Mechanical Engineering and Mechanics
Core concepts
Understand statics (forces, moments, equilibrium, free-body diagrams), dynamics (mass, acceleration, inertia), and strength of materials (stress, strain, Hooke's law, safety factor). Know standard machine elements: bearings, shafts, keys, couplings, screws, springs and seals. Cover tolerances and fits (clearance, transition, interference), surface roughness, and the basics of technical drawing including sectional views and geometric dimensioning.
Traps in questions
- Forgetting to convert units (N·mm to N·m, mm² to m²).
- Ignoring the difference between static and dynamic load capacity of a bearing.
- Confusing stress (force per area) with force.
- Applying a safety factor to the load instead of to the strength.
- Misreading a fit designation such as H7/h6.
Worked example
A shaft of diameter 20 mm carries a bending moment of 150 N·m. Section modulus for a solid round shaft: W = π·d³/32 = π × 0.02³/32 ≈ 7.85 × 10⁻⁷ m³. Bending stress σ = M/W = 150 / 7.85 × 10⁻⁷ ≈ 191 MPa. If the material's allowable stress is 120 MPa, the design fails and the diameter must increase. Rule of thumb: doubling diameter reduces bending stress by a factor of eight (d³ relationship).
Practical competence
Be able to read and produce simple technical drawings, specify fits and tolerances, and reason about lubrication, wear and fatigue. Understand that fatigue failure occurs below yield strength after many load cycles.