Knowledge of hydraulic equipment: Explanation of cavitation phenomenon in engineering hydraulic machinery system

Due to the high precision of the hydraulic components of the construction machinery and the small fit gap between the relative moving parts, the mating surface will become black and even small pits will appear after cavitation occurs, which will cause the min core to jam and the pressure will be out of adjustment. When the cylinder liner is heavily corroded, deep and irregularly shaped dents will appear in the damaged area. During the H-level maintenance of a ZL50C loader, it was found that the diesel engine cylinder liner, especially its thrust side, had honeycomb-shaped holes. When the hydraulic valves such as the variable speed control valve and the distribution room were removed and washed, the matching surface was found to be black. And there are scratches and small pits, which are cavitation phenomena. Cavitation occurs because when the oil pressure is lower than the oil-gas separation pressure, the gas dissolved in the oil forms "cavities" in the oil. When the pressure reaches a certain value again, these bubbles are It bursts immediately under the action, resulting in high temperature and high pressure. When it acts on the surface of the part, the part will be damaged.

Due to the high precision of the hydraulic components of the construction machinery and the small matching gap between the relative moving parts, the mating surface will become black and even small pits will appear after cavitation occurs, which will cause the min core to jam and pressure imbalance. When the cylinder liner is heavily corroded, the damaged area will have deep irregularly shaped dents, as if the cylinder liner surface has been corroded by strong acid; when the cavitation phenomenon is the most serious, four points will penetrate the cylinder liner wall. The cooling liquid in the machine body enters the cylinder, causing a serious accident in the engine. When cavitation is combined with other types of corrosion, the damage rate will increase exponentially or even dozens of times.

The main factors affecting the cavitation of the hydraulic system of construction machinery

(1) Oil quality. If the oil has poor foam resistance and is easy to vaporize and form foam, it 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 mechanically stirred and the gas is easy to dissolve in the oil; if the oil level is too low, the working pump is easy to absorb air, resulting in insufficient circulating oil flow, which increases the air or water in the oil to form bubbles Probability. For example, if the transmission oil level is too low, the pressure at the inlet of the variable speed pump will be lower than the vaporization pressure of the oil, and the oil will vaporize or evaporate, and bubbles will be precipitated. These bubbles will burst quickly when they flow to the high pressure area and cause 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 frequency of oil pressure changes is high. The frequency of pressure changes directly affects the formation and rupture speed of air bubbles. The parts with high frequency of pressure changes have a fast cavitation rate. For example, when the construction machinery is operating, the boom and the bucket need to be continuously manipulated, so the position of the distribution valve spool and the oil passage is the most serious due to the high frequency of pressure changes, all of which turn black to form carbon deposits and There are small pits and scratches.

(5) Poor quality of cooling water. When the cooling water contains corrosive media (such as acid radical ions), it will form a combined effect of corrosion and cavitation, accelerating the rate of cavitation.

(6) Air and moisture invade the oil. The more air and moisture invade the oil system, the wider the range of cavitation. The main channel of air intrusion is that the suction port and pipe joints of the pump are not tightly sealed, which causes air to enter the system. The main channel of moisture intrusion is fear of internal leakage of the cooler.

(7) The cooling system is poorly maintained. The radiator pressure cap and thermostat that have a greater impact on cylinder liner cavitation. A good radiator pressure cap can keep the pressure of the coolant higher than the vapor pressure, thereby reducing the occurrence of cavitation. The thermostat keeps the coolant in a suitable temperature range, which can reduce the energy when the bubble bursts. Therefore, if the temperature and pressure are abnormal due to poor maintenance, the cavitation rate will be accelerated.

(8) Conditions of use. The rough working process of construction machinery, especially diesel engines, has high explosive pressure, which causes a corresponding increase in lateral percussive force, which causes an increase in cavitation.

Main measures to prevent cavitation

(1) The selected oil should ensure oil quality and proper viscosity. Refuel in strict accordance with the oil dipstick standard to keep the system clean. When adding oil, you must pay attention to cleaning, do not bring in moisture and impurities; when adding cooling water, the water must not contain corrosive substances. The various oil dipsticks of construction machinery are marked with upper and lower limit scales, and the normal standard for controlling the oil volume is conducive to reducing cavitation. The magnetic coarse filter should be cleaned regularly, and various filter elements should be replaced to keep the oil clean. Pay attention to check the oil level. Oil quality and oil color. If you find water droplets in the hydraulic system of the construction machinery, the oil turns milky white or the oil is foamy, you should carefully find the source of water and air, and check the oil cooler and the seals at the pipe joints Sex.

(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 a failure occurs, it should be analyzed and eliminated in time. If the oil circuit is not unblocked, the cooling water is too small or the internal or external leakage occurs, it should be eliminated immediately.

(3) Reduce hydraulic shock. It is not advisable to operate the hydraulic control valves and distribution rooms too fast or too hard, nor to increase the throttle too frequently to reduce hydraulic shock.

(4) Maintain the normal gap between the joint surfaces. When manufacturing or repairing, assembling according to the lower limit of assembly tolerance can reduce the influence of cavitation. If cavitation has occurred, only use metallographic paper to polish to remove carbon deposits, and do not use ordinary fine sandpaper to polish.

(5) Maintain the cooling system correctly. Ensure that the radiator pressure cover is intact and can work normally, which can reduce the generation of cavitation; keep the temperature of the cooling system in an appropriate range to reduce the energy of bubble burst; use an appropriate amount of coolant additives to inhibit rust.

(6) The operation process should be stable to avoid pressure shock.