Excavator Bucket Cylinder Sliding Fault

1. Fault Phenomenon

A brand excavator developed a bucket cylinder sliding fault. Initially subtle, the bucket would slightly descend when the boom was raised. Initially suspected to be cylinder leakage, the issue was not addressed immediately. After over a month of continued operation, the sliding worsened, rendering normal digging impossible.

Technical observations:

• With the boom raised and bucket tilted downward (in reversal state), and the control lever in neutral, the bucket cylinder rod retracted at 20 cm/min, causing immediate bucket descent.

• When the bucket was tilted upward (forward state) under the same conditions, the cylinder rod extended at 20 cm/min, also causing bucket descent.

2. Troubleshooting & Resolution

Initial Hypothesis: Internal cylinder damage causing leakage and sliding.

Initial Checks & Findings:

1. Hydraulic Oil & Filters: The return filter element contained iron particles. The hydraulic oil was dark brown with a reddish tint. Although changed at the recommended 1000-hour interval, the oil showed signs of degradation, potentially accelerated by high system operating temperatures.

2. Cylinder Inspection: The bucket cylinder was disassembled. The piston, piston rod, seals (piston and barrel), and cylinder bore wall were found to be undamaged with no scoring.

3. Seal Replacement Test: After replacing all cylinder seals, testing confirmed the sliding fault persisted, ruling out cylinder leakage as the root cause.

Further Analysis & Final Diagnosis:

1. Hydraulic Circuit Review: Analysis of the bucket cylinder circuit (involving pump, flow control valve, bucket directional control valve, and cylinder) indicated that the flow control valve only affects cylinder speed during combined operations, not neutral position stability.

2. Directional Control Valve Inspection: The bucket spool valve was removed. Significant damage was found:

   • Scoring on the spool surface.

   • Deep grooves worn into the spool.

   • The centering spring was checked but not found broken or stuck.

3. Root Cause: The damage to the directional control valve spool caused internal leakage within the main control valve. This leakage bypassed pressure in neutral, allowing the bucket cylinder to drift (slide) under load.

Conclusion: The bucket cylinder sliding was conclusively attributed to internal leakage within the main control valve's bucket spool assembly, caused by severe scoring and grooving of the spool valve. Replacing the damaged spool valve was the necessary repair.