Trouble shooting of weak lifting and slow left slewing of excavator boom

(1) Fault phenomenon

During the construction of a R942 hydraulic excavator, the following faults occurred: the boom was weak in lifting and the left slewing was slow; When the operation continues for 0.5h, the oil temperature rises, the boom cannot be lifted, and the left slewing can hardly rotate.

(2) Troubleshooting

When other actions are normal and the main pump and the main relief valve are also in good condition, the reason for the boom weakness may be that the clearance between the main control valve and the spool valve and the valve body is too large due to wear, resulting in large internal leakage; Overload valve and oil make-up valve are damaged or the pressure is too low; Damage of boom cylinder seal leads to increased internal leakage; The pilot control circuit has failed.

When the oil temperature is 45~55 °, the above four reasons were checked item by item, and the first three items were found to be normal. When checking the pilot oil circuit, it was found that there was oil leakage under the handle of the pilot valve controlling the lifting boom; When the engine is running at high speed, the measured pilot oil pressure for boom lifting is only 2MPa (the standard value is 3.3MPa), and the main control valve spool cannot be pushed to the position of large stroke Z, resulting in weak boom lifting.

After disassembling the pilot valve, it was found that the O-ring in the plunger sleeve was damaged. Test the machine after replacement, and the boom lifting returns to normal.

After item by item inspection, it was found that the core of the pilot cone valve on the overload valve was seriously worn (there was about 0.10 mm wear mark). After grinding, the installation test showed that both left and right rotations returned to normal.