Abstract: When we are repairing the hydraulic cylinder of construction machinery, we often see some honeycomb holes on the inner wall of the hydraulic cylinder, the surface of the piston or the piston rod, which are caused by cavitation.
The damage caused by cavitation in the hydraulic cylinder is quite large. It will cause the mating surface to become black, and even the phenomenon of scorching of the support ring and sealing ring, resulting in internal leakage of the hydraulic cylinder. When cavitation and other types of corrosion work together, it will accelerate the corrosion rate of the main parts of the hydraulic cylinder several times or even tens of times, thereby seriously affecting the normal use of construction machinery. Therefore, it is necessary to prevent cavitation in hydraulic cylinders.
1. Main cause of cavitation
(1) Substantial analysis of cavitation
The occurrence of cavitation is mainly due to that a certain amount of air is mixed in the oil between the piston and the guide sleeve during the operation of the hydraulic cylinder. With the gradual increase in pressure, the gas in the oil will become bubbles. When the pressure rises to a certain limit, these bubbles will burst under the action of high pressure, thereby quickly acting on the high-temperature, high-pressure gas On the surface of the part, it causes cavitation of the hydraulic cylinder, causing corrosive damage to the part.
⑵ Unqualified hydraulic oil leads to cavitation
Ensuring the quality of hydraulic oil is an important factor to prevent cavitation. If the oil has poor anti-foaming properties, it is easy to generate foam, which can cause cavitation. Secondly, the frequency of oil pressure change too fast and too high will also directly cause the formation of bubbles and accelerate the bursting speed of bubbles. Tests have proved that the rate of cavitation in parts with high frequency of pressure change will be accelerated. For example, at the inlet and return ports of hydraulic cylinders, due to the relatively high frequency of pressure changes, the degree of cavitation is also relatively higher than other parts. In addition, oil overheating will also increase the chance of cavitation.
⑶ Improper manufacturing and maintenance lead to cavitation
During assembly or maintenance, the hydraulic system is not fully vented, which results in the existence of gas in the system, and cavitation can occur under the action of high temperature and high pressure.
⑷ The quality of the coolant is bad, causing cavitation
When the coolant contains corrosive media, such as various acid ions, oxidants, etc., it is prone to chemical and electrochemical corrosion, etc. Under their combined action, the speed of cavitation will also be accelerated; if the cooling system is well maintained, it can be Prevent cavitation. For example, if the pressure cover of the cooling system radiator is maintained properly, the coolant pressure of the radiator can always be higher than the vapor pressure, thereby preventing the occurrence of cavitation. As another example, the thermostat of the cooling system; a thermostat with good performance can keep the cooling liquid within a suitable temperature range, which can reduce the energy released when the bubbles burst.
2. Measures to prevent cavitation
Although the causes of cavitation are multi-faceted, as long as the necessary measures are taken to actively prevent them, cavitation can be avoided. Let's talk about the preventive measures that should be taken for the causes of cavitation.
(1) Strictly control the selection of hydraulic oil
Select hydraulic oil in strict accordance with oil standard. The use of good quality hydraulic oil can effectively prevent bubbles from appearing in the hydraulic system during work. When selecting oil, select according to the minimum temperature in different regions, and add hydraulic oil according to the dipstick standard. At the same time, the hydraulic system should also be kept clean. ), Often check the oil quality, oil level and oil color of the hydraulic oil. If blisters, foam, or milky white appear in the hydraulic oil, you should carefully find the source of the air in the oil and eliminate it in time.
⑵Prevent excessive oil temperature and reduce hydraulic shock
Reasonable design of the heat dissipation system to prevent the oil temperature from being too high is the key to maintaining the normal temperature of the hydraulic oil. If an abnormality occurs, you should find the cause and eliminate it in time. When operating the hydraulic control lever and the distribution valve, we must strive for stability, not too fast, too fierce, nor frequently increase the engine throttle to minimize the impact of hydraulic oil on hydraulic components. At the same time, the cooling system should be maintained in time to keep the temperature of the cooling system within an appropriate range to reduce the energy released when the bubbles burst. While not affecting the normal circulation of the coolant, a certain amount of anti-corrosion additives can be added appropriately to suppress rust.
⑶Maintain the normal clearance of the joint surface of each hydraulic component
When manufacturing or repairing the main parts of the hydraulic cylinder (such as cylinder block, piston rod, etc.), the assembly should be assembled according to the lower limit of the assembly size tolerance. Practice has proved that this can reduce the occurrence of cavitation. If the hydraulic components have cavitation, you can only use metallographic sandpaper polishing technology to remove cavitation pits and surface area carbon, and you can't use ordinary fine sandpaper for grinding.
⑷ Pay attention to exhaust during maintenance
After the hydraulic cylinder is repaired, the hydraulic system should be run smoothly for a certain period of time, so that the hydraulic oil in the hydraulic system can be fully circulated; if necessary, the hydraulic cylinder inlet pipe (or return pipe) can be disassembled to make the hydraulic oil overflow To achieve the effect of exhausting a single hydraulic cylinder.
