Pressure control valve classification and structure

- Oct 15, 2019-

A pressure control valve is a valve used to control and regulate the pressure of a liquid flow in a hydraulic system. Such valves operate on the principle that the pressure of the liquid acting on the spool is balanced with the spring force.


The pressure control valve is a general term for a valve that controls and regulates the flow pressure, referred to as a pressure valve. It is a method of balancing the hydraulic pressure acting on the spool with the force of the wide core spring to establish and maintain the working pressure of the controlled liquid. If the spring force is adjustable, the pressure of the controlled liquid can also be changed to achieve the purpose of controlling and regulating the flow pressure. The pressure valves are all used in parallel in the oil circuit system. When the pressure of the controlled liquid rises above the spring pre-adjustment pressure due to external reasons, the balance relationship between the valve core and the spring is broken, and the elbow, the spool will be forced to move, and the opening passage drains (overflow) to the return line. The pressure of the controlled oil is maintained at the level of the spring pre-regulation pressure; sometimes the movement of the spool is not to open the oil return passage, but to change the flow cross section of the dedicated throttle decompression port, that is, to change its pressure drop, so that The working pressure of the pre-regulated decompression oil line remains unchanged; sometimes it is intentionally to increase the oil pressure to enter another working oil path for the purpose of sequential action.


The pressure control valve is a general term for a valve that makes pressure. Divided into overflow valve, pressure reducing valve and sequence valve according to the purpose.


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Overflow valve


The relief valve is a pressure control valve that stabilizes the pressure of the controlled system or circuit through the valve port overflow to achieve voltage regulation, pressure regulation or pressure limiting.


Features According to the structure, the relief valve has two types: direct acting and pilot operated.


For the direct-acting relief valve, when the pressure is high and the flow rate is large, the pressure-regulating spring is required to have a large rigidity and the adjustment performance is poor, and the leakage of the spool valve makes the high-pressure control difficult to realize, so it can only be used for the relief valve. Low pressure and small flow occasions.


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The direct-acting relief valve consists of a valve core, a valve body, a spring, an upper cover, an adjusting rod, an adjusting nut and the like. The oil inlet of the valve body is connected to the outlet of the pump, and the outlet is connected to the oil tank. In the original state, the spool is in the lowermost position under the action of the spring force, and the inlet and outlet ports are cut off. The imported oil acts on the bottom end of the valve core through the radial hole and the axial hole of the valve core. When the hydraulic pressure is equal to or greater than the spring force, the valve core moves up, the valve port opens, and the inlet pressure oil overflows to the oil tank through the valve port.


The pilot type relief valve is characterized by the use of a pilot valve to control the main valve. When the main valve of the relief valve overflows, the inlet pressure of the relief valve can be maintained at a constant value set by the pilot valve spring. Another feature of the pilot-operated relief valve is its remote control port for easy system unloading or remote pressure regulation.

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The pilot overflow is composed of a pilot valve and a main valve. The pilot valve is actually a small flow direct acting relief valve with a spool valve. The main valve core has a damping hole, and the upper cavity action area is slightly larger than the lower cavity action area, and the spring only acts as a reset when the valve port is closed.



The proportional relief valve is a closed-loop intelligently controlled relief valve whose actuator can adjust the valve opening according to the analog signal from the controller, so that the control parameters of the system are automatically kept within the set range.


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The ordinary relief valve, like the proportional relief valve, has a spool with one end of the spool being the pressure generated by the hydraulic oil and the other end being the mechanical force. The ordinary relief valve adjusts the hydraulic pressure by adjusting the spring force. The proportional relief valve is an electromagnet that directly generates thrust and acts on the spool. The input voltage on the electromagnet can be varied between 0 and 24 volts, and the generated thrust changes accordingly, resulting in continuously varying hydraulic pressure.


Because the thrust of the proportional electromagnet is not large, the flow rate of the direct-acting proportional relief valve is very small, and the flow rate is only about 1 liter/min at a pressure of 70 MPa. When a large flow proportional valve is required, the proportional valve should be used as a pilot valve, and a large-diameter relief valve should be provided below.


The function of the relief valve


The relief valve is connected to the outlet of the pump to ensure a constant system pressure, called a constant pressure valve.



The relief valve is connected to the outlet of the pump to limit the maximum system pressure and protect the system. It is called a safety relief valve (safety valve).


System unloading or remote voltage regulation can be achieved by using the remote control port of the pilot operated relief valve. The remote pressure regulating valve is actually a separate pressure pilot valve, which is connected to the remote control port of the pilot type relief valve for remote pressure regulation. The set pressure must be lower than the set pressure of the pilot valve.


Pressure reducing valve


The pressure reducing valve is a pressure control valve that uses a liquid flow to flow through a gap to generate a pressure loss such that its outlet pressure is lower than the inlet pressure.


According to different adjustment requirements, there are fixed value pressure reducing valve, differential pressure reducing valve and fixed ratio pressure reducing valve. Among them, the fixed value pressure reducing valve is the most widely used, and it is also referred to as a pressure reducing valve.


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The pressure reducing valve consists of a pressure pilot valve and a main valve. The outlet pressure oil is led to the upper chamber of the main spool and the front chamber of the pilot valve. When the outlet pressure is greater than the set pressure of the pressure reducing valve, the pilot valve is opened, the main spool is moved upward, the pressure reducing gap is closed, and the pressure reducing valve is started. Decompression and ensure that the outlet pressure is constant.


The main differences between the pressure reducing valve and the relief valve are:


(1) At rest, the valve port of the pressure reducing valve is normally open, and the valve port of the relief valve is normally closed;


(2) The pressure reducing valve controls the outlet pressure to be stable, and the overflow valve controls the inlet pressure to be stable;


(3) The valve port of the pressure reducing valve is closed with the increase of the outlet pressure, and the valve port of the relief valve is opened with the increase of the inlet pressure;


(4) The pressure inlet and outlet of the pressure reducing valve are all pressure oil circuits. The oil return through the pilot valve must be separately led back to the oil tank, and the overflow valve is merged with the outlet to return to the oil tank.


Sequence valve


The function of the function sequence valve is to control the sequential action of multiple actuators by controlling the pressure change in the hydraulic system and controlling the oil passage.



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Features According to the structure, the sequence valve also has two types: direct-acting and pilot-operated. The direct-acting sequence valve has a simple structure and a sensitive action, but the set pressure is low, and a pilot-type sequence valve should be used at a higher pressure. According to the different sources of control pressure, the sequence valve has internal control and external control. Please pay attention to the difference between the two graphic symbols. The internal control type sequence valve is closed before the inlet pressure reaches the set pressure. After the set pressure is reached, the valve port is opened to make the pressure oil enter the oil passage; the external control type valve valve opening and closing and oil The inlet pressure of the road is irrelevant and depends only on the pressure of the control oil passage.


The main difference between the sequence valve and the relief valve is:


(1) The sequence valve has only two states of opening and closing, and does not have the function of automatic voltage regulation as the relief valve;


(2) The inlet and outlet ports of the sequence valve are all pressure oil circuits, so its drain port must be led back to the tank separately, and the overflow valve is merged with the outlet to return to the tank.


(3) Once the inlet and outlet ports of the sequence valve are connected, the pressure loss of the oil through the valve is small, and when the overflow valve is in the overflow state, the pressure loss is generally large.