ROCKING TEST OF A DECK ELECTRO-HYDRAULIC CRANE - PROCEDURE, CORRECT READINGS, INTERPRETATION

Procedure for the rocking test

1. Preparation: With the crane in full working order and hoisted clear of personnel, check the area below is clear and secured; confirm the safe working load (SWL), test load and the maker's procedure. Fit a certified proof load (typically 1.25 x SWL for the structure test).
2. Raise the load: Hoist the test weight a short distance; check the hoist/luff/ slew brakes hold the load and the crane is stable.
3. Carry out rocking: With the load suspended, rotate (slew) the crane to the extreme positions and rock the load by alternately raising and lowering the load and/or luffing the jib, so the jib and slew structure are alternately loaded and unloaded (rocking). This imparts the fatigue/ dynamic loading representative of service.
4. Observe and measure: Watch every structural member, the pivot/heeling pins, the slew bearing, the welds and the hydraulic system for abnormal deflection, noise, movement, cracking or oil leaks. Measure the jib-head deflection (below the load, from a fixed datum / taut wire) before and during the test.
5. Record: Note the deflection readings and any observations on the test sheet; compare against the maker's allowable deflection.
6. Complete: Lower and remove the load, re-inspect the structure (welds and joints) for cracks/deformation, and perform the operational checks (limit switches, load indicator, brakes, pressure relief) before the crane returns to normal SWL service.

How to ensure correct readings

• Use a calibrated/ certified test load and certified slings; check the load certificate.
• Use a properly calibrated measuring device (DTI, taut wire with rule, laser) mounted on a stable datum, not influenced by the crane frame movement.
• Zero the gauge at a defined reference (jib at the test position, no load) and read at the same position under load.
• Take readings in calm/ still conditions to avoid wind-loading error; weigh and confirm the exact applied load.
• Record at consistent points (same slew position, same load height) each time so the results are comparable.
• Repeat readings to confirm repeatability; use more than one observer/ position where possible.
• Ensure the test load is exactly the proof value and the crane is on a stable, level mooring.

Interpretation of readings for planning future overhauls

• If the deflection is within the maker's limit and returns to zero on removal of load (no permanent set): the crane is healthy; the readings establish a baseline for trending.
• If permanent set (residual deflection) or increasing deflection occurs with each rocking: indicates plastic deformation/fatigue, and the crane must be taken out of service.
• A consistently higher deflection than the baseline suggests loss of stiffness (crack, loosening of the slew pins/bearing, weld weakness) - schedule structural inspection/ NDT and plan for repair in the overhaul schedule.
• Increasing/ uneven deflection on one side may indicate bearing wear or a loose pivot, guiding overhaul of the slew/luff bearings.
• Repetitive cracking metal sounds or oil leaks signal the need for hydraulic seal/bearing overhaul.

The test thus provides a condition baseline and a trend against the maker's limits to schedule the crane's structural and mechanical overhauls (pins, bearings, slew ring, hydraulic seals) rather than awaiting a failure.