Reduction gear bearings must support the weight of the gears and their shafts. They must also hold the shafts in place against tremendous mechanical forces. These forces are exerted by the shafts and gears when they are transmitting power to the propeller shaft. (NOTE: It takes a lot of horsepower to move a 5,000-ton ship through the water at 15 knots.) Like other radial bearings in main engine installations, these bearings are of the babbitt-lined split type. However, the bearings are not spherically seated and self-aligning. Instead, they are rigidly mounted into the bearing housings by dowels or by locking screws and washers. The angular direction of the forces acting upon a main reduction gear bearing changes with the amount of propulsion power being transmitted by the gear.

To avoid wiping, reduction gear bearings must be positioned so that the heavy shaft load is not brought against the area where the bearing halves meet (the split). For this reason, most bearings in reduction gears are placed so that the split is at an angle to the horizontal plane.

MAIN THRUST BEARINGS

The main thrust bearing is usually located in the reduction gear casing. It absorbs the axial thrust transmitted through the shaft from the propeller. In fact, in the main thrust bearing for a propeller shaft, the only thing that prevents metal-to-metal contact between the fixed and moving parts of the bearing is a thin, wedge-shaped film of oil! Kingsbury bearings, also called segmental pivoted-shoe thrust bearings (fig. 12-14), are commonly used for main thrust bearings. This bearing consists of pivoted segments or shoes (usually six or eight) against which the thrust collar revolves. The action of the thrust shoes against the thrust collar restrains the ahead or astern axial motion of the shaft to which the thrust collar is secured. These bearings operate on the principle that a wedge-shaped film of oil is more readily formed and maintained than a flat film, and that it can, therefore, carry a heavier load for any given size.

The upper leveling plates upon which the shoes rest and the lower leveling plates equalize the thrust load among the shoes (fig. 12-15). The base ring, which supports the lower leveling plates, holds the plates in place. This ring transmits the thrust on the plates to the ships structure via housing members that are bolted to the foundation. Shoe supports (hardened steel buttons or pivots) located in the shoes separate the shoes and the upper leveling plates. This separation enables the shoe segments to assume the angle required to pivot the shoes against the upper leveling plates. Pins and dowels hold the upper and lower leveling plates in position. This design allows for ample play between the base ring and the plates and ensures freedom of movement (oscillation only) of the leveling plates. The base ring is prevented from turning by its keyed construction, which secures the ring to its housing.