How do gears, cams and pulleys transmit and change motion, and how are their ratios calculated?
Gears and gear trains, the velocity ratio and gear ratio, idler gears, compound gear trains, cams and followers, pulleys and belts and their ratios, and how these mechanisms change the speed, force and type of motion.
A focused answer to Eduqas A-Level Product Design on gears, cams and pulleys: gear trains, gear ratio and velocity ratio, idler and compound gears, cams and followers, and pulley and belt systems, with a worked gear ratio calculation and how each changes speed, force and motion.
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What this dot point is asking
Eduqas wants you to explain how gears, cams and pulleys transmit and change motion, and to calculate gear ratio and velocity ratio. These mechanisms change the speed, force (torque) and type of motion, combining technical knowledge with applied maths, so they are examined as recall, a ratio calculation and applied reasoning about a product.
Gears and gear ratio
Velocity ratio and torque
Cams and followers
Pulleys and belts
A pulley and belt system transmits rotary motion between shafts, often across a distance, using a belt running over two or more wheels. The speed change is set by the ratio of the pulley diameters (the velocity ratio is the driven diameter over the driver diameter), exactly as gears use teeth: a small driver pulley turning a large driven pulley slows the output and increases torque. Belts are quieter and cheaper than gears, tolerate misalignment, and can slip under overload, which protects the machine (and is used deliberately in some drives). They cannot transmit large forces without slipping, and they can stretch and wear. A toothed (timing) belt prevents slip where exact timing matters (a car camshaft). Pulleys, gears and cams are often combined to route and transform motion through a product.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20204 marksA driver gear has 20 teeth and meshes with a driven gear of 60 teeth. Calculate the gear ratio, and state what happens to the speed and the torque of the driven gear.Show worked answer →
A Component 1 gear ratio calculation. Marks for the ratio and the effect on speed and torque.
The gear ratio is the driven (output) teeth divided by the driver (input) teeth: , written as 3:1. Because the driven gear has three times as many teeth, it turns three times more slowly than the driver, so the output speed is reduced to one third, and the torque (turning force) is increased about three times.
Award marks for the correct ratio (3:1) and the link: more teeth on the driven gear means slower speed and greater torque. A common dropped mark is inverting the ratio or saying speed increases.
Eduqas 20216 marksDiscuss how gears, cams and pulleys each change motion in a product, giving an example of each, and explain how a designer would calculate or estimate the change. Refer to ratios where relevant.Show worked answer →
A Component 1 extended question marked by levels of response. Reward each mechanism, its effect, an example and the ratio.
Gears transmit rotary motion between shafts and change speed and torque by their tooth ratio (a gear ratio of 3:1 slows the output and increases torque); an idler reverses direction without changing the ratio; compound gears give large ratios. A cam converts rotary motion into reciprocating or oscillating motion of a follower, the shape of the cam setting the movement (a car engine valve, a toy). A pulley and belt transmits rotary motion between shafts, often at a distance, with the speed change set by the pulley diameter ratio (and belts can slip, which protects the system).
A top answer explains each mechanism, gives an example, and calculates or estimates the change using the tooth or diameter ratio, concluding that the designer chooses the mechanism and ratio to get the required output speed, force or motion type.
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Sources & how we know this
- Eduqas A Level Design and Technology specification (Product Design) — Eduqas (2017)