Power Transmission and Machine Elements
Power Transmission and Machine Elements
This domain covers how rotation is transferred and changed: belt, chain and gear drives, gear trains, worm drives, rack and pinion, screw threads, clutches and brakes. Expect velocity-ratio calculations and questions on slip, module and efficiency.
Core rules
- Belt drive ratio: N1/N2 = D2/D1. Slip reduces driven speed: N2 = N1 × D1/D2 × (1 − slip). Creep is elastic slip from belt stretch. Crossed belts reverse rotation.
- Gear: module m = pitch diameter / number of teeth, so d = mz. Ratio = z2/z1 = N1/N2. Centre distance = (d1 + d2)/2.
- A simple gear train ratio is the product of driven teeth over driving teeth; an idler does not change the ratio, only direction.
- Worm drive gives high reduction and self-locking in one direction, but efficiency is low (often 40–70 %) because of sliding contact.
- Screw: lead = pitch × number of starts. Rack and pinion converts rotary to linear motion; v = π d N.
Worked example
A pinion of 20 teeth, module 3 mm, drives a 60-tooth gear. Pitch diameters: d1 = 3 × 20 = 60 mm; d2 = 3 × 60 = 180 mm. Centre distance = (60 + 180)/2 = 120 mm. Ratio = 60/20 = 3:1, so a pinion at 1440 rpm drives the gear at 480 rpm. If the belt driving the pinion slips 2 % (100 mm driver at 1440 rpm onto a 250 mm pulley), N2 = 1440 × 100/250 × 0.98 = 564.5 rpm, not the ideal 576 rpm.
Traps
Using outside diameter instead of pitch diameter in gear ratios; mixing module units (mm) with centre distance in cm; forgetting slip in belt calculations; assuming a worm drive is efficient because the ratio is high.