Boom Drift Plus Slow Functions on a 22-Tonne Excavator
A 22-tonne excavator with 8,700 h has three complaints from the same operator: the boom drifts down about 150 mm in 10 minutes with the engine off and the oil at 55 °C; the arm and both travel functions are slow; and the hydraulic oil runs about 15 °C hotter than on the identical sister machine. On a calm day the drift is measured with the bucket empty and the arm fully retracted. With the boom supported, the two boom cylinder lines are capped at the cylinder ports: over 10 minutes there is no further drift. Pilot pressure at the arm spool measures 28 bar at 2,000 rpm against a 39 ± 2 bar specification. Main relief pressure at boom-up stall measures 300 bar against a 343 bar specification. Boom-up cycle time is 4.6 s against 3.5 s. The machine has a quick coupler, and the operator mentions he has been using the boom to sweep the stockpile face. Hoses and fittings show no external leaks.
1. Explain what each of the three measurements proves and what it rules out. State the most likely root cause of the drift and of the slow functions, and whether they share a cause.
Read the worked answer
- Capped-cylinder drift test: the boom cylinder is not the leaker. With the lines capped at the cylinder ports, the only path for oil to leave the bore is past the piston seal to the rod side. The boom held for 10 minutes, so the piston seal and bore are serviceable. Do not quote a cylinder reseal on this evidence - the leakage is downstream, past the boom spool in the control valve back to tank.
- Main relief 300 bar vs 343 bar is either the relief valve weeping or out of adjustment, or the pump not making full flow at pressure. It explains reduced force and slow boom-up, but 43 bar alone does not explain the heat. The distinction matters: if you simply wind the relief up you mask a worn pump and overload the boom cylinder, hoses and structure.
- Pilot pressure 28 bar vs 39 ± 2 bar is the likely cause of the slow arm and travel. Low pilot pressure means spools are not fully shifted, so main oil is throttled across a partly open land - a pressure drop converted to heat, which fits the 15 °C higher oil temperature and the 4.6 s boom cycle. Verify by measuring pilot pressure at the pilot pump outlet and at each spool: a drop between them points to a leaking pilot hose or swivel; a low reading at the pump points to the pilot relief valve, a clogged pilot filter or a worn pilot pump.
Two separate faults, not one. Drift is control-valve spool leakage; slow functions and heat are low pilot pressure; the main relief is a third, possibly related, finding. The operator's habit of sweeping with the boom is a plausible contributor to spool and bore wear, since it side-loads the cylinder and shocks the valve.
2. The operator wants to keep running it because 'it only drifts when it's parked'. Give your risk assessment and recommendation.
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Recommendation: take it out of production, and do not work under the boom.
- The drift is a suspended-load hazard, not a convenience issue. 150 mm in 10 minutes is a slow but continuous descent of a loaded boom. Anyone rigging, greasing boom pins or changing a bucket under a raised boom is relying on the cylinder holding. Typical acceptance is a few tens of millimetres per 10 minutes, and this machine is well past it. Any intervention under the boom requires a mechanical restraint or the bucket on the ground, regardless of the drift number.
- The quick coupler raises the stakes. A descending boom with an attachment that has its own release mechanism is a two-failure scenario. Confirm the secondary lock functions and the attachment is grounded before any work near it.
- Low pilot pressure degrades controllability. A spool that does not fully shift gives the operator less proportional control - a real risk in a stockpile with people on foot. Combined with 15 °C elevated oil temperature the machine is heading for a hose or seal failure.
- The heat is the cost argument. Oil running 15 °C hot shortens seal and hose life, so 'keep running it' is buying a bigger repair.
If it must run while parts are on order: restrict it to a defined low-duty task with a spotter, no personnel under the boom, an agreed shutdown temperature, and a written record that the customer accepted the risk. That is a commercial decision, not a technical one, and should be documented as such.
3. Lay out the repair and verification plan, including whether to simply raise the main relief setting.
Read the worked answer
Repair plan, in the order that lets you verify each step.
- Baseline before disassembly. Record pilot pressure at the pump and at each spool, main relief at stall, pump flow at relief versus no-load flow (a large drop means a worn pump, not the relief valve), and pump case-drain flow. A flow meter here saves a needless pump replacement.
- Fix the pilot circuit first. Replace the pilot filter, inspect the pilot relief valve, and pressure-check hoses and the swivel. Re-test - this is cheap and may recover most of the slow-function complaint on its own.
- Main relief. Bench-test or substitute the relief valve. If pressure reaches 343 bar with good flow, the pump is fine. Do not simply wind the relief up to compensate: that raises system pressure above what the boom cylinder, hoses and structure were designed for and turns a wear problem into a failure.
- Control valve. Remove the boom section, inspect spool and bore for scoring and wear, reseal, and check the load-hold check valve. A bore worn past limits means replacing the section, not polishing it.
- Verification: warm to 55 °C, set pilot pressure to spec, set main relief to 343 bar, confirm boom-up cycle near 3.5 s, and repeat the drift test with a loaded bucket against the manufacturer's limit - not just 'better than before'. Confirm oil temperature stabilizes under load.
- Operator and PM: coach against using the boom to sweep, and add pilot pressure and drift checks to the PM sheet.