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Medium speed engines. Crankshafts and main bearings

Crankshafts may be under slung to protect the bedplates from firing stresses. The crankshaft is supported underneath the crankcase framework by bearing housings bolted up into the frame. There is therefore no real need for a bedplate, and frequently a simple sheet metal sump is sufficient. This means that the frame provides the required longitudinal and transverse stiffness of the engine and the omission of the bedplate reduces the overall weight of the engine.

There are some models that have bedplates, usually cast structures, similar to the general layout of a slow speed engine. The other major difference is that the crankshaft is normally a solid unit, forged, or even cast on smaller engines. Forged shafts have to be carefully constructed to provide a reasonable grain flow along the length of the shaft. In many cases these shafts are ‘stiff’ enough to span worn down bearings without showing up on the deflection bridge gauge readings. Care should be taken when recording del flections to see that such spanning is checked for.
Main bearing are usually of the shell type; white metal lined (copper lead or aluminium tin) with a flashing of lead indium or lead tin approx. 0,0005 inch thick for running in. This flashing may be slightly thicker for a fuller life. The steel backing shells are held in place and shape by the bore of the housing and will be designed to provide ‘nip’ sufficient interference fit to provide adequate grip on the bearing shell, preventing it from turning in the housing. 
It is important to appreciate that only one such ‘locating’ bearing should be fitted to any one shaft. Otherwise the differing thermal expansion of frame and crankshaft may cause problems.

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