Similarities and Differences of Three Pressure Control Valves

Hydraulic transmission is the transmission of energy conversion, transmission and control using pressure oil or compressed air as working medium. Hydraulic transmission system generally consists of four parts: power part - hydraulic pump; executive part - hydraulic cylinder; control part - various hydraulic valves; auxiliary part - oil tank, pressure gauge, pipeline and so on.

In the hydraulic transmission system, the control valve used to control the oil hydraulic pressure or to realize some action by the change of pressure is called pressure control valve, which is called pressure valve for short. There are many kinds of pressure control valves, which can be divided into relief valves, relief valves, sequential valves and pressure relays according to their different uses. Generally, they work by using the basic principle of balance between hydraulic pressure and spring force.

Because the working principle, internal structure and function symbols of relief valve, relief valve and sequential valve have great similarities, it is easy to confuse students when they study and analyze the hydraulic system diagram and detect the system failure. Therefore, a correct understanding of the working principle and function symbols of the three valves and an effective fault analysis method are of great significance to the overall study of the hydraulic system.

1) All three kinds of valves are composed of main valve and pilot valve. 2) Damping holes are found in the main spool, which is the direct cause of the difference of opening pressure P between the top and bottom of the main spool. 3) When Δp exceeds the spring elasticity of the main valve (the self-weight and resistance of the valve are neglected), the main valve core is opened. 4) When the pressure condition changes, the displacement of the three spools is the same, but the valve opening changes differently. 

(1) First, identify which valve is the relief valve (the outlet is connected to the tank) from the way the valve discharges oil; then, in the remaining valves, the valve spool is usually open, and the valve spool is usually closed by the sequential valve. (2) First of all, it can identify which valve is the relief valve (the outlet is connected to the tank); then, in the remaining valves, the sequence valve is connected to the oil inlet by dotted line on one side of the main valve core, and the pressure relief valve is connected to the oil outlet by dotted line on the other side of the main valve core. (3) Firstly, the initial position of the main spool is used to identify which valve is a pressure relief valve (normally open); then, compared with the discharge mode of the valve, the relief valve is directly connected to the tank at the outlet, and the sequential valve is the feeding branch behind the outlet.

The failure of three kinds of pressure control valves, i.e. pilot relief valve, pilot relief valve and pilot sequential valve, is manifested by inaccurate action of the spool. Therefore, in the process of analyzing the failure, the first step is to determine which kind of pressure control valve is, considering the function, working principle and structure of the valve. Secondly, according to the characteristics of the fault, the position change of the valve core in the valve body (the opening of the valve becomes larger or smaller) is analyzed. Finally, the reason that causes the valve core to be in the position is analyzed. Then, the possible conditions leading to the valve failure can be inferred, and the corresponding troubleshooting can be carried out one by one according to various possible conditions.