An idler gear is a gear that is inserted between two other gears. Idler gears do not affect the gear ratio between the input and output gears. The gear ratio would be computed just the same if there were no idler gear.


However, idler gears DO affect the direction of rotation. Recall that when using spur gears, the output axle rotates in the opposite direction as the input axle. By adding an idler gear between the two, the output axle rotates in the same direction as the input axle.You can also use idler gears to change the spacing between the input and output axles. Remember, idler gears do not change the gear ratio.
What is the gear ratio of this gear train? The input axle is on the gear on the far left, and the output axle is on the gear on the far right. (Hint: You don't even need to count the teeth on the other gears). Does the output axle rotate in the same or the opposite direction as the input axle?

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See also: Gears, Spur gear