Key Stage 2 design and technology · Technical knowledge

KS2 cams, levers and linkages: a guide for parents

In Key Stage 2 design and technology, children learn to understand and use mechanical systems in their products. Cams, levers and linkages are the mechanisms behind pop-up cards, moving toys and models with figures that bob, nod or wave. Children learn how each one changes the movement put in (the input) into a different movement that comes out (the output), and practise marking and making the parts accurately enough for them to work.

Cams, levers and linkages, explained

The national curriculum asks pupils to understand and use mechanical systems in their products and lists gears, pulleys, cams, levers and linkages as examples. These are examples that schools may choose, not a required list, so one class may spend a term on cams and linkages while another focuses on gears. The thread running through all of them is input and output: a hand turns a handle or pushes a lever (the input), the mechanism changes the movement, and something moves in a different way (the output). Describing that change precisely is the main skill.

A cam is a shaped wheel, or sometimes a shaped piece of card, that turns on an axle. A part called the follower rests on its edge, and as the cam turns the follower is pushed up and down. So the turning (rotary) movement of the handle becomes an up-and-down (linear) movement. The shape of the cam decides the movement. A circle fixed off-centre gives a smooth rise and fall. A snail or drop cam gives a slow rise and a sudden drop, and only works when it is turned one way. A pear-shaped cam keeps the follower still at its lowest point for about half of each turn, then lifts it and lets it fall. The follower needs a guide, such as a slot or a tube, so it moves straight up and down rather than wandering.

A lever is a stiff bar that turns around a fixed point called the pivot or fulcrum. You push or pull on one part of it (the effort), and another part moves the load. Scissors, see-saws and bottle openers are all levers, although the pivot is in a different place on each. Where the pivot is placed matters: if it is close to the input end, a small push moves the output end a long way but with less force, and if it is close to the output end, the output moves less but with more force. Children test different pivot positions and see for themselves.

Linkages join levers together with pivots and bars to change the direction or the type of movement. A reverse-motion linkage makes the output move the opposite way to the input. A bell crank, which is an L-shaped lever, turns the direction of movement through a right angle. A parallel-motion linkage makes the output move in the same direction as the input. Some pivots are fixed to the base card, others are loose and move with the linkage, and children learn to tell the two apart.

The second statutory objective is about choosing and using tools and equipment accurately for cutting, shaping, joining and finishing. For these models that means measuring strips to the nearest millimetre, using a template, making holes exactly where they are marked, using a hole punch or bradawl with care and fixing pivots with paper fasteners so the parts turn freely without wobbling. A hole that is a few millimetres out can make a linkage jam or the follower slip. Teachers want children to test, adjust and explain what they changed.

Words your child will hear

Cam: A shaped wheel that turns on an axle and pushes a follower, changing turning movement into up-and-down movement.
Follower: The part that rests on a cam and moves as the cam turns.
Lever: A stiff bar that turns around a pivot to move a load.
Pivot: The fixed point that a lever or linkage turns around. Also called a fulcrum.
Linkage: A set of levers and bars joined by pivots that changes the direction or kind of movement.
Input and output: The input is the movement you put in. The output is the movement that results.
Bell crank: An L-shaped lever that changes the direction of a movement by a right angle.

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: a cam to make a figure rise slowly and drop quickly

Design a cam mechanism so that a card figure slowly rises out of a box and then drops back down. Which cam should you use, and how far will the figure rise?

Describe the movement you need. The figure should go up slowly and drop suddenly, then repeat. The input is turning a handle, and the output is a straight up-and-down movement.
Choose the cam. A circle fixed off-centre rises and falls smoothly and evenly, so there is no sudden drop. A pear cam would pause at the bottom for half of each turn. A snail (drop) cam rises gradually along its curve and then drops at the straight edge, which is the movement you want.
Mark out and make the cam. Draw the snail shape on thick card using a template, so that its edge is 2 cm from the centre hole at the nearest point and 4 cm at the furthest. Cut it out and make the axle hole exactly at the marked centre.
Build the frame. Fix the axle through two holes at exactly the same height on opposite sides of the box, and attach the cam firmly to it, with a handle at one end. Make the follower from a card strip with a flat bottom, and fit a guide, such as a slot in the lid, so that it can only move up and down.
Test and adjust. Turn the handle in the direction that lets the follower climb the slope and drop off the straight edge. Turned the other way, the follower catches on the straight edge. If the follower jams, the guide is too tight or not straight. If it jumps off the cam, the cam is too small or the follower too light. Adjust, test again and check how far the figure rises.
```
Furthest edge:  4 cm
Nearest edge:   2 cm
Rise:  4 − 2 = 2 cm
```

Use a snail cam. The follower rises gradually as the edge gets further from the axle, and drops suddenly at the straight edge. The figure rises by the difference between the furthest and nearest distances from the axle, 4 cm minus 2 cm, which is 2 cm.

Common mistakes and how to help

Thinking a cam makes the follower travel in a circle

The cam is a turning part, so it is natural to assume everything connected to it turns too.

What helps: Hold the follower in a guide and watch it. The cam turns, but the follower only goes up and down. That change from turning to straight-line movement is what the mechanism is for.

Placing the pivot of a lever anywhere that is convenient

Children focus on getting the parts to fit on the card rather than on how the position changes the movement.

What helps: Make a simple lever from a strip and try pivots at different distances along it. Measure how far each end moves. The end further from the pivot always moves further.

Assuming the output of a linkage moves in the same direction as the input

Pushing something usually makes it move away from you, so a linkage that reverses or turns the movement is surprising.

What helps: Build a reverse-motion linkage with a strip pivoted in the middle: push one end down and the other goes up. Then a bell crank, where an up-and-down push becomes a side-to-side one.

Believing small errors in hole positions do not matter

A hole that is a couple of millimetres out looks fine to the eye, so measuring and marking feel like unnecessary effort.

What helps: Show how a loose or misplaced hole makes a linkage wobble, jam or lose some of its movement. Using thicker card, a ruler, a template and a washer under the fastener usually fixes it.

How to help at home

Find levers around the home. Scissors, tongs, pliers, a stapler and a bottle opener are all levers. For each one, ask your child where the pivot is, where you push and what moves. Sorting them by where the pivot sits is a good challenge.
Make a pop-up card. A strip of card with a paper fastener as a pivot can make a character nod, wave or pop out when you push a tab. Try moving the pivot and see how the movement changes.
Build a simple cam toy. Push a drinking straw or pencil through two holes in a small box as an axle, fix an off-centre card circle to it, and drop a card strip through a slot in the top of the box so that it rests on the circle as the follower. An adult should make the holes and the slot. Turn the axle and watch the follower rise and fall.
Spot linkages on a walk or at home. A folding clothes airer, an adjustable desk lamp, a folding pushchair and the wipers on a parked car use linkages. Look at them while they are parked or still, and ask what movement goes in and what comes out.
Make a character that moves. Challenge your child to build a card character that does one movement of their choice, such as an arm that waves or a mouth that opens. Ask them to explain which mechanism they used and why, and how they would improve it.

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