Mechanical Systems, Mechanisms and Power Transmission
Mechanical Systems, Mechanisms and Power Transmission
The exam tests levers, pulleys, gears, belts, chains, screws, cams and linkages, using mechanical advantage, velocity ratio and efficiency.
Core rules
- Mechanical advantage MA = load / effort. Velocity ratio VR = distance moved by effort / distance moved by load. Efficiency = MA / VR × 100%.
- Gear ratio = driven teeth / driving teeth. Speed ratio is the inverse: a 20-tooth gear driving a 60-tooth gear gives one-third the speed and, ideally, three times the torque.
- Belts and chains transmit power over distance; belts can slip, chains do not. Tension and alignment matter.
- A lead screw converts rotation to linear motion: distance per revolution equals the lead.
- Cams convert rotation into timed linear motion; the follower profile controls the motion law.
Worked example
A 20-tooth gear drives a 60-tooth gear at 1500 rpm with an input torque of 5 N·m. Gear ratio = 60/20 = 3. Output speed = 1500/3 = 500 rpm. Ideal output torque = 5 × 3 = 15 N·m. At 80% efficiency, actual output torque = 15 × 0.80 = 12 N·m. Reasoning: speed falls by the ratio, torque rises by the ratio, and losses reduce the delivered torque.
Traps
Confusing MA and VR; ignoring efficiency; using the wrong gear as driven; forgetting that belt slip lowers output speed; assuming torque and speed both increase.