How do gears, cams and pulleys transmit motion, and how are gear and velocity ratios calculated?
Rotary mechanisms: gears and gear trains (gear ratio and output speed), cams and followers (converting rotary to reciprocating motion), and pulleys and belt drives (velocity ratio and speed), with calculations of ratio and speed.
A focused answer to OCR A-Level Product Design on rotary mechanisms: gears and gear trains with gear-ratio and output-speed calculations, cams and followers converting rotary to reciprocating motion, and pulleys and belt drives with velocity-ratio and speed calculations.
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What this dot point is asking
OCR wants you to know how gears, cams and pulleys transmit motion, and to calculate gear ratios, velocity ratios and output speeds. These rotary mechanisms change the speed, direction, torque or type of motion, and they carry calculation marks in Component 01.
Gears and gear trains
The exam discriminator is the direction of the ratio: driven over driver, and more teeth on the driven gear means slower and stronger.
Cams and followers
Pulleys and belt drives
Why these mechanisms matter
Gears, cams and pulleys let a single motor or input drive a product at the right speed and force and in the right type of motion. Gearing down trades speed for torque (a cordless drill); a cam turns a motor's spin into the reciprocating action a product needs; a belt drive transmits motion between separated shafts and can be changed easily.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20204 marksA driver gear has 20 teeth and meshes with a driven gear of 60 teeth. The driver turns at 300 rpm. Calculate the gear ratio and the output speed of the driven gear.Show worked answer →
A Component 01 gear calculation. Marks for the ratio, the method and the output speed with units.
Gear ratio = teeth on the driven gear divided by teeth on the driver: (a ratio of ). The output speed = input speed divided by the gear ratio: rpm. So the driven gear turns slower (100 rpm) but with three times the torque.
A common dropped mark is inverting the ratio or forgetting that a larger driven gear turns slower; more teeth on the driven gear means a lower output speed and higher torque.
OCR 20224 marksA motor pulley of 30 mm diameter drives a belt to a pulley of 90 mm diameter. The motor runs at 1500 rpm. Calculate the velocity ratio and the output speed of the larger pulley.Show worked answer →
A Component 01 pulley calculation. Marks for the velocity ratio, the method and the output speed with units.
For a belt drive, velocity ratio = driven (output) pulley diameter divided by driver (input) pulley diameter: . The output speed = input speed divided by the velocity ratio: rpm. The larger output pulley turns slower (500 rpm) with greater torque, the belt-drive equivalent of gearing down.
A common dropped mark is using radius for one pulley and diameter for the other, or inverting the ratio; use the same measure for both, and a larger output pulley turns slower.
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