(a) Manufacturing Process for Semi-Built Crankshaft

The semi-built crankshaft is manufactured by joining separate sections. The crank throws, which consist of the crankpin and the two webs, are typically forged as a single piece. The journal pins (the parts that rest in the main bearings) are separate components. The assembly process involves a shrink fit where the journal pins are inserted into the crank webs. This is accomplished by cooling the journal pins, often using liquid nitrogen, which causes them to contract. They are then fitted into the webs. Once the pins return to ambient temperature, they expand, creating a strong, interference-fit joint. The shrinkage allowance for this process is typically 1/500th to 1/600th of the journal pin's diameter. The crank throws themselves are often produced using continuous grain forging to enhance their fatigue resistance. After assembly, the entire crankshaft is tested for structural integrity using methods like ultrasound or radiology.

(b) Composition of Material Used

The material used for crankshaft construction is typically carbon steel with the following composition:

• Carbon: 0.4%
• Manganese: 0.6%
• Silicon: 0.1%
• Sulphur: 0.04%
• Ultimate Tensile Strength (UTS): 500–550 MN/m²

This material offers a tensile strength (UTS) of approximately 500-550 MN/m². However, for optimal performance, the material selection prioritizes high fatigue strength. To mitigate wear and damage, the bearing surfaces of the crankpin and main journals are hardened to around 220 Brinell Hardness Number (BHN). For medium-speed engines, a 3% Nickel-Chromium (Ni-Cr) steel alloy with 1% Copper (Cu) and Molybdenum (Mo) might be used, exhibiting a UTS of approximately 700 MN/m² and a hardness of around 480 BHN.

(c) Stress for the Web of a Crankshaft

The webs of a crankshaft are subjected to a combination of bending stress and torsional stress.

• Bending stress is caused by forces acting perpendicular to the crankshaft axis, such as combustion pressures, inertia forces from the piston and connecting rod, and the static weight of the moving components. This stress is highest near the fillets connecting the webs to the crankpin and journal.
• Torsional stress is caused by the twisting action from the engine's power output being transmitted through the shaft. This stress is highest when the engine is delivering peak torque.