Main parameters and common problems of hydraulic pump
Main parameters and common problems of hydraulic pump
1.6.4 cavitation and suction performance of hydraulic pump
As we all know, high pressure and high speed is an important way to reduce the size and weight of hydraulic pump. However, one of the obstacles of high speed is cavitation. The cavitation resistance of various pumps is evaluated by suction performance.
(1) Cavitation is a common problem. A small amount of air is dissolved in hydraulic fluid. When the hydraulic system works, in the liquid flow, when the pressure (absolute pressure) is lower than the air separation pressure of the oil at the corresponding temperature, the gas will precipitate and form bubbles, which is called cavitation phenomenon. These bubbles are brought to the high pressure area by the liquid flow. Under the action of high pressure, the bubbles burst rapidly, and the volume is greatly reduced and condensed. The high pressure oil around fills the volume with high speed, forming a hydraulic shock (also known as water hammer). The impact pressure can reach several hundred MPa, thus causing vibration. When the impact pressure is greater than the elastic limit of the material in contact with the liquid flow, mechanical damage will occur on the metal surface. The acidic gas may be separated from the liquid flow, which may cause oxidation and even electrochemical action, thus accelerating the corrosion damage of the metal surface. There are small caverns on the surface of the damaged area, and the damage is several millimeters deep. After cavitation, the liquid will become turbid and accompanied by noise, and even break off in serious cases.
In the hydraulic system, where the pressure is lower than the air separation pressure, cavitation may occur. For example, all kinds of hydraulic valve throttle, small diameter pipe, etc. may produce cavitation phenomenon, but the most noteworthy is the hydraulic pump, which is the heart of the hydraulic system.
Cavitation in the hydraulic pump occurs in the process of oil suction, because the absolute pressure in the oil suction port and oil suction chamber of the pump is generally lower than 1 atmospheric pressure (0.1MPa). When the total pressure in the suction chamber of the hydraulic pump overcomes all kinds of resistance losses such as filters and pipes, and makes the liquid flow accelerate to keep up with the movement of the squeezer in the pump, the remaining pressure is easily lower than the air separation pressure, resulting in cavitation. Bubbles then condense in the process of oil drainage in the high pressure area, resulting in cavitation.
Cavitation not only shortens the service life of hydraulic pump and reduces its efficiency, but also has adverse effects on the whole hydraulic system and other components. Therefore, it should be avoided as far as possible.
(2) Measures to improve the anti cavitation performance and the suction performance of hydraulic pump in the manufacturing and repair of hydraulic pump, measures to improve the anti cavitation performance of hydraulic pump are as follows: increase the suction arc length or change the oil inlet direction to reduce the loss of oil suction resistance or make centrifugal force help to absorb oil; adopt materials with relatively stable mechanical and chemical properties (such as copper and stainless steel), and improve the surface roughness of parts It can improve the quality of work and increase the hardness of some materials (such as carbon steel and stainless steel).
In the use and operation of the hydraulic pump, in order to avoid cavitation of the hydraulic pump, the hydraulic pump with strong self-priming ability should be selected as far as possible, and try to make the minimum suction pressure (limit suction pressure) of the oil suction chamber greater than the air separation pressure of the liquid (the air separation pressure is related to the type, temperature and air solubility of the liquid). The measured results show that the higher the oil temperature and the more air solubility, the higher the air solubility The higher the separation pressure is. The minimum suction pressure and self suction capacity are the two suction performance indexes of hydraulic pump, which are defined as follows.
① The minimum suction pressure is to ensure that the hydraulic pump can normally absorb oil at the highest speed. The minimum pressure allowed in the suction is called the minimum suction pressure of the hydraulic pump.
② The self-priming capacity of the hydraulic pump can absorb oil by itself with the help of atmospheric pressure. The lowest suction pressure (absolute pressure) of a self-priming pump must be less than atmospheric pressure. The self-priming capacity is usually expressed by vacuum degree (the difference between atmospheric pressure and absolute pressure). The higher the vacuum degree is, the stronger the self-priming capacity of the hydraulic pump is.
In order to ensure the normal operation of the hydraulic pump, the vacuum degree of the oil suction port of the hydraulic pump should not be too large, that is, the absolute pressure P2 of the oil suction port of the pump should not be too low, otherwise when the absolute pressure is lower than the air separation pressure PG of the oil, the air dissolved in the oil will separate and precipitate to form cavitation, leading to cavitation. Therefore, it is necessary to limit the vacuum degree of the suction port of the hydraulic pump or increase the pressure of the suction port. Therefore, it is easy to see that the measures to limit the vacuum degree of the oil suction port or increase the pressure of the oil suction port of the hydraulic pump should not only increase the diameter of the oil suction pipe, shorten the length of the oil suction pipe and reduce the local resistance so as to reduce (ρ v2g2) / 2 and △ P, but also limit the oil suction height HS of the hydraulic pump. The oil suction height of various types of hydraulic pumps is different, generally taking HS ≤ 0.5m. If the hydraulic pump is installed below the liquid level of the oil tank to form a backflow (when HS is negative), it is more advantageous to reduce the vacuum degree of the oil suction port of the hydraulic pump.
It can be concluded from the above example that under the condition of a certain speed of hydraulic pump, in order to avoid cavitation, the total pressure at the suction port of hydraulic pump should be increased as much as possible. The specific measures can be taken from the following aspects.
① Increase the diameter of the suction pipe of the pump to reduce the liquid flow speed.
② Try to shorten the height between the hydraulic pump and the liquid level of the oil tank.
③ A large capacity filter is used at the end of the oil suction pipe, and the hydraulic pump is immersed in the oil of the oil tank (Fig. n) to reduce the resistance loss.
④ The high flow pump adopts an elevated oil tank, that is, the oil tank is installed above the hydraulic pump (Figure o), forming a backflow.
⑤ The auxiliary pump is set to deliver the oil with a certain pressure to the suction port of the main hydraulic pump. For example, in the hydraulic system shown in Figure P, the auxiliary pump 1 supplies pressure oil to the suction of the main hydraulic pumps 2 and 3 (both driven by the same hydraulic motor 4), and the suction pressure is set by the relief valve 5. The maximum discharge pressure of pump 2 and pump 3 is set by relief valve 6 and 7 respectively.
⑥ A pressurized oil tank is used, that is, the oil tank is closed and low pressure air is introduced into the oil tank. As shown in Figure Q, the composition and principle of the pressurized oil tank: the hydraulic pump 2 sucks oil from the completely closed oil tank 9, and the oil tank is filled with filtered and dry air. The charging pressure (slightly higher than the atmospheric pressure) is set by the pressure reducing valve 5, which is usually 0.05 ~ 0.07MPa. In order to prevent improper pressure, the air pressure safety valve 4, electric contact pressure gauge 3 and alarm are set. Since increasing the pressure of the fuel tank increases the air content in the oil, the pressurized fuel tank is only used for special occasions (such as the hydraulic system of jet passenger aircraft, etc.).
⑦ For the pump used in low temperature occasions, heating measures should be taken to the oil in the oil tank to avoid cavitation due to low oil temperature and high viscosity.
It should be noted that the suction performance of the hydraulic pump is only related to the structure of the pump itself. The inlet passage of gear pump and screw pump is relatively smooth, so the suction performance is better. The suction performance of vane pump and plunger pump is poor due to the resistance of inlet valve distribution mechanism. The flow resistance of the inlet valve is the largest and the suction performance is the worst.