Gears & Gear Ratios

Why this matters in HCIC: every paper so far has 1–2 gear questions — counting teeth, working out speeds, or spotting jammed gears.

A gear is a wheel with teeth. When two gears mesh, the teeth push each other, so turning one turns the other — in the opposite direction. Think of two people facing each other holding hands and spinning: one goes clockwise, the other must go counter-clockwise.

Speed comes from counting teeth. When A turns once, its 30 teeth push past the meshing point, and B (20 teeth) must turn 30÷20 = 1.5 times to keep up.

Rules to memorise

Meshed gears spin in opposite directions.
driven speed = driver speed × (driver teeth ÷ driven teeth)
Small gear = fast but weak. Big gear = slow but strong (more torque).
Small driver → big driven = torque (hauling, climbing). Big driver → small driven = speed.

Exam trap: in a chain A→B→C, the middle gear cancels out: C's speed = A × (A teeth ÷ C teeth). And a loop of gears with an odd number of gears (3, 5…) jams — nothing can move!

HCIC Trainer