When two gears are directly next to one another, they move in opposite directions to create motion. When both gears need to move in the same direction, an idler gear is used between the two. It transfers motion from one gear to the other while affecting nothing other than the direction in which the second gear spins. The gears the idler connects may be referred to as the "driving gear," which imparts motion, and the "driven gear," which accepts motion. The gear ratio between the driving and driven gears is calculated in the same manner as it would be were the idler gear not present.
Idler gears can also be used to create space between the driven and driving gears, as well as between the shafts that attach to those gears. In such a situation, the size of the idler gear will not affect the speed at which the driven gear spins, so the size of the idler gear can be determined simply by the space to be bridged. Such a configuration can also be set up when the use of larger driving and driven gears is impractical or impossible because of space constraints. In this manner, the idler may be used to achieve the same results as a belt or chain strung between the driving and driven gears.
Multiple idler gears may also be used to span a gap, paying attention only to their number so the driven gear spins in the desired direction. An odd number of idlers will cause the driving and driven gears to spin in the same direction, and an even number will cause them to spin in opposite directions. This method also may be employed to connect driving and driven gears when there is a physical obstruction that would prevent them from making direct contact. This is accomplished by arranging a series of idlers so they go around the obstruction.
In the case of a manual vehicle transmission, the reverse idler allows the shaft that drives the forward gears to also drive the gear that propels the vehicle in reverse. All the other gears in the transmission make direct contact with the countergear, the gear that allows the transfer of power to the axle that moves the wheels and, in turn, moves the vehicle forward. The reverse gear, however, has the reverse idler gear between it and the countergear. The presence of an idler gear between the reverse gear and the countergear causes the axle and wheels to spin in a different direction than they do when the other gears are engaged, propelling the vehicle in reverse. The term "reverse idler gear" may also be used to simply refer to a single idler in any scenario.