Key Stage 3 design and technology · Technical knowledge

KS3 design and technology: gears, levers and mechanisms

In Years 7 to 9, pupils study how mechanical systems change movement and force. They learn that a mechanism can turn a spinning motion into a back-and-forth one, reverse a direction, speed something up or give extra turning force at the cost of speed. They use simple calculations, such as gear ratios, to predict what a system will do before they build it.

Gears, levers and mechanisms, explained

In Key Stage 2 children usually meet gears, pulleys, cams, levers and linkages one at a time and describe what they do. At Key Stage 3 the question changes to why a designer would choose one. Every mechanical system has an input, a process and an output. A motor or a hand-turned crank provides the input, the mechanism changes the movement or force, and the output does the useful job. Pupils learn to describe movement in four types: rotary (turning round), linear (in a straight line), reciprocating (backwards and forwards in a straight line) and oscillating (swinging to and fro in an arc).

Gears are the best-known example of changing speed and force. When a small gear (the driver) turns a larger one (the driven gear), the output turns more slowly but with more turning force. The gear ratio is the number of teeth on the driven gear divided by the number of teeth on the driver. Meshing gears turn in opposite directions, and an extra gear between them, called an idler, makes both end gears turn the same way without changing the ratio. Pulleys with a belt work in a similar way, with diameters in place of teeth, although both pulleys turn the same way unless the belt is crossed. A belt can also slip, which is a useful safety feature in some machines. A worm gear gives a very large reduction in a small space and, in most designs, cannot be driven backwards by the load.

Other mechanisms change the type of movement. A cam turning on a shaft pushes a follower up and down, so rotary motion becomes reciprocating, and the shape of the cam decides the pattern. A crank and slider turns rotary motion into reciprocating motion, as in a sewing machine needle. A rack and pinion turns rotary motion into linear motion, and is used in some steering systems. Linkages made of levers joined by pivots can reverse a direction, change it by 90 degrees (a bell crank) or make two parts move together (a parallel motion linkage). A ratchet and pawl lets something turn in only one direction.

Levers show most clearly the trade-off that runs through this topic: a mechanism can increase force, but not for free. If a lever lets you lift a heavy box with a small effort, you must move your end a longer way than the box moves. Schools sometimes teach the three classes of lever by where the pivot, effort and load sit. Pupils also consider materials and structure: gears might be nylon, which is quiet and needs little oil, or metal where more strength is needed, and the frame holding the mechanism must be stiff enough that the shafts stay aligned. Friction, wear and lubrication become practical design questions.

Words your child will hear

Driver and driven: The driver is the part that supplies the motion, and the driven part is the one it moves.
Gear ratio: Teeth on the driven gear divided by teeth on the driver. A ratio of 3:1 means the driver turns three times for each turn of the driven gear.
Idler gear: A gear placed between two others to change the direction of the output. It does not change the ratio.
Follower: The part that rests against a cam and moves as the cam turns.
Reciprocating motion: Movement backwards and forwards in a straight line, such as a piston.
Torque: Turning force. Gears and levers can increase it, but only by reducing speed or distance.
Linkage: A set of levers joined at pivots that changes the direction or size of a movement.

What the National Curriculum says

The programme of study for Key Stage 3 says:

When designing and making, pupils should be taught to understand how more advanced mechanical systems used in their products enable changes in movement and force
When designing and making, pupils should be taught to understand and use the properties of materials and the performance of structural elements to achieve functioning solutions

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

A worked example

A motor turns a 20-tooth driver gear at 120 revolutions per minute (rpm). It meshes directly with a 60-tooth driven gear that moves a model crane’s hook. What is the gear ratio, the speed of the output, and what happens to the turning force and direction?

Work out the gear ratio. Gear ratio = teeth on driven gear ÷ teeth on driver.
```
Ratio = 60 ÷ 20
      = 3, written 3:1
```
Say what the ratio means in words. The driver must turn 3 times for the driven gear to turn once. The driven gear is therefore 3 times slower.
Calculate the output speed by dividing the input speed by the ratio.
```
Output = 120 ÷ 3
       = 40 rpm
```
Describe the force. The speed has been reduced by 3, so the turning force at the output is about 3 times larger, ignoring friction. This is why a crane’s drive uses a reduction gear train: slower but stronger.
State the direction. Two gears in direct contact turn in opposite directions, so the hook’s drum turns the opposite way to the motor. Adding one idler gear between them would make the drum turn the same way as the motor, and the ratio would stay 3:1.

The gear ratio is 3:1, the output turns at 40 rpm, the turning force is about three times greater (less in practice, because of friction), and the direction is reversed. The trade-off to remember: speed is lost when force is gained.

Common mistakes and how to help

A bigger gear always turns faster

A large gear has a larger outer edge, which looks as if it is moving more quickly, so children assume it spins faster.

What helps: Count the teeth. Each tooth that passes the meshing point must be matched by one on the other gear, so a gear with three times as many teeth makes one turn in the time the small one makes three. Try it with two cogs from an old toy, marking one tooth with a pen.

A machine such as a lever or gear train makes extra energy

It feels as though a small effort lifts a big load, so something must have been gained.

What helps: Show that the small effort has to move much further. A long-handled tool needs a bigger arc than the load travels. Force is traded for distance or speed, never created.

An idler gear changes the speed

Adding another gear seems as if it must change something about the output speed.

What helps: The idler only reverses the direction of the last gear. The ratio depends only on the first driver and the last driven gear. Use three gears and compare with two to see this.

A gear ratio of 3:1 means the output goes three times faster

A bigger ratio number sounds like a bigger result, and ratios can be written in either order.

What helps: Always check which gear is the driver. When the driven gear has more teeth, the output is slower and stronger. Write what happens in a sentence each time: “three turns in, one turn out”.

How to help at home

Bicycle gears. If your child has a bike with gears, ask which gear makes the pedals easiest to turn on a hill and why. Look together at the front and rear sprockets and count the teeth, then compare the ratio with how quickly the wheel turns. Do this with the bike standing still.
Spot the types of motion. Look at a bicycle pump, a sewing machine needle, windscreen wipers and a playground see-saw. Ask your child which move in a straight line (reciprocating) and which swing in an arc (oscillating), and, for the machines, what movement goes in and which mechanism does the changing. Sketch two.
Build with a construction kit. If you have a building set with gears or pulleys, challenge your child to build a mechanism that makes a hand-turned crank spin an output three times slower, then three times faster. Ask them to predict before they test.
Lever experiments. Use a wooden spoon or ruler on a pencil as a pivot to lift a small book, and compare how hard it is to push when the effort is near and far from the pivot. Ask your child where the extra distance went.

Builds on

Key Stage 2: Strengthening and reinforcing structures
Key Stage 2: Gears and pulleys
Key Stage 2: Cams, levers and linkages
Key Stage 2: Series circuits in products
Key Stage 2: Programming and controlling products