When we repair the hydraulic cylinder of construction machinery, we can often see some honeycomb holes on the inner wall of the hydraulic cylinder and the surface of the piston or piston rod, which are all caused by cavitation. The harm of cavitation in the hydraulic cylinder is quite great. It will lead to the blackening of the mating surface and even the scorching of the support ring and seal 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 main parts of hydraulic cylinder several times or even dozens of times, which will seriously affect the normal use of construction machinery. Therefore, it is very necessary to prevent the cavitation of hydraulic cylinder.
1. Main causes of cavitation
(1) essence analysis of cavitation
Cavitation is mainly caused by a certain amount of air mixed in the oil between the piston and the guide sleeve during the working process of the hydraulic cylinder. With the gradual increase of pressure, the gas in the oil will become bubbles. When the pressure rises to a certain limit value, these bubbles will rupture under the action of high pressure, which will quickly act the high-temperature and high-pressure gas on the surface of the parts, resulting in gas corrosion of the hydraulic cylinder and corrosive damage to the parts.
⑵ cavitation caused by unqualified hydraulic oil quality
Ensuring the quality of hydraulic oil is an important factor to prevent cavitation. If the foam resistance of the oil is poor, it is easy to produce foam, resulting in cavitation. Secondly, the change frequency of oil pressure is too fast and too high, which will also directly cause the formation of bubbles and accelerate the rupture speed of bubbles. The test shows that the cavitation speed will be accelerated in the parts with high pressure change frequency. For example, at the oil inlet and return port of the hydraulic cylinder, due to the relatively high frequency of pressure change, the degree of cavitation is also relatively higher than that of other parts. In addition, oil overheating will also increase the probability of cavitation.
(3) cavitation caused by improper manufacturing and maintenance
Due to the failure to fully exhaust the hydraulic system during assembly or maintenance, there is gas in the system, which can produce cavitation under the action of high temperature and high pressure.
(4) cavitation is caused by the quality problem of coolant
When the coolant contains corrosive media, such as various acid radical ions and oxidants, it is prone to chemical and electrochemical corrosion, which will also accelerate the speed of cavitation under their combined action; If the cooling system is well maintained, cavitation can be prevented. For example, if the pressure cap of the radiator of the cooling system is well maintained, the coolant pressure of the radiator can always be higher than the vapor pressure, so as to prevent cavitation. Another example is the thermostat of the cooling system; A thermostat with good performance can keep the coolant in the appropriate temperature range and reduce the energy released when the bubble breaks.
2. Measures to prevent cavitation
Although there are many reasons for cavitation, cavitation can be avoided as long as necessary measures are taken to actively prevent it. The following is about the preventive measures that should be taken according to the causes of cavitation.
(1) strictly control the selection of hydraulic oil
Select hydraulic oil in strict accordance with the oil standard. The selection of high-quality hydraulic oil can effectively prevent bubbles in the working process of the hydraulic system. When selecting the oil, it shall be selected according to the minimum temperature in different regions, and the hydraulic oil shall be filled according to the dipstick standard. At the same time, the hydraulic system shall be kept clean (when filling the hydraulic oil, moisture and other impurities shall be prevented from being brought in). The oil quality, oil level and oil color of the hydraulic oil shall be checked frequently. If bubbles and foam are found in the hydraulic oil, or the oil turns milky white, the source of air in the oil shall be carefully found, And eliminate it in time.
(2) prevent excessive oil temperature and reduce hydraulic impact
The key to keep the hydraulic oil temperature normal is to reasonably design the heat dissipation system and prevent the oil temperature from being too high. In case of abnormality, find out the cause and eliminate it in time. When operating the hydraulic control lever and distribution valve, we should strive to be stable, not too fast, too fierce, or increase the engine throttle frequently, so as to minimize the impact of hydraulic oil on hydraulic components. At the same time, the cooling system shall be maintained in time to keep the temperature of the cooling system within an appropriate range, so as to reduce the energy released when the bubble breaks. Without affecting the normal circulation of coolant, a certain amount of anti-corrosion additives can be appropriately added to inhibit corrosion.
(3) maintain the normal clearance of the joint surface of each hydraulic element
When manufacturing or repairing the main parts of the hydraulic cylinder (such as cylinder block, piston rod, etc.), the assembly should be carried out according to the lower tolerance limit of the assembly size. Practice has proved that this can well reduce the occurrence of cavitation. If cavitation has occurred to hydraulic components, only metallographic sandpaper polishing technology can be used to remove cavitation pitting and surface carbon deposition, and ordinary fine sandpaper cannot be used for grinding.
(4) pay attention to exhaust during maintenance
After the maintenance of the hydraulic cylinder, the hydraulic system shall operate stably for a certain time to fully circulate the hydraulic oil in the hydraulic system; If necessary, disassemble the oil inlet pipe (or oil return pipe) of the hydraulic cylinder to overflow the hydraulic oil, so as to achieve the effect of exhaust of a single hydraulic cylinder.
