Measures to reduce the failure rate of hydraulic servo valve
Measures to reduce the failure rate of hydraulic servo valve
1. Introduction
In the hydraulic servo system, the servo valve is a high-precision control element for electro-hydraulic speed, electro-hydraulic pressure, electro-hydraulic position and electro-hydraulic control and adjustment. It is the bridge between electrical signal and hydraulic signal and the heart of the hydraulic servo system. The working condition of servo valve directly affects the operation stability of the whole servo system.
In the hydraulic servo system, about 70% of servo valve faults are caused by oil pollution, and 30% are caused by pressure impact and short circuit of electrical system. Therefore, the fault analysis and troubleshooting of servo valve is actually how to control the cleanliness of hydraulic oil, reduce the impact of hydraulic system and prevent short circuit of electrical system.
2. Common faults and cause analysis of servo valve
(1) The servo valve has no pressure output.
1) The pilot valve element is stuck. According to the power output specification, the servo valve is generally divided into one, two and three levels of control. In the two and three levels of control servo valve, the main valve core of the servo valve is driven by the pilot valve core. The pilot valve core is usually stuck because the hydraulic oil is too dirty, or the sealing ring of the valve core is worn out, and the pilot valve core and valve sleeve are roughened by metal particles, so that the valve core is stuck and cannot slide.
2) No control signal input. The possible causes are that the UC module controlling the servo valve crashes or is damaged, the 24V power supply is open or short circuited, the signal transmission line is disconnected or the signal line falls off with the servo valve plug, the servo valve signal line plug has poor contact with the servo valve jack, the servo valve signal line plug pin is damaged, and the servo valve jack is damaged.
3) The main valve element is stuck. Generally, the main valve core is stuck in the valve sleeve and cannot act due to large particles of metal pollutants entering the servo valve.
4) The built-in signal amplification board of the servo valve is burnt out. The servo valve signal amplification board is integrated in the servo valve body and installed on the servo valve by a separate junction box. Due to the poor working environment of the servo valve, water may enter the servo valve amplification board or water vapor may form condensation on the servo valve amplification board, resulting in short circuit of the servo valve amplification board, Damage the electrical components (resistance, capacitance, signal line plug, etc.) on the amplification board; If the 24V (12V) DC power supply voltage exceeds the allowable range of the servo valve and the power line is connected reversely, the built-in signal amplifier of the servo valve will also be damaged. As shown in Figure 194, the water vapor forms condensation on the resistance of the servo valve amplification plate, resulting in the short circuit and burning of the resistance on the amplification plate, resulting in the damage of the whole amplification plate.
5) The pilot stage is damaged. In the jet pipe servo valve, generally, the jet pipe is blocked due to the low cleanliness of the oil, or the jet pipe is cracked and broken due to the pressure impact of the oil, and the jet pipe is sprayed, and the hydraulic oil cannot be accurately sprayed into the receiving device; In the nozzle baffle servo valve, the feedback rod is usually broken due to the pressure impact of the oil, or the control oil orifice is blocked due to particulate pollutants in the oil.
6) The feedback device is damaged. In the force feedback servo valve, the coil of the force feedback motor is burnt out due to the entry of water in the feedback device, and the guide force feedback rod is broken after the force feedback rod is deformed under the action of external force; In the electric feedback servo valve, the feedback ruler is damaged and the wiring of the feedback magnetic ruler is broken due to the entry of water in the feedback device.
(2) The pressure output of servo valve lags behind and oscillates, while the actual pressure is inconsistent with the given pressure.
1) The control oil filter is blocked. Due to the low cleanliness of the oil and the high content of various impurities in the oil, when the oil flows through the filter, the impurities are filtered out by the filter, and a large number of impurities accumulate in the interior and surface of the filter, resulting in the blockage of the filter screen of the control oil filter, the poor oil flow of the control oil, and the lagging action of the valve core. In Figure 195, the upper left figure shows that the filter is blocked by oil sludge due to the oxidation and deterioration of oil; The figure below on the left shows that there are many impurities in the oil, especially some cloth scraps are filtered out by the oil filter when passing through the oil filter, and the cloth scraps gradually block the filter; The following figure on the right shows that there are plastic impurities in the hydraulic oil. Generally, the plastic impurities are in flakes, which will block the filter when passing through the filter.
2) The servo valve spool does not move smoothly. Due to the poor cleanliness of the oil and the high content of oil sludge and solid impurities in the oil, especially the long-term accumulation of oil sludge in the valve core and valve sleeve, the movement of the valve core in the valve sleeve is blocked, and the output pressure cannot change in time with the change of the given value.
3) The ball head of the feedback rod in the force feedback servo valve is worn. As shown in Figure 196, the left figure shows the new feedback rod ball joint with smooth surface, and the right figure shows the worn ball joint with unsmooth surface and obvious wear marks. The wear of feedback dry ball joint is generally due to the long service time of servo valve, resulting in excessive wear of ball joint after extended service; The high water content in the hydraulic oil makes the lubrication performance of the hydraulic oil worse and accelerates the wear of the ball joint.
4) The sharp edge of valve element throttling is worn or damaged. As shown in Figure 197, the upper left figure shows the throttling sharp edge of the new valve, and the throttling sharp edge coincides; The following figure on the left shows the worn throttling sharp edge. When the servo valve is in the middle position, the leakage of the worn throttling sharp edge increases, resulting in unstable output pressure.
5) The control feedback circuit is improperly adjusted and the zero of the servo valve spool is too large.
(3) The outlet pressure of the servo valve is constant and cannot be adjusted.
1) As soon as there is pressure oil input at the P port of the servo valve, there is pressure output at the outlet of the servo valve. Repeatedly adjust the given value of the servo valve, but the pressure at the h{port of the servo valve does not change, but there is an adjustment signal input when detecting the input signal. The reason is that the servo valve spool is stuck at a certain opening position, resulting in the output pressure constant at a certain value and can not be adjusted.
2) The zero point of the control feedback circuit is not adjusted properly, resulting in high zero point.
3. Measures to reduce servo valve failure
(1) Improve the cleanliness of the oil.
1) Improve the cleanliness of hydraulic fluid.
① In the hydraulic servo system, the cleanliness of the oil is required to be controlled within nas4 ~ nas7 levels, and the cleanliness of the new oil is generally nas8 ~ nas11 levels. Therefore, it is necessary to set up a standby tank to add the new oil into the standby tank for circulation in advance. When the cleanliness of the oil reaches the level required by the servo system, it can be used in the reuse tank of the system; A high-precision filter with a filtering accuracy of 3 ~ 5 µ m shall be set on the servo valve control oil circuit. The filter shall be equipped with a blockage display alarm device. The replacement cycle of the filter element is preferably 3 ~ 4 months; In the circulating system of the servo system, the circulating filter element should preferably use a high-precision filter element, and the filter element with a filtering accuracy of 3 µ m is generally used.
② Regularly sample and inspect the working oil in the system, usually once a month. If the physical and chemical properties of the oil exceed the specified range, the oil must be replaced; The oil tank of the servo system shall be cleaned regularly, generally once a year. During cleaning, it is better to use dough to clean the inner wall of the oil tank; In order to prolong the service life of the oil, the oil temperature should be controlled at about 40 ℃ to avoid long-term use when it exceeds 50 ℃. The long-term use of the oil under high temperature conditions will oxidize and deteriorate the oil, and the oil will produce putty after oxidation and deterioration. The putty entering the servo valve will lead to the jamming of the pilot stage of the servo valve, the blockage of the damping hole and the blockage of the built-in filter element.
③ When using the electro-hydraulic servo valve, it shall be installed and maintained according to the product manual. Before installation, do not remove the base protection plate of the valve and the upper cover of the pilot level control valve, and do not move the zero adjustment mechanism of the servo valve at will, so as to avoid pollution in the valve, damage of parts and zero position deviation of the valve. Before installing the servo valve, first check whether the surface of the base connecting plate is smooth and flat, whether there are pollutants attached, whether the oil inlet and outlet are connected properly, whether the O-ring seal is intact, whether the locating pin hole is correct, etc. the surface of the valve block on which the servo valve is installed shall be cleaned with special detergent to prevent dirt from entering the servo valve.
2) Cleanliness of hydraulic lines.
The oil pipe of the hydraulic servo system can be copper pipe or stainless steel pipe. The pipeline shall be pickled in advance to remove the sundries in the pipeline. After pickling, it shall be dried with clean nitrogen; When the pipeline welding needs to break, the breaking machine must be used. After breaking, the pipeline must be cleaned. During welding, the pipeline must be filled with argon before welding, so as to prevent the welding flow in the pipeline from entering the system; If there is a threaded pipe joint on the pipeline, adhesive or raw material belt shall not be used at the joint of the pipe joint. When installing flanges and joints, do not wipe the connecting surface with cotton wires that are easy to fall off, so as to prevent foreign matters from entering the system and blocking the servo valve.
(2) Reduce system pressure shock.
1) In order to improve the control accuracy of the servo valve and reduce the pressure impact of the pilot stage of the servo valve, a pressure compensator can be set in front of the servo valve control oil to stabilize the pressure of the control oil within a specified range, or the pressure of the control oil can be adjusted through the pressure compensator to meet the requirements of the servo valve for the control oil pressure; In the general servo system, the pressure of the servo valve control oil is equal to the system pressure. In fact, appropriately reducing the pressure of the control oil will not have a great impact on the step response of the servo valve. On the contrary, after the control oil pressure is reduced, the failure rate of the pilot stage can be reduced, so as to reduce the failure rate of the servo valve. Now take Moog d662 P.. Taking M series servo valve as an example, it can be seen from figure 198 that when the valve core opening of servo valve is 50%, the step response of control oil pressure decreases by 0.2ms for every 10bar reduction. For the servo system in metallurgical industry, the step response time decreases by 1 ~ 5ms, which has little impact on the control accuracy and fully meets the control requirements of field equipment, Therefore, properly reducing the control oil pressure can greatly reduce the failure of the servo valve (see Figure 198).
2) In the servo system, the pressure impact can lead to the damage of the pilot stage of the servo valve, such as the fracture of the jet pipe, the cracking of the end cover of the pilot stage and so on. In order to reduce the pressure impact of the system, one or more groups of accumulators can be set in the hydraulic station to absorb the pressure fluctuation of the system, and a shock-absorbing accumulator can be set in front of the oil inlet pipeline of the servo valve to reduce the impact of pressure impact on the servo valve.
(3) Check the control cable and plug regularly.
Regularly check whether the servo valve plug is loose and whether the wiring of the plug is loose. If necessary, wrap the servo valve wiring and plug with electrical insulating tape to prevent water and steam from entering the servo valve plug and causing short circuit of the electrical system; Regularly check whether the surface of the control cable of the servo valve is intact. If it is damaged, replace it in time. Do not have a butt joint for the control cable of the servo valve. It must be ensured that the cable from the electrical control box to the servo valve is a whole cable. If there is an interface, it may affect the accurate transmission of the control signal.
(4) Improve the working environment. As the electric amplification circuit is integrated in the servo valve, if water or steam enters the electric amplification circuit, it will cause short circuit damage to the electric amplification board. Therefore, the valve station equipped with servo valve should be placed away from water and steam, and the valve station should be placed in a closed box, with ventilation facilities in the box to ensure air circulation; The desiccant can also be placed in the junction box equipped with the electrical amplifier board, and the desiccant can be changed regularly to ensure that the surface of the electrical amplifier board is dry.
(5) Reasonable use of servo valve. When the servo valve works normally, it is generally required that the valve core opening shall not exceed 75%. If it exceeds 75%, it will cause damage to the valve core of the servo valve. Therefore, on the premise of not affecting the equipment control requirements, controlling the valve core opening of the servo valve within 75% can effectively reduce the failure rate of the servo valve; No matter the servo valve controlled by current or voltage, the input current and voltage are not allowed to exceed 25% of the rated value, otherwise the built-in signal amplification board of the servo valve will be damaged. Therefore, it is necessary to ensure that the input signal and voltage of the servo valve are within the specified range.
Through the above analysis of the failure causes of the servo valve, several main reasons affecting the performance of the servo valve are obtained. Countermeasures are formulated for various reasons. The servo valve should be used in strict accordance with the service requirements of the servo valve, strictly control the cleanliness of the hydraulic oil, minimize the pressure impact of the servo system, and ensure that the servo valve works under a good working condition; Regularly check the cleanliness of the oil of the servo system, regularly check the zero deviation of the valve core of the servo valve, and timely replace the servo valve with poor performance.
