the motion pressure until it reaches the workpiece. Once the piston rod reaches the workpiece, the pressure rises and acts on the regulating piston through the control line at the outlet of the pressure reducing valve. The regulating piston begins to close off the flow. This is how the pressure reducing valve (PRV) has a throttling effect. Beyond the PRV, pressure keeps rising until it reaches the set value. Once it reaches the set value, the valve closes completely. Upstream of the valve, pressure keeps rising until the set system pressure is reached

When the 4/3 directional control valve (2.1) is actuated for the piston rod’s forward motion, the system pressure drops to the motion pressure of the drilling cylinder’s piston. This also causes the pressure ahead of the PRV to drop. Pressure can also drop ahead of the PRV due to leakage in the valves. A drop in clamping pressure acts on the regulating piston, causing the valve to open. To prevent this, a pressure higher than the desired clamping pressure needs to be guaranteed ahead of the PRV.

Since the drilling cylinder’s piston moves at low pressure, a pressure drop occurs ahead of the pressure reducing valve. Another option to consider, for keeping the system pressure high during the forward motion of the drilling operation, is placing a restrictor ahead of directional control valve 2.1.