Created on:2021-09-09 14:09

Construction machinery_ Hydraulic system failure of SD7 bulldozer

Construction machinery_ Hydraulic system failure of SD7 bulldozer

The chassis hydraulic system of SD7 bulldozer is mainly composed of chassis, transmission control valve, steering brake control valve, torque converter outlet overflow valve and pipeline accessories. In use, if a hydraulic element fails, the oil temperature of the chassis hydraulic system will rise. If the system oil temperature rises to 120 ℃, the system will automatically alarm to attract the operator's attention. The causes of high oil temperature are as follows.

① When the oil temperature gauge shows abnormal temperature alarm of chassis hydraulic system, first determine whether the oil temperature gauge is damaged. The simplest way to judge is to feel the approximate temperature near the rear axle box without touching it. If the temperature is not high, it can be judged that it is caused by the damage of the thermometer. Replace the thermometer.

② There is too little or too much hydraulic oil in the rear axle box. When the oil level in the rear axle box is too low, the hydraulic pump will suck air intermittently. Due to the reduced efficiency of the hydraulic pump, the temperature of the chassis hydraulic system will rise. When the oil level in the rear axle box is too high, too much hydraulic oil enters the transmission, causing oil stirring, increasing power loss and increasing the oil temperature of the chassis hydraulic system. The fault can be eliminated by adjusting the hydraulic oil in the rear axle box to the required position.

③ The oil suction pipeline leaks oil. The oil inlet pipeline of the chassis hydraulic pump leaks oil. When the pump is running, it will suck in air, reduce the efficiency of the pump, and increase the temperature of the chassis hydraulic system. When this failure occurs, the hydraulic pump will have suction noise after the bulldozer starts, and foam in the rear axle box can also be observed.

④ The oil filter is blocked, and the filter elements of the primary and secondary oil filters are blocked, so that the oil absorption and oil inlet are not smooth, resulting in the increase of system pressure and temperature.

⑤ The working pressure of the torque converter is abnormal, and the failure of the outlet or inlet pressure valve of the torque converter will cause abnormal pressure. The outlet pressure valve of the torque converter limits the working pressure of the torque converter, and its setting value is 0.29mpa. When the actual pressure value is lower than 0.29mpa, cavitation will be caused and the system oil temperature will rise due to the slow heat dissipation cycle of the working oil; When the outlet pressure is higher than 0.29mpa, the power loss increases and the system oil temperature increases due to the increase of internal leakage of torque converter and the operation of pump wheel and turbine under high pressure. The judgment method of this fault is to detect the outlet pressure of the torque converter. If the pressure value is abnormal, check whether the spring of the outlet pressure valve of the torque converter is broken and whether the valve rod is flexible. The pressure can be increased or reduced by adjusting the adjusting gasket at the spring end. The pressure setting value of the torque converter inlet pressure valve shall be less than 0.7MPa. If the chassis system is heated due to the failure of the valve, the cause, inspection method and pressure adjustment method are the same as those of the torque converter outlet pressure valve.

⑥ The failure of transmission valve is easy to cause transmission heating. The failure of the variable speed valve generally shows that the pressure will be reduced due to the softening of the spring in the variable speed valve, the reduction or increase of the orifice and the increase of the valve stem clearance, resulting in the reduction of the compression force of the variable speed clutch, the slip of the friction plate and the increase of the system oil temperature. The method to judge the fault of the speed change valve is to connect a pressure gauge behind the transmission. If the pressure of the clutch in the direction of high-speed operation of the engine is lower than 2.22mpa and the pressure of the clutch for speed is lower than 2.6MPa, check the speed change valve of the transmission. The inspection items include whether each valve spring is broken, whether each valve rod moves flexibly and whether the assembly position is correct, whether the throttle hole on the valve rod or valve body is blocked, whether the filter screen and throttle hole of the throttle element become larger or blocked, and whether the sealing ring is intact. When the above items are normal, the pressure of the variable speed valve can be adjusted by the following methods: when the pressure loss is less than 0.4MPa, the system pressure can be increased by adding the adjusting pad in the pressure regulating sleeve; When the pressure loss is greater than 0.4MPa, the position of the screw (at the pressure regulating sleeve) on the valve body can be changed to adjust the pressure, and the pressure difference at each position is about 0.4MPa. Through the above methods, the fault of the variable speed valve can be eliminated after the pressure of the variable speed valve is adjusted to the direction control pressure of 2.22mpa and the speed control pressure of 2.6MPa.

⑦ The steering brake valve fails. The steering brake valve controls the maximum pressure of steering and brake clutch (2.245 ~ 2.75mpa respectively). When the pressure controlled by the steering brake valve decreases, the steering clutch does not get the proper compression force, the brake clutch does not get enough separation force, and the slip between the friction plates increases the system oil temperature. In normal bulldozing operation, when the system temperature exceeds the normal value and the bulldozer deflects or walks weakly when bulldozing, check the steering pressure; If the bulldozer does not move and moves slowly or does not move when bulldozing, and the engine shows heavy load, check the braking pressure. The pressure adjustment method of the steering brake valve is to remove the left and right steering and brake spring seats from the front end of the valve, adjust the number of adjusting gaskets inside, increase the gasket to increase the pressure, and reduce the gasket to reduce the pressure. If the valve rod is not flexible, grind the valve rod with metallographic sandpaper.

⑧ The hydraulic pump is damaged. If the hydraulic pump works under the pressure of 3Mpa, it will lose some power, but it will not cause the system to heat up. If the pump is worn or damaged due to other reasons, its efficiency will be greatly reduced, and the reduced power will be converted into heat energy, which will also increase the temperature of the system.

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