Answer EKM Question 24

Q24. Sketch and describe the -different types of Crankshafts used in Marine engines. Also describe the process of Induction Hardening performed on crankshafts and give the advantages of this process for the crankshafts.

Answer: Crankshaft manufacturing is a complex and elaborate process and the exact Procedure varies with the type and size of the engine.
Based on the construction method crankshaft can be broadly divided into the following four categories:
1. Fully built crankshaft
2. Semi-built crankshaft
3. Fully welded crankshaft
4. Solid forged crankshaft 

1. Fully built crankshaft
(a) In fully built crankshaft, all parts are manufactured separately by steel casting or forging and then assembled together using shrink fitting.
(b) Very simple construction and design also replacement of damaged parts is very simple. All individual components (i.e., Main shaft, Crank web and crank pin) are made and machined separately for the fitting faces.
(c) For proper shrink fitting, calculation must be done with special care so that the friction between the pin and web is sufficient enough to transmit the torque without stressing the pin and web.
(d) Very few marine engines use fully built crankshaft due to the concern of lack of grain flow and its web, which is designed with two holes and must have the considerable strength to allow two shrunk fits.

2. Semi-built crankshaft

(a) Semi-Built crankshafts have the crank throws (crank-pin and crank web forged together) as a single component and the main shaft general is shrunk fit to the web.
(b) As compared to fully build crankshaft, this has continuous gain flow across the crank pin and web. It gives better fatigue strength with small webs and lighter shaft weight.
(c) Very in large slow speed 2 stroke engine and large 4 stroke medium speed engines due to their reliability.

3. Fully welded crankshaft

(a) Fully welded crankshaft is made up of a series of forgings each comprising of half a main journal, web, crank-pin, second web, and half a main journal.
(b) These forgings were then welded together using a submerged arc welding process to form the crankshaft. After welding the journals were stress relieved and Machined.
(c) Has advantage of continuous grain flow, the webs could be made a thinner (no shrink fit to accommodate), leading to a lighter shorter crankshaft.

4. Solid built crankshaft:

(a) In solid forged crankshaft, the whole crankshaft is forged or cast as on single piece.
(b) Highly suitable for very small engines.
(c) This kind of forging provides continuity of material grain flow which allows the smooth transmission of stress and better fatigue resistance.
(d) Such crankshafts are limited to the smaller engines.because there is a limit to the size of forging equipment and the size of steel bar which can be produced.