(1) There are two sources of gas in the hydraulic closed system: one is that it is inhaled into the system from the outside, which is called mixed air; the other is that the air dissolved in the hydraulic oil is separated due to the cavitation phenomenon.

① The compressibility of the oil increases (1000 times), which leads to the action error of the actuator, resulting in crawling, destroying the stability of the work, resulting in vibration, and the normal work of the hydraulic equipment. b. The noise and vibration of the hydraulic pump and pipeline are greatly increased, and the wear is intensified. The bubble becomes a "spring" in the high pressure area, and the system pressure fluctuates greatly, When the pressure oil bubbles are compressed, a large amount of heat is released, and the local combustion oxidizes the hydraulic oil, resulting in the deterioration of the hydraulic oil. Air bubbles enter into the lubricating part, cut the oil film, gear pump causes the phenomenon of burn and wear of sliding surface and increase of friction (air mixing, oil viscosity increasing); air bubbles accumulate in the oil tank, increase the volume, oil leaches from the oil tank, polluting the ground; e. air bubbles cause cavitation.

② The harm of cavitation. The so-called cavitation refers to the phenomenon that when the pressure of the flowing pressure oil drops at the local position (high flow rate, low pressure) and reaches the saturated vapor pressure or air separation pressure, a large number of bubbles are formed due to the separation of the dissolved air. When it flows from the local low pressure area to the high pressure area again, the bubbles break and disappear, and local high pressure and high temperature are formed in the process of breaking and disappearing, resulting in vibration and instability Regular noise, metal surface is oxidized and eroded, this phenomenon is called cavitation, also called cavitation. Cavitation often occurs at the inlet of hydraulic pump and near the throttle of control valve. In addition to the hazards of air mixing, cavitation also causes pitting corrosive wear on the metal surface. Because the bubbles generated in the low-pressure area will burst into the high-pressure area and generate a pressure of several iompa, push the metal particles and repeatedly make the metal wear sharply. Because of the bubbles (holes), the effective suction flow of the pump is reduced. In addition, due to the deterioration of cavitation oil, bubbles are adiabatically compressed in the high-pressure area, resulting in higher temperature, which aggravates the chemical reaction speed between oil and air, and even causes combustion, emission and smoke, and carbon element dissociation, resulting in oil blackening.