Repair and maintenance of the damaged shift mechanism of the loader transmission

The power shift transmission of the loader adopts a shift and speed change mechanism with a hydraulic press multi-plate wet clutch. Each gear is composed of 2 to 5 friction plates, corresponding pressure plates, hydraulic cylinders, etc. When the gearshift lever is placed in a certain position, the pressure oil flows through the control valve into the corresponding hydraulic cylinder, pushing the piston to press the driving plate and the driven plate tightly, thereby achieving the purpose of power transmission. In actual use, it is often not until the machine fails to work properly that maintenance is carried out, thus missing the best maintenance opportunity and shortening the service life of the transmission. This article analyzes the causes of clutch damage in power shift transmissions, introduces a method for detecting the sealing performance of clutch pistons and measures for preventing transmission faults, in order to increase the service life of power shift transmissions.

The causes of clutch damage

When the clutch of the power shift transmission is engaged, the pressure of the hydraulic oil overcomes the spring force to press against the friction plates. As the hydraulic oil pressure rises, the friction plates come into contact with the metal discs and gradually tighten, enabling the clutch to smoothly transmit the flywheel power to the input shaft of the transmission. Each time a gear is shifted, the friction plates come into contact with the steel plates, and the heat generated is dissipated by the coolant. When the friction plates and steel plates become thinner, more hydraulic oil is needed to ensure their full engagement. The engine has to accelerate further, and the slippage time of the friction plates on the steel plates also increases. As a result, more frictional heat is generated, causing the temperature of the hydraulic oil to rise. When the temperature reaches a level sufficient to change the performance of the seals in the transmission, the transmission will experience internal leakage. The internal leakage further causes an increase in heat from two aspects: high-pressure oil leaks through the damaged seal, causing the oil temperature to continue to rise; Due to oil leakage, the hydraulic pump has to supply more oil to ensure the pressure required when engaging the clutch, allowing the clutch to engage fully. This also demands that the engine continue to accelerate to generate a greater flow rate for the hydraulic pump. Such a vicious cycle will eventually lead to the clutch overheating or burning out, until it completely fails.

2. Detection of the sealing performance of the clutch piston

When the loader is weak in movement or the oil temperature is too high, it is necessary to determine whether it is caused by poor sealing of the clutch piston or insufficient oil pressure. However, conventional detection methods sometimes find it difficult to determine the cause of the fault, especially when there is no power, it is even more difficult to check the sealing rings of the assembled transmission assembly. At this point, a pneumatic valve can be used for testing.

First, remove the transmission control valve (for assembled transmissions, it should be done before installing the control valve), and then successively introduce 0.6 to 0.8MPa of gas into each oil hole of the transmission. If a "bang" sound is heard when gas is introduced into a certain hole without any or very little leakage sound, it indicates that the seal between the corresponding clutch piston and the cylinder barrel is good. If the sound of the piston pressing against the friction plate is not heard and the air leakage is relatively loud, it indicates that the seal between the clutch piston and the cylinder barrel has failed, and the seal should be replaced.

3. Prevention of Transmission faults

The bearings and gear pairs of the power shift transmission are vulnerable parts, so the transmission should be inspected and maintained regularly. The typical maintenance interval for transmissions abroad is 5,000 to 10,000 hours. According to China's national conditions, it is recommended that the wear of the clutch be checked every 2,500 to 5,000 hours, and any abnormal signals be detected at any time. Since the power shift transmission is unlikely to fail quickly, it can usually continue to be used for 750 to 1000 hours after the first wear marks appear on the friction plates. If regular inspections and maintenance are carried out during this period, and the friction plates, seals and individual bearings are replaced in a timely manner, the damage to undamaged parts (such as steel plates, gears, shafts, hydraulic cylinders and most bearings, etc.) can be avoided.

Before a clutch fails, the following abnormal conditions often occur:

(1) Although the machine is still working, the oil in the transmission has deteriorated, with a decrease in viscosity and containing metal impurities. Therefore, the oil sample of the transmission should be analyzed more frequently before the recommended oil change interval. Because clutch wear is a sign of the expansion of transmission faults, if metal impurities enter the gear pair or jam the gears and prevent them from centering, if the machine does not stop, it will crush other components in the transmission.

(2) A decline in shifting performance is a sign of clutch damage. When the clutch starts to slip, even if the machine accelerates smoothly, it will still rush forward. At this point, the machine should be stopped for maintenance.

(3) When the clutch control oil level drops and the machine has to accelerate to engage the clutch, it indicates that the clutch has suffered excessive wear or the seal has failed. Therefore, measuring the oil pressure of the transmission and observing whether the oil temperature rises can determine the wear of the clutch. At the same time, the quality of the oil should also be analyzed. If iron or chromium particles are found in the oil, it indicates that the gear pair has begun to wear.

To sum up, if the seals and friction plates are replaced before the clutch fails, the maintenance cost will be 30% lower than that after the clutch fails. Similarly, if the planetary gear or gear pair is repaired before it fails, the cost will be two-thirds lower than that of repairing a completely damaged transmission.