According to the principle of "from easy to difficult, from the outside to the inside", we should first ask about the use and maintenance, then check whether there is leakage in the oil cylinder, whether there is air in the system, and then measure the pressure of the pilot system, the pressure of the main safety valve, the flow of the variable displacement pump and the pressure of the starting point.
1. Check the quality of hydraulic oil and oil tank
First, ask about the replacement time of hydraulic oil and equipment maintenance, check the permeability of hydraulic oil tank filter, and check whether the vent hole of oil tank cover is blocked. All of these can cause the difficulty of oil absorption of the main pump, and the pressure and flow of the whole hydraulic system will be reduced, resulting in the phenomenon of slow and weak boom lifting.
Through the boom in the process of lifting whether there is a pause stuck phenomenon, accompanied by sound, to judge whether the hydraulic system into the air. There are two main air intake channels in the system. ① if the pump or oil pipe joint in the low-pressure oil circuit is not tightly sealed, the oil tank can be sealed first, and then the air pressure of 0.2-0.4mpa can be applied in the oil tank, so that the air intake point in the low-pressure oil circuit can be easily found, and then it can be replaced or repaired by welding. ② During the process of replacing the hydraulic oil and disassembling the pump and valve, air will be mixed in the oil. The machine can be operated for half an hour with small load. After the air in the oil is exhausted, the work can be carried out with load.
2. Check the hydraulic cylinder for leakage
There are usually two kinds of hydraulic cylinder leakage: external leakage and internal leakage, which can be directly determined by appearance inspection. The inspection method of internal leakage can extend the piston rod of the oil cylinder to the limit position, loosen the oil return pipe joint, pull the control lever to continue to pressurize the oil cylinder to see if there is obvious oil leakage. In case of oil leakage, replace the oil seal on the piston, repair the cylinder or replace the cylinder assembly.
3. Check and adjust the pilot system pressure
The normal pilot pressure is 4MPa. The inspection method is to connect the pressure gauge to the pressure tap at the outlet of the pilot pump, then start the diesel engine to run at the rated speed, and observe the pressure of the pressure gauge. When the data on the gauge is less than 4MPa, screw off the pressure regulating bolt cap of the pilot relief valve, loosen the lock nut of the pressure regulating bolt, use a driver to make small angle adjustment clockwise, and observe whether the indication value of the pressure gauge changes at the same time. If there is any change, the pilot pressure can be adjusted to 4MPa. If there is no change, there may be serious wear of the pilot relief valve spool, poor sealing between the spool and the valve seat, and the spool needs to be replaced. The pilot pump has serious internal leakage and needs to be removed for repair or replacement.
4. Check and adjust the pressure of main safety valve
If the pilot pressure is normal or after adjustment, but the boom lifting is still slow and weak, the main relief valve should be measured. The main safety valve has the function of overload protection for the system. When the system pressure exceeds 32Mpa, the safety valve will open, and part or all of the hydraulic oil will flow back to the oil tank through the safety valve, so that the pressure will not increase. If the pressure setting value is too low, it will lead to insufficient pressure in the system.
First, connect the two pressure gauges to the pressure taps of the two oil outlets of the main pump, then start the engine to run at the rated speed, operate the pilot valve to make the cylinder extend or receive the limit position, and keep the pilot handle stationary. At the same time, observe the pressure of the pressure gauge. Since the two pumps flow together during the boom lifting process, the values of the two pressure gauges are the same. If the value is low, loosen the lock nut of the adjusting bolt of the main safety valve, and screw the adjusting bolt to adjust the pressure value. If the pressure remains unchanged after adjustment, the main safety valve and main working pump shall be disassembled for maintenance.
5. Check and adjust the flow rate and starting point pressure of variable displacement pump
During the inspection, connect the flowmeter in series with the main oil circuit, connect two pressure gauges at the oil outlet of two pumps, then start the engine and increase the speed to the rated speed, operate the pilot valve to make the boom rise and fall, and observe the pressure gauge and flowmeter at the same time.
① Check the maximum flow. When the oil outlet pressure of the variable displacement pump is lower than the starting point pressure, the oil output is the maximum. The starting point pressure of the variable displacement pump is 18Mpa, so when the value of the pressure gauge is lower than 18Mpa, the value of the flowmeter should be the maximum flow value. If it is less than the specified value, check whether the oil level of the hydraulic oil tank is too low or whether the viscosity of the hydraulic oil meets the requirements. If there is no problem, adjust the large flow limit screw. Loosen the lock nut of the large flow limit screw, and adjust it anticlockwise with the hexagon wrench. The maximum flow increases, otherwise, the maximum flow decreases. Tighten the lock nut after adjustment.
② Check that the minimum flow and maximum flow meet the requirements or after adjustment, continue to operate the pilot valve to make the actuator cylinder extend or contract to the limit position, and keep the pilot handle still. At the same time, observe the pressure gauge and flowmeter. At this time, the pressure gauge indicates 30MPa, and the flowmeter should display the minimum flow value. If the value does not meet the requirements, the small flow limit screw should be adjusted in the same way as the large flow limit screw.
③ Check the starting point pressure. If the maximum and minimum flow meet the requirements, according to the principle of variable oil supply, as the pressure increases, the maximum flow will start to decrease under a certain pressure, that is, variable oil supply will start. If the pressure value is lower than 18Mpa, it is necessary to adjust the pressure at the starting point. Loosen the lock nut and adjust it clockwise with an Allen key. The pressure will increase, otherwise it will decrease. Adjust and tighten the lock nut.
If the above parts are repaired and still have faults, the clearance between the stem and the valve hole of the control valve group should be carefully measured. This phenomenon is not common in actual maintenance.