How should a Nippon Sharyo plunger pump that is severely worn be repaired?

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During the maintenance of Nippon Pump Company’s piston pumps, it is common to encounter severe wear of these pumps. So, how should we address this issue? Let’s analyze together the causes and consequences of wear in the Niigata piston pumps. 1 Fault phenomena and cause analysis: The ZL04 type wheel loader produced by our factory often exhibits the fault phenomenon of the boom rising slowly when the throttle is opened fully at idle conditions, and failing to move under heavy load. The machine’s hydraulic system features a single-pump series circuit. During maintenance, we identified the causes of the faults from the following three aspects and improved the system; the details are as follows. (1) Check for internal leakage in the boom cylinder. The simplest way is to raise the boom and see if it drops freely significantly. If the drop is significant, remove the cylinder for inspection; if the seal ring is worn, it should be replaced. (2) Check the control valve. First, clean the safety valve and check whether the valve core is worn; if it is worn, it should be replaced. If there is still no change after installing the safety valve, then check the wear condition of the control valve spool; the allowable gap is generally 0.06 mm, and it should be replaced if the wear is severe. (3) Measure the pressure of the Niigata plunger pump. If the pressure is low, make adjustments; if the pressure still cannot be increased, it indicates that the Niigata piston pump is severely worn. Upon inspection, the pressure of the Niigata plunger pump was found to be only 5–7 MPa, which is significantly lower than the rated pressure of the hydraulic system. After disassembling the Fujioka plunger pump, it was found that the shaft sleeve was worn out, the inner wall of the pump casing in the low-pressure area had been severely damaged by the gears, and the side walls were also slightly worn. It can be seen that the main reason for the boom being unable to lift under load is: a. Severe wear of the Niigata piston pump. Severe leakage occurs inside the pump at low speeds ； At high speeds, the pump pressure increases slightly, but due to wear and internal leakage in the pump, the volumetric efficiency drops significantly, making it difficult to reach the rated pressure. Prolonged operation of the Niigata piston pump exacerbates wear, leading to an increase in oil temperature. This results in wear of hydraulic components and aging or damage of seals, causing them to lose their sealing capacity. The hydraulic oil also deteriorates, ultimately leading to malfunctions. b. Improper selection of hydraulic components. The boom cylinder is of the 70/40 non-standard series, and the seals are also non-standard; this results in high manufacturing costs and difficulties in replacing the seals. A small bore in the boom cylinder will inevitably result in a high set pressure for the system. c. The hydraulic system design is unreasonable. As can be seen from Figure 1, the control valve and the fully hydraulic steering gear are connected in series with a single pump; the setting pressure of the safety valve is 16 MPa, and the rated operating pressure of the Nippon Sharyo piston pump is also 16 MPa. Niigata piston pumps often operate under full load or for extended periods under overload (high pressure), and hydraulic shocks occur in the system. Failing to change the oil over time and contamination of the hydraulic oil exacerbate wear on these pumps, which can lead to the pump casing bursting (such failures have been reported in the past). 2 Improvements and Effects (l) Improve the design of the hydraulic system. After multiple demonstrations, an advanced priority valve and load-sensing fully hydraulic steering gear were ultimately adopted, as shown in Figure 2. The new system is capable of allocating flow preferentially to the steering system according to its requirements; it ensures an adequate supply of fuel regardless of the load level or the speed of rotation of the steering wheel. The remaining fuel can be used entirely by the working device circuits, thereby eliminating power losses caused by an excessive supply of fuel to the steering circuit, improving system efficiency, and reducing the operating pressure of the Niigata piston pump. (2) Optimize the design of the boom cylinder and the Fujikoshi piston pump to reduce the system operating pressure. Through optimized calculations, the boom cylinder uses the standard series 80/4. The displacement of the Niigata plunger pump has been increased from 10 ml/r to 14 ml/r, and the system set pressure is 14 MPa, which meets the requirements for the lifting force and speed of the boom cylinder. (3) During use, it is also necessary to pay attention to the proper operation and maintenance of the loader, regularly add or replace hydraulic oil, maintain the cleanliness of the hydraulic oil, and carry out regular inspections and maintenance. Thanks to the improved hydraulic system design, the gear pump operates more smoothly without any overload occurrences. At the same time, the rational oil circuit design improves system efficiency and fully utilizes the overall performance of the machine, thereby enhancing work efficiency. The steering action is smooth and reliable, preventing the steering from becoming unstable at high speeds.