Power Transmission and Simple Mechanisms
Power transmission and simple mechanisms asks you to calculate speed ratios in belt, chain and gear drives and to identify a mechanism from its motion. Show the ratio, then the answer with units.
Belt drives
Flat belt for long centres, V-belt for grip at short centres, timing belt for exact speed, rope drive for large power over long distance. For a simple belt, N1 D1 = N2 D2, so N2 = N1 D1 / D2. An open belt keeps direction; a crossed belt reverses the driven pulley. Slip reduces the driven speed, so N2 = (N1 D1 / D2) × (1 − s).
Gears
Simple train: speed ratio follows the tooth numbers, T2/T1. Compound train: velocity ratio = product of driven teeth / product of driver teeth, and direction alternates with each mesh. An idler changes direction only.
Mechanisms
Four-bar linkage converts rotation to oscillation; slider-crank converts rotation to reciprocation (engine, shaper); crank and slotted lever gives quick return; cam and follower gives timed lift; ratchet and pawl gives intermittent motion; toggle gives a large clamping force; bell crank changes the direction of motion; pantograph copies drawings to scale.
Worked example
A 100 mm pulley running at 1440 rpm drives a 250 mm pulley through an open belt. N2 = 1440 × 100/250 = 576 rpm. With 5 % slip, N2 = 576 × 0.95 = 547 rpm. For a compound train with drivers of 20 and 30 teeth and driven gears of 60 and 90 teeth, ratio = (60 × 90)/(20 × 30) = 9, so the output turns at one-ninth of the input speed.
Traps
Forgetting slip. Mixing radius and diameter within one calculation. Confusing a compound train with a simple train. Saying a crossed belt reverses direction without reading the question wording carefully.