First, the introduction Pressure relief valve is a control element. It has two main functions: ①by overflowing excess oil flow to the tank to maintain a constant pressure of the hydraulic system, the pressure regulator. For example in a variable displacement pump system. ② only when the pressure exceeds a certain pre-set pressure value, open the overflow, the system pressure will not increase, to prevent system pressure overload, play a safety role. For example in a variable displacement pump hydraulic system. Play the role of safety valve. Commonly used relief valve has two kinds of direct acting and pilot, direct acting for low pressure, pilot for medium and high pressure. Direct-acting spring force is directly balanced with the hydraulic force of the oil inlet to control the pressure. Therefore, the spring is hard and the adjusting torque is large, which can not be used for medium and high pressure. Pilot relief valve structure can be divided into two parts, the lower part is the main part of the spool, the upper part of the pilot pressure regulator. The characteristics of this valve is the use of the main spool valve pressure difference between the upper and lower ends to move the main valve spool, which pressure control. With this structure, the use of high pressure, pressure overshoot small, under the same pressure, the handle of the adjustment torque much. Second, the failure principle (A) the pressure rise is very slow, even a little attrition This failure phenomenon refers to: when tightening the pressure screw or handle, from the relief to the pressure relief state, the pressure should be followed Rise, but in the event of malfunction, the pressure rises slowly, or even go up. Even if the rise also lags for a long time. Analysis of the state of pressure regulation shows that the moment from the pressure relief state into the pressure regulator state, the main valve against the valve cover, and the main valve is fully open overflow. When the boost pressure is adjusted, the pressure in the main valve cavity increases. When the pressure rises to open the pilot pressure regulator, the pressure relief valve enters into the pressure increasing and boosting state. The main valve core and the valve seat (or valve body) maintain a small opening , Relief valve main spool from the unloading position drops to the pressure required to open the required time, that is relief valve back to the lag time. This period of time is long, the pressure rises slowly. Many factors affect the lag time, mainly with the main spool relief valve spool travel distance and the closure speed. The closing speed depends on the main spool orifice flow through the pilot flow and the size of the main spool diameter. Pilot flow again with the orifice aperture and hole length related. (B) although the pressure may rise but rose less than the maximum regulatory pressure This failure phenomenon, although the total tension regulator hand wheel, the pressure rose to a certain value will not be able to continue to rise, especially when the oil temperature is high , Especially notable. (C) do not come under pressure This failure phenomenon, even if the whole loose pressure regulating hand wheel, but the system does not come under pressure. (D) large pressure fluctuations This failure phenomenon manifested as relief valve pressure fluctuations in the range of 0.2Mpa and 0.3Mpa fluctuations, more than this indicator is called the pressure fluctuations. (E) Vibration and noise, vibration and noise accompanied by impact relief valve Valve components in the first place and second only to the pump, vibration and noise is a pair of twin brothers, often simultaneously disappear at the same time. Both the main and pilot spools have a "liquid-mass-spring" oscillating system. The causes of vibration and noise are similar to those of "large pressure fluctuations," but only to a greater extent. This phenomenon should be considered from the entire hydraulic system. The noise of the piping system includes the low-frequency sound of the pump source and the high-frequency noise of the relief valve. High-frequency noise relief valve is due to the instability of the pilot valve generated, that is, leading pilot pressure before the cavity oscillation caused by high-frequency mechanical vibration noise, the general harsh whistle. (F) out of pressure, the pressure offset large This symptom is manifested as: a pre-adjusted a certain pressure during use, but slowly decline (and sometimes rise) to another pressure value, and then slowly boost , Restore the regulator value, this phenomenon cycle or repeated. This phenomenon can be observed through the pressure gauge. (G) opening and closing characteristics Poor opening and closing characteristics refers to the opening and closing relief valve characteristics. The opening characteristic is expressed as a percentage of the opening pressure to the rated pressure; the closing characteristic is expressed as a percentage of the closing pressure to the rated pressure. The higher the percentage, the more the relief valve can maintain the pressure of the hydraulic system under different overflow. Both the opening pressure and the closing pressure during the bench test are measured as the flow rate of the relief valve is reduced or increased to 1% of the nominal flow rate, which is an indication of static characteristics. Is a seized item. For the relief valve, the opening and closing characteristics of the differences, both determined by the design parameters, but also depends on the quality of processing, especially the first deputy needle valve processing quality. Operating conditions (such as back pressure, oil cleanliness, etc.) also have a certain impact on the opening and closing characteristics. Specific instructions are as follows: ①needle valves and valve seat processing quality good opening and closing characteristics. On the Y-valve, when the set pressure is 6.3Mpa, the closing pressure> 5.5Mpa, otherwise the closing pressure is very low. ② Relief valve outlet back pressure effects: high back pressure, opening and closing characteristics of the poor, the general export back pressure should not exceed 0.1Mpa. High pressure relief valve can be used alone to return oil, can reduce the back pressure, but also improve stability. The main valve hydraulic power: the main hydraulic power, opening and closing characteristics of poor, in order to reduce the main valve hydraulic power, the main valve may be appropriate to increase the diameter, change the direction of oil into and out of measures. Needle valve hydraulic power effects: not very obvious. Cone valve cone angle α small, hydraulic power is small; but 2α ⑤ needle valve regulator spring stiffness reduction, opening and closing better.
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