Design & Technology
Where very large speed reductions are required, several pairs of gears can be used in a compound gear train. A small gear drives a large gear. The large gear has a smaller gear on the same shaft. This smaller gear drives a large gear. With each transfer, the speed is significantly reduced.
Another method of making large speed reductions is to use a worm gear. This is a shaft with a thread like a screw. This connects at 90° to a large gear (the thread shaft points along the outside edge of the larger gear). Each time the shaft spins one revolution, the gear turns forward by only one tooth. If the gear has 50 teeth, this creates a gear ratio of 50:1. The worm can drive the worm gear round, but the worm gear cannot drive the worm. This means that worm gears are good to use in hoists, the load will not fall back when the motor stops. Worm gears are a good option when you wish to alter direction or rotary motion through 90° and reduce the speed. The photograph to the left shows a worm gear powered by a motor.
Bevel gears, like worm gears, change the axis of rotation through 90°. The teeth have been specially cut so the gears will mesh at right-angles to each other, where spur gears must be parallel.
A pinion is a round cog and the rack is a flat bar with teeth. The driver cog either moves along the rack, as in a rack and pinion (funicular) railway - or else the driver cog moves the rack, as in the steering system in cars. Rack and pinion changes rotary motion into linear motion - as shown in the diagram below.
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