(2) the machining accuracy reduces the amount of air in the hydraulic system in the form of free fluid in the hydraulic oil, which increases the compressibility of the oil. When the hydraulic system load fluctuation, actuator oil pressure pulsation occurs, resulting in gas expansion or contraction caused by actuator oil flow rate change, caused by hydraulic machinery under low velocity crawling phenomenon. The transmission stiffness is an important index for the high precision hydraulic machinery which requires fast response and accurate positioning. The transmission stiffness is related to the liquid bulk elastic modulus. When the hydraulic system is working, the actual oil contains both the dissolved gas and the mixed gas. The gas dissolved in the oil has no effect on the elastic modulus of the liquid, and the gas mixed with the oil will decrease the bulk elastic modulus of the oil, so that the pressure wave in the oil can be slowed down. Thus leading to the reduction of hydraulic machining accuracy.

(3) to accelerate the oxidative deterioration of oil hydraulic system when working with liquid gas in the bubble along with the development of liquid machinery 2000 supplement? Equipment maintenance?

Moving to the high pressure area, bubbles around the oil pressure under the impact of the volume decreased rapidly until the collapse. At this point in the center of the bubble will produce high pressure. The experimental results show that the bubble collapse in the center of pressure can be as high as 150 ~ 200M Pa. in addition to the liquid particle kinetic energy into pressure energy, and partly into heat, the local temperature and local temperature is 1000 degrees centigrade. The instantaneous temperature of the bubble burst depends on the rate of change of the local pressure of the liquid and the specific heat of the liquid and gas. When the local temperature of the liquid reaches the degree that the oxygen in the gas can be combined with the liquid molecules, that part of the liquid is oxidized, thus causing the deterioration of the oil. In addition, this phenomenon occurs ifSealing elementNear the support ring, the seal and support ring will be burnt. In this way, in addition to the direct failure of the component, the support ring or seal burned hard particles will also cause the hydraulic system failure.

     2, air pollution hydraulic system identification of air pollution caused by the hydraulic system, often with the following one or more additional features. (1) the sound of random changes in hydraulic pump or safety valve occasionally abnormal noise (2) actuator occasional beating or crawling (3) oil within the tank low transparency, oil return opening is the emergence of the bubble (4) oil filter with black residue oil after oxidation. If there is one or more of these phenomena, which means that the hydraulic system has been polluted by air.

     3 hydraulic system, hydraulic system and prevention of air pollution is a closed system, is not easy to know the pollution status of the early system, often only through the noise of hydraulic system, vibration and crawling to determine whether the contaminated air pollution control, therefore, should be based on prevention. Hydraulic system prone to air pollution in the main parts of the hydraulic pump suction port, control valve near the valve port, hydraulic cylinder two cavity. For this reason, the corresponding preventive measures should be taken in the design and application of hydraulic system.

（1) the prevention of air pollution of the hydraulic pump suction port in the volume of the oil tank design and determine the oil consumption of machinery and equipment, should ensure that the minimum oil level when there is enough oil, the hydraulic pump suction nozzle with oil and air. The oil tank should be provided with a separator to separate the suction pipe and the oil return pipe, so that the liquid flow can be used to separate the bubbles from the box. Or in the tank to set up metal mesh to eliminate bubbles.

When used, should always check the oil outlet pipe at the junction of the seal, so as not to seal the air suction. Should also be regularly check the oil suction filter oil absorption situation, if necessary, timely cleaning or replacement filter, in order to ensure the smooth oil pump suction, to avoid the pump suction mouth to produce cavitation.

(2) the design of hydraulic circuit to control air pollution prevention valve is not reasonable, control the oil resistance valve area is too large, the pressure drop is too large or hydraulic mechanical impurity after use in a long time, the fluid through the narrow gap have a sharp drop, will produce cavitation. Especially in the flow control valve throttle valve prone to this situation. In order to prevent cavitation near the valve port of the control valve, the pressure difference between the front and the back of the control valve should be made to meet the performance requirements. The cavitation phenomenon caused by the accumulation of impurities can be eliminated by cleaning the control valve.

(3) the prevention of air pollution caused by the two chamber of the hydraulic cylinder is due to the closed cavity of the cylinder of the hydraulic cylinder, and the original air in the cylinder and the air brought by the oil from the pipeline of the system will be stored in the cavity of the cylinder of the. As the executive component of hydraulic system, hydraulic cylinder will directly reflect the harm of air pollution, if the air is not discharged in time, it will cause the hydraulic cylinder to crawl at low speed, which will affect the normal work. In order to avoid the occurrence of this phenomenon, in addition to take measures to prevent the air into the hydraulic system, in order to be able to eliminate the accumulation of air in the cylinder, the hydraulic cylinder can be set on the valve. For the hydraulic cylinder with large inertia and fast running speed, a buffer device should be installed at both ends of the cylinder.

4 it is not difficult to see that the air in the hydraulic system is a kind of pollutant of hydraulic machinery. Because the air bubbles occupy a certain space, the air in the hydraulic system will destroy the continuity of the oil, which will result in the decrease of the output flow of the pump. In the pump pressure pipe will also cause the flow and pressure fluctuations, so that the hydraulic machinery is not stable, processing accuracy. At the same time, it will make the hydraulic machinery produce strong noise and accelerate the deterioration of the oil pressure in the hydraulic system. Therefore, we must pay attention to the harm to the air in the hydraulic system of hydraulic machinery, to take effective measures to control the content of oil in the air in the practical work, to improve the working performance of the hydraulic machinery, meet the requirement of processing precision.

Ji Guang guo. Fault diagnosis and elimination of hydraulic system [M]. Beijing: Ocean Press, 1998

Lei Tianjue. Hydraulic Engineering Handbook [M]. Beijing: China Machine Press, 1990

First author: Du Qiaolian, female, born in 1966, lecturer. Research interests: mechatronics. 15 papers have been published.

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