The function and characteristics of the relief valve

- Oct 14, 2019-

The role of the relief valve

Constant pressure overflow: In the metering pump throttle regulation system, the metering pump provides a constant flow. When the system pressure increases, the flow demand is reduced. At this point, the relief valve opens, allowing excess flow to overflow back to the tank, ensuring the inlet pressure of the relief valve, ie the pump outlet pressure is constant (the valve port often opens with pressure fluctuations).


Regulating effect: The relief valve is connected in series on the return line, the relief valve generates back pressure, and the smoothness of the moving parts increases.


System unloading function: a small flow solenoid valve is connected in the remote control port of the overflow valve. When the electromagnet is energized, the remote control port of the overflow valve is connected to the fuel tank, and the hydraulic pump is unloaded. The relief valve is now used as an unloading valve.


Safety protection: When the system is working normally, the valve is closed. Only when the load exceeds the specified limit (system pressure exceeds the set pressure), the overflow is turned on, and the overload protection is performed, so that the system pressure is no longer increased (usually the set pressure of the relief valve is 10% to 20% higher than the maximum working pressure of the system). ).


In practical applications, it is generally used as an unloading valve, as a remote pressure regulating valve, as a high and low pressure multi-stage control valve, as a sequence valve for generating back pressure (string on the return road).


The relief valve generally has two structures: 1. A direct-acting relief valve. 2. Pilot operated relief valve.


The main requirements for the relief valve are: large pressure regulation range, small pressure regulation deviation, small pressure oscillation, sensitive action, large overload capacity and low noise.


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Performance characteristics of direct-acting relief valve

1. High sensitivity Since the valve core surface of the control opening is a tapered surface, when the axial direction of the valve core moves slightly, it can have a larger opening; the spool has a small volume, small inertia and flexible movement.


2. Flow capacity The large-end deflector on the left end of the valve core has an annular groove. When the oil flows through the groove, the flow direction changes to form a hydraulic power in the opposite direction to the spring force. When the spool is opened, the amount of spring compression increases, and the flow rate through is also increased, thereby increasing the hydraulic power generated, thereby offsetting the increase of the spring force, so that the opening stability of the spool is increased, and the flow capacity is increased. .


3. Wide range of pressure regulation Due to the above two reasons, the spring stiffness required for the valve core is greatly reduced, thereby increasing the pressure regulation range of the valve.


4. Stability increases The appropriate clearance is set between the damping piston at the lower end of the valve core and the valve body, so that the valve core is damped by the hydraulic oil when moving, the damping piston does not directly contact the valve body, and the friction force when the valve core moves is reduced. At the same time, the fluctuation of pressure can be timely fed back to the valve core to make it move flexibly and smoothly, so the stability of the pressure is greatly increased.


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Relief valve use precautions

1. The type of relief valve should be selected according to the working conditions and specific requirements of the hydraulic system. Generally, the direct-acting relief valve has a fast response and should be used as a safety pressure-keeping valve. The pilot relief valve has good opening and closing characteristics and should be used as a pressure-regulating constant pressure valve.


2. The relief valve with good opening and closing characteristics should be selected as much as possible to improve the speed load characteristics and loop efficiency of the actuator. In terms of dynamic characteristics, the selected relief valve should have good stability at the same time as the response speed is fast.


3. Correctly use the connection mode of the relief valve, correctly select the connecting piece (installation bottom plate or pipe joint), and pay attention to the seal at the joint; the oil ports of the valve should be correctly connected to the system, and the external drain port must be directly connected to the fuel tank. .


4. According to the working pressure and flow of the system, the rated pressure and flow rate (diameter) specifications of the relief valve are selected reasonably.


For the relief valve used as the remote pressure regulating valve, the flow rate is generally 0.5% to 1% of the flow rate of the overflow valve where the remote control port is located.


5. The set pressure should be determined and adjusted according to the purpose and function of the system in which the relief valve is located, especially as the relief valve used for the safety valve. The initial set pressure must not exceed the maximum pressure of the hydraulic system.


6. When adjusting the pressure, pay attention to the correct rotation direction to adjust the pressure regulating mechanism. When the pressure is adjusted, the lock nut should be fixed.


7. If the system needs to be remotely regulated, unloaded or multi-stage pressure controlled by the remote control port of the pilot type relief valve, the screw plug of the remote control port should be unscrewed and connected to the control oil circuit; otherwise, the remote control port should be Sealed tightly.


8. If the pressure regulation range of the relief valve needs to be changed, it can be realized by replacing the pressure regulating spring of the relief valve, but at the same time, it should be noted that the set pressure of the spring may change the opening and closing characteristics of the valve.


9. For electromagnetic spill valves, the voltage, current and wiring form must be correct.


10. The oil return chamber of the unloading relief valve should be directly connected to the fuel tank to reduce back pressure.


11. When the relief valve has a fault such as pressure regulation failure or excessive noise, it is necessary to find out the cause and repair it in time. The relief valve assembly that has been removed and repaired during repair should be properly installed and taken care of to prevent secondary pollution.