Discussion on the Maintenance and Repair of Hydraulic Systems in Loaders

Abstract: The application of hydraulic technology in loaders is becoming increasingly sophisticated, but customers' requirements for the reliability of the system are also getting higher and higher. In order to ensure that loaders meet the technical indicators and working performance requirements of the hydraulic system, especially for the maintenance and repair of the hydraulic system, a comprehensive analysis of the hydraulic system must be conducted, and the methods for testing hydraulic components and systems must be mastered. Further enhance maintainability and efficiency.

Key words: Maintenance and repair of loader hydraulic system

1. Basic requirements for the hydraulic system

The hydraulic system can adjust pressure, flow rate, working position, etc., to make the components or system meet the specified technical indicators. Maintenance and repair are to ensure that all technical indicators of components or systems meet the requirements for the normal operation of loaders and to eliminate or avoid potential failure points.

The hydraulic system mainly consists of hydraulic components and the hydraulic system. The maintenance and inspection of hydraulic components should be carried out in a laboratory or workshop with complete testing equipment and a good environment to ensure the accuracy and reliability of the test. The overall commissioning of the hydraulic system is usually carried out on the main machine to make full use of the original devices and equipment of the machinery, and at the same time ensure the adaptability of the hydraulic system to the overall main machine.

2. Testing of hydraulic components

2.1 Testing of power components

The main power component of a loader is the hydraulic pump. The inspection of hydraulic pumps mainly focuses on two performance parameters: working pressure and flow rate. The test should be conducted at the nominal rated speed of the hydraulic pump. The output pressure and flow rate of the tested hydraulic pump at the rated speed should be measured specifically through a pressure gauge and a flow meter. The specific testing methods are as follows

Open the safety valve and let the hydraulic pump run no-load at the rated speed for 5 to 10 minutes. During the test, pay attention that the rotation direction of the pump should be consistent with the specified direction.

② Close the safety valve to an appropriate pressure (not exceeding the maximum working pressure of the hydraulic pump), adjust the throttle valve, observe the pressure gauge, and make the hydraulic pump reach the rated pressure.

③ Observe the flow meter and measure the flow rate of the hydraulic pump at the rated pressure.

The hydraulic pumps of loaders mostly adopt CB series gear pumps. During use and testing, there are no adjustable components. By measuring the pressure and flow rate at the rated speed, the specific performance indicators of the hydraulic pump can be understood. When the pressure or flow rate fails to reach the specified indicators, it indicates that the hydraulic pump does not meet its nominal parameters and should be repaired or replaced.

2.2 Control Components

The control components mainly include variable-speed control valves, steering servo valves, steering flow control valves, control valves of the hydraulic system of the working device, as well as relief valves and safety valves of each part.

(1) Relief valves and safety valves should act according to the pressure of the liquid and control the pressure of the liquid. Their main performance parameter is to control the pressure. Except for the steering relief valve of the ZL40 loader which is a pilot-operated relief valve, all other relief valves and safety valves are direct-acting. Each valve is equipped with an adjustment handwheel or an adjustment nut. During the test, the working pressure of the relief valve or safety valve is adjusted by constantly adjusting the control spring pressure through the handwheel or nut. The method is as follows:

Close the speed control valve to allow the test valve to pass through the maximum flow rate.

② Adjust the pressure of the test valve and observe the pressure gauge to reach the specified pressure.

③ Adjust the speed control valve to make the flow through the test valve change from small to large, and observe whether the opening pressure of the test valve is the specified pressure. Otherwise, the adjusting element of the test valve should be repeatedly adjusted.

(2) Directional control valve

The directional control valves used in loaders mainly include variable speed control valves, steering servo valves, steering flow control valves, and working device hydraulic system control valves, which are used to control the direction of the fluid flow in the hydraulic system. The adjustable components in the directional control valve of the ZL40 loader mainly include the pressure regulating valve in the variable speed control valve and the safety valve in the hydraulic control valve of the working device, with control pressures of 1.5 and 15Mpa respectively. Among them, the pressure regulating valve in the variable-speed control valve is direct-acting, and the safety valve in the working device hydraulic control valve is pilot-operated. Both adjust their working pressure through the adjusting nut. The test method is as follows:

① Repeatedly operate the control lever of the test subject's reversing valve to check if it moves smoothly and if there is any jamming.

② Operate the reversing valve of the test subject. Through the movement of the hydraulic cylinder, test the reversing function of the reversing valve and observe whether the movement of the hydraulic cylinder is smooth and without any pause.

③ Place the directional control valve at each passage position and let it pass through the specified flow rate. Test whether the pressure values at its inlet and outlet reach the specified values. Otherwise, the corresponding adjusting elements should be repeatedly adjusted. For faulty directional control valves, if adjustment is ineffective, replacement or repair should be carried out.

2.3 Actuator

The actuating components of a loader mainly refer to hydraulic cylinders. The main functional parameters of a hydraulic cylinder are its action performance and pressure resistance. The debugging method is as follows:

Open the two one-way throttle valves, operate the reversing valve, and test the movement of the tested hydraulic cylinder under no-load conditions to see if there is any obstruction or pause.

② Position the piston rods of the test hydraulic cylinder at both ends of the cylinder respectively, and measure whether the maximum distance that the piston rods can move meets the usage requirements.

③ Completely close the two one-way throttle valves, and position the piston rods of the test hydraulic cylinder at one end of the cylinder respectively. Apply the rated pressure, turn on the switch at the measuring cup, and measure the internal leakage with the measuring cup.

④ Position the piston rods of the test hydraulic cylinder at both ends of the cylinder respectively, apply 1.5 times the rated pressure to each of the two working oil chambers, maintain for several minutes, and observe whether all components are damaged or deformed, and whether there is any oil leakage at each joint surface.

Hydraulic cylinders with problems should be replaced or repaired again.

3. Testing of the hydraulic system

To ensure the overall performance and technical indicators of the loader's hydraulic system, after testing and adjusting the hydraulic components, a comprehensive and meticulous debugging of the entire hydraulic system should also be carried out to achieve the reasonable coordination of each component's operation and guarantee the stable and reliable operation of the hydraulic system.

The overall inspection and commissioning of the hydraulic system include no-load commissioning and load commissioning. Before the specific inspection, a comprehensive check should be conducted to ensure that the installation of the hydraulic system of the loader to be debugged is correct, that the hydraulic pipelines are correct and reliable, that the grade of the hydraulic oil is correct and meets the cleanliness requirements, and that the height of the oil in the oil tank meets the requirements. Only after confirming that there are no errors can the overall debugging be carried out.

3.1 No-load monitoring and debugging

The purpose of no-load debugging is to check whether each hydraulic component is working normally in the system and adjust the parameters of each hydraulic component in the system to the specified technical requirements, so as to ensure the stable and reliable performance of the entire machine.

During debugging, start the hydraulic pump without load first and run it at the rated speed to check if there are any abnormal sounds from the transmission devices of the hydraulic system. At the same time, observe whether the oil pressure and oil temperature readings on the instrument panel are normal. After everything is normal, proceed with the following operations.

After confirming that the hydraulic pump is working properly, operate each control lever in sequence to make each actuator run under no-load conditions respectively. The speed should gradually increase from slow to fast, and the stroke should also be gradually increased until it runs at a low speed throughout the entire process to expel the air present in the system. Then, under no-load conditions, make each actuator operate without load under the normal working procedure. Check the correctness and coordination of each action, the smoothness during the start, stop and speed transition of each action, and whether there are any incorrect actions or "crawling" and impact phenomena. Generally, after no-load operation for 1 to 2 hours, check whether the oil pressure, oil temperature, leakage and other conditions meet the requirements.

Although various parameter adjustments and tests were carried out during the component debugging process, during the no-load debugging process, the pressure values of each voltage regulating component in the system still need to be reasonably adjusted to ensure the normal and stable operation of the entire system. Improper adjustment of the system pressure value not only leads to the loss of hydraulic energy, causing the oil temperature to rise, but also affects the coordination of actions, eventually resulting in operational failures of the machinery. The pressure value should be adjusted in accordance with the technical regulations for use or the actual usage conditions. At the same time, a specific analysis should be conducted in combination with the specific structure, quantity and pipeline conditions of various hydraulic components actually used to determine the pressure adjustment range.

3.2 Load Monitoring and Commissioning

The purpose of load testing is to check whether the hydraulic system can achieve the predetermined working requirements after being subjected to load, such as speed load characteristics, whether leakage is severe, whether the working capacity meets the design or maintenance requirements, and whether the oil temperature of the hydraulic system is within the allowable range, etc.

Load commissioning should be carried out under multiple possible working conditions to test its adaptability to various working conditions. The entire debugging process should be carried out in accordance with the principle of progressing from simple to complex and from single actions to compound actions, and all actions of the working device should be tested. During the testing process, the pressure, flow rate, temperature, noise, leakage and other parameters should be monitored throughout. Any problems found should be recorded in a timely manner. When serious problems occur, the machine should be stopped immediately for inspection and troubleshooting.

During the debugging process, for performance parameters with inaccurate values, the relevant adjustment devices should be adjusted according to their working principles to gradually bring the performance parameters of the entire machine to normal during the debugging process. If the adjustment is ineffective, a check and analysis should be conducted to identify the cause and repair or replace the relevant components.

References

Edited by Lei Tianjue, Manual of Hydraulic Engineering, Machinery Industry Press

CAI Zuguang, Leakage and Prevention of Common Seals for Hydraulic Cylinders, Hydraulics and Pneumatics

Wang Jian, Construction Machinery Institution, Shijiazhuang Railway University