Explain briefly how the different moving parts of a main engine are subjected to various stresses.

Name the stresses and justify by stating how they originate.

The main moving parts af the engine are:-

1.crankshaft consisting of several journals, crank webs and crankpins all of which are
rotating members.

2.piston with rings and skirt, piston rod and crosshead which are vertically reciprocating
members; and

3.Connecting rod with bearings which both rotate and reciprocate.

The movemant of the above creates centrifugal and vertical inertia forces causing stress
on the parts.

The other major force causing stress is gas pressure in the cylinder which varies during the

Stress is also caused by thermal loading of parts when they are not uniformly heated. The
piston crown upper surface which is exposed to high combustion temperatures tends to expand more than the inner surface which is cooled by circulating oil or water. The differential thermal expansion force sets up internal stresses in the piston crown.


Crankshaft is subject to turning moment resulting from gas pressure causing shear stresses
on the journal, webs and crankpin. The journal stresses may further increase du e to torsional vibrations especially at or near resonant conditions. The crankshafts also subject to bending moment between main bearing supports due to gas and inertia forces of the reciprocating parts. The bending moment causes tensile and compressive stresses alternately as the shaft rotates.

Centrifugal forces produce tensile stresses on the crank webs, shear stress
pin and journal as well as bending stresses on the journals.


Gas pressure forces cause bending and shear stresses on top of the piston crown and
compressive stress on he wall. Thermal loading causes stresses in the upper part af the
crown. Effects of inertia forces due to reciprocating parts are rather smai”, but thay do
cause some compressive stresses during acceleration and tensile stresses during retardation
on the piston wall.

Piston rod

The piston rod is subject mostly to compressive stresses originating from gas pressure.
und some compressive and tensile stress due to inertia forces.

Connecting Rod

Connecting rod stresses are of complex nature. These consist of compressive and bending
stresses due to gas pressure, inertia and centrifugal forces.