Machine Elements, Power Drives and Transmission Systems
What this domain tests
Belt, chain and gear drives, gear terminology, velocity ratio, couplings, clutches, keys and brakes – with simple numericals.
Core rules
- Velocity ratio of a belt drive = N₁/N₂ = D₂/D₁. V-belts transmit more power than flat belts because of wedge action; timing belts give a fixed ratio with no slip.
- Belt slip reduces output speed; creep is the differential stretch on the slack side. Belt speed v = πDN/60 m/s.
- Gear: module m = d/z (mm), so d = mz; circular pitch p = πm; addendum = 1m; dedendum = 1.25m; centre distance = m(z₁ + z₂)/2.
- Velocity ratio of a gear pair = z₂/z₁ (reduction when the driven gear is larger). Worm drive gives high reduction (ratio = number of worm-wheel teeth ÷ number of worm starts) and is self-locking at low lead angles.
- Chain drives need correct sag (about 2 % of centre distance), alignment and lubrication.
- Keys: parallel key transmits torque by shear and crushing; never fit a key that is slack or too tall, which prevents the hub seating.
Worked example
A pinion of 20 teeth drives a 60-tooth gear; module = 3 mm. Pitch diameters: d₁ = 3 × 20 = 60 mm, d₂ = 3 × 60 = 180 mm. Centre distance = (60 + 180)/2 = 120 mm. Velocity ratio = 60/20 = 3:1 reduction. Circular pitch = π × 3 = 9.42 mm.
Traps that cost marks
- Confusing module (d/z, mm) with diametral pitch (z/d, reciprocal), which are inverse.
- Using radius instead of diameter in v = πDN/60.
- Ignoring slip when a belt-driven spindle runs slow.
- Over-tightening a chain or belt, which overloads bearings.