– If there is any hydraulic drift in the work equipment (cylinders), check as follows to determine if the cause is in the cylinder packing or in the control valve.
1. Checking for defective cylinder packing
1) Checking boom and bucket cylinders
1] Set in the same posture as when measuring hydraulic drift, and stop the engine.
2] Operate the boom control lever to RAISE or the bucket control lever to CURL.
– If the lowering speed increases, the packing is defective.
– If there is no change, the control valve is defective.
2) Checking arm cylinder
1] Operate the arm cylinder to move the arm in fully, then stop the engine.
2] Operate the arm control lever to IN.
– If the lowering speed increases, the packing is defective.
– If there is no change, the control valve is defective.
– Set safety lock lever OFF position.
– If it is difficult to judge the condition, fill the bucket with soil or apply the rated load when carrying out the test.
[Reference]
If the cause of the hydraulic drift is in the packing, and the above operation is carried out, the downward movement becomes faster because of the following reasons.
1) If the work equipment is set to the above posture (holding pressure applied to the bottom end), the oil at the bottom end leaks to the head end. However, the volume at the head end is smaller than the volume at the bottom end by the volume of the rod, so the internal pressure at the head end increases because of the oil flowing in from the bottom end.
2) When the internal pressure at the head end increase, the balance is maintained by a certain pressure (this differs according to the amount of leakage) in proportion to it.
3) When the pressure is balanced, the downward movement becomes slower. If the lever is then operated according to the procedure given above, the circuit of the head end is opened to the drain circuit (the bottom end is closed by the check valve), so the oil at the head end flows to the drain circuit and the downward movement becomes faster.