Sterntube failures on the rise

His colleague Beatriz Garcia manages SLS’ Bearing Centre in Vigo, Spain, where many damaged bearings end up for repair. Speaking to The Motorship, she suggested that excessive speed, causing high temperatures in the bearings, is behind some of the damage that comes into the Bearing Centre, but she also reported another trend: some bearings are being sent for repair that, in the past, might have simply been replaced, perhaps because the laser technology can complete repairs without the risk of deformation from pre-heating.

Among the tools in her centre is a patented laser system that can apply white metal to new or damaged bearing surfaces in a precisely-controlled sequence. It can work on bearings up to 4m external diameter and 2.5m long, weighing up to 10 tonnes and reliably achieves a bond strength between the new bearing surface and the bearing’s steel base of 100MPa. This has enabled the plant to win contracts for bearings to be used in nuclear power stations, for which a bond strength of at least 90MPa is demanded.

Mr Vazquez identified some operational factors that can lead to the metal-to-metal contact that results in bearing damage. Among them is ‘hammering’, which can happen if the propeller is not fully submerged, which can happen if the ship is not fully-loaded and “the operators and crew are under pressure to deliver,” he said. This hammering can destroy the bond that secures the bearing material resulting in cracks that will force the ship into dock for repairs, incurring large costs and losses.

Shaft alignment is also critical – particularly at high speeds – otherwise bearing damage can occur and even the shaft might be damaged beyond repair because it could overheat and crack, putting the ship out of action for months, he suggested.