How do gears change speed, torque and direction, and how is the gear ratio found?
Spur, idler, compound, bevel and worm gears, the gear ratio, and the trade-off between speed and torque.
A CCEA A-Level Technology and Design answer on gear types - spur, idler, compound, bevel and worm - calculating the gear ratio from tooth numbers, and the trade-off between output speed and torque.
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What this dot point is asking
CCEA expects you to know gear types (spur, idler, compound, bevel, worm), calculate the gear ratio from tooth numbers, find the output speed, and explain the speed-torque trade-off. Gear-ratio and output-speed calculations are very common, including compound gear trains.
The answer
Gear types
The gear ratio
The speed-torque trade-off
Worked example: a compound gear train
Examples in context
Example 1. Bicycle gears. A small front and large rear sprocket give a low gear (high torque for hills, low speed); the reverse gives a high gear (high speed, low torque), the speed-torque trade-off you feel directly.
Example 2. Electric drill. A compound gear train reduces the high-speed motor to a slower, high-torque chuck speed, exactly the multiply-the-stages reasoning used above.
Try this
Q1. A driver of 25 teeth meshes with a driven of 75 teeth. State the gear ratio. [1 mark]
- Cue. (3:1).
Q2. If the driver in Q1 turns at 600 rev/min, find the output speed. [2 marks]
- Cue. .
Q3. What is the main purpose of an idler gear? [2 marks]
- Cue. To reverse the direction of the driven gear (and bridge a gap) without changing the overall gear ratio.
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 20196 marksA driver gear has 20 teeth and meshes with a driven gear of 60 teeth. The driver turns at 300 rev/min. Calculate the gear ratio and the output speed, and state the effect on torque.Show worked answer →
The gear ratio is the ratio of the driven (output) teeth to the driver (input) teeth:
The output speed is the input speed divided by the gear ratio:
Because the output turns slower (one third of the input speed), the torque is increased (roughly three times, ignoring losses). This is a reduction gear: it trades speed for turning force.
Markers reward gear ratio = driven/driver = 3:1, output speed = 100 rev/min, and the statement that lower speed means higher torque.
CCEA 20214 marksExplain the function of an idler gear, and state one use of a worm and wheel.Show worked answer →
An idler gear is a gear placed between the driver and the driven gear. It does not change the gear ratio (the overall ratio still depends only on the driver and final driven gears), but it reverses the direction of the driven gear so it turns the same way as the driver, and it can bridge a gap between two gears that are too far apart to mesh directly.
A worm and wheel is used where a large speed reduction (and large increase in torque) is needed in a compact space, for example in a winch, a hoist or the tuning mechanism of a guitar/stringed instrument. It also tends to be self-locking (the wheel cannot drive the worm back), which is useful for holding a load.
Markers want the idler's role (direction reversal / bridging, no change to ratio) and a valid worm-and-wheel use (large reduction, often self-locking).
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Sources & how we know this
- CCEA GCE Technology and Design specification — CCEA (2016)