The U.S. Navy currently uses a rubber stave and bearing shell configuration for most main, intermediate, and stem tube strut bearing configurations. The rubber stave consists of two parts, the soft wear rubber and a harder non-metallic backing. The main and intermediate strut bearings are lubricated by means of seawater flow, as a result of ship movement through the bearing channels between the staves. The stem tube bearings are lubricated by pressurized seawater from the ship's firemain system.

Beginning in December 1999, increased oversight focused on problems associated with staves that resulted in unscheduled replacements and disruptions to several ships' schedule. A government investigation team, consisting of representatives from various commands, was formed to identify the problem, determine the root cause, and develop corrective action. Bearing staves from various ships of different classes were identified for examination and testing. The results indicated: a lack of bond adhesion between the wear rubber and its backing, leading edge (LE) debonding, delamination, bubbles, dimples, installation damage, cracking or bulging of the stave ends; and damage caused by obstructed water channels. Discussions with various shipyards and other organizations indicated that this was not a new problem but rather an existing problem that was detected during bearing removal. It was remedied simply by replacing the staves with new ones, therefore, no corrective action or concerns were raised.

During the investigation, several problems associated with the waterborne bearing military specification were noted. These problems included: areas of unclear guidance or requirements, allowance for variation within the manufacturing process, and inconsistent sampling size requirements.

In addition to problems noted above, the dry-docking and subsequent maintenance philosophy are beginning to change that will adversely affect the life of a stave and increase the demand for performance. Past practice has been to dry-dock the ship every 5 years; pull the shafting every 10 years, at which time, the staves are replaced. Dry-dockings are beginning to be extended for between 10 to 16 years, which will increase the in-service time for staves. The current practice is switching from a time-based to a condition-based maintenance philosophy. This will probably entail periodic measurement of stave wear to determine replacement. This approach will only be successful with an increase in stave durability.

The results of the investigation team were changes to the waterborne bearing military specification to clarify requirements and guidance, modification of the sampling requirements, as appropriate, to match the manufacturing process, and verification that stave material requirements are being met in order to better survive increased in-service life expectancy. Additional guidance to shipyards and the fleet was issued to provide better instructions for stave and bearing installation and direction for "zero" thrust controllable pitch propeller (CPP) ship operation.

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