1 Failure Phenomenon and Cause Analysis
The ZL04 wheel loader produced by our factory often experiences a failure when the boom is slowly lifted, but the heavy load does not move even when the throttle is fully open. This issue is typically associated with the hydraulic system, which is a single pump series circuit, as shown in Figure 1.
During maintenance, we investigated the root causes of this problem from three main aspects and made necessary improvements to the system.
(1) Inspect the internal leakage of the boom cylinder. A simple way to check this is to lift the boom and observe if it drops freely. If there's a noticeable drop, the cylinder should be removed for inspection, and the seals should be replaced if they are worn.
(2) Examine the control valve. Start by cleaning the safety valve and checking the spool for wear. If the spool is worn, it must be replaced. If there’s no improvement after replacing the safety valve, inspect the valve stem of the control valve. The clearance should generally not exceed 0.06mm. If significant wear is found, the component should be replaced.
(3) Measure the pressure of the hydraulic pump. If the pressure is below normal, adjustments should be made. However, if the pressure still doesn't meet specifications, it indicates that the hydraulic pump is severely worn.
After testing, the pump pressure was only 5–7 MPa, significantly lower than the system's rated pressure. Upon disassembling the pump, we found the sleeve was worn, and the inner wall of the pump housing in the low-pressure area showed severe damage caused by the gears. The side walls were also slightly worn. From these findings, we identified the main reasons for the failure:
a. The hydraulic pump is severely worn. At low speeds, the pump leaks significantly, and although the pressure increases slightly at high speeds, the volumetric efficiency drops due to internal leakage, making it difficult to reach the rated pressure. Prolonged operation increases wear and raises oil temperature, leading to further wear of components, seal aging, loss of sealing ability, and degradation of hydraulic oil, ultimately causing system failure.
b. The selection of hydraulic components was unreasonable. The boom cylinder uses a non-standard 70/40 series, and the seals are also non-standard, increasing manufacturing costs and making replacement inconvenient. Additionally, the small diameter of the boom cylinder forces the system to operate at higher pressures.
c. The hydraulic system design is flawed. As shown in Figure 1, the control valve and full hydraulic steering gear are connected in series with a single pump. The safety valve is set to 16 MPa, and the pump’s rated pressure is also 16 MPa. This means the pump often works under full load or prolonged overload, leading to hydraulic shocks, long-term oil contamination, and increased wear, which can cause the pump casing to rupture.
2 Improvements and Results
(1) Improve the hydraulic system design. After extensive discussions, we introduced an advanced priority valve and a load-sensing full hydraulic steering gear, as shown in Figure 2. The new system can prioritize flow distribution based on steering needs, ensuring sufficient oil supply regardless of load or steering speed. The remaining oil is directed to the working device circuit, reducing unnecessary power loss and improving system efficiency while lowering the operating pressure of the hydraulic pump.
(2) Optimize the design of the boom cylinder and hydraulic pump to reduce system working pressure. Through detailed calculations, we replaced the boom cylinder with a standard 80/4 series and increased the pump displacement from 10ml/r to 14ml/r. The system setting pressure was adjusted to 14MPa, which meets the lifting force and speed requirements of the boom cylinder.
(3) During use, proper operation and maintenance of the loader are essential. Regularly add or replace hydraulic oil to maintain its cleanliness and perform daily inspections and maintenance.
With the improved hydraulic system, the gear pump operates more smoothly without overloading. The oil circuit design is reasonable, system efficiency is enhanced, and the overall performance of the machine is fully utilized, resulting in improved work efficiency. The steering action becomes smooth and reliable, eliminating the “floating†phenomenon at high speeds.
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Zhoushan Golden Pard Machinery Co., Ltd.  , https://www.goldenpardmachines.com