The working principle of direct acting relief valve is simply designed for controlling hydraulic pressure. The hydraulic direct acting relief valve is often equipped in various hydraulic systems as pressure regulation or keeping system pressure in normal operation stage. –


– Closed-Loop Automatic Control Theory Of PressDirect Acting Relief Valve Working Principle:

From the control theory point of view, the direct acting relief valve is a hydraulic component with closed-loop automatic control, the picture A is simply description of closed-loop automatic control for pressure relief valves.

The input pressure is the spring presetting pressure, the output pressure is controlled pressure (Inlet pressure), the hydraulic force is generated by the pressure being controlled is back on the valve spool of effective area, the force is compared with the spring force, which is automatically adjust throttle opening area of ​​the pressure relief valve, so that the controlled pressure is substantially in a constant status.


– Construction Schematics Of Direct Acting Relief Valve –
1. Orifice | 2. Valve Housing  |  3. Poppet  |  4. Valve cover  |  5. Adjusting screw
6. Spring seat  |  7. Adjustment Spring  |  8. Drain port

The picture B is the generally structure of direct acting relief valve, for example, direct-acting relief valve is composed of valve housing, valve spool and regulating mechanism (Including: pressure adjusting screw, pressure regulating spring) and other main components. There are inlet port P (Connection to hydraulic pump and controlled hydraulic oil circuit) and outlet port T (The port connects to oil reservoir) are designed on left and right side of valve housing, in addition to above parts, the damping hole and inner drain port is required sometimes.

 The fluid force on the valve spool is direct equilibrium with the spring force in direct-acting relief valve. Please refers to picture B, the valve spool is in the close position against the spring force, the oil port P and T is each isolated. When the incoming hydraulic pressure rises to higher than the presetting pressure of spring force, the valve spool lifting and open the P port and the hydraulic oil is pressurized out from the outlet port T. The valve spool is in different position as per varies with fluid flow rate, but there is a very small distance of movement for poppet, therefore, the hydraulic oil flows into the direct acting relief valve if only open a little gap on inlet port. The pressure on inlet port P is essentially constant of direct acting relief valve.

When the pressure on inlet port drops, the spring force is larger enough to set the valve spool in the close position, but the spring force is available to be altered by adjustment of presetting pressure and changes different stiffness spring in order to alter the pressure adjustment range. The orifice is as a dynamic hydraulic damping, used for minimizing the spool vibration caused of various pressure in order to improve valve stability. There is a little leakage comes from the small gap between the surface of spool and the hole of valve housing, the leaking oil in spring chamber flows directly to outlet T port through inner channel 8 , this type of drain is named inner drain.

Below Picture C is the picture of direct acting relief valve with different inner structures. Picture C-1, C-2 are the lifting type and poppet type of direct acting relief valve; Picture C-3, C-4 and C-5 are the spool type, spool with orifice type and differential spool type of direct acting relief valve.


– Direct Acting Relief Valve with different spool –
1. Valve spool with different type | 2. Valve Body  |  3. Spring | 4. Adjusting screw


The characteristics of direct-acting relief valve is simple structure, high sensitivity, but the pressure is greatly influenced by the overflow rate volume, hence static adjustment deviation(The difference between the set pressure and open pressure ) is large, dynamic characteristics due to different structural configuration, such as the poppet type, ball type of direct acting relief valve which is response faster, sensitive action, but poor stability, loud noise, often used as pilot valve of safety valve and pressure relief valve. The spool type direct acting relief valve is slow to pressure response, large pressure regulating range and good operation stability.

– Direct Acting Relief Valve Animation –