Fabricating New Thordon Bearings

2014 June 10
by Chris Brown

While performing routine maintenance on a 120 foot Bennetti yacht, we needed to pull the propeller shafts for inspection. Once the shafts were removed, we discovered that the Thordon plastic shaft bearings needed to be replaced.

Worn Thordon bearing with new unfinished material

Worn Thordon bearing with new unfinished material

Thordon material is a hard compound that is an alternative to rubber cutless bearings.  Thordon bearings are common in large commercial vessels (over 1,000 ton) and we see them in our yard on Trinity’s, Bennetti’s and other vessels from 120 foot and larger.

One advantage with Thordon bearings is the elimination of bearing squeal or, more technically, “stick-slip”.  Bearing squeal is sometimes found with rubber bearings on stainless prop shafts when the shafts are turning extremely slowly.  The shaft is not spinning fast enough to get the hydrodynamic lift of water between the rubber and the stainless steel.  The result is a squealing sound.

Thordon bearings are custom machined to the vessel’s exact tolerances. The strut barrel or stern tube is measured by our machinist to 0.001” in many places. The shaft diameter must be measured as well. We then use a bearing calculator to get the exact inside and outside tolerances. Those tolerances are adjusted for an interference (liquid nitrogen) fit or a glue (epoxy) fit. They are also adjusted for the temperature of the machine shop at the time of the machining.

A great deal of care goes into the measurements and calculations. If the bearings are a few thousandths too tight on the shaft it will start an over heat condition that is a chain reaction and causes bearing failure. These bearings cannot be purchased off the shelf and should not come from a machine shop that did not come to the vessel to take careful measurements before machining.

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