The ZL50 loader utilizes a hydraulic and hydro-mechanical transmission system, offering stable gear shifting, a large transmission ratio, high operational efficiency, and the ability for stepless speed variation. It is widely applied in various industrial settings. The transmission mechanism employs a planetary gear-type power shift system, with the shifting process controlled hydraulically. During operation, there are instances where shift shock occurs—this refers to the loader failing to start smoothly after a gear change, resulting in a brief interruption of power followed by a sudden engagement that causes a jarring load impact. The complexity of the hydraulic transmission system, which combines both hydraulic and mechanical components, makes fault diagnosis challenging. Based on an analysis of the transmission control system's working principle, this paper presents a practical method for identifying and resolving similar issues in real-world applications.
1. Failure Analysis
The hydraulic system responsible for gear shifting in 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 correct oil pressure (1.1–1.5 MPa) is maintained within the transmission control valve. It directs pressure oil to both the shift control valve and the torque converter, while also providing protection if the oil pressure becomes too high. The reversing valve controls the operation of two brakes and one clutch, enabling gear changes based on operational requirements. The brake release valve automatically disengages the transmission during braking, enhancing braking performance and reducing energy consumption.
The smoothness of gear shifts depends largely on the spring accumulator and the main pressure valve. Here’s how they work together: the piston at the end of the accumulator is placed inside a cylinder, with the right side resting on a spring. The ends of the large and small springs are positioned against the main pressure valve and the housing boss, respectively. An oil chamber A is formed between the left side of the piston and a screw plug at the end of the cylinder. This chamber communicates with the reversing valve through an oil passage that includes a check valve and an orifice. During a gear shift, hydraulic pressure flows into the clutch cylinder, lowering the pressure in the oil passage. The oil chamber A then replenishes the oil via the check valve, allowing the brake or clutch to engage quickly. At the same time, as oil leaves chamber A, the main pressure valve’s control passage (ab) moves the valve stem to the left, reducing the overall system pressure. Once the friction plates make contact, the cylinder stops moving, and the pressure rises again. Some 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 stem to the right. This gradual increase in pressure allows the main and driven components to engage smoothly, resulting in a seamless and shock-free gear shift.
The check valve plays a crucial role in ensuring timely oil replenishment to the clutch or brake cylinder, facilitating a quick shift. It also helps reduce system pressure at the start of the shift. The orifice, on the other hand, allows for a gradual rise in pressure after shifting, ensuring that the friction plates press together smoothly without causing shocks. Without the spring accumulator, check valve, and orifice, gear shifts can still occur, but they will likely be abrupt due to the absence of the pressure drop and subsequent rise.
In practice, if a shift shock occurs, it’s important to inspect the orifice of the check valve located between the oil chamber A and the reversing valve for blockage. This can often be cleared using compressed air or a fine copper wire. Additionally, since only the piston of the spring accumulator and the valve stem of the main pressure valve affect the system pressure, it’s necessary to check whether these components are stuck. From experience, improper cleaning of the oil circuit over time can lead to excessive impurities, which may cause the orifice to clog or the piston to become stuck—both of which are common causes of shift shocks.
2. Key Points to Consider
To maintain the optimal performance of the ZL50 loader, certain precautions should be taken during operation:
1) The cleanliness of the transmission fluid directly affects the performance of the transmission control valve. Therefore, it is essential to replace the hydraulic oil according to the manufacturer's recommended schedule and clean the oil sump regularly.
2) The main pressure valve is pre-adjusted at the factory, so care must be taken not to disturb its balance. If any part is replaced, it must be re-adjusted carefully to ensure proper function.
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