China Wheel (Taishan) Co.,LTD , https://www.tcwwheels.com
1 Failure phenomenon and cause analysis
The ZL04 wheel loader produced by our factory often has a failure phenomenon when the boom is slowly lifted and the heavy load does not move when the large throttle is empty. The hydraulic system of the machine is a single pump series circuit, as shown in Figure 1.
We looked for the cause of the failure from the following three points during the repair and improved the system.
(1) Check the internal leakage of the boom cylinder. The easiest way is to raise the boom to see if it has a clear free fall. If the drop is obvious, remove the cylinder and check that the seal should be replaced if it is worn.
(2) Check the control valve. First clean the safety valve and check if the spool is worn. If it is worn, it should be replaced. If there is no change after the safety valve is installed, check the wear of the valve stem of the control valve. The clearance limit is generally 0.06mm. If the wear is serious, it should be replaced.
(3) Measure the pressure of the hydraulic pump. If the pressure is low, the adjustment is made, and the pressure is still not adjusted, indicating that the hydraulic pump is seriously worn.
The pressure of the hydraulic pump is only 5-7 MPa after inspection, which is significantly lower than the rated pressure of the hydraulic system. After disassembling the hydraulic pump, the sleeve was found to be worn. The inner wall of the pump casing in the low-pressure zone was severely “swept†by the gears, and the side walls were slightly worn. It can be seen that the main reasons for the failure of the boom load are:
a. The hydraulic pump is severely worn. The pump leakage is severe at low speed operation; the pump pressure is slightly increased at high speed operation, but the volumetric efficiency is significantly reduced due to wear and internal leakage of the pump, and it is difficult to reach the rated pressure. The long-term operation of the hydraulic pump intensifies the wear and the oil temperature rises, thereby causing the wear of the hydraulic components and the aging and damage of the seals, the loss of the sealing ability, the deterioration of the hydraulic oil, and finally the failure.
b. The selection of hydraulic components is unreasonable. The boom cylinder is 70/40 non-standard series, and the seals are also non-standard parts. The manufacturing cost is high and the seal replacement is inconvenient. The cylinder diameter of the boom cylinder is small, which will inevitably make the system set high pressure.
c. The hydraulic system design is unreasonable. It can be seen from Fig. 1 that the control valve and the full hydraulic steering gear are connected in series with a single pump, the safety valve is set to a pressure of 16 MPa, and the rated working pressure of the hydraulic pump is also 16 MPa. Hydraulic pumps often work under full load or long-time overload (high pressure), and the system has hydraulic shock, long-term oil change, hydraulic oil contamination, and the hydraulic pump wear is aggravated, causing the hydraulic pump casing to burst. Such a fault).
2 Improvements and effects
(l) Improve hydraulic system design. After many arguments, the advanced priority valve and load sensing full hydraulic steering gear are used, as shown in Figure 2. The new system can preferentially distribute the flow according to the steering requirements. The oil supply is sufficient regardless of the load size and the steering wheel speed. The rest can be supplied to the working device circuit, thus eliminating the power caused by excessive oil supply to the steering circuit. Loss increases system efficiency and reduces the working pressure of the hydraulic pump.
(2) Optimize the design of the boom cylinder and hydraulic pump to reduce the system working pressure. Through optimization calculations, the boom cylinders use the standard series 80/4. The displacement of the hydraulic pump is increased from 10ml/r to 14ml/r, and the system setting pressure is 14MPa, which meets the lifting force and speed requirements of the boom cylinder.
(3) During the use, attention should also be paid to the correct use and maintenance of the loader. Regularly add or replace hydraulic oil to maintain the cleanliness of the hydraulic oil and strengthen daily inspection and maintenance.
With the improved hydraulic system design, the gear pump works more smoothly and no overload occurs. At the same time, the oil circuit design is reasonable, the system efficiency is improved, and the performance of the whole machine is fully utilized, and the work efficiency is improved. The steering action is smooth and reliable, avoiding the phenomenon of “floating†at high speed.