A ZL50 loader with hydraulic and hydraulic mechanical transmission, with a stable speed, large transmission ratio, high efficiency and continuously variable speed and other characteristics, the application is very wide. The transmission planetary gear-type power shift transmission, shift operating system for the hydraulic. In use sometimes shift shock failure, that is, after the shift loader can not be a smooth start, but there is a brief power transmission interrupt and then suddenly make the whole machine load shock phenomenon. Hydraulic transmission involves the coupling of hydraulic transmission and mechanical transmission, the analysis of the cause of the failure is more difficult. Based on the analysis of the operating principle of the transmission control system, this paper puts forward the methods of analysis and judgment of faults, and resolves many similar faults in practical application. 1, fault analysis ZL50 loader transmission speed operation hydraulic system shown in Figure 1. Variable speed control valve is mainly composed of the main pressure valve, spring accumulator, reversing valve and brake off valve. The role of the main pressure valve is to ensure that the transmission control valve of the appropriate hydraulic pressure (1.1-1.5MPa) on the one hand the pressure oil to the variable speed control valve, on the other hand leading to the torque converter, when the oil pressure is too high Play a security role. The reversing valve is used to control the operation of two brakes and one clutch, so as to change different gears according to the needs of use. Brake off the valve used to brake the transmission off automatically, thereby enhancing the braking effect and reduce power consumption. The key components that ensure a smooth shift of loader are spring accumulator and main pressure valve. Its working principle: the piston at the end of the accumulator is installed in the piston cylinder, the right end (the bottom in the figure) is on the spring, the right end of the size spring is on the boss of the main pressure valve and the housing, respectively. The left end of the piston and the screw plug between the end of the formation of oil chamber A, and through the oil passage and the connecting valve connected to the oil passage. In this section of the oil filled with check valve and throttle. When the gear shift hydraulic pressure into the shift clutch cylinder, so that the hydraulic oil pressure decreases, the accumulator oil chamber A through the check valve to replenish the oil, the brake or clutch quickly combined. At the same time as the oil out of the oil chamber A, under the action of the main pressure control valve (ab), the valve stem moves left to lower the hydraulic pressure of the system. When the main and driven discs are in close contact, the cylinder stops moving and the hydraulic pressure rises , Part of the flow of oil through the orifice to the oil chamber A, the pressure of the oil chamber A gradually increased, pushing the piston to the right, compressing the spring, the valve stem of the main pressure valve to the right, so that the hydraulic system will gradually increase, The main, driven components combined with a smooth, smooth and * shift. One-way valve is the role of timely shift to the brake or clutch cylinder oil, so shift quickly. At the same time after the charge, the main pressure valve stem left to reduce the pressure at the beginning of the shift system. The role of the orifice is to shift the system pressure gradually increased, so that the main brake disc or clutch, driven friction plate gradually compressed, so that the shift gentle without impact. From the above shift when the shift control valve action analysis can be seen to achieve a smooth shift requires spring accumulator and the main pressure valve with the hydraulic pressure in the shift gradually increased. If there is no spring accumulator and the check valve and throttle on the oil passage, but also shift, but the shift process as there is no system of hydraulic first drop, then there must be an impact. In practice, if a shift shock occurs, first check the orifice of the check valve located on the valve body between the ends of the oil chamber A for blockage. Can use compressed air or thin copper wire to clear. In addition, because only the spring accumulator piston and the main pressure valve stem movement in order to achieve system hydraulic pressure changes, it is also necessary to check the piston and stem stuck without stuck phenomenon. According to practical experience, if the oil system is not in accordance with the provisions of time cleaning, excessive fluid impurities, easily lead to the throttle clogging and the piston stuck. This is a common cause of shift shocks. 2, Matters needing attention In order to ensure the smooth performance of the loader can *, in the use of the following matters need attention. 1) The fluid level of the transmission has some influence on the performance of the transmission control valve. Therefore, it is necessary to change the hydraulic oil according to the specified period and clean the oil pan. 2) The main pressure valve in the mechanical factory has been commissioned, be careful not to undermine the balance. If you change parts, you must carefully re-pressure.