The split inflatable rubber seal ring is widely used in Navy ships. It is installed aft of the prime seal assembly rig (packing box or mechanical seal). It is used when it is necessary to repair or replace the prime sealing elements when the ship is waterborne.

When the seal is needed, it is inflated with nitrogen and expands against the shaft to form a watertight seal. Nitrogen is used because it is dry and will not deteriorate the seal as rapidly as compressed air, which often contains moisture, oil, and dirt.

The operation and testing of the split inflatable ring seal should be done according to ships instructions and the applicable MRC. Never exceed the maximum pressure designated for inflating the seal. You can find the specified pressure designations in the Naval Ships Technical Manual, chapter 9430 (243). Over-pressurization can rupture the seal.

Strut Bearings

Strut bearings, as well as the stern tube bearings, are equipped with composition bushings that are split longitudinally into two halves. (See fig. 12-21.) The outer surface of the bushing is machined with steps to bear on matching lands

Figure 12-21.-Details of a strut bearing.

in the bore of the strut. One end is bolted to the strut.

Since it is usually not practical to use oil or grease as a lubricant for underwater bearings, some other frictionless material must be used. There are certain materials that become slippery when wet. They include synthetic rubber; lignum vitae, a hard tropical wood with excellent wearing qualities; and laminated phenolic material consisting of layers of cotton fabric impregnated and bonded with phenolic resin. Strips made from any of these materials are fitted inside the bearing. Most Navy installations use rubber composition strips (view C of fig. 12-21).

Figure 12-22.-Controllable pitch propeller components.