Exam Question

(a) Fatigue in Crankshafts
Fatigue failure occurs when a crankshaft section is subjected to cyclic stress reversals . Over time, the material's properties deteriorate, weakening its ability to withstand tensile stress, leading to crack initiation . With continued operation and stress reversals, this crack progressively grows until complete failure occurs.
Smaller crankshafts , such as those in 4-stroke medium-speed engines, are typically solid forged , with a continuous grain line structure . This enhances fatigue resistance and overall strength.
Larger crankshafts , as used in 2-stroke slow-speed crosshead engines, are often semi-built or fully built . These are fabricated from separate forged crank throws, with main journals shrink-fitted into machined holes in the webs. While the crank throw may have continuous grain flow, the shrink-fit areas are potential weak spots .