The ZL50 loader utilizes a hydraulic and hydro-mechanical transmission system, known for its stable shifting, large gear ratio, high work efficiency, and stepless speed control. It is widely used in various industrial applications. The transmission system employs a planetary gear-type power shift mechanism, with the shifting controlled hydraulically. However, during operation, occasional shift shock failures may occur, where the loader fails to start smoothly after shifting, resulting in a brief interruption of power followed by a sudden engagement that causes a load shock. Due to the integration of hydraulic and mechanical components, diagnosing such issues can be complex. Based on an analysis of the transmission control system’s working principle, this paper introduces a fault diagnosis method that effectively resolves many similar issues encountered in real-world scenarios.
1. Failure Analysis
The hydraulic shifting system of the ZL50 loader's transmission is illustrated in Figure 1. The shift control valve consists of several key components: the main pressure valve, the spring accumulator, the reversing valve, and the brake release valve. The main pressure valve ensures that the control valve receives the correct oil pressure (1.1–1.5 MPa) and also directs pressure oil to the torque converter. It acts as a safety device when the pressure exceeds safe limits. The reversing valve controls the operation of two brakes and one clutch, allowing gear changes according to operational needs. The brake release valve automatically disengages the transmission during braking, improving braking performance and reducing energy consumption.
The smooth shifting process relies heavily on the spring accumulator and the main pressure valve. The accumulator contains a piston that sits within a cylinder, with the right end resting on a spring. The left side of the piston forms an oil chamber A, which communicates with the reversing valve via an oil passage equipped with a check valve and orifice. During shifting, hydraulic pressure flows into the clutch cylinder, causing the pressure in the oil passage to drop. The oil chamber A then refills through the check valve, enabling quick engagement of the brake or clutch. As oil flows out of chamber A, the main pressure valve’s control passage moves the valve stem to the left, lowering the system pressure. Once the friction discs make contact, the cylinder stops, and pressure begins to rise again. Part of the oil flows through the orifice back into chamber A, gradually increasing its pressure and pushing the piston to the right, compressing the spring and moving the main pressure valve’s stem to the right. This gradual increase in pressure allows for smooth engagement of the gears, minimizing shocks during shifting.
The check valve plays a critical role in quickly replenishing oil to the clutch or brake cylinder, ensuring a rapid shift. At the same time, it helps lower the system pressure at the start of the shift. The orifice, on the other hand, ensures a gradual pressure rise after shifting, allowing the friction plates to engage smoothly without impact. Without the spring accumulator, check valve, and orifice, shifting would still be possible but would likely result in noticeable shocks due to the absence of a controlled pressure drop and rise.
In practice, if shift shocks occur, it’s important to inspect the orifice and check valve for blockage. These can often be cleared using compressed air or a fine copper wire. Additionally, since only the piston of the spring accumulator and the stem of the main pressure valve affect the system pressure, it’s essential to check for any sticking or malfunction in these components. From practical experience, failure to clean the hydraulic system according to schedule can lead to excessive contamination, causing orifice blockage and piston sticking—common causes of shifting shocks.
2. Key Points to Note
To maintain the optimal performance of the ZL50 loader, the following points should be considered during use:
1) The cleanliness of the transmission fluid significantly affects the performance of the control valve. Therefore, it is crucial to replace the hydraulic oil on schedule and clean the oil reservoir regularly.
2) The main pressure valve is pre-adjusted at the factory, so care must be taken not to disturb its balance. If any parts are replaced, careful re-adjustment is necessary to ensure proper function.
Shijiazhuang Zhongjia Slurry Pump Co.,Ltd. , https://www.zhongjiapump.com