How do gears, pulleys and cams change speed, force, direction and type of motion?
Rotary motion systems: gears and gear trains, gear ratios and how they change speed and torque, pulley and belt systems, and cams and followers that convert rotary motion into reciprocating or oscillating motion.
A focused answer to OCR GCSE Design and Technology J310 on gears, gear ratios, pulleys and cams: how each changes speed, force, direction and type of motion, with a worked gear-ratio calculation.
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What this dot point is asking
OCR J310 expects you to understand rotary motion systems: gears, pulleys and cams. You must know how each changes speed, force (torque), direction or type of motion, and you must be able to calculate a gear ratio, one of the set J310 maths skills. In the written exam this is tested by a gear-ratio calculation and by explaining how a cam and follower converts motion.
Gears and gear ratios
A larger driven gear (more teeth) turns more slowly than the driver but with more torque (turning force); a smaller driven gear turns faster with less torque. Two meshing gears also turn in opposite directions; an idler gear between them keeps the direction the same and does not change the ratio.
Pulleys and belts
A small driver pulley turning a large driven pulley slows the output (and increases torque); a large driver turning a small driven speeds it up. Belts can slip (a safety feature, and a drawback), and a crossed belt reverses the direction of rotation. Pulleys are used where shafts are far apart, as in many machines and engines.
Cams and followers
The shape of the cam controls the movement: a pear cam gives a brief rise then a long dwell (still period), a circular eccentric gives smooth continuous rise and fall, and a snail cam gives a sudden drop. Cams are used in engines (valves), toys and automata to time movements from a single rotating input.
Try this
Q1. A driver gear of 15 teeth meshes with a driven gear of 45 teeth. Calculate the gear ratio. [2 marks]
- Cue. , so 3:1.
Q2. State the type of motion a cam and follower produces from a rotating shaft. [1 mark]
- Cue. Reciprocating (up and down) or oscillating motion.
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 J310/01 20193 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 output speed compared with the input.Show worked answer →
A 3-mark calculation: marks for the ratio, simplifying it, and the effect on speed.
Gear ratio is the driven (output) teeth divided by the driver (input) teeth: 60 divided by 20 equals 3, so the ratio is 3:1.
A ratio of 3:1 means the driven gear turns once for every three turns of the driver, so the output is slower than the input (one third of the speed), and the torque (turning force) is increased.
Markers reward the calculation (60 over 20 equals 3:1), and the effect: the output turns slower (and with more torque). Inverting the ratio, or saying the output speeds up, loses marks.
OCR J310/01 20224 marksExplain how a cam and follower works, and describe the type of motion it produces from a rotating input.Show worked answer →
A 4-mark Explain wants the mechanism and the motion change described.
A cam is a specially shaped piece (often an off-centre or pear shape) fixed to a rotating shaft. A follower rests against the cam's edge. As the cam rotates (rotary input), its changing radius pushes the follower up and lets it fall, so the follower moves up and down. This converts the continuous rotary motion of the shaft into reciprocating (up-and-down) motion of the follower.
The shape of the cam controls the timing and size of the movement (a pear cam gives a brief rise and dwell). Markers reward: the cam is shaped and rotates, the follower tracks its edge, and rotary input becomes reciprocating output. Confusing the input and output motions loses marks.
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