1 Failure Phenomenon and Cause Analysis
The ZL04 wheel loader produced by our factory frequently experiences a problem where the boom lifts slowly, and when the engine is revved at high throttle with no load, the heavy weight does not move. This issue is related to the hydraulic system, which uses a single pump in a series circuit, as illustrated in Figure 1.
During maintenance and troubleshooting, we investigated the root cause of this failure from three main aspects and implemented improvements to enhance the system’s performance.
(1) Checking for internal leakage in the boom cylinder: One of the simplest ways to identify this is by raising the boom and observing if it drops freely. If it falls significantly, the cylinder should be removed and the seals inspected. If the seals are worn, they need to be replaced.
(2) Inspecting the control valve: First, clean the safety valve and check the spool for wear. If the spool is damaged, it should be replaced. If there is no improvement after replacing the safety valve, the control valve stem should be checked for wear. The clearance limit is typically around 0.06mm. If the wear is severe, the entire component must be replaced.
(3) Measuring the hydraulic pump pressure: If the pressure is low, an adjustment is made. However, if the pressure still does not reach the required level, it indicates that the hydraulic pump is severely worn.
After inspection, the hydraulic pump pressure was found to be only 5–7 MPa, far below the system's rated pressure. Upon disassembling the pump, it was discovered that the sleeve was worn, and the inner wall of the pump casing in the low-pressure area had been heavily scraped by the gears. The side walls were also slightly worn. These findings indicate the primary causes of the boom's failure:
a. Severe wear in the hydraulic pump leads to significant internal leakage, especially at low speeds. Although the pressure may increase 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 damage of hydraulic components, seal aging, and ultimately, system failure.
b. The selection of hydraulic components was not optimal. The boom cylinder used is a non-standard 70/40 series, and the seals are also non-standard, making replacement inconvenient and increasing manufacturing costs. Additionally, the small diameter of the boom cylinder forces the system to operate at higher pressures than necessary.
c. The hydraulic system design was flawed. As shown in Figure 1, the control valve and full hydraulic steering gear are connected in series with a single pump, and the safety valve is set at 16 MPa, matching the rated working pressure of the hydraulic pump. This results in the pump often operating under full load or long-term overload, causing hydraulic shocks, prolonged oil change intervals, contamination, and accelerated pump wear. In some cases, this can even lead to a ruptured pump housing.
2 Improvements and Results
(1) Upgrading 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. This new system prioritizes flow distribution based on steering requirements, ensuring sufficient oil supply regardless of load or steering speed. Any excess flow is then directed to the working device circuit, eliminating unnecessary power loss in the steering circuit and improving overall system efficiency while reducing the working pressure of the hydraulic pump.
(2) Optimizing the boom cylinder and hydraulic pump design to lower the system’s working pressure: Through detailed calculations, we redesigned the boom cylinder to use a standard 80/4 series. We also increased the hydraulic pump displacement from 10 ml/r to 14 ml/r, and set the system pressure to 14 MPa, which meets the lifting force and speed requirements of the boom cylinder.
(3) Ensuring proper usage and maintenance during operation: Regularly adding or replacing hydraulic oil helps maintain its cleanliness, and daily inspections and maintenance are essential to ensure the system runs smoothly.
With the improved hydraulic system, the gear pump operates more smoothly without overloading. The oil circuit design is now more rational, resulting in higher system efficiency and better utilization of the machine’s overall performance, thereby increasing work efficiency. The steering action has become smooth and reliable, preventing issues such as “floating†at high speeds.
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Zhoushan Golden Pard Machinery Co., Ltd.  , https://www.goldenpardmachines.com