Forces on the Crankshaft
Due to its nature of operation, there are several types of forces which come to act upon the crankshaft of engines used in marine propulsion. You will get a better idea about these forces if you take a close look at the image shown below which shows the various twisting and bending forces.
As can be seen from the figure, these forces are due to a variety of factors including but not limited to the weight of the pistons, combustion loads, the axial load from the propeller which is immersed in the sea, compressive loads of webs on journals and so forth.
Most of these forces have alternating patterns which gives rise to fatigue and the materials used for construction need to have substantial Ultimate Tensile Strength. Apart from that the other properties required in the material of a crankshaft are wear resistance, tensile strength, and ductility.
The material for construction also depends on the speed on the engine and slow speed marine diesel engines have crankshafts fabricated out of plain carbon steel with a percentage of carbon lying between 0.2 & 0.4%, while the alloy steels are used for engines having a relatively higher speed.
A stress diagram of a particular crankshaft would also help to show the stresses in a web fillet in a Sulzer RND 10 crankshaft as follows