Sketch and describe the different types of crankshafts used in marine engines:

1. Solid (one-piece) crankshaft: The crankshaft is forged or cast as a single piece, with the crank throws (webs) and journals integral. Used for smaller engines (medium-speed and some large engines). Advantages: strong, simple, no joints. Disadvantages: difficult to manufacture in very large sizes.
2. Built-up (semi-built) crankshaft: The crankshaft is built up from separate components - the crank throws (each consisting of a pin and two webs) are shrunk onto the main journals (or the webs are shrunk onto the pins). Used for very large slow-speed engines where a one-piece forging is impractical. Advantages: allows very large crankshafts to be made; the components can be heat-treated and inspected separately. Disadvantages: the shrink-fit joints must be reliable; there is a risk of slippage.
3. Welded crankshaft: The crankshaft is fabricated by welding the webs to the journals. Used for some medium-speed engines. Advantages: allows the use of different materials and reduces weight. Disadvantages: the welds must be of high quality and are subject to fatigue.

The most common for large slow-speed marine engines is the semi-built (built-up) crankshaft, where the crank throws are shrunk onto the main journals.

Describe the process of induction hardening performed on crankshafts and give the advantages:

Induction hardening is a surface-hardening process applied to the crankshaft journals and fillets. The process:

1. The crankshaft journal is placed in an induction coil which generates a high-frequency alternating magnetic field.
2. The field induces eddy currents in the surface of the journal, which heat the surface rapidly to the hardening temperature (above the transformation temperature).
3. The heated surface is then quenched (cooled rapidly, e.g. by water spray), transforming the surface to a hard martensitic structure.
4. The journal is then tempered (reheated to a lower temperature) to reduce brittleness and internal stress.

The process hardens only the surface (a case) of the journal, leaving the core tough.

Advantages of induction hardening for crankshafts:

1. High surface hardness and wear resistance of the journals, reducing wear in the bearings.
2. High fatigue strength: the compressive residual stress in the surface and the hard case improve the fatigue resistance of the crankshaft, which is important because the crankshaft is subject to bending and torsional fatigue.
3. The core remains tough, giving the crankshaft strength and resistance to impact.
4. The process is fast, controllable, and can be applied to selected areas (the journals and fillets) without affecting the rest of the shaft.
5. It improves the service life and reliability of the crankshaft.