How do belt and chain drives transmit rotary motion, and how is the velocity ratio found?
Belt and pulley drives, and chain and sprocket drives: transmitting rotary motion over a distance, the velocity ratio, and choosing between them.
A CCEA GCSE Technology and Design answer on belt and pulley drives and chain and sprocket drives: transmitting rotary motion over a distance, calculating the velocity ratio from pulley or sprocket sizes, slip, and choosing between belts and chains.
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What this dot point is asking
CCEA wants you to know how belt and pulley drives and chain and sprocket drives transmit rotary motion over a distance, how to calculate the velocity ratio from the pulley or sprocket sizes, the problem of slip, and how to choose between a belt and a chain. The velocity-ratio calculation mirrors the gear-ratio one.
The answer
Transmitting rotary motion over a distance
Gears must mesh directly, but sometimes the input and output shafts are far apart. A belt or a chain carries the rotary motion across the gap.
With a belt over two pulleys, the driven pulley turns the same way as the driver (unless the belt is crossed, which reverses it). Like gears, a larger driven wheel turns more slowly with more torque.
The velocity ratio
So a small driver pulley turning a large driven pulley gives a reduction (slower, stronger output), exactly like a gear reduction.
Slip and why chains are positive drives
Choosing between a belt and a chain
Worked example: a belt-drive calculation
Examples in context
- Example 1. A bicycle
- A chain and sprocket drive transmits the rider's effort to the rear wheel with no slip and an exact ratio, and changing sprocket sizes changes the gear.
- Example 2. A washing machine
- A belt drive turns the drum quietly and cheaply, and the belt can slip if the drum jams, protecting the motor.
- Example 3. A drill press
- A belt over stepped pulleys lets the user change the spindle speed by moving the belt to different pulley sizes, using the velocity ratio directly.
Being able to calculate the velocity ratio and output speed, and justify a belt versus a chain, lets you answer both the calculation questions and the comparison questions.
Try this
Q1. State the equation for the velocity ratio of a belt drive. [1 mark]
- Cue. VR = diameter of driven pulley / diameter of driver pulley.
Q2. A 25 mm driver pulley drives a 100 mm pulley. Calculate the velocity ratio. [2 marks]
- Cue. , i.e. .
Q3. For Q2, if the driver turns at 800 rev/min, find the output speed. [2 marks]
- Cue. .
Q4. Give one advantage of a chain drive over a belt drive. [1 mark]
- Cue. No slip (positive, exact drive), or it can transmit a larger force.
Q5. Why might belt slip be a useful feature? [1 mark]
- Cue. The belt slips instead of overloading the motor if the driven part jams, acting as a safety feature.
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 pulley of diameter 40 mm drives a larger pulley of diameter 120 mm by a belt. Calculate the velocity ratio and state the effect on output speed.Show worked answer →
Velocity ratio , i.e. (1, 1).
A velocity ratio of means the larger driven pulley turns three times more slowly than the driver (1), so the output speed is reduced to one third of the input with greater torque (1).
CCEA style4 marksGive two advantages of a chain and sprocket drive over a belt and pulley drive.Show worked answer →
A chain does not slip because it has links that engage with the sprocket teeth (1), so the drive is positive and the speed ratio is exact (1).
A chain can transmit a larger force or power than a flat belt (1), which is why it is used on bicycles and motorbikes (1). Markers also accept that it can drive heavier loads reliably.
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Sources & how we know this
- CCEA GCSE Technology and Design specification — CCEA (2017)