Bedplates are the “foundation” of the engine, without the support of which the shaft alignment in particular, and engine structure as a whole, would inevitably be lost.

The original bedplate design followed the practices involved in the steam reciprocating engines, but it soon became apparent that the magnitude of the forces generated inside a diesel were substantially greater than those in a steam engine, and of a different nature, with the consequence that bedplates had to be developed which would accommodate the ‘cyclic’ peak pressures, and at the same time maintain the alignment for these longer engines. As power weight ratios became more competitive, the fabricated bedplate evolved to provide a light but strong and stiff foundation. The fabricated bedplate has subsequently undergone several refinements.

The traditional basic structure of two longitudinal girders tied together with transverse girders has stood the test of time. There have been some modification to accommodate the ever increasing mean effective pressures; a consequence of the ongoing search for greater powers whilst still minimising weight.
The top and bottom plates of the longitudinal girder are substantially thicker than their supporting plates, because they are the ones upon which the longitudinal and transverse alignment of the engine depend. It is therefore necessary for there to be adequate thickness in these plates to allow for a final machining across them once the whole bedplate is assembled, thereby recovering any distortion that may have occurred during the fabrication process. This machining, performed in a huge milling/planning machine, that both spans and travels the length of the bedplate, leaves the bedplate ready to accept, and align on its upper face, the rest of the engine structure. The lower face provides a flat surface for the mating of the supporting chocks.
The chocks are used to compensate for the natural unevenness of the tank tops and in so doing give the bedplate a seating as close as possible to the stiff and flat bed where the engine was originally built and tested. The chocks are carefully hand scraped and fitted at their location and the engine pulled down onto them using ‘holding down bolts’.
The longitudinal girders are tied together by ‘transverse girders’ to make the bedplate. Until recently, these were of two plate construction. 
The search for increasing power and, possibly more importantly, fuel economy has led to the development of the super long stroke engines with stroke bore ratio 2.5:1. Such engines require large throws from the crankshaft. To accommodate these, if the traditional double plate box type longitudinal girders were to be used, the bedplate would need to be much wider. However, a solution has been devised such that the increased throw is accommodated, to a large extent, within the bedplate itself. This has been made possible by making the bedplate much deeper. In addition, the shaftcentre line has been dropped and the horizontal forces generated by the crankshaft and its running gear are thus taken directly into the transverse girder instead of along the line of the interface between the A-frames and the bedplate.

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