Fretting Failure In Gearbox


A bad night 

When a live fish carrier’s gearbox failed, it was a bad night with strong winds and choppy seas. Without propulsion, the ship laid broadside upon the water and started to roll violently, making life difficult for the four crew members. The yacht was blowing swiftly ashore by the prevailing wind and was less than two nautical miles (nm) from land. The distressed vessel’s master transmitted a VHF distress signal. Fortunately, another fish carrier who had heard the distress call had been close enough to the drifting ship to be able to attach a towline before it hit the rocky shore. A short while later, a lifeboat and an emergency towing vessel showed up and escorted the two boats to a harbour.


The main input shaft (see figure) along the top half of the gearbox had failed, according to an inspection the next day. The damaged parts were taken apart and sent to the gearbox manufacturer, who determined that fretting (micro-movement) between the gearwheel and shaft was the most likely reason for failure. This movement caused a crack to form in the area and led to failure. A closer look revealed that the failure happened at the point where the shaft changed into the gearwheel flange. The vessel was out of commission during the roughly month-long fabrication and installation of a new shaft.

The Lessons

1. Maintain → The vessel had a history of propeller fouling and minor groundings. The controllable pitch propeller had developed a malfunction, during which the response was erratic and jerky. □ Intermittent problems with propulsion should not be left unattended. Defective propulsion could lead to navigational accidents if vessel control is erratic, especially in shallow and busy waterways. □ Events that stress the propeller can cause significant loading on the driving gear, resulting in relative movement between the gearwheel and the shaft. Failure is almost inevitable when a crack develops.

2. Check → Since its delivery in 1996, the manufacturer had not serviced or inspected the vessel’s gearbox. Routine inspections and regular gearbox oil analysis were not conducted. □ The gearbox is a complex item of machinery. It needs routine inspection and frequent monitoring to ensure that incipient failures are detected early and corrective actions are taken. Regular gearbox oil analysis can detect particles and provide an early indication of excessive or uneven wear on the components.


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Source: MAIB


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