Types of motion in mechanisms
Design & Technology · Key Stage 3 · The School
Four types of motion
ROTARY motion turns continuously round a fixed point, like a wheel. LINEAR motion moves in a straight line, like a drawer sliding out. OSCILLATING motion swings back and forth around a fixed point, like a pendulum, without completing a full turn. RECIPROCATING motion moves straight back and forth repeatedly, like a sewing machine needle.
Mechanisms convert between them
A MECHANISM's real job is usually converting one type of motion into another. A motor typically produces rotary motion; a mechanism might convert that rotary motion into the linear motion needed to open a drawer, or the reciprocating motion needed to drive a saw blade back and forth.
Naming the type before naming the mechanism
Before asking "what mechanism is this?", it helps to ask "what motion goes in, and what motion comes out?" A rack and pinion converts rotary motion (the pinion gear turning) into linear motion (the rack sliding). A crank and slider converts rotary motion into reciprocating motion. Naming the motion types first makes the mechanism's actual job clear before its name is even known.
A simple hand-cranked drill converts motion through several stages. Turning the hand crank produces ROTARY motion at the handle. Gears inside the drill transfer that rotary motion, often changing its speed, while it STAYS rotary all the way to the drill bit — a rotary-to-rotary conversion, changing speed rather than motion type. Compare this to a hand-operated water pump, where turning a rotary handle drives a piston in and out — a rotary-to-RECIPROCATING conversion, genuinely changing the type of motion, not just its speed. Two mechanisms, both starting from a hand turning a rotary crank, doing genuinely different jobs — one keeps the motion type the same throughout, the other deliberately changes it. Spotting which is which is exactly what naming the motion types is for.