Downloaded from https://www.everyspec.com FM 1-506 Dimensional interchangeability does not necessarily indicate functional interchangeability. Therefore, some bearings may be suited for thrust or axial loads while others are not. Bearing design (size and number of balls, depth and type of groove, width and thickness of races, construction of the separators) will determine the load and speed for which the bearing can be used. Bearings made to take both radial and thrust loads will be in- creased on one side of the outer race and will usually be stamped “thrust.” placed in a split-type bearing housing produce a unit which is both efficient and easily maintained.
Bearing inserts can be mass-produced to very close tolerances and require little or no fitting during assembly. The bearing is renewed by merely taking the housing apart, replacing the inserts, and bolting it up again. Inserts on aviation engines are often of the tri-metal variety. The steel shell will have a layer of silver on its inner surface and a layer of babbitt on top of the silver. This bearing is durable and provides excellent heat trans- fer. It is typically used for crankshaft and rod end bear- Plain Bearings ings (Figure1-3). Plain bearings are classified as split type or solid type. Split Type. The lower end of an engine connecting rod is a good example of a split bearing.
It may be installed and used on a shaft in locations that preclude use of other bearing types. Another example of split bearings is the main bearing mounts supporting the en- gine crankshaft. Solid Type. An engine connecting rod also fur- nishes a good example of a solid friction bearing the piston pin bearing or bushing. Bearings of this type can be used only where it is possible to slip them over the end of the shaft on which they run. Bearing inserts must always be secured in the hous- ing they must never turn with the shaft. Split-type inserts will have a tang on each half which will fit into a notch in the housing. Solid-type inserts (bushings) are pressed into the housing.
Either method will preclude the chance of turning in the housing. Inserts that must absorb thrust or axial loads are designed with flanges along the sides to take this side load. Bearings without this side flange must never be subjected to thrust or axial loads. Construction Features. Refer to Figure 1-2. A basic requirement for the plain bearing is that it and the shaft be made of dissimilar metals. A steel bearing surface could never be used with a steel shaft. The bearing metal is always softer than the steel shaft, yet it must be hard enough to provide adequate heat transfer and possess good wear qualities. The metal used in bearing is known as babbitt.
Teflon-Lined Bearings A new type of bearing finding increasing use is the Teflon-lined bearing. It offers high reliability and easy maintenance. It is self-lubricating, chemically inert, and shock-resistant; and it has a low coefficient of friction. It is designed as a spherical or as a journal bearing and also as a rod end bearing. Refer to Figures 1-4 and 1-5. A typical friction bearing contains babbitt metal inserts in the bearing housing. A bearing insert consists of a steel shell on which a layer of babbitt has been applied The shell is actually two half shells that when 1-9