Polymer Alloy Extends Icebreaker's Propeller Shaft Bearing Life
Under severe Arctic operating conditions, a relatively new bearing material has outlasted traditional laminated-phenolic propeller shaft bearings by a ratio of about nine to one. The longer-lasting bearing was made from Thordon XL, an alloy of polymer and synthetic rubber, manufactured by Thomson-Gordon Limited of Burlington, Ontario, Canada.
These findings were verified by the U.S. Coast Guard following two seasons of testing aboard the 9,500- ton Polar Sea, one of the service's largest icebreakers, which operates in the Antarctic and East and. West Coast Arctic waters. These Operations are exceptionally hard on propeller shaft bearings. They required annual replacement because of the pounding they take due to frequent impact with ice. The traditional bearings lacked the impact loading and deflection capabilities required for the job.
On the advice of Lockheed Shipbuilding of Seattle, which had some experience with Thordon bearings, the Coast Guard agreed to a comparison test of the traditional material with the new. In July of 1983, a new laminated phenolic, micarta bearing was installed in the Polar Sea's starboard propeller shaft, while a Thordon XL bearing was fitted in the identical port shaft.
Though both shafts were nomically 36 inches OD, careful measurements were taken of each finished bearing ID and of initial bearing/shaft clearances.
The Polar Sea then took up her customary schedule of duties.
One year later, clearance readings were taken at Todd's Seattle shipyard by Terp Christianson, outside machinist, and witnessed by L.A. Herbert, ship superintendent, and CWO Menge, USCG inspector.
These readings indicated a weardown of 0.026 inch on the port shaft bearing, the one using Thordon XL, and a weardown of 0.234 on the starboard shaft, the laminated phenolic bearing. Wear of the phenolic bearing exceeded wear of the Thordon XL many times, this being validated and documented by Coast Guard records on file at Lockheed Shipbuilding.
The phenolic bearing had to be replaced at this docking, with another phenolic bearing immediately available from stock. A second year of testing was undertaken with the same Thordon XL bearing remaining in the port shaft.
At the beginning of May this year the Polar Sea underwent another drydock bearing inspection at the Todd shipyard. Total weardown of the Thordon XL bearing after two seasons was found to be 0.067 inch, still less than one-third the wear of the phenolic bearing in a single season.
The Polar Sea is scheduled for a major re-fit this month. Depending on Congressional approval and funds being allocated, the icebreaker may receive a full complement of Thordon bearings in all propeller shafts.: Each propeller shaft is fitted with two bearings—one forward and one aft. The forward bearings are 66 inches long, the aft bearings 133.5 inches long. Both are built up from 33-inch-long dovetail staves, freezefitted into slots in the bearing carriers.
The forward bearing requires 64 staves while the aft bearing requires 128, for a full complement of 192 staves per shaft set. Thomson- Gordon supplied Thordon XL molded staves to fit the carrier and supplied computer-generated final dimensions for the convenience of final machining by the shipyard.
The Thordon material combines the known advantages of rubber bearings with increased rigidity, permitting higher pressures to be carried at lower dry and boundary friction levels. The major reason for its success is a low coefficient of friction— 0.188 against machined bronze. It also has a constant, predictable water swell of 1.3 percent of the wall thickness.
Thordon has a low modulus of elasticity, which prevents deformation of the bearing housing under severe operating conditions, and allows the bearing to return to its original configuration once excessive stress is removed. It has the ability to deform but continue to provide a bearing surface, such as when a propeller shaft is bent or otherwise damaged.
Thordon can be run with grease, mineral oil, or water as a lubricant, and it thrives even in polluted or abrasive environments. It is sometimes used to convert from oil-lubricated to water-lubricated bearings, thus eliminating an expensive seal and reducing the possibility of pollution from leaks.
For further information and free literature on Thordon bearing material, C i r c l e 14 on Reader Service Card
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