Key Stage 2 design and technology · Technical knowledge

KS2 gears and pulleys: a guide for parents

In Key Stage 2 design and technology, children learn to understand and use mechanical systems in their products. Gears and pulleys are two of the most common examples, because they show clearly how turning movement can be passed on, sped up, slowed down or reversed. Children also practise choosing and using tools accurately, since a gear that is badly marked out will not turn.

Gears and pulleys, explained

The national curriculum says that pupils should understand and use mechanical systems in their products, and gives gears, pulleys, cams, levers and linkages as examples. Those examples are non-statutory, so schools choose which mechanisms to teach and some will cover only two or three in a year. Gears and pulleys are popular because they are easy to see and test. Every mechanical system has an input, such as turning a handle, and an output, such as a figure moving or a load rising, and children learn to describe what happens in between.

A gear is a wheel with teeth. When two gears mesh, their teeth interlock, so turning one makes the other turn. The gear the handle turns is the driver, and the gear that is turned is the driven gear. Two meshing gears always turn in opposite directions. If you want the driven gear to turn the same way as the driver, you put a third gear, called an idler, between them. The number of teeth matters: when a small driver turns a large driven gear, the large gear turns more slowly but with more turning force. When a large driver turns a small gear, the small one turns faster but with less force. A bicycle uses the same idea, with a chain linking toothed wheels of different sizes.

A pulley is a wheel with a groove for a belt or rope. Children usually meet two kinds. In a belt drive, a belt loops around two pulley wheels on separate axles, so turning one turns the other, in the same direction if the belt is straight and in the opposite direction if it is crossed. Different sizes of wheel change the speed in the same way as gears do. In a lifting pulley, a rope runs over a wheel. A single fixed pulley only changes the direction of the pull, so pulling down lifts the load up, but it does not make the lifting easier. A pulley system with a moving wheel shares the load between sections of rope, so the pull is smaller, but you pull a longer length of rope.

Because these mechanisms depend on accurate parts, they link to the second objective: selecting from and using a wider range of tools and equipment, such as cutting, shaping, joining and finishing tools, accurately. Gear teeth must be evenly spaced, centre holes must be in the exact middle and axles must be parallel and the right distance apart, or the gears jam or slip. Teachers show children how to measure and mark out carefully, use templates, cut and drill safely with supervision, and test and adjust. Which tools and kits a school uses varies, from card and split pins to wood, dowel and ready-made gear sets.

Gears and pulleys are all around us in bicycles, clocks, hand whisks, can openers, cranes, blinds and washing lines. Children are expected to link what they see in their own models to these everyday uses, and to make decisions about which mechanism suits a job. Do they want speed or force, the same direction or the opposite? These decisions come back in more detail in Key Stage 3, so the aim here is for children to see a mechanism as a design choice that they can predict, test and explain.

Words your child will hear

Mechanism: A set of moving parts that changes one kind of movement or force into another.
Input and output: The input is the movement you put in, such as turning a handle. The output is the movement that comes out.
Driver and driven: The driver gear is the one you turn. The driven gear is the one it turns.
Mesh: When the teeth of two gears fit together so that one turns the other.
Gear ratio: The comparison of the number of teeth on two gears, which tells you how much faster or slower one turns.
Idler gear: A gear placed between two others to make them turn the same way. It does not change the speed.
Pulley: A wheel with a groove for a rope or belt, used to change direction, pass on turning or lift loads.

What the National Curriculum says

The programme of study for Key Stage 2 says:

When designing and making, pupils should be taught to understand and use mechanical systems in their products [for example, gears, pulleys, cams, levers and linkages]
When designing and making, pupils should be taught to select from and use a wider range of tools and equipment to perform practical tasks [for example, cutting, shaping, joining and finishing], accurately

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

Try it: counting the turns of two gears

A driver gear with 24 teeth turns a driven gear with 12 teeth. How many times does the driven gear turn when the handle turns the driver once? What happens if the driven gear has 48 teeth instead?

Count the teeth that pass the meeting point. When the driver turns once, all 24 of its teeth pass the point where the gears mesh, so 24 teeth of the driven gear must pass it too.
Work out the turns of the driven gear. It has 12 teeth, so 24 teeth is two complete turns, because 24 ÷ 12 = 2. The gear ratio from driver to driven is 24 : 12, which simplifies to 2 : 1. The driven gear turns faster, but with less turning force. It also turns in the opposite direction.
```
Driver:  24 teeth, 1 turn
Driven:  12 teeth
24 ÷ 12 = 2 turns
```
Change to a bigger driven gear. Now the driven gear has 48 teeth. When 24 teeth pass the meeting point, the driven gear has gone only part of the way round: 24 ÷ 48 = ½ turn.
```
Driver:  24 teeth, 1 turn
Driven:  48 teeth
24 ÷ 48 = ½ turn
```
Describe the effect. The ratio is now 24 : 48, which is 1 : 2. The driver must turn twice to turn the driven gear once. The output is slower, but it has more turning force, which is what you want for a gear that lifts something heavy.
Add an idler. If a third gear is placed between the driver and the driven gear, the driver and driven gears now turn the same way. The driven gear’s speed stays the same as before, because the idler passes the movement on without changing the ratio.

With 12 teeth the driven gear turns twice for each turn of the driver, so it is faster and weaker. With 48 teeth it turns half a turn, so it is slower but stronger. An idler changes the direction but not the speed.

Common mistakes and how to help

Thinking the bigger gear always turns faster

Big things seem more powerful, and a big wheel covers more ground.

What helps: Put a pen mark on one tooth of each gear and turn the driver once, counting how many times the marks come back to the start. The gear with fewer teeth turns more often.

Expecting two meshing gears to turn the same way

Children picture the second gear as a copy of the first rather than being pushed by its teeth.

What helps: Put your fingers on the two teeth at the meeting point and turn slowly. They move in opposite directions. Then add a third gear in the line and see the first and last turn the same way.

Assuming every pulley makes lifting easier

Pulleys are linked in people’s minds with cranes and lifting heavy things.

What helps: Compare a fixed pulley, which only changes the direction of the pull, with a system that has a moving wheel, which cuts the pull but needs more rope. Using a small bag of weights and a spring balance (or just your hands) makes the difference clear.

Blaming the gears themselves when the mechanism jams or slips

It is easy to think that the parts are faulty, when the gears are in fact too close together, too far apart or not lined up on their axles.

What helps: Check the distance between the axles, the centre holes and the alignment of the gears. Mark out again, adjust and retest. Accurate marking and measuring is part of the skill.

How to help at home

Look at a hand whisk. A hand-cranked whisk or egg beater is a ready-made gear train. With an adult, turn the handle slowly (keeping it out of the bowl) and watch how one turn of the handle turns the beaters several times. Count the teeth if you can.
Make a belt drive. Ask an adult to make two holes in a cardboard box, push a pencil through each, slide a cotton reel onto each pencil and loop a rubber band over both reels. Turn one pencil and watch the other follow. Try crossing the band, and using reels of different sizes.
Bottle-top gears. Fix ridged plastic bottle tops to a piece of card with a paper fastener through the centre of each (an adult can make the holes), placed so their edges press together. The ridges grip like small teeth. Turn one and see which way the other goes, then add a third.
Spot gears and pulleys. Look at a clock, a hand-cranked pencil sharpener, a bicycle (with an adult, keeping fingers away from the chain), a blind cord or a clothes airer. Ask where the input is and what the output does.
Use tools carefully. When making any model with moving parts, encourage your child to measure and mark holes carefully before they cut or pierce, and to check that the axles line up. Small errors make mechanisms jam, which is a very good reason to take care.

Builds on

Key Stage 1: Strong, stiff, stable structures
Key Stage 1: Levers and sliders
Key Stage 1: Wheels and axles

Leads on to

Key Stage 3: Properties of materials
Key Stage 3: Beams, frames and forces
Key Stage 3: Gears, levers and mechanisms
Key Stage 3: Electronic circuits and systems
Key Stage 3: Sensors, actuators and microcontrollers