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.
Fabrication of Crankshaft
Crankshaft manufacture is a complex and elaborate process and the exact procedure would vary with the type and size of the crankshaft under consideration but a few things would be good to know
- Fully-built Crankshafts are those in which all the various components are shrink-fitted after separate fabrication
- Semi-built Crankshafts are those in which several parts such as crank-throw and pins are case out of a single piece.
- Welded Crankshafts are those in which the crank-shaft is made by welding case web crank pins and half journal units.
- Flanged Coupling Crankshafts are made out in two pieces joined together by flanged couplings
There is a lot more to learn about marine diesel engine crankshafts which we will study in the next article such as taking crankshaft deflections.
References
Image of Forces on Crankshaft - Marine Engineering Knowledge by Brian
Image of Stresses on Crankshaft - Sanyal, D.K. (1998) Principles & Practice of Marine Diesel Engines. Mumbai: Bhandarkar Publications.
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