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Research on automatic deflation valve for hydraulic system
Abstract: This paper systematically analyzes the ways and hazards of oil mixed with gas in hydraulic system, designs an automatic deflation valve and optimizes the structure of hydraulic system. A hydraulic system of gas hazards Hydraulic fluid as the working medium of the hydraulic system not only transmits power, but also on the system or other devices play a role in lubrication and cooling, the work of oil-containing gas hazards here are briefly described. Due to the existence of gas during the operation of the system, the oil flow is turbulent, which is unfavorable to heat dissipation and increases the pressure loss, accelerating the temperature rise and oxidation of the oil liquid, shortening the service life of the oil liquid, affecting the rigidity and response of the system Characteristics, so that system pressure pulsation, components, pipes and other connections loose, the actuator output does not meet the rated value and produce creeping, chattering, and even produce misoperation; system pressure changes, due to the bubble suddenly shrinking and amplification, that is produced Cavitation and cavitation, the system produces noise or even destroyed, oil and gas mixed to a certain extent, the fuel injection tank may increase. Of particular concern should be the constant pressure system with a load, the presence of air threatens it even more. After the system stops working, the oil contained in the oil will be released under normal pressure and will gather at the height of the pipeline. When the pump restarts, the gas in the pipeline on the pump outlet and the gas discharged from the pump will be compressed and then returned to the oil absorption Side, resulting in dry friction or hydraulic pump running in the critical state of lubrication and produce suction, causing difficulties to start, reduce the volumetric efficiency, serious or even make the hydraulic pump burned. Oil and gas mixture will also cause the actuator dry friction damage, if at the same time other devices (such as transmission) need hydraulic system oil for lubrication, due to the presence of gas in the oil, when starting this device not only failed to timely lubrication, And the compressed air delivered at this time also destroyed the original oil film, there is the risk of burning scrapped. Therefore, some pump manufacturers put forward special requirements on pump start-up: In order to eliminate stagnant air, start the pump outlet connector must be loosen or plug to allow air to escape. 2 gas mixed into the hydraulic system 2.1 man-made factors The system does not have a good oil precipitation, filtration, resulting in excess air into the system start debugging and before each use of no-load operation is not very good Gas, resulting in the original pipeline, the actuator chamber air trapped in the system; pipeline, especially the suction pipe leaks, resulting in outside air fleeing into the system. 2.2 System Factors Experiments show that the normal amount of dissolved oil in mineral oil up to 6% to 12%. Commonly used hydraulic oil dissolved in the air is generally about 9%, which means that under normal circumstances the system of oil is mixed with a certain gas. According to Henry's law, the solubility of gas in the oil is proportional to the absolute pressure. When the system is running, the oil will have a large pressure drop through the valve, filter and other components, causing the air to be precipitated and suspended in the oil in the form of tiny bubbles. System back to the oil (some through the filter) in the tank to produce spray, foam, while inevitably stirring oil in the tank, also make the air mixed, the oil in the gas is sucked into the system cycle, resulting in oil gas content Increasing. For the low tank system, because there is no suction check valve (hydraulic system of construction machinery common), the system stops working after the influx of gas from the suction tube, rose to the hydraulic pump inlet, if the hydraulic pump static sealing performance is not good, the gas also After the pump rises to the pressure pipeline. 3 traditional deflation method from the gas into the way, the human factor caused by easier to exclude, as long as the increase in assembly quality, strengthen system commissioning and pay attention to each start after no-load operation can be a period of time; due to system causes Deflate regularly. The traditional method of deflation is to unblock the plug at the outlet and the top of the hydraulic pump during start-up to deflate it, which can bring great inconvenience to the system and can not be deflated by users. This paper presents a method of automatic deflation, the effect is good. 4 automatic deflation valve works Pump starts, the gas pressure chamber from the lower end of the valve 2 into the lower part of the valve core 2 through the horizontal hole, the valve body 2 and the valve body 4 of the annular space and the formation of the opening, and then After the orifice on the spool 2 to spring chamber, into the tank. Due to the function of the orifice d, when the gas passes through the assembly, an air pressure is generated at the lower end of the valve core. When the flow rate of the gas passing through is smaller, the force formed by the air pressure on the area A of the valve core 2 is smaller than the pre-pressure KX0 of the spring. 2 still remain in the original position does not move. When the gas flow through the larger, then the gas pressure generated at the lower end of the valve spool to the spool 2 force greater than the spring pre-pressure KX0 that reaches the valve starting pressure pm (relative pressure), the spool 2 will be up Moves and gradually closes the opening formed by the valve core 2 and the valve body 4, which finally acts as a throttling effect to raise the pressure again, and the valve core 2 rapidly moves upward until the opening is closed. Through the gas flow is not large, the resulting pressure does not reach the valve starting pressure, the spool 2 does not move, so that when the gas is discharged, the hydraulic pump to work normally, the liquid through the orifice d back pressure, the pressure The role of the same gas pressure effect, when the oil pressure reaches the starting pressure pm, the spool 2 quickly move up, the pressure continues to rise to close the pressure pc, the spool moved upward X, and ultimately close the spool 2 and the valve body 4 The formation of the opening, to achieve a leak-tight cone mouth seal. From the above process, it can be found that the small hole d on the valve core 2 controls the throttle and controls the closing pressure and the flow rate. Pressure flow formula describes the above process: Starting pressure pm = KX0 / A Closing pressure pC = K (X0 + X) / A Where K - spring stiffness X0 - spring pre-compression X - the amount of spool openings. The gas flow rate that makes the valve core move can be calculated as the gas velocity through the throttle orifice d: Qg = 113 CVÏ€d2 / 4 (pm + 0.1) (273 / T) 1/2 where pm + 0.1 - pm absolute Pressure value to make the spool closed liquid flow, according to the liquid through the orifice for the thin-walled hole calculation: Q1 = CqÏ€d2 / 4 (2pc / Ï) 1/2 Where CV - gas cross-section contraction factor, 0.6 d - the diameter of the control orifice T - the absolute temperature of the gas Cq - the fluid flow coefficient, generally 0.7 Ï - the density of the fluid In general, X / X0 >> 1, so pm ≈ pC . Parameter settings: First, determine the valve closing pressure and the required bleed flow based on the system minimum pressure and maximum flow, and then calculate the d and Q1 values. For example, when the closing pressure is 0.1 Mpa and the spool deflates to 200 L / min, the orifice d = 2 mm and the liquid flow Q1 is closed = 1 L / min. Automatic bleed valves for this type of valve are available for systems with a minimum flow rate of more than 1 L / min, a maximum flow rate of less than 200 L / min and a minimum pressure of more than 0.1 Mpa. 5 Test 5.1 The gas passes the performance test to connect the unit with compressed air. When the pressure is lower than 0.1 Mpa, the gas is discharged through the unit. When the inlet pressure reaches 0.1 Mpa, the valve is closed. 5.2 Load System Shut-off Test Mount the assembly to the pump outlet with a large amount of gas evolved at low start-up, followed by a small amount of oil spill, eventually closing and no oil infiltration. 6 practical analysis â‘ automatic deflation valve The ideal installation position is the pump outlet of the highest pressure pipeline, the gathering of air quickly back to the tank, is conducive to rapid oil absorption, to avoid dry friction, play a protective effect on the pump, while giving Other lubrication devices to be lubricated quickly. â‘¡ vent valve outlet connected back to the tank, make up for gas, reduce pump vacuum suction, pump a virtuous circle of oil.