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Medium speed engines. Connecting rods

Connecting rods in medium speed engines are usually of the ‘marine type’. The bottom end is separate from the palm of the connecting rod, thereby allowing the fitting of compression plates (these in turn control the compression ratio and compression pressure). These connecting rod palm may be of reduced width to allow its withdrawal through the cylinder as the piston is lifted.

The removal of a piston is a problem with medium speed engines or trunk engines (trunk engines being those in which there is no crosshead, the piston being directly connected to the connecting rod), as the connecting rod can not be detached from the piston whilst the piston is in the cylinder. The bottom of the connecting rod is often larger than the bore of the cylinder, mainly because it has to carry the bottom end bearing which runs on a shaft which is as large as the cylinder bore in diameter, or even larger. Thus the bottom end must be constructed cither to be removable or possibly obliquely cut. There are also known as fixed centre connecting rods as it is not possible to fit various thicknesses of compression shim.
Vee type engines provide another problem. If the cylinder centres are transversely in line then the bottom end will also be in line which leads to two bearings wanting to run on the same section of crankshaft. This may be overcome by adopting the fork and blade; method, or using articulated connecting rods. In either case, problems arise when overhaul is required, and spare gear levels are increased as the variety of bearing increases. Where vee engines don’t have their cylinder centres transversely in line, the connecting rods can run side by side on the same throw of a crank, the crank pin having to be extended to accommodate the two bottom ends. This leads to a heavier crankshaft as well as a slightly longer engine, but it simplifies both access and overhaul.
The lubrication of the top end (gudgeon pin) is usually achieved by oil being fed from the main bearing supply along drillings in the crankshaft to the bottom end, and then up through borings in the connecting rod to the top end. Most medium speed engines are of the 4-stroke type which naturally provides an alleviation of the downward load on the gudgeon pin during the induction stroke, which helps lubrication of the top end bearing. In 2-stroke medium speed engines, this reversal of load does not occur and it is therefore prudent to increase the load carrying area of the gudgeon pin and at the same time maximise the lubrication flow to that area. The reversal of thrust in the 4-stroke causes greater cyclic fatigue problems with the bottom end bearing bolts. For safety’s sake, therefore, the running hours given by the manufacturers for their bottom end bolts should be strictly adhered to. Failure of one of these bolts could lead to total engine failure; expensive, undesirable and usually avoidable if bolt tensions, firing pressures and quoted running hours are conformed with.

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