Changing the Kind and Plane of Motion

Turn a knob, slide a part

Look at a microscope that has a focusing knob. You turn the knob (rotary motion), and part of the microscope slides up or down (linear motion). The mechanism has transformed the motion: it changed one kind of motion into another kind!

In the last lessons, gears and belts transmitted motion and changed its speed and direction. In this lesson, mechanisms change the kind of motion and the plane it happens in.

Rack and pinion: rotary ↔ linear

A pinion is a small round gear. A rack is a straight bar with teeth. When the pinion turns, its teeth push the rack's teeth, and the rack slides in a straight line.

It works the other way too: push the rack, and the pinion turns.

Where you find it: the steering of many cars (turn the steering wheel, and a rack slides sideways to point the front wheels), the focusing knob of some microscopes, and some toy cars.

Cam and follower: rotary → up and down

A cam is a wheel that is not round, or a round wheel with its axle off-centre. A rod called the cam follower rests on top of it. As the cam turns, its bumpy edge lifts the follower up, and then lets it drop back down. Rotary motion becomes reciprocating (up-and-down) motion.

The shape of the cam controls how the follower moves. An egg-shaped cam lifts the follower once per turn. A cam with two bumps lifts it twice.

Where you find it: car engines (cams open and close valves), wind-up toys with bobbing parts, and moving toys and displays.

Crank and slider: back and forth ↔ rotary

A crank turns in a circle. A connecting rod joins it to a part that can only slide back and forth (the slider).

• Turn the crank: the slider moves back and forth (rotary → reciprocating). A sewing machine uses rotary motion from its motor to move the needle up and down.
• Push the slider back and forth: the crank turns (reciprocating → rotary). In a car engine, pistons move up and down and turn the crankshaft.

Your legs on a bicycle work a bit like this: they push up and down, and the pedal cranks turn round and round.

Changing the plane of motion

Imagine the flat surface that a motion happens in. That is its plane.

• A Ferris wheel turns in an upright plane, like a door.
• A merry-go-round turns in a flat, level plane, like a tabletop.

Some mechanisms change the plane:

• Bevel gears have slanted teeth and mesh at a right angle. In a hand egg beater, you turn the crank wheel in an upright plane. Bevel-type gears make the beaters spin around in a level plane in the bowl.
• A worm gear is shaped like a screw. When the worm turns, it slowly turns a regular gear set at a right angle. Worm gears change the plane and slow the motion down a lot. The tuning pegs on many guitars use a worm gear: you turn the peg, and the post that the string winds around turns slowly, so you can tune carefully.