The phenomenon of cavitation in the hydraulic system of construction machinery

Due to the high precision of hydraulic components in construction machinery and the small fit clearance between relatively moving parts, cavitation can cause the mating surfaces to turn black or even develop small pits, leading to the jamming of the core and pressure imbalance. When the cavitation of the cylinder liner is severe, deep and irregularly shaped concave points will appear in the damaged area.

During the H-level maintenance of a ZL50C loader, it was found that there were honeycomb-like holes in the diesel engine cylinder liner, especially on the side subjected to thrust. When disassembling and cleaning the hydraulic valves such as the transmission control valve and the distribution chamber, it was discovered that the mating surfaces had turned black with scratches and small pits. This is a cavitation phenomenon. Cavitation occurs because when the oil pressure is lower than the oil-gas separation pressure, the gas dissolved in the oil and released forms "cavities" in the oil. When the pressure rises again to a certain value, these bubbles immediately burst under the action of pressure, thereby generating high temperature and high pressure. When they act on the surface of the parts, they will cause damage to the parts.

Due to the high precision of hydraulic components in construction machinery and the small fit clearance between relatively moving parts, cavitation can cause the mating surfaces to turn black or even develop small pits, leading to the jamming of the core and pressure imbalance. When the cavitation of the cylinder liner is severe, deep and irregularly shaped concave points will appear in the damaged area, as if the surface of the cylinder liner has been corroded by strong acid. When the cavitation phenomenon is at its most severe, the four points will penetrate the cylinder liner wall, allowing the cooling liquid inside the engine block to enter the cylinder and causing a serious engine accident. When cavitation combines with other types of corrosion, the rate of damage will increase several times or even dozens of times.

The main factors influencing cavitation in the hydraulic systems of construction machinery

(1) Oil quality. If the oil has poor anti-foaming properties and is prone to vaporization and foam formation, it is likely to cause cavitation.

(2) The oil level is either too high or too low. If the oil level is too high, the oil will be mechanically stirred, which may cause gases to dissolve in the oil. If the oil level is too low, the working pump is prone to drawing in air, resulting in insufficient flow of the circulating oil and increasing the probability of air or water forming bubbles in the oil. For instance, if the oil level in the transmission is too low, the pressure at the inlet of the transmission pump will be lower than the vaporization pressure of the oil. As a result, the oil will vaporize or evaporate, releasing bubbles. These bubbles will quickly burst when they reach the high-pressure area, causing cavitation.

(3) The oil is overheating. When the oil temperature is too high, the oil is prone to vaporization, water evaporation, an increase in foam and air bubbles, and an increase in cavitation.

(4) The frequency of oil pressure changes is high. The frequency of pressure changes directly affects the formation and rupture rate of air bubbles. In areas with a high frequency of pressure changes, the cavitation rate is faster. When construction machinery is in operation, the boom and bucket need to be constantly controlled. Therefore, at the position corresponding to the oil passage of the distribution valve core, due to the high frequency of pressure changes, the cavitation phenomenon is the most severe. The entire area turns black, forms carbon deposits, and has small pits and scratches.

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

(6) Air and moisture invade the oil. The more air and moisture invade the oil system, the wider the range of cavitation will occur. The main channels for air intrusion are the suction ports of the pump and the pipe joints, where air enters the system due to poor sealing. The main channel for water to seep in is the fear of internal leakage in the cooler.

(7) Poor maintenance of the cooling system. The radiator pressure cap and thermostat have a significant impact on cylinder liner cavitation. A good radiator pressure cap can maintain the coolant pressure higher than the vapor pressure, thereby reducing the occurrence of cavitation. The thermostat keeps the coolant within an appropriate temperature range, which can reduce the energy when bubbles burst. Therefore, if it is not well maintained and causes abnormal temperature and pressure, it will accelerate the rate of cavitation.

(8) Conditions of use. The working process of construction machinery is rough, especially diesel engines, which have a large maximum burst pressure, causing the lateral striking force to increase accordingly and thus leading to an increase in cavitation.

The main measures for preventing cavitation

(1) The selected oil should ensure its quality and appropriate viscosity. Add oil strictly in accordance with the dipstick standard and keep the system clean. When refueling, it is essential to pay attention to cleanliness and avoid bringing in water and impurities. When adding cooling water, there must be no corrosive substances in the water. All kinds of oil dipsticks for construction machinery are marked with upper and lower limit scales. Controlling the normal standard of oil quantity 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 checking the oil level. When water droplets are found in the hydraulic system of construction machinery, the oil turns milky white or appears foamy, the sources of water and air should be carefully investigated, and the sealing of the oil cooler and the joints of each pipe should be checked.

(2) To prevent the oil temperature from being too high, design the heat dissipation system reasonably to ensure the normal oil temperature. If a malfunction occurs, it should be analyzed and eliminated in a timely manner. If there is an unobstructed oil passage, insufficient cooling water or internal or external leakage, it should be immediately eliminated.

(3) Reduce hydraulic shock. When operating each hydraulic control valve and distribution room, it is not advisable to do so too quickly or forcefully, nor should the throttle be increased too frequently to reduce hydraulic shock.

(4) Maintain normal clearances at each mating surface. Assembling in accordance with the lower limit of the assembly tolerance during manufacturing or repair can reduce the impact of cavitation. If cavitation has occurred, only gold paper can be used for polishing to remove the carbon deposits. It is strictly forbidden to use ordinary fine sandpaper for grinding.

(5) Properly maintain the cooling system. Ensuring that the radiator pressure cap is intact and functioning properly can reduce the occurrence of cavitation. Keep the temperature of the cooling system within an appropriate range to reduce the energy required for bubble rupture. The use of an appropriate amount of coolant additive can inhibit rusting.

(6) The operation process should be smooth and avoid pressure shock.