NON-DESTRUCTIVE TESTS ON A TAILSHAFT SUSPECTED DAMAGED IN A GROUNDING
The tailshaft (propeller shaft) is a critical component; after a grounding, NDT is carried out to ascertain the extent of damage. The specific parts and the NDT methods:
- Visual inspection: A thorough visual examination of the whole shaft, the propeller boss, the coupling, the keyway, and the bearing/ seal areas for obvious damage (bending, scoring, cracks, corrosion, fretting). This is the first step and guides the NDT.
- Dye-penetrant (liquid penetrant) testing: Used to detect surface-breaking cracks on the shaft surface, particularly at the keyway, the coupling, the fillet radii, and the propeller boss. The surface is cleaned, a penetrant is applied, allowed to dwell, then a developer is applied; cracks show as coloured indications. Suitable for detecting surface cracks.
- Magnetic particle testing (MPI): Used on the ferromagnetic shaft to detect surface and near-surface cracks, especially at the keyway, fillets, and the coupling. The shaft is magnetised and iron particles are applied; cracks cause flux leakage and particle build-up. Effective for surface/ near-surface defects.
- Ultrasonic testing (UT): Used to detect internal/ subsurface defects (cracks, inclusions, voids) and to measure the shaft wall thickness/ detect corrosion or wastage. A transducer sends ultrasonic pulses into the shaft and reflections from defects/ the far wall are analysed. Used to check the shaft for internal damage and to measure the shaft diameter/ wall thickness.
- Radiographic testing (RT): Used to detect internal defects (cracks, voids, inclusions) in the shaft, particularly at the coupling, keyway and the propeller boss. X-rays/ gamma rays pass through the shaft and are recorded on film/ detector; defects show as density variations. Used where internal integrity must be confirmed.
- Eddy-current testing: Used to detect surface/ near-surface cracks and corrosion, particularly on the shaft surface and in the keyway.
- Dimensional/ straightness check: The shaft is checked for straightness (bending) by measuring the run-out/ deflection (dial indicator) and the diameter (micrometer) at several positions, to detect bending or ovality from the grounding.
- Hardness testing: A hardness check (e.g. Brinell/ ultrasonic hardness) may be carried out to detect localised work-hardening or damage from the grounding.
The specific NDT applied depends on the part:
- Shaft body: visual, UT (internal), straightness/ dimensional check.
- Keyway/ coupling: dye-penetrant, MPI, UT, RT.
- Propeller boss/ cone: dye-penetrant, MPI, UT.
- Bearing/ seal areas: visual, dimensional check, eddy-current.
The results are compared with the maker's/ Class limits, and the shaft is repaired (machining, re-metalling) or renewed as required, with Class approval.