Cavitation in hydraulic system of construction mac

2022-06-17
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Cavitation erosion of hydraulic system of construction machinery

during the h-level maintenance of a ZL50C loader, it was found that there were honeycombed cavities in the cylinder liner of the diesel engine, especially on the thrust side. 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 small pits. This is cavitation erosion. 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 reaches a certain value again, these bubbles will burst immediately under the action of pressure, resulting in high temperature and high pressure. When the pressure acts on the surface of the parts, the parts will be damaged

due to the high precision of hydraulic components of construction machinery and the small fit gap between relative moving parts, the mating surface will become black or even small pits will appear after cavitation, which will cause the core to be stuck and the pressure to be out of balance. When the cylinder liner is seriously cavitating, 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, making the cooling liquid in the engine enter the cylinder, resulting in serious engine accidents. When pitting corrosion is combined with other types of corrosion, the damage rate will be doubled or even tens of times

the main factors affecting cavitation erosion in the hydraulic system of construction machinery are:

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

(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; If the oil level is too low, 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 transmission oil level is too low, the pressure at the inlet of the transmission pump will be lower than the vaporization pressure of the oil, the oil will vaporize or evaporate, and bubbles will be separated out. These bubbles will burst rapidly when flowing 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 cavitation erosion speed is fast in the parts with high frequency of pressure change. For example, the boom and bucket need to be operated continuously during the operation of construction machinery, so the position corresponding to the oil passage of the distribution valve spool has the most serious cavitation due to the high frequency of pressure change. All of these materials have blackened and formed carbon deposits and small pits and scratches on SL solar aircraft for the first time

(5) poor quality of cooling water. When the cooling water contains corrosive media (such as acid radical ions), the combined effect of corrosion and pitting corrosion will be formed to accelerate the pitting corrosion rate

(6) air and moisture intrusion due to the long-term use of the spring fatigue testing machine, some oil problems will inevitably occur. The more air and water intruded into 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 to avoid 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 with two backup rollers in a proper temperature range, which can reduce the energy when bubbles break. 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

the main measures to prevent cavitation erosion are:

(1) the selected oil shall 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 for controlling oil volume is conducive to reducing cavitation erosion. Clean the magnet primary filter regularly and replace various filter elements to keep the oil clean. Pay attention to check the oil level. Oil quality and color: if water drops are found in the hydraulic system of construction machinery, the oil turns milky white or the oil turns into foam, the source of water and air shall be carefully found, and the tightness of oil cooler and pipe joints shall be checked

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

(3) reduce hydraulic shock. Operate each hydraulic control valve and distribution room should not be too fast or too fierce, nor should the accelerator be increased too frequently to reduce the hydraulic impact

(4) maintain the normal clearance of each joint surface. During manufacture or repair, assembly according to the lower tolerance limit of assembly can reduce the impact of cavitation. If pitting has occurred, the carbon deposit can only be removed by polishing with metallographic paper, and it must not be polished with ordinary fine sandpaper

(5) properly maintain the cooling system. Ensure that the radiator pressure cap is intact and can work normally, 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; Use an appropriate amount of coolant additives to inhibit corrosion

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

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