The bedplate acts as the main strength member, maintains correct alignment and supports the weight of the components. it must be capable of withstanding the fluctuating forces created during operation and transmit them to the ships structure. In addition it may also collect lubricating oil. In slow speed engine design, it consists of a deep longitudinal box section with stiffening in the form of members and webs.

Transverse members are fitted between each throw of the crankshaft. These support the main bearing saddles and Tie -rod connection. They are attached to the structure by substantial butt welds.

To reduce the engine height the sump of the bedplate may be sunken allowing it to fitted into a recess in the ships structure.

Plate and weld preparation is required with welds of the double butt type if possible. Regular internal inspection of the parts especially the transverse girder is required for fatigue cracking. Tie bolts should be checked for tighteness.

Box girders-A box girder is stronger and more rigid then I or H section girder of the same c.s.a.

From the simple beam bending equation we have;

M /I = s /y = E/R

M=Bending moment
I=2nd moment of area of the cross section
s =Stress
y=distance from the axis of bending to the outer face
E= modulus of elasticity
R-radius of curvature of the bending.

This can be arranged into

s = (M/I) . y

It can be seen that for the same bending moment on a symmetrical shape of same size, the stress is reduced on the increasing 2nd moment of area. The second moment of area increase with moving of material away from the axis of bending towards the extremes of the section.

Because of this the commonest way of construction a fabricated bedplate is by creating two box section girders and tie them using transverse girders.

Welded bedplate

The advent of the small bore slow speed has seen the use of single side bedplates. A box section is then created by using a box section crankcase structure rather than the more traditional A-frame.This has the advantages of reducing width as well as weight and increasing the amount of fabrication so reducing assembly times.

Modern welded bedplate

Due to the weight penalty, the use of cast iron is generally limited to smaller units where fabrication becomes impractical. However, cast iron has internal resilience allowing it to dampen down vibrations, this has led to its usage on some medium speed installations, especially passenger carriers, where noise and vibration suppression is important. .

The most highly loaded pat of a bedplate is the transverse girder. Classification societies require that residual stress is removed after construction.

The transverse girder acts as a simple beam with the forces of combustion acting on the piston passing down through the bearing. The forces acting on the head are passed through the Tie rods.

Forces acting on transverse girder

It can be seen that to reduce the bending moment the tie rods have to be brought closer to the crankshaft. The limit to this is the securing arrangement required for the main bearing keep. One method is to use two instead of one bolts which can be made of smaller diameter. Sulzer use an alternative and very successful method in the form of jacking bolts. These jack against the bottom of the A-frame.

Sulzer amin bearing arrangements .