Created on:2022-03-30 13:17

Cavitation of hydraulic valve of construction machinery and Its Countermeasures

Cavitation of hydraulic valve of construction machinery and Its Countermeasures

1. Introduction

Hydraulic valve is the most used component in the hydraulic system of construction machinery. Its function is to control the pressure, flow and flow direction of hydraulic oil in the hydraulic system, so as to meet the requirements of force (or torque), speed and direction required by the actuator, so that the whole hydraulic system can work in coordination as required. In engineering practice, some faults often occur in the hydraulic system: for example, the one-way valve does not play a one-way control role, the pressure fluctuation of the hydraulic system is large, the system produces noise, etc. during maintenance, it is often found that there are small pits and honeycomb holes formed by material peeling off on the inner surface of the valve hole or the outer surface of the valve core of the hydraulic valve, that is, the so-called cavitation phenomenon of the hydraulic valve, This phenomenon will have a great impact on the stability, reliability and accuracy of the hydraulic system. The research on the causes of cavitation of hydraulic valve is of great significance to the detection and prevention of cavitation failure and the reliability and stability of hydraulic system.

2. Formation analysis of cavitation of hydraulic valve

In the hydraulic system, although the functions of various hydraulic valves are different, their essence is the same. They all control the liquid flow by changing the size of the valve hole and changing its overflow area and pressure difference. Due to the pressure difference between the inlet and outlet of the hydraulic valve, cavitation will occur as long as the pressure is lower than the critical pressure of air separation dissolved in oil at the working temperature.

During the use of construction machinery, in order to regulate the pressure, flow or flow direction of oil to meet the needs of the action of the actuator of the hydraulic system, the hydraulic valve should act frequently. When the hydraulic system changes the flow rate of the hydraulic oil due to the action of the hydraulic valve, resulting in the pressure drop, and the pressure of a local oil is lower than the critical pressure of air separation dissolved in the oil at the working temperature, a large number of dissolved air in the hydraulic oil will be separated to form air bubbles. When the pressure drops below the saturated vapor pressure of the oil, the oil will boil and produce a large number of bubbles. These bubbles are mixed in the working oil, making the oil originally filled with the hydraulic valve into a discontinuous state and forming cavitation. When the valve core impacts the hydraulic oil again, these bubbles move with the oil to the place where the oil pressure rises sharply, and the formed bubbles will burst suddenly due to impact and compression. The gas in the bubble state is re liquefied or dissolved in the hydraulic oil, and the volume suddenly decreases. The hydraulic oil moves towards the bubble center at a high speed to produce an impact phenomenon, produce great impact force and high temperature, and quickly spread around in the form of pressure transition, resulting in great impact and extrusion on the outer surface of the valve core and the inner surface of the valve hole. This process is repeated, which makes the materials on the outer surface of the valve core and the inner surface of the valve hole fatigue, the surface protective film is damaged, and the material particles gradually fall off from its surface, so that the outer surface of the valve core and the inner surface of the valve hole become rough, and then there are more and more small pits and honeycomb holes. With the continuous occurrence of cavitation, it eventually leads to the failure of the matching between the valve core and the valve hole of the hydraulic valve, which deteriorates the working performance and reliability of the hydraulic system.

The cavitation of hydraulic valve can be analyzed as follows from the perspective of Mechanics:

As shown in Figure 189, when the valve core moves to one side with acceleration a, a local oil with sectional area s in contact with the valve core also moves with the valve core with acceleration a under the action of oil pressure P. at this time, the oil pressure at L from the surface of the valve core will drop to P0 because

p0S=pS+ ρ LSa

Then p = P0- ρ La

Where ρ Is the density of the hydraulic oil.

When p is lower than the critical pressure of air separation dissolved in oil at working temperature, the air dissolved in hydraulic oil will be separated in the form of bubbles. When the spool moves to the other side with acceleration a, the oil pressure ratio P0 at l increases ρ La。 When the acceleration is large, so that there is a great impact on the outer surface of the valve core and the inner surface of the valve space, resulting in the destruction of the surface material and cavitation.

3. Cause analysis of cavitation of hydraulic valve

From the formation of hydraulic valve cavitation, regardless of the structural factors of hydraulic valve, the generation of cavitation is mainly related to the bubbles generated by hydraulic oil and the frequent action of hydraulic valve.

(1)       Frequent action of hydraulic valve. In the hydraulic system, all kinds of hydraulic valves control the hydraulic flow pressure, flow and flow direction by changing the size of the valve hole to change its overflow area and pressure difference, so as to meet the requirements of force (or torque), speed and direction required by the actuator. The frequent action of hydraulic valve makes the change frequency of oil pressure too fast and too high, which will directly cause the formation of bubbles and accelerate the rupture speed of bubbles. The test shows that the cavitation speed will be accelerated in the parts with high pressure change frequency.

(2)       The hydraulic oil produces bubbles. In hydraulic transmission, hydraulic oil always contains a certain amount of air. Air can be dissolved in hydraulic oil or mixed in hydraulic oil in the form of bubbles. For mineral hydraulic oil, there is 6% ~ 12% dissolved air at one atmospheric pressure at room temperature. The critical pressure of air separation into bubbles in oil is related to the type of oil, oil temperature and air dissolution. Prevent the hydraulic oil from generating bubbles, which can slow down the occurrence of cavitation.

4. Countermeasures to prevent cavitation of hydraulic valve

In addition to reasonable design, scientific material selection and improvement of processing technology, the rational use of hydraulic equipment is very important to reduce hydraulic valve cavitation. In the process of use, it is necessary to reduce the occurrence of cavitation of the hydraulic valve, mainly to minimize the generation of bubbles in the hydraulic oil and correctly operate the hydraulic valve.

(1)       Operate the hydraulic valve correctly. The frequency of oil pressure change directly affects the formation and rupture speed of bubbles. The cavitation speed is fast in the parts with high pressure change frequency. Therefore, the operation of each hydraulic valve should be stable, and the operation process should not be too fast or too fierce, so as to reduce the hydraulic impact and reduce the occurrence of cavitation.

(2)       Reduce bubbles in hydraulic oil. To reduce the generation of bubbles in hydraulic oil, important considerations in use are as follows:

1) Prevent excessive hydraulic oil temperature. The cooling system shall be maintained in time to keep the temperature of the cooling system within an appropriate range, so as to reduce the energy released in case of bubble rupture.

2) Reasonably select and use hydraulic oil. The hydraulic system has strict requirements for the varieties, grades and anti foaming properties of hydraulic oil, and should be added or replaced as required. If the oil resistance to foam is not good, easy to vaporize and form foam, it is easy to cause cavitation.

3) Pay attention to exhaust after maintenance and oil change. If the hydraulic system is not fully vented after maintenance and oil change, there is gas in the system, which is easy to produce cavitation under the action of high temperature and high pressure. Therefore, after the maintenance and oil change of the hydraulic system, the air in the system should be removed according to the provisions of the attached "operation manual".

4) Prevent the phenomenon of "empty suction" of hydraulic pump. "Empty suction" of hydraulic pump refers to that the oil sucked by the pump is mixed with air (not the air dissolved in the oil). The air mixed into the oil is usually suspended in the oil in the form of small bubbles. These bubbles will increase the generation of cavitation as the oil continues to flow. Therefore, the connection of oil suction pipe of hydraulic pump should be strictly sealed; The oil in the oil tank shall be filled up as required; The oil suction pipe must be immersed into the specified position of the oil tank, which can effectively prevent the "empty suction" phenomenon of the hydraulic pump.

In short, based on the analysis of the formation process of hydraulic valve cavitation, it is considered that without considering the structural factors of hydraulic valve, the generation of cavitation is mainly related to the generation of bubbles in hydraulic oil and the frequent action of hydraulic valve. Therefore, it is proposed to reduce hydraulic valve cavitation in addition to reasonable design, scientific material selection and improving processing technology, The rational use of hydraulic equipment is very important to reduce the cavitation of hydraulic valve. In the process of use, it is necessary to reduce the occurrence of cavitation of the hydraulic valve, mainly to minimize the generation of bubbles in the hydraulic oil and correctly operate the hydraulic valve.

 

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