Mechanical Systems
What the exam tests
This domain covers power transmission, support, and fastening: belts, chains, gears, couplings, bearings, shafts, lubrication, torque, and vibration. The exam expects you to calculate ratios and speeds, recognise wear modes, and choose safe corrective actions rather than simply replacing parts.
Core rules
- For pulleys and gears, speed ratio is inversely proportional to diameter or tooth count: n1/n2 = D2/D1 or Z2/Z1. Torque changes in the opposite direction in an ideal drive.
- Belt speed is π × D × n / 60 with D in metres and n in rpm. Compare it with the belt's rated speed and the small pulley's minimum diameter.
- Bearing life falls sharply with load and misalignment. Over-greasing can damage seals and cause heat; under-lubrication causes metal contact and noise.
- Fasteners need the specified grade, clean threads, correct lubrication, and a calibrated torque wrench. Torque is not a substitute for the correct preload method where stretch control is specified.
- Vibration analysis distinguishes imbalance, misalignment, looseness, and bearing defects by frequency pattern, not by amplitude alone.
Worked example
A 1450 rpm motor with a 100 mm pitch-diameter driver pulley drives a 250 mm pulley. Ratio = 250/100 = 2.5, so driven speed = 1450/2.5 = 580 rpm. Belt speed = π × 0.100 × 1450/60 = 7.59 m/s. Check that the belt rating covers 7.59 m/s and that the small pulley is not below its minimum diameter, which would shorten belt life.
Traps
Mixing radius and diameter; using motor rpm as driven rpm; over-tensioning belts to stop slip; greasing a sealed bearing; and aligning a coupling before checking soft foot or pipe strain.