Operational Information

The Two Stroke Crosshead Engine

The Camshaft
 

The Camshaft carries the cams which operate the fuel pumps and exhaust valves. Because these operate once every cycle of the engine, the camshaft on a two stroke engine rotates at the same speed as the crankshaft.

It is also very important that the fuel pump and exhaust valve operate at exactly the right time, so the camshaft is driven by the crankshaft. Two methods are used, a geared drive and a chain drive.

 

Chain drives are relatively light, narrow in width and flexible; however they elongate in service due to wear, which will affect the camshaft timing, and they have a limited life - 15 years.

Gear drives should last the life of the engine; However, a gear train  is heavier, and more expensive. If the gear wheels are misaligned, shock loading will result, which can  lead to broken teeth.

The Camshaft is made in sections, each section spanning one or two cylinders. The individual cams are manufactured from steel which are heat treated to give them a very hard surface, but retaining a tough interior. The cams are expanded onto the camshaft in the correct position using either heat or hydraulic means. The couplings for joining the camshaft segments together are also expanded hydraulically, allowing for adjustment and to facilitate the replacement of a cam if necessary.

The camshaft runs in underslung white metal lined bearings, lubricated in most cases by the main engine LO system. Older B&W engines used a separate LO system for the camshaft because of the possibility of contamination by fuel oil leaking past the fuel pumps.

On engines with chain driven camshafts, as the chain elongates, the timing of the fuel pumps and exhaust valves is retarded. When this retardation reaches a certain point, the camshaft must be retimed. This is done by expanding the coupling between camshaft and drive using high pressure oil, and turning the camshaft to the correct position using a large spanner and chain block.

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