BedplateThe bedplate is foundation on which the crosshead diesel engine is built. It must be rigid enough to support the weight of the rest of the engine and maintain the crankshaft, which sits in the bearing housings in the transverse girders, in alignment. At the same time, it must be flexible enough to hog and sag with the tank top plate to which it is attached and which forms part of the ships structure.

If the bedplate is too rigid, then as the hull flexed, the holding down bolts, which secure the engine to the tank top plate, is likely to break and there will be a danger of cracking of the bedplate.

Basically the bedplate consists of two longitudinal girders running along the length of the engine. Connecting these two longitudinal girders are transverse girders, which are positioned between each crankshaft throw and on either side of the thrust collar. Built into these transverse girders are main bearing pockets for crankshaft to run in.

On smaller engines, the bedplate can be made from cast iron as a single casting. Larger engines have a fabricated bedplate. This means that it is welded together from steel sections, steel castings and plates. The steel is mild carbon steel with carbon percentage less than 0.23%.

Earlier fabricated bedplates had box section longitudinal girders and box section fabricated transverse girders. Problems were encountered with cracking of transverse girders, which increased as engine powers and crankshaft throws got larger.

The modern bedplate is constructed from fabricated longitudinal girders with cast steel transverse sections having bearing pockets and tie bolt holes welded into place. After manufacture, the bedplate is stress relieved, the bearing pockets are line bored and landing surfaces machined.

The bedplate should be inspected regularly for evidence of cracking. This can occur in the welds joining the transverse girders to the longitudinals and under the bearing pockets, where the cracks can be radial or follow the line of the pocket. The cause may be faulty manufacturing process, uneven loading, overloading of engine, loose tie bolts and loose holding bolts.