Cause analysis and Countermeasures of cavitation e

2022-08-08
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Cause analysis and Countermeasures of cavitation erosion in the hydraulic system of construction machinery

due to the high accuracy of hydraulic components of construction machinery and the small fit gap between relative moving parts, cavitation erosion will lead to the blackening of the fit surface and even the appearance of small pits, which will make the core stuck and the pressure maladjusted. When the cylinder liner cavitation is heavy, deep irregular pits will appear in the damaged area

during the h-level maintenance of a ZL50C loader, it was found that there were honeycombed holes in the cylinder liner of the diesel engine, especially on the side under the thrust. When the hydraulic valves such as the variable speed control valve and the distribution room were disassembled and cleaned, it was found that the mating surface was blackened and there were scratches and pits, which was the phenomenon of cavitation. Cavitation occurs because when the oil pressure is lower than the oil-gas separation pressure, the gas dissolved in the oil will form cavities in the oil. When the pressure rises to a certain value again, these bubbles will burst immediately under the action of pressure, resulting in high temperature and high pressure. When it acts on the surface of the parts, it will damage the parts

because the hydraulic products of construction machinery have fully passed the ROHS environmental protection certification of the international alliance and the green supplier certification of overseas high-end customers, the components have high precision and small fitting gap between relative moving parts. After cavitation, the fitting surface will become black or even appear small pits, which will make the Min core stuck. The force beam drives the heavy head 1 to move downward at the required speed to complete the experimental pressure imbalance. When the cylinder liner is heavily pitted, deep irregular pits will appear in the damaged area, as if the cylinder liner surface has been corroded by strong acid; When cavitation is the most serious, the four points will penetrate the cylinder liner wall, causing the cooling liquid in the engine body to enter the cylinder, resulting in serious engine accidents. When pitting corrosion is combined with other types of corrosion, the damage rate will increase exponentially or even dozens of times

main factors affecting cavitation erosion of hydraulic system of construction machinery

(1) oil quality. If the oil has poor foam resistance, it is easy to vaporize and form foam, which is easy to cause cavitation

(2) the oil level is too high or too low. If the oil level is too high, the oil is easily dissolved in the oil due to mechanical agitation; The oil level is too low, and the working pump is easy to absorb air, resulting in insufficient circulating oil flow, which increases the probability of air or water bubbles in the oil. For example, if the oil level of the transmission is too low, the pressure at the inlet of the transmission pump will be lower than the vaporization pressure of the oil, and the oil will vaporize or evaporate, releasing bubbles. These bubbles will burst rapidly when they flow to the high-pressure area, resulting in cavitation

(3) the oil is overheated. If the oil temperature is too high, the oil is easy to vaporize, the water evaporates, the foam and air bubbles increase, and the cavitation phenomenon increases

(4) the change frequency of oil pressure is high. The frequency of pressure change directly affects the formation and rupture speed of air bubbles. The parts with high frequency of pressure change have fast cavitation speed. For example, during the operation of construction machinery, the boom and bucket need to be operated constantly, so the position of the distribution valve spool corresponding to the oil passage is also the most serious cavitation due to the high frequency of pressure change, all of which are blackened to form carbon deposits and small pits and scratches

(5) poor quality of cooling water. When the cooling water contains corrosive media with a carrier mass less than 5000kg (such as acid radical ions), the combined effect of corrosion and cavitation will be formed to accelerate the cavitation rate

(6) air and moisture invade the oil. The more air and water invading the oil system, the wider the range of cavitation erosion. The main channel of air intrusion is that the oil suction port and pipe joint of the pump are not tightly sealed, resulting in air entering the system. The main channel of water intrusion is fear of internal leakage of cooler

(7) poor maintenance of the cooling system. The radiator pressure cap and thermostat have a great impact on cylinder liner cavitation. A good radiator pressure cap can keep the coolant pressure higher than the steam pressure, thus reducing cavitation. The thermostat keeps the coolant in a proper temperature range, which can reduce the energy when bubbles burst. Therefore, if the temperature and pressure are abnormal due to poor maintenance, the speed of cavitation will be accelerated

(8) service conditions. The working process of construction machinery is rough, especially the diesel engine. The maximum explosion pressure is large, which causes the corresponding increase of lateral knocking force, resulting in the increase of cavitation erosion

main measures to prevent cavitation erosion

(1) the selected oil should ensure the oil quality and appropriate viscosity. Fill oil in strict accordance with the dipstick standard to keep the system clean. Pay attention to cleaning when refueling, and do not bring in water and impurities; When adding cooling water, the water must not contain corrosive substances. All kinds of oil dipsticks of construction machinery are marked with upper and lower limit scales. The normal standard of controlling oil volume is conducive to reducing cavitation erosion. Clean the magnet primary filter regularly, replace various filter elements, and keep the oil clean. Pay attention to check the oil level. Oil quality and oil color. If water drops, oil turns milky white or oil turns into foam in the hydraulic system of construction machinery, carefully find the source of water and air, and check the tightness of oil cooler and pipe joints

(2) prevent the oil temperature from being too high, and reasonably design the heat dissipation system to ensure that the oil temperature is normal. If there is a fault, it should be analyzed and eliminated in time. If the oil circuit is blocked, there is too little cooling water or internal and external leakage, it should be eliminated immediately

(3) reduce hydraulic shock. Operate each hydraulic control valve and distribution room should not be too fast, too fierce, or increase the throttle too frequently to reduce the hydraulic impact

(4) maintain the normal clearance of each joint surface. During manufacture or repair, the influence of cavitation can be reduced by assembling according to the lower limit of assembly tolerance. If pitting has occurred, you can only polish with metallographic paper to remove carbon deposits, and you must not polish with ordinary fine sandpaper

(5) maintain the cooling system correctly. Ensure that the radiator pressure cap is intact and normal. Too fast or too slow will cause the test results to work differently, which can reduce the occurrence of cavitation; Keep the temperature of the cooling system in an appropriate range to reduce the energy of bubble rupture; Using an appropriate amount of coolant additives can inhibit corrosion

(6) the operation process should be stable to avoid pressure impact

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