Several Points of Analysis on Faults in the Hydraulic System of Excavators

Taking the domestic YW-100B hydraulic excavator, which is widely used in engineering construction, as an example, this paper analyzes the phenomena, causes and troubleshooting methods of common faults in its hydraulic system.

1) The two main oil pipes of the walking motor burst. The main reason why the two main oil pipes (A and B) of the walking motor of the hydraulic excavator often burst is that the valve core of the two-speed valve fails to return to its original position in time after changing speed, resulting in the inability to connect the oil inlet and the oil return port, causing the oil pressure to be excessively high instantaneously. At this point, one should:

(1) Appropriately increase the elastic force of the spring in the two-speed valve. After the operating pressure is released, the valve core can smoothly return to its original position under the action of the spring force. However, it should be noted that increasing the elastic force of the spring cannot be simply done by adding a growth rate and a gasket; it must be tested based on the elastic force of the spring.

(2) Grind the dual-speed valve to remove burrs, allowing the valve core to move freely within the valve body without any jamming.

(3) Operations should be standardized. For instance, the correct operation for shifting gears during the movement of an excavator is: no matter what speed is changed, the excavator must first come to a stop, then shift gears and move again.

2) Deviation while walking. When an excavator deviates during movement and it is confirmed that the technical condition of the traveling motor is normal and the tension of the tracks has been adjusted correctly, the following segmented checks can be carried out:

(1) Check the working pressure of the walking hydraulic circuit. First, check the side with a slower walking speed. Starting from the combination valve, remove the two oil pipes on the pressure-limiting valve and block the oil outlet with a plug. Remove the two traveling oil pipes on the other combination valve again, start the engine, pull the traveling control handle of the combination valve with the plug, and observe whether the oil pressure meets the requirements. Then, observe whether there is oil flowing out of the A and B holes of the other combination valve (pressure limiting valve). If the oil pressure is normal and there is no oil flowing out of the A and B holes of the other combination valve, it can be considered that the fault does not lie in the combination valve. If the oil pressure fails to reach the normal value, it indicates a serious internal leakage phenomenon. If oil flows out of the A and B holes of the other combination valve, it indicates that the two combination valves are in series. The only way to make them connect is through the shuttle valve. Check the sealing performance of the shuttle valve or replace it.

(2) If the combination valve is working properly, the central rotary joint can be inspected in sequence. There are many oil channels at the central rotary joint, and they are prone to intercommunication with each other. The inspection method is as follows: Disassemble the A and B oil pipes of the two walking motors, and then block the A and B oil pipes of one of the hydraulic motors. Start the engine, pull the reversing valve of the hydraulic motor that has blocked the oil pipe, and check whether the oil pressure is normal (make sure that the working performance of the two oil replenishment valves is intact). Observe again whether the A and B oil holes of the hydraulic motor that are not blocked are discharging oil. If the oil pressure is abnormal and oil is flowing out from the unblocked A and B oil holes, it is considered that the oil passage seal of the central rotary joint is not good and the oil passages have been interconnected. The sealing parts of the central rotary joint need to be repaired or replaced. The second and fourth oil grooves of the central rotary joint are connected to the A and B oil holes of the left traveling motor, while the third and fifth oil grooves are connected to the A and B oil holes of the right traveling motor. If the second and third oil grooves or the fourth and fifth oil grooves are connected in series, the movement of the excavator will be abnormal.

3) Crawling of the hydraulic cylinder. The main reason for the crawling of the piston rod of the hydraulic cylinder when it extends is that the fit between the sealing parts inside the hydraulic cylinder and the cylinder wall is not good, which causes a large change in resistance due to static friction. This phenomenon usually occurs when a hydraulic cylinder is newly assembled. After a period of use, the crawling phenomenon will naturally ease and eventually disappear. If the crawling phenomenon of the hydraulic cylinder still exists after being used for a period of time, it is necessary to check whether all the seals are intact and correctly assembled.

4) The short oil inlet pipe of the combination valve is cracked. Apart from the working pressure of the combination valve exceeding the specified value, the main reason is that the valve core of the speed-limiting valve does not move flexibly within the valve body, resulting in jamming. This causes the working oil coming from the shuttle valve to fail to push open the speed-limiting valve normally, leading to poor oil return and increased oil return resistance, which in turn causes an instantaneous pressure increase and oil pipe rupture. As long as the speed-limiting valve is polished or ground to make its valve core move flexibly, the fault can be eliminated.

5) The piston rods of the power arm cylinder, bucket rod cylinder and bucket cylinder only extend and do not retract. The return oil pressure rises, the oil pipe joint breaks, and the engine load increases. The above-mentioned faults, apart from the valve core being in the normal position, were mainly caused by the reverse installation of the shuttle valve. From the analysis of its working principle, it can be known that the front supply circuit combination valves, according to their different working conditions, respectively draw their own working pressure oil. Through the shuttle valve, the limit valve pushes open the speed limit valve, opening the main return oil circuit, so that the entire hydraulic system is in a normal working state, that is, each valve group and all actuating components can normally draw in and return oil. When the Angle valve is installed in reverse (the oil inlet of the rear group valve on the shuttle valve is mistakenly connected to the oil outlet of the shuttle valve, while the oil outlet is connected to the oil inlet of the rear group valve), the speed-limiting valve on the main return oil circuit cannot obtain the working pressure oil brought by the shuttle valve to overcome the spring force of the speed-limiting valve, and cannot push the speed-limiting valve core to the return oil position. At this time, the speed-limiting valve core is in the position of closing the main return oil circuit under the action of the spring pressure. The pressure oil output from the hydraulic pump, due to the lack of a circuit, abnormally increased the pressure in each pipeline of the valve group, causing the oil pipe to rupture. If the directional control valve is pulled in this situation, the rodless chamber of the hydraulic cylinder has a large volume and high pressure, and the piston rod can still be pushed with difficulty but cannot be retracted. Due to the blocked main return oil passage, the pressure of the valve group and oil pipe abnormally rose, and the load on the diesel engine increased. As long as the shuttle valve is correctly assembled, the above-mentioned faults can be eliminated.

6) The hydraulic pump cannot achieve converging, and the excavator does not move fast. When the position of the valve for changing the valve is correct and the combined valve of the solenoid valve is not stuck, the main cause of this fault is that the spring of the back pressure valve is broken or has too little elasticity. From the analysis of the structure and principle of the hydraulic system of this excavator, it can be known that a back pressure valve is set at the last checkpoint of the total return oil circuit of the hydraulic system. The return oil system has a return oil pressure of 1.2MPa, and the converging valve utilizes this pressure to push the valve core to achieve converging. So when the back pressure valve spring breaks and the pressure in the return oil system drops, the command system does not have sufficient pressure to push the converging valve or the double-speed valve, thus the converging cannot be achieved and the traveling speed of the excavator cannot be increased. When the back pressure valve spring is replaced and the return oil pressure reaches the normal value, the above-mentioned faults can be eliminated.