Tandem Roller. Monday , June 04th , 2018 - 15:14:31 PM
The third gear in the picture has 42 teeth. The gear ratio between the idler and third gear is thus 42/21 or 2:1 and hence the final gear ratio is 1.62x2≈3.23. For every 3.23 revolutions of the smallest gear the largest gear turns one revolution or for every one revolution of the smallest gear the largest gear turns 0.31 (1/3.23) revolution a total reduction of about 1:3.23 (Gear Reduction Ratio (GRR) = 1/Gear Ratio (GR)).
It is essential to have two coupled gears of different sizes on the intermediate layshaft. If three gears were used the overall ratio would be simply that between the first and final gears the intermediate gear would only act as an idler gear: it would reverse the direction of rotation but not change the ratio.
Idler gears can also transmit rotation among distant shafts in situations where it would be impractical to simply make the distant gears larger to bring them together. Not only do larger gears occupy more space the mass and rotational inertia (moment of inertia) of a gear is proportional to the square of its radius. Instead of idler gears a toothed belt or chain can be used to transmit torque over distance.
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