Machine Design and Mechanical Components
Core content
Design follows a chain: load estimate, material selection, stress calculation, failure criterion, factor of safety, geometry. For ductile static loading use the von Mises (distortion energy) criterion; Tresca is more conservative. Fatigue governs rotating machinery: alternating and mean stresses combine through the S–N curve, the endurance limit, and a mean-stress correction such as Goodman (σa/Se + σm/Sut = 1/FS). Apply stress concentration factors to the alternating component. Bearings are rated by basic dynamic load: L10 = (C/P)^p × 10⁶ revolutions, with p = 3 for ball bearings and 10/3 for roller bearings; convert to hours by dividing by 60n. Gears: ratio = N2/N1, torque scales inversely with speed at constant power, and P = 2πnT. Also know fits and tolerances, belt and chain drives, shaft shoulder design, bolted joint preload and spring rate.
Traps: using yield strength instead of the endurance limit in a fatigue calculation; forgetting the revolutions-to-hours conversion; treating L10 as a guaranteed life (it is a 10 % failure probability); confusing pitch-line velocity with rotational speed; and ignoring service factors for shock loading.
Worked example. A ball bearing with C = 30 kN carries P = 6 kN at 1500 rpm. L10 = (30/6)³ = 125 × 10⁶ revolutions, so life = 125 × 10⁶/(60 × 1500) ≈ 1390 hours. Doubling the load to 12 kN cuts life by a factor of eight - the cube law is the most useful single fact in this topic.
Rule of thumb: every doubling of bearing load removes about 87.5 % of the rating life.