ROCKING TEST OF DECK CRANE

What is the rocking test?

The rocking test (also called "rocking" of the slew/derrick or crane jib test) is a structural load test carried out on a deck crane (or derrick) to verify the integrity and safe working load of the crane's jib and the slew/ luffing structure. The crane jib is "rocked" (swung) under load through a controlled arc or the crane is loaded to a proof-load angle, and the deflections/ soundness of the structure are observed. It demonstrates that the crane will safely sustain its working load and that the structural members, welds, pivot and slew components are sound.

Procedure of the rocking test

1. Preparation: Ensure the crane is in good order, the area is clear of personnel, and the deck is clear below the crane. Check the safe working load (SWL), proof load, and the maker's test procedure; confirm the crane is secured/moored so the deck can accept the load.
2. Select the test load: hoist a certified test weight (typically the proof load = 1.25 x SWL, or as the maker/Class require) onto the head block; attach with correct slings.
3. Raise the load a short distance and check the hoist/ lowering brakes and that the crane holds.
4. Carry out the rocking motion: with the load suspended, the crane is slewed/luffed (rotated) through the extreme positions and rocked by applying the lowering/luffing brakes so the jib is alternately loaded and unloaded (rocking action) - this flexes the jib and slew structure, reproducing service fatigue loading.
5. Observe during rocking: watch for abnormal deflection, distortion, cracking sounds, permanent set of the jib, and inspect the pivot, heel pins, structural members and welds after rocking.
6. Measure and record deflections: take deflection readings at the jib head with a taut wire/ DTI before and during the test; compare against the maker's allowable deflection.
7. On completion: Lower and remove the load, re-examine the structure (welds/joints) for any new cracks or deformation, grease/re-secure as appropriate, and record the results on the test sheet.
8. Carry out the load test of the safety/load indicators and perform the electrical/hydraulic functionality checks (limit switches, pressure, etc.) as a final check.

Fault finding procedure (tabulated)

• Excessive deflection / jib sagging: damage/overload/worn pivot - check for permanent set; STOP test, relieve load, inspect.
• Cracking sound or visible crack: material fatigue/fracture - stop, empty the load, inspect with NDT (dye penetrant/Magnaflux).
• Slew does not move freely under load / binds: slew pin bearing worn or tight - stop, inspect bearing.
• Hoist/luffing grabs or slips: brake or overload issues - stop, inspect brakes/clutch.
• Hydraulic leaks/pressure drop on deck (if electro-hydraulic): oil seal / valve fault - stop, isolate, repair.
• No abnormal reading: healthy - record as passed.

Action if deviation out of limit

If the deflection or any observed parameter exceeds the maker's limit, stop the test immediately, relieve/remove the load, and the crane is withdrawn from service (tagged out-of-use, "DO NOT USE"). Investigate the cause - structural deformation, fatigue, or defective components - using NDT (magnetic particle/cracks), and report to the Chief Engineer/ machinery superintendent. The crane must not return to service until the defect is rectified (repair/renewal of the affected member/bearing), and a re-proof or re-test carried out to the maker's/Class requirements. A survey/ approval may be needed before returning to service.