How do gears change speed, torque and direction, and how is gear ratio calculated?
Gears and gear trains: simple gear trains, gear ratio, the effect on speed, torque and direction, idler gears, and compound gear trains.
A CCEA GCSE Technology and Design answer on gears and gear trains: simple gear trains, calculating gear ratio from teeth, the effect on output speed, torque and direction, the role of an idler gear, and compound gear trains for large ratios.
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What this dot point is asking
CCEA wants you to understand gears and gear trains: how meshed gears change speed, torque and direction, how to calculate the gear ratio from the number of teeth, the role of an idler gear, and how a compound gear train gives a large ratio. Gear-ratio calculations are a guaranteed exam topic.
The answer
Gears and how they mesh
A gear is a toothed wheel. When two gears mesh, the teeth interlock so that one (the driver, connected to the input) turns the other (the driven, connected to the output). The driver and driven turn in opposite directions.
Calculating the gear ratio
The output speed follows from the ratio:
So a small driver turning a large driven gear gives a reduction (slower, stronger output), while a large driver turning a small driven gear gives a speed increase (faster, weaker output).
Idler gears and direction
Compound gear trains
Compound trains are used where a big speed reduction and a big torque increase are needed, such as in a clock or a powered winch.
Worked example: an output-speed calculation
Examples in context
- Example 1. A hand whisk
- A large hand-cranked gear drives small beater gears, so the beaters spin much faster than the handle - a speed increase using the gear ratio in reverse.
- Example 2. A bicycle in low gear
- A small front gear driving a large rear gear gives a reduction, so the wheel turns slowly with high torque, making it easy to climb a hill.
- Example 3. A clock
- A compound gear train links the fast-spinning escapement to the slow-moving hour hand, achieving a huge reduction in a small mechanism.
Being able to calculate the gear ratio and output speed, and explain the idler and compound train, lets you answer both the calculation questions and the "explain the purpose" questions.
Try this
Q1. State the equation for the gear ratio of a simple gear train. [1 mark]
- Cue. Gear ratio = teeth on driven gear / teeth on driver gear.
Q2. A 16-tooth driver meshes with a 48-tooth driven gear. Calculate the gear ratio. [2 marks]
- Cue. , i.e. .
Q3. For the gears in Q2, if the driver turns at 600 rev/min, find the output speed. [2 marks]
- Cue. .
Q4. What is the purpose of an idler gear? [2 marks]
- Cue. It makes the driven gear turn the same way as the driver, without changing the gear ratio.
Q5. Why is a compound gear train used? [1 mark]
- Cue. To achieve a large gear ratio (big speed reduction and torque increase) in a small space.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA style4 marksA driver gear has 20 teeth and meshes with a driven gear of 60 teeth. Calculate the gear ratio and state the effect on the output speed.Show worked answer →
Gear ratio , written as (1, 1).
A ratio of means the driven gear turns once for every three turns of the driver (1), so the output turns three times more slowly than the input but with greater torque (1).
CCEA style3 marksExplain the purpose of an idler gear in a gear train.Show worked answer →
An idler gear is a gear placed between the driver and the driven gear (1). Its purpose is to make the driven gear turn in the same direction as the driver (1), reversing the direction that two meshed gears alone would give. It does not change the overall gear ratio (1).
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Sources & how we know this
- CCEA GCSE Technology and Design specification — CCEA (2017)