In an effort to find a solution, the USCG hired engineering consulting firm LamaLo Technology to investigate the problem. A project was undertaken to develop a permanent repair to prevent the ‘loss of fit’ between the bearing housings and the stern frame castings.

‘We commissioned the study because we had a problem with maintaining the bronze carriers’ fit, said USCG staff engineer James Happe. ‘Ice-breaking operations and ice impact pounded the carrier shells and when they are loose, this can be a real problem.’

The original arrangement consisted of a bearing housing with dovetail slots in which Thordon XL bearing staves were inserted. The study concluded that the best approach would be to permanently fix the bronze carrier shells into the frame of the stern tube, so they effectively became part of the ship’s structure. Such a change would, however, still have to facilitate bearing removal with the shaft still in place. After investigating various alternative arrangements a shortened, double keyed, segmented, split tube, fully grooved type bearing was recommended.

These bearing types offer a larger surface area that supports the shafting, resulting in better distribution of the ice loads to the bearing housing and bossing. In addition, these bearing designs provide for a much better condition for hydrodynamic lift than the existing dovetail slot design.

‘This would provide the required high ratio of bearing surface to water groove and provide more support for the shaft, said LamaLo’s Bruce Cowper. ‘We also shortened the housing/bearing length in the bossing to ease alignment requirements. But it was Thordon who came up with the double-key bearing design.’

Based on over 20 years experience with the existing Thordon XL staves on the Polar vessels and acceptable performance in other applications in the USCG fleet, XL was chosen as the preferred material for the new bearings. Thordon XL is approved down to 2:1 L/D ratio and comes in a variety of configurations including staves, full form tube and split tube. Thordon also provided the bronze double key sets for this project.

Since no work on the new bearings is expected for 8 to 10 years, annual savings in repair/maintenance costs of about $230,000 per bearing are expected. In addition, there will be a reduced risk of unscheduled dockings due to premature bearing or seal failures.

Thordon estimates that over a 20 year period, potential savings of $13m can realistically be gained by implementing the proposed modifications.

MJ Information No: 22623