Boom drift on a 22-tonne excavator
A 22-tonne hydraulic excavator (8,600 h) is on a residential groundworks site. The operator reports that with the engine stopped, oil at 55 °C and the boom raised about 1.2 m, the boom settles 35–45 mm in five minutes. When the oil is cold the drift is less than 10 mm. Pilot pressure is 3.8 MPa at the accumulator. The boom cylinder rod seal was replaced 300 h ago after a scoring repair. The holding valve was last tested 2,000 h ago. There is no visible external leak, the boom will hold a 1.5 t bucket load at idle, and the machine is needed for a concrete pour tomorrow. The site supervisor wants the fitter to ‘just change the holding valve’ because it is quicker than removing the cylinder. The fitter has a 25 L/min flow meter, caps and a calibrated pressure gauge. Describe how to separate cylinder internal bypass from holding-valve leakage and what decision to take.
1. Outline a safe test sequence to distinguish cylinder internal bypass from holding-valve leakage, including the readings you would accept or reject.
Read the worked answer
Support the boom mechanically, not on the hydraulic holding valve. Warm the oil to 50–55 °C, relieve pressure, and disconnect both cylinder hoses at the cylinder. Cap the cylinder ports and plug the hoses. Lower the support so the boom weight loads the cylinder, then measure rod creep for 10 minutes with a dial gauge. If the rod continues to creep with both ports capped, oil is crossing the piston seal - internal bypass. If creep stops, the cylinder is sealed and the holding valve is the leak path. A second check is a flow-meter test on the rod-end line: with the boom held against a stop, leakage above about 0.2 L/min at 35 MPa and 55 °C is rejectable for this size of cylinder. Repeat cold and hot; a leak that grows markedly when hot points to piston/seal clearance, not the holding valve. Record pressures and temperatures, and do not use the holding valve as a support.
2. The isolation test shows 0.7 L/min from the rod-end port when the boom is loaded and hot; the holding valve bench-leaks 0.05 L/min at 35 MPa. What do you conclude, and what would you release to site?
Read the worked answer
The cylinder is the dominant fault. A hot leakage of 0.7 L/min is well above the 0.2 L/min acceptance limit and explains why drift worsens with warm, thin oil. The holding valve leak of 0.05 L/min is minor by comparison and would not produce the reported 35–45 mm in five minutes on its own. Replacing the holding valve alone would leave a 0.7 L/min internal bypass and the boom would still settle. Release the machine only after the cylinder is removed, the barrel and rod are checked for scoring, a seal kit is fitted and the reassembled cylinder is pressure-tested and drift-tested hot. Do not release it for lifting or for work over people. If the pour cannot wait, hire a replacement machine; a temporary external check valve is not a substitute for repairing the cylinder because it does not restore controlled lowering.
3. The pour is in 14 hours. What temporary measures and communications are appropriate?
Read the worked answer
Tell the supervisor the holding-valve-only repair is not technically justified and would not fix the measured bypass. With 14 hours to the pour, the practical options are: repair the cylinder now if a seal kit and rod inspection can be completed and tested, or bring in a replacement excavator. If repair is attempted, pressure-test the reassembled cylinder and repeat the hot drift test before release. Restrict the machine from lifting duties until then and barricade the boom area. Record the decision, the test readings and the supervisor’s instruction. Do not allow the machine to work over the pour area with a known 0.7 L/min bypass, because a sudden drift under a suspended skip can injure people and damage formwork.