Grade by Grade
D. Structures and Mechanisms · Machines and Mechanisms

Mechanisms and Their Parts

D2.2 A1.1 A1.4About 40 minutes
Learning goals
  • I can name the parts of common mechanisms, like axles, cranks, gears, chains, belts, springs and levers.
  • I can describe the purpose of each part.
  • I can find the input and the output of a mechanism.
  • I can investigate a mechanism safely.

Words to know

input
Where you put force or motion into a machine, like the pedals of a bike or the handle of a can opener.
output
The part of a machine that does the job, like the back wheel of a bike or the cutting wheel of a can opener.
axle
A rod that a wheel or gear turns on or with.
crank
A handle joined to an axle at a right angle. Turning the crank turns the axle.
gear
A wheel with teeth around its edge that fits into another gear or a chain.
chain
A loop of linked metal pieces that joins two gears (sprockets) so they turn together.
belt
A loop of rubber or other flexible material that joins two pulleys so they turn together.
spring
A coil or strip that stretches or squeezes and then goes back to its shape. It can push a part back into place.
frame
The part that holds the moving parts in place.

Look inside!

If you could shrink down very small and walk inside a bicycle, a clock or a can opener, what would you see? You would see parts: wheels, rods, teeth, chains and springs. Every part has a purpose. When one part moves, it pushes or pulls the next part. Together they make a mechanism.

Input and output

Every mechanism has an input and an output.

  • The input is where the force or motion goes in. It is usually where you push, pull or turn.
  • The output is where the motion or force comes out to do the job.

On a bicycle, the pedals are the input. The back wheel is the output. In between, the parts pass the motion along.

Close-up of a bicycle's pedal, crank arm, toothed front sprocket and chain.
On a bicycle the pedal is the input. Your push turns the crank and the toothed sprocket, which pulls the chain to carry the motion back to the wheel, the output. AI-generated image
pedalchainring (front gear)sprocketrear axlechain
The drive mechanism of a bicycle. Your foot pushes the pedal, the crank arm turns the chainring, the chain pulls the sprocket, and the sprocket turns the rear axle and back wheel.
PartWhat it looks likeIts purpose
pedalflat place for your footthe input: your foot pushes here
crank armbar joining the pedal to the centre of the chainringturns the chainring when you push the pedal
chainring (front gear)big wheel with teethgrabs the chain and pulls it along
chainloop of metal linkscarries the motion from the front gear to the back gear
sprocket (rear gear)small wheel with teeth on the back wheelis pulled around by the chain and turns the back wheel
axlerod through the middle of a wheelthe wheel turns on it
brake leverhandle you squeezea lever that pulls a cable to press the brakes on the wheel
framemetal tubesholds all the parts in the right places
Parts of a bicycle and their purposes

Parts you will find in many mechanisms

  • Axle: a rod that a wheel or gear turns on.
  • Crank: a handle on an axle. You turn it in a circle to turn the axle (like the handle on a pencil sharpener or an egg beater).
  • Gear: a wheel with teeth. The teeth fit into another gear or a chain.
  • Pulley wheel: a wheel with a groove for a rope or a belt.
  • Belt or chain: a loop that joins two wheels so they turn together.
  • Lever: a bar that turns on a fulcrum (pivot).
  • Spring: pushes or pulls a part back into place.
  • Frame or case: holds the moving parts where they belong.
Worked example

Let's look at a hand-crank egg beater

Name the parts of a hand egg beater and the purpose of each part.

  1. Crank handle (input): you turn it in a circle.
  2. Big gear: it is joined to the crank. Its teeth push on the small gears.
  3. Small gears: they are joined to the beaters. The big gear makes them spin.
  4. Beaters (output): they spin in the bowl and mix the eggs.
  5. Frame and handle: you hold the handle; the frame holds the gears in place.

Input: the crank. Mechanism: a big gear turning small gears. Output: the spinning beaters. Each part passes the motion to the next one.

Worked example

Let's look at a clothespin

A wooden clothespin has three parts. What are they, and what does each one do?

  1. Two wooden arms: each arm is a lever. You squeeze one end (the input) and the other end opens (the output).
  2. A metal spring: it is the fulcrum (the pivot) and it pushes the arms closed again when you let go.
  3. The output is the grip at the tips that holds the clothes on the line.

Two levers turn on a spring that also squeezes them closed. Squeeze to open; let go to grip.

squeeze here (input)spring = fulcrumjaws open(output)
A clothespin is two levers joined by a spring. Squeeze the input end, the arms turn on the spring, and the jaws at the output end open. Let go, and the spring snaps them shut.

Safety first

Scientists follow safety rules. Only look at mechanisms a grown-up says are safe. Never take apart anything that plugs in or uses batteries, and keep fingers away from blades, teeth on gears and chains. Moving gears and chains can pinch!

Just for fun

Why did the bicycle fall over?

It was two-tired!

Hands-on activity

Mechanism detective

You need: one or two safe mechanisms from home (choose with a grown-up), such as a clothespin, a click pen, a hand can opener, a manual pencil sharpener (no blade showing), a bicycle, a hand egg beater, or a wind-up toy; paper and a pencil.

  1. Pick one mechanism. Make it move slowly while you watch closely.
  2. Draw it. Label every part you can see.
  3. Beside each label, write the part's purpose.
  4. Draw an arrow to show the input (where you push or turn) and the output (what does the job).
  5. Tell your grown-up the path the motion takes from input to output: "I turn the ___, which turns the ___, which ___."

What to look for: labels such as handle, crank, axle, gear, spring, lever; a purpose for each part; a correct input and output. Safety: a grown-up handles anything with a blade; no taking apart electrical devices; keep fingers clear of gear teeth and bike chains.

Your turn

1

What is the input of a mechanism?

2

On a bicycle, what is the input and what is the output?

3

What is the purpose of a bicycle chain?

4

What is a crank?

5

What is the purpose of the spring in a clothespin?

6

What is the purpose of the frame of a machine?

7

Name the output of a hand egg beater.

More practice: warm-up

1

What is an axle?

2

What do we call a wheel with teeth?

3

What part of a bicycle do you squeeze to stop?

4

What can join two pulley wheels so they turn together?

5

What does a spring do after you squeeze it and let go?

More practice: on your own

1

Trace the motion in a bicycle from the input to the output.

2

A pencil sharpener has a crank. What is the purpose of the crank?

3

In a click pen, a spring pushes the tip back in. What would happen if the spring broke?

4

Why do you think a bicycle chain has many small links instead of being one stiff piece?

5

How is a belt like a chain? How is it different?

Challenge

1

A hand can opener has two long handles, a turning key, a toothed wheel and a sharp round cutting wheel. Give the purpose of each part.

2

A mechanism has these parts: a crank, an axle, a rope and a bucket. What could it be for, and what does each part do?

3

Your bike chain fell off. Which part can no longer do its job, and why?

Check your understanding

Quiz

1.

Where you put force into a mechanism is called the...

2.

What is the purpose of a bicycle chain?

3.

A handle on an axle that you turn in a circle is a...

4.

What is the output of a hand egg beater?

5.

Which part pushes a part back into place?

6.

Which is a safe rule when you investigate a mechanism?

For parents and teachers

Big idea (D2.2): Mechanisms are made of parts, and each part has a purpose. Following the motion from input to output is the key skill. Observing and labelling a real mechanism is research (A1.1); following safety rules is A1.4.

How to help: Turn a bicycle upside down (a grown-up holds it steady) and turn the pedal slowly by hand. Ask your child to say what each part does. Keep fingers away from the chain and spokes.

Common difficulties:

  • Naming parts without purposes. Ask "What would happen if that part were missing?"
  • Mixing up input and output. Input: where you push. Output: what does the job.
  • Safety: never let children take apart electrical devices. Supervise anything with blades or pinch points.