The bedplate is the massive base foundation of a large slow-speed marine diesel engine, forming the engine's main load-bearing and alignment structure.

Construction and features:

• It is a heavy fabricated welded structure (or in some large engines a single cast/grey-iron or a fabricated steel box-girder structure) with two longitudinal side girders (double webs) and cross-members/transverse girders, forming a strong box section. The whole assembly sits on the tank top/engine seating.
• Materials: high-grade cast iron for fabrications or high-tensile (fine-grain) steel plate with a certified grade for welding. Cast iron is used where good damping (vibration absorption), machinability and casting of the bearing housings/entablature feet is required; steel plate fabrication is lighter and stronger for the large low-speed diesel bedplates. Main and cross bearings are seated on the bedplate.
• The bedplate carries the line of bearing housings for the main (crankpin) bearings and crosshead columns (A-frames/entablature), which are bolted on top, and integral seats/chocks/pockets to locate them. It incorporates the main bearings and the crankcase oil return/tank (scavenge-oil tank with oil drain openings and separation), and the engine's seating feet with holding-down bolts into the tank top.
• It is machined flat and level so that all bearings sit on one true plane, and is designed with sufficient stiffness/low deflection that the engine maintains alignment under all loads.

Why selected: cast iron/steel gives high compressive strength, stiffness, damping, machinability and cheapness/castability; the box-section design gives the required rigidity without excessive weight, and it must be able to absorb and transmit the huge vertical and horizontal forces without deflecting or resonating.

The bedplate is the foundation on which the whole engine is aligned: the crankshaft centre line, the crosshead/entablature columns and the main bearings are aligned relative to it, and the holding-down feet set the engine axial and transverse location on the tank top. Any distortion or misalignment of the bedplate would transmit directly into the crankshaft (bending, torsional and whirling stresses), the bearing housings and the entablature, causing overheating, rapid wear and possible crankshaft or bearing failure. Its correct flatness and rigidity are therefore essential.

Loads and stresses it handles:

• The full weight of the engine (frame, running gear, crankshaft, covers) - dead weight carried on the seating.
• The vertical gas/bearing loads and inertia forces transmitted through the crossheads and bearings - cyclical up/down forces.
• Horizontal forces: the reaction of the engine against its own rocking couple and the horizontal inertia of the piston/connecting rod assembly.
• Bending moment of the crankshaft ends and the torque reaction of the engine on the seating.
• The whole engine's foundation reaction and vibration-resonance forces (damping).

The bedplate distributes these loads over the large seating area on the tank top so that the hull is not overloaded locally, and it stays rigid to hold all bearings in line under every crank angle, absorbing vibration and preventing misalignment, keeping the main bearing clearances and the main bearing loads correct throughout operation. The holding-down bolts and seating chocks secure it against transverse movement and the rocking moment.