Idler Gears

Idler gears are those type of intermediate gears that are inserted between two other types of gears. Typically an Idler Gear performs three functions. Firstly Idler Gear facilitates to change the direction of rotation of output shaft. Secondly Idler gear typically help in reducing the size of the input/output gears while continuing to maintain the spacing of the shafts. Thirdly, Idler gears can also make changes in the spacing between the output and input axles. Interestingly, Idler gears do not have any impact on the gear ratio between the output and input gears. The gear ratio is calculated as if there was no Idler Gear. Services of Idler Gears are also utilized for transmitting rotation among distant shafts in cases where it would not be wise to just, make the distant gears bigger in order to bring them together.

Functioning of the Idler Gear
The important function performed by the idler Gear can be understood from the working of a typical gear, considering what happens when there is no Idler Gear.

As is clear from the next image, the direction of rotation of output axle is in opposite direction to that of the input axle. Now see what happens if in between the two, an idler gear is added. The output axle stats rotating in the same direction as that of the input axle. Now another interesting tale concerning Idler Gears, if a second idler gear is added the input and output axle would again start rotating in opposite directions.

Application of the Idler Gears
An idler gear working in the manual transmission system plays a vital role. Internally a manual transmission system looks like this.

Let us take a close look at the Idler Gear now:
The reverse gear on the manual transmission system typically uses an Idler gear. It helps in the following way. In the above diagram the blue coloured reverse gear turns in a direction that is opposite to all other blue coloured gears. So, it is not possible to throw the transmission into reverse position as long as the car is moving ahead. The dog teeth would never engage, now the Idler Gear helps the reverse gear to slide into a mesh with the other gears for reversing the direction of rotation.

Let us the application of Idler Gear in the application related to Automotive differential.

Idler Gears in Automotive Differential
The image shown above is of an Automotive differential, typically using a bevel-gear epicyclic train. It is the pinion which, drives the ring gear (crown wheel). The Crown Gear is able to rotate freely, and carries the Idler Gears. Though only one Idler Gear is required here, but extra Gears can give a better symmetry. In the epicyclic chain, the ring gear is able to correspond to the planet carrier, while the Idler Gears correspond to the planet gears. The function of the Idler Gears is to drive the side gears on the half-axles. This corresponds to the sun and ring gears, and in effect are the output gears. As the two half-axles start revolving at a similar speed, there is no revolving of Idler Gears. But as soon as the half-axles start moving at different speeds, the idlers also start revolving. The differential puts same amount of torque to the side gears (idlers drive them at equal distances), but allows them to rotate at varying speeds.