 A case where numerous idler gears might be used is as described above, where there are a number of output gears that need to be driven simultaneously.Caterpillar track idler wheels

A tracked vehicle uses a combination of wheels and rollers, including "drive sprockets", "idler wheels", "track return rollers" and "road wheels".It is quite similar in concept to a conveyor belt, only instead of a machine carrying objects on top of a powered continuous belt, it's a machine that moves itself over a continuous belt.