1. Failure analysis and elimination of bucket cylinder and left walking motor work failure
A Hitachi excavator, after 8500 hours of operation, appeared a failure of the bucket cylinder and the left walking motor to work weakly, but the slewing motion and right walking were normal, and the remaining motions were slightly slow.
1. Possible cause of failure of bucket cylinder work failure
(1) The pilot oil path of the control bucket is faulty.
(2) The control valve core is stuck or severely worn.
(3) The charge valve of the bucket circuit is stuck.
(4) The bucket cylinder, piston or oil seal is seriously damaged.
(5) The main unloading valve is stuck.
(6) The rear pump or its control system is faulty.
2. Possible causes of the malfunction of the left walking motor
(1) The pilot oil circuit that controls left travel is faulty.
(2) The control valve core is stuck or severely worn.
(3) The traveling motor is faulty.
(4) Serious oil leakage from the center rotary joint.
(5) The main unloading valve is stuck.
(6) The rear pump or its control system is faulty.
It is known from the principle of the hydraulic system of the machine that the bucket cylinder and the left travel motor are separately supplied by the rear pump. Therefore, the bucket cylinder and the left travel motor work weak at the same time, the most likely reason is the main unloading valve Or after the pump and its control system. So the high-pressure oil pipes of the front pump and the rear pump were exchanged with each other. When the machine was tested again, it was found that the bucket cylinder and the left travel motor had been working normally. On the contrary, the swing motor and the right travel motor were weak. This shows that the bucket cylinder and the left travel motor and their control systems are normal. The fault should be on the rear pump or its control system that supplies oil to the bucket cylinder and the left travel motor separately.
3. Check the pump control system and analyze the results according to Figure 1-11
(1) Since the front pump works normally, it is proved that the control pilot pump and TVC valve shared by the front pump and the rear pump work normally.
(2) Install an oil pressure gauge with a measuring range of 6 MPa at the outlet of the NC valve, and measure the oil pressure at this location as p, (When the outlet pressure of the CO valve is not measured, the QO valve adjustment bolt is tightened by 2 to 4 turns. When p is found, the value increases, and when the adjustment bolt is returned to the original position, p; drops to the original value. The test result is in accordance with the operating characteristics of the CO valve, indicating that the CO valve is working properly.
(3) When tightening the NC valve adjustment bolt 2 to 4 turns, p is found; the value rises, and when the adjustment bolt is returned to the original position, p; drops to the original value. The test result is consistent with the operating characteristics of the NC valve, indicating that the NC valve is working properly.
(4) After disassembling and inspecting the servo mechanism, it was learned that the return spring was not broken and had good elasticity, the connecting rod mechanism did not fall off, and the valve core was not stuck or worn. This shows that the servo mechanism works normally.
Knowing from the above inspection results, the control system of the rear pump works normally, and the weak operation of the bucket and the left travel motor can only be caused by the failure of the rear pump itself.
Remove the hydraulic pump assembly. After disassembly inspection, it was found that the hydraulic components of the front pump are intact and the rear pump is damaged more seriously. There are several deep grooves in the oil seal of the valve plate. There is a slight strain on the end surface of the plunger cylinder , The remaining hydraulic components have no obvious wear phenomenon. Obviously, the rear pump does not work properly because the contact surface of the plunger cylinder and the valve plate is severely worn, causing serious leakage of hydraulic oil, which prevents the hydraulic pressure from being established, resulting in the bucket cylinder and the left travel motor working weakly.
4. Troubleshooting
In view of the fact that the end surface of the plunger cylinder is not damaged and the valve plate is seriously damaged, the maintenance program of grinding the plunger cylinder and updating the valve plate is adopted. That is, first use a surface grinder to finely grind the worn end surface of the plunger cylinder, then polish it with chromium oxide, and finally manually grind the plunger cylinder and the valve plate to ensure that the contact area is more than 95%.
