Analysis and Elimination of Shift Impact Fault of ZL50 Loader

The ZL50 loader adopts hydraulic and hydrodynamic mechanical transmission, featuring smooth speed change, large transmission ratio, high working efficiency and stepless speed change, and is widely applied. Its transmission adopts a planetary gear type power shift transmission, and the shift operation system is hydraulic. Sometimes, during operation, shift shock faults occur. That is, after shifting gears, the loader cannot start smoothly but experiences a brief interruption in power transmission and then suddenly engages, causing the entire machine to experience load shock. The hydraulic transmission mode involves the coupling of hydraulic transmission and mechanical transmission, making the analysis of fault causes rather difficult. Based on the analysis of the working principle of the transmission control system, this paper proposes the methods for analyzing and judging faults, and has solved many similar faults in practical applications.

1. Fault analysis

The hydraulic system for the variable speed operation of the ZL50 loader transmission is shown in Figure 1. The variable speed control valve is mainly composed of a main pressure valve, a spring accumulator, a directional control valve and a brake disengagement valve. The function of the main pressure valve is to ensure the appropriate oil pressure (1.1-1.5MPa) of the transmission control valve. The pressure oil flows to the transmission control valve on one hand and to the hydraulic torque converter on the other. When the oil pressure is too high, it can also play a safety protection role. The directional control valve is used to control the operation of two brakes and one clutch, thereby changing different gears as required by usage. The brake disengagement valve is used to automatically disengage the transmission during braking, thereby enhancing braking effect and reducing power consumption.

The key components that ensure smooth gear shifting of a loader are the spring accumulator and the main pressure valve. Its working principle: The piston at the end of the accumulator is installed in the piston cylinder, and the right end (shown at the bottom in the figure) is pressed against the spring. The right ends of the large and small springs are respectively pressed against the main pressure valve and the convex platform of the housing. An oil chamber A is formed between the left end of the piston and the end plug, and it is connected to the oil passage of the directional control valve through the oil passage. A check valve and a throttle hole are installed on this oil passage. When shifting gears, the hydraulic pressure in the oil circuit flows into the cylinder of the shift clutch, thereby reducing the oil pressure in the oil circuit. The oil chamber A of the accumulator is replenished with oil through the check valve, enabling the brake or clutch to engage quickly. At the same time, due to the oil flowing out of oil chamber A, under the action of the main pressure valve controlling the oil passage (A-B), the valve stem moves to the left, causing the oil pressure of the system to drop. When the driving and driven discs are in close contact, the oil cylinder stops moving, and the oil pressure rises. Some of the oil flows into oil chamber a through the throttle hole, and the pressure in oil chamber A gradually increases, pushing the piston to move to the right and compressing the spring. As A result, the valve stem of the main pressure valve moves to the right. In this way, the oil pressure of the system gradually increases, enabling the driving and driven components to combine smoothly and achieving smooth gear shifting.

The function of the check valve is to replenish oil to the cylinder of the shift brake or clutch in a timely manner, enabling rapid gear shifting. At the same time, after refueling, move the valve stem of the main pressure valve to the left to reduce the system pressure at the beginning of the gear shift. The function of the throttle hole is to gradually increase the pressure of the system after gear shifting, thereby gradually pressing the driving and driven friction plates of the shift brake or clutch, making the gear shifting smooth and shock-free.

From the above analysis of the action process of the gear shifting control valve during gear shifting, it can be seen that achieving smooth gear shifting requires the cooperation of the spring accumulator and the main pressure valve, so that the oil pressure gradually rises after gear shifting. Even without a spring accumulator, a check valve on the oil passage and a throttle hole, gear shifting is still possible. However, during the shifting process, due to the absence of the system's oil pressure dropping first and then rising, there is bound to be an impact.

In practical use, if shift shock occurs, it is necessary to first check whether the throttle hole of the check valve on the valve body located between the ends of oil chamber A is blocked. It can be unblocked with compressed air or a thin copper wire. In addition, since only the movement of the piston of the spring accumulator and the valve stem of the main pressure valve can achieve the change of system oil pressure, it is also necessary to check whether the piston and valve stem are stuck. Based on practical experience, if the oil circuit system is not cleaned at the prescribed time and there are too many impurities in the oil, it is easy to cause the blockage of the throttle hole and the jamming of the piston. This is a common cause of shift shock.

2. Precautions

To ensure the good and reliable variable speed performance of the loader, the following matters should be noted during use.

The cleanliness of the transmission fluid has a certain impact on the performance of the transmission control valve. Therefore, the hydraulic oil must be replaced and the oil pan cleaned at the prescribed cycle.

2) The main pressure valve has been debugged at the factory of the machinery. Be careful not to disrupt its balance. If the parts are replaced, the voltage must be carefully readjusted.