ME or MEng Master of Engineering Master of Engineering Machine Design 3 — Questions and Answers
Question 1: A ball bearing has a basic dynamic load rating C = 25 kN and carries a radial load of 5 kN. The expected life at 90% reliability in millions of revolutions is:
- 125 (Correct answer)
- 3125
- 25
- 625
Correct answer: 125
L10 = (C/P)³ = (25/5)³ = 5³ = 125 million revolutions for ball bearings (exponent = 3).
Question 2: In the design of a power screw, self-locking occurs when the lead angle λ satisfies:
- λ < φ (friction angle) (Correct answer)
- λ > φ
- λ = 45°
- λ > 30°
Correct answer: λ < φ (friction angle)
A power screw is self-locking when the lead angle is less than the friction angle (λ < φ = arctan μ).
Question 3: Which type of gear provides the highest power density and is most efficient for parallel shaft applications?
- Worm gear
- Bevel gear
- Helical gear
- Spur gear (Correct answer)
Correct answer: Spur gear
Spur gears have the highest efficiency (~99%) for parallel shafts due to pure rolling contact and no axial thrust component.
Question 4: The endurance limit of a steel part is reduced by surface finish, size, and reliability factors. If Se' = 300 MPa and ka=0.8, kb=0.9, kc=0.897, the corrected endurance limit Se is:
- 194 MPa (Correct answer)
- 270 MPa
- 300 MPa
- 215 MPa
Correct answer: 194 MPa
Se = ka·kb·kc·Se' = 0.8 × 0.9 × 0.897 × 300 ≈ 194 MPa.
Question 5: A thin-walled pressure vessel has internal pressure p, radius r, and wall thickness t. The hoop (circumferential) stress is:
- pr/(2t)
- pr/t (Correct answer)
- 2pr/t
- pr/(4t)
Correct answer: pr/t
Hoop stress σ_θ = pr/t is twice the longitudinal stress pr/(2t) in a thin-walled cylindrical vessel.
Question 6: In Hertzian contact between two cylinders with parallel axes, the contact stress distribution across the contact width is:
- Uniform
- Parabolic
- Elliptical (Correct answer)
- Linear
Correct answer: Elliptical
Hertzian contact between cylinders produces an elliptical (semi-elliptical) pressure distribution across the contact width.
Question 7: When designing a welded joint under fatigue loading, AWS and AISC specifications reduce allowable stress primarily because:
- Welds introduce stress concentrations and residual stresses (Correct answer)
- Welding reduces material hardness
- Weld geometry reduces cross-sectional area
- Filler material has lower yield strength
Correct answer: Welds introduce stress concentrations and residual stresses
Weld toes and roots act as stress raisers, and tensile residual stresses from cooling significantly reduce fatigue strength.
ME or MEng Master of Engineering Master of Engineering Machine Design 3
A ball bearing has a basic dynamic load rating C = 25 kN and carries a radial load of 5 kN.
The expected life at 90% reliability in millions of revolutions is: