Shell Bearings

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Thin shell bearings are used for most bearing applications in the main engine. They consist of a steel backing strip coated with a layer of white metal. Bearings may be bi-metal or tri-metal. typical materials are steel-babbitt, steel-bronze or steel-tin/aluminium ( tin-aluminium has slightly greater load bearing capacity than white metal and maintains its fatigue strength over a greater range of temperatures. The bearing metal thickness is 0.5 to 3mm. An Overlay of 20 -40 micrometersmay be applied to improve conformity . This is generally a ductile coating of lead and tin. In addition new bearings may have a flashlayer of a few microns of tin to prvent oxidation
An intermediate layer may be used between the overlay and main bearing metal to avoid diffusion. This is particularly found where bearing loads are very such as in the lower half of the cross head bearing. The layer is galvanically applied.

Compared with the traditional cast bearing they have a number of advantages.
1. Shells are prefinished thus allowing for quicker and easier replacement.
2. The bearings are made under strict controlled conditions giving consistent high quality products
3. In many cases the top and bottom halves are interchangeable in an emergency.
4. Thin layer of white metal cools quickly giving fine grain structure which has high strength and fatigue resistance.

Shell bearings sometimes have a layer of copper or bronze between the steel and the white metal in order to improve adhesion of the white metal. This layer will also provide safe guard in the event that the white metal being worm away.
For camshafts shell bearings are still used in preference to ball or roller race, the action of the cam followers provides impact loading which can must be supported by the bearing. Ball or roller race would tend to suffer fatigue or brinelling damage. In addition to this replacement is simpler with the plain bearing.

Bearing wear must be checked as this can allow the camshaft to drop thus altering the timing.

WHITE METAL BEARING CORROSION

White metals are tin based, that is they contain a higher proportion of tin then other compounds. A typical composition might be 86% tin, 8.5% Antimony, 5.5% lead. In the presence of an electrolyte corrosion of the tin can occur forming extremely hard, brittle, stannous and stannic oxides (mainly stannic oxide Sn₂0) normally in the presence of moisture. These oxides are usually of a grey to grey black coloured surface layer on the white metal, either in local patches or completely covering the bearing. The hardness of this brittle oxide layer could be as high as twice that of steel and if it became detached, possible due to fatigue failure, serious damage to bearing and journal surfaces could occur. The formation of the oxide layer is accompanied by an upward growth from the white metal, which can considerably reduce clearances and could lead to overheating and seizure etc.

Factors which appear to contribute towards the formation of tin oxides are
1. Boundary lubrication e.g starting conditions
2. Surface discontinuities
3. Concentration of electrolyte e.g. fresh or salt water or other contamination
4. Oil temperature
5. Stresses in the bearing metal

Additives in the lubricating oil can add some degree of protection as can efficient centrifuging.

Stannic Oxide being much harder than the white metal causes two problems:-
1. Prevents absorption of dirt particles.This is normally carried out when abrasive particles are stuck to the surface of the white metal. Local overheating and melting occurs and the particle falls into the white metal
2. The oxide is brittle and can crack with the piece edge projecting out ( causing machining type failure of thrust collars especially.)

Both these result in scoring of the journal, these are normally considered as a low temperature type of failure. In addition the presence of water in the lub oil can cause the oxidation of the metals in the bearing causing the metal to grow. This reduces clearance and can lead to bearing failure.