Cause analysis and troubleshooting of common fault

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Analysis and troubleshooting of common faults of overflow valve

the components that are easy to produce noise in hydraulic devices are generally considered to be pumps and valves, and the valves are mainly overflow valves and electromagnetic directional valves. There are many factors that produce noise. The noise of overflow valve includes flow velocity sound and mechanical sound. The flow velocity noise is mainly caused by oil vibration, cavitation and hydraulic shock. Mechanical noise is mainly caused by the impact and friction of parts in the valve

(1) noise caused by uneven pressure

the pilot valve part of the pilot relief valve is a vibration prone part, as shown in Figure 3. When overflowing under high pressure, the axial opening of the pilot valve is very small, only 0.003 ~ 0.006 cm. The flow area is very small, and the flow rate is very high, up to 200m/s, which is easy to cause uneven pressure distribution and vibration caused by unbalanced radial force of the cone valve. In addition, the ovality produced during the processing of cone valve and cone valve seat, the sticking of dirt at the pilot valve port and the deformation of pressure regulating spring will also cause the vibration of cone valve. Therefore, it is generally believed that the pilot valve is the vibration source of noise

due to the existence of elastic elements (Springs) and moving masses (cone valves), a condition for oscillation is formed, and the front cavity of the pilot valve acts as a resonant cavity, so the vibration of the cone valve is easy to cause the resonance of the whole valve and produce noise, "The noise in the new Utilization Center built by us with millions of dollars is usually accompanied by violent pressure beating.

(2) Noise generated by cavitation

when air is sucked into the oil for various reasons, or when the oil pressure is lower than atmospheric pressure, part of the air dissolved in the oil will separate out and form bubbles. These bubbles are large in the low-pressure area, and when they flow with the oil to the high-pressure area, they are compressed, and the volume suddenly becomes smaller or the bubbles disappear; On the contrary, if the volume was originally small in the high-pressure area, but when it flowed to the low-pressure area, the volume suddenly increased, and the bubble volume in the oil changed rapidly on May 9, 2012. The sudden change of bubble volume will produce noise, and because this process takes place in an instant, it will cause local hydraulic shock and vibration. The pilot valve port and main valve port of the pilot relief valve have great changes in oil flow rate and pressure, which is prone to cavitation, resulting in noise and vibration

(3) noise caused by hydraulic shock

when the pilot relief valve is unloaded, pressure shock noise will occur due to the sudden drop of pressure in the hydraulic circuit. The higher the working conditions of high pressure and large capacity, the greater the impact noise is. This is due to the hydraulic shock caused by the short unloading time of the overflow valve. During unloading, due to the sharp change of oil flow rate, the sudden change of pressure is caused, resulting in the impact of pressure wave. The pressure wave is a small shock wave, and the noise generated by itself is very small, but it is transmitted to the system with the oil. If it resonates with any mechanical part, it may increase the vibration and enhance the noise. Therefore, when hydraulic shock noise occurs, it is usually accompanied by system vibration. (4) Mechanical noise

the mechanical noise emitted by the pilot relief valve generally comes from the impact of parts and the friction of parts due to machining errors

among the noise emitted by the pilot type overflow valve, sometimes there is a mechanical high-frequency vibration sound, which is generally called self-excited vibration sound. This is the sound of the main valve and pilot valve due to high-frequency vibration. Its occurrence rate is related to the configuration, flow, pressure, oil temperature (viscosity) and other factors of the oil return pipeline. In general, the occurrence rate of self-excited vibration is high due to small pipe diameter, small flow, high pressure and low oil viscosity

the measures to reduce or eliminate the noise and vibration of the pilot relief valve are generally to add vibration elimination elements to the pilot valve

the vibration damping sleeve is generally fixed in the front cavity of the pilot valve, that is, the resonant cavity, and cannot move freely

various damping holes are set on the vibration damping sleeve to increase damping to eliminate vibration. In addition, due to the addition of parts in the resonant cavity, the volume of the resonant cavity is reduced, and the stiffness of the oil increases under negative pressure. According to the principle that the components with high stiffness are not easy to resonate, the possibility of resonance can be reduced

the vibration damping pad is generally matched with the resonant cavity and can move freely. There is a throttling groove on the front and back of the vibration damping pad, and the oil can produce damping effect when flowing, so as to change the original flow situation. Due to the addition of vibration damping pad, a vibration element is added, disturbing the original resonance frequency. The vibration damping pad is added to the resonant cavity, which also reduces the volume and increases the stiffness when the oil is pressurized, so as to reduce the possibility of resonance

an air storage hole and a throttling edge are set on the vibration elimination screw plug. Because there is air in the air storage hole, the air is compressed when pressed, and the compressed air has a vibration absorption effect, which is equivalent to a miniature vibration absorber. When the air in the small hole is compressed, the oil is filled in, and when it expands, the oil is pressed out, which adds an additional flow to change the original flow. Therefore, noise and vibration can also be reduced or eliminated

in addition, if the overflow valve itself is improperly assembled or used, it will also cause vibration and noise. For example, the three section concentric overflow valve is improperly matched during assembly, the flow is too large or too small during use, and the cone valve is abnormally worn. In this case, the adjustment should be carefully checked or the parts should be replaced

(II) radial clamping of valve core

due to the influence of machining accuracy, the main valve core is radially clamped, so that the main valve cannot be opened or closed without pressure relief, and the radial clamping is caused by pollution

(III) pressure regulation failure

pressure regulation failure sometimes occurs in the use of overflow valve. There are two cases of pressure regulation failure of pilot relief valve: one is that the pressure cannot be established by adjusting the pressure regulating handwheel, or the pressure cannot reach the rated value; The other way is to adjust the pressure of the handwheel without decreasing or even increasing. In addition to the radial clamping of the valve core due to various reasons, there are also the following reasons for the failure of pressure regulation:

the first is that the damper of the main valve body (2) is blocked, and the oil pressure cannot be transmitted to the upper chamber of the main valve and the front chamber of the pilot valve, so the pilot valve loses the function of regulating the pressure of the main valve. Because there is no oil pressure in the upper chamber of the main valve and the spring force is very small, the main valve becomes a direct acting overflow valve with very small spring force. When the pressure in the oil inlet chamber is very low, the main valve opens the overflow and the system cannot establish pressure

the reason why the pressure cannot reach the rated value is that the pressure regulating spring is deformed or selected incorrectly, the compression stroke of the pressure regulating spring is not enough, the internal leakage of the valve is too large, or the conical valve of the pilot valve is not specially picked up, and so on

the second is that the damper (3) is blocked, and the oil pressure cannot be transmitted to the cone valve, so the pilot valve loses the regulating effect of the main valve pressure. After the damper (small hole) is blocked, the cone valve will not open the overflow oil under any pressure. There is no oil in the valve all the time, which completely solves the influence of fluid flow caused by vibration or direction change at low speed. The pressure in the upper and lower chambers of the main valve is always equal. Because the annular pressure bearing area at the upper end of the main valve core is larger than the annular pressure bearing area at the lower end, the main valve is always closed and will not overflow. The pressure of the main valve rises with the increase of load. When the actuator stops working, the system pressure will increase infinitely. In addition to these reasons, it is necessary to check whether the external control port is blocked and whether the cone valve is installed well

(IV) other faults

during the assembly or use of the overflow valve, due to the damage of the O-ring and combined seal ring, or the loosening of the mounting screws and pipe joints, undue leakage may be caused

if the cone valve or main valve core is worn too much, or the sealing surface is in poor contact, it will also cause excessive internal leakage, and even affect the normal work

common faults of electromagnetic relief valve include pilot solenoid valve failure, main valve pressure regulation failure and impact noise during unloading. The latter can be reduced or eliminated by adjusting the added buffer. If there is no buffer, a back pressure valve can be added at the overflow port of the main valve. (the pressure is generally adjusted to about 5kgf/cm2, that is, 0.5MPa)

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