Class 4 (4th Engineer) MEKM 📅 Dec 2022

Exam Question

(a) With reference to crankshafts and piston crowns outline the main stresses to which these parts are subjected. (4)

(b) In each case explain how the stresses are generated and how they vary in magnitude and direction during one cycle. (6)

(c) With respect to the whole engine describe how stresses may be accommodated by good design and influenced by poor maintenance. (6)

Reference Answer

### (a) Main Stresses on Crankshafts and Piston Crowns
**1. Crankshaft Stresses:**
* **Bending Stresses (Alternating Tensile & Compressive):** Arise from the gas pressure and inertia forces of the reciprocating parts acting on the crankpin, which is supported by the main bearings. The crankshaft effectively acts as a beam, causing it to bend. As it rotates, any given point on the journal surface alternates between tension and compression.
* **Shear Stresses:** Generated by the transmission of torque (turning moment) from the crankpin through the webs to the main journals. Centrifugal forces also contribute to shear stress on the crankpin and journals.

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