Idler Gear

What Are Idler Gears?

An idler is a type of gear wheel that is inserted among multiple other gear wheels. There are primarily two purposes of an idler gear. First of all, the idler gear changes the direction of the output shaft rotation. Second of all, an idler gear serves as a mean to help reduce the size of input and output gears while retaining the spacing of the shafts.

To illustrate more, we’ve all been taught mostly that two gears will move in the opposite direction to create motion when placed directly together. But when both gears are required to move in the same direction, an idler gear needs to be used between the two to achieve so. It simply allows motion to be transferred from one gear the other without affecting the elements other than the direction in which the second gear rotates. The gears that are connected to the idler gear are often referred to as the “driving gears” which transfers motion, while the motion is accepted by the “driven gears”. Whether or not an idler gear is present, the gear ratio is calculated the same way (we’ll talk more about this in the following section). 

Idler Gears: Gear Ratio

Following the above, the gear ratio between the input and output shafts are not affected by an idler gear. Take note of the fact that the ratio depends only on the number of teeth on the first and last gear in a series of gears chained together. The intermediate gears do not alter the overall gear ratio of the chain regardless of their size, except when the direction of rotation of the final gear needs to be adjusted.

Similarly, gear ratio between the input and output shafts is also not affected by the size of the idler gear used in a non-geared friction drive system. The input shaft’s surface speed is imparted directly to the surface speed of the idler gear, and from the idler gear to the output shaft. The surface speed remains the same whether or not the idler gear is smaller or larger, which enables the output shaft to be driven at a constant speed no matter the size of the idler gear. Furthermore, there are instances where ratio of rotations between the gears may vary, such as when there is an abruptly excessive load on the system. 

Applications of Idler Gears

The primary function of an idler gear is to revise the direction of the spinning gears. In practice, the typical automobile manual transmission achieves gear reversal by utilizing a reverse idler gear between two gears. To illustrate better of the aforementioned, a driven gear will typically rotate clockwise as the driving gear rotates counter-clockwise. By adding a third gear (the idler), it will spin the same direction as the driven gear. A typical automobile transmission is fundamentally configured with such setup, so when the engine runs, the output shaft spins in the opposite direction, driving the vehicle forward.

Another application is for paper pressing using a series of rollers. Every motor needs to be powered, but coupling a motor with each of them is not cost-effective. In this case, simply adding an idler gear onto the end of the shaft of each roller can solve the problem, as each roller would be rotating the opposition direction of the previous one. The result of adding a small idler gear between each larger gear is that the rollers will be powered to operate in the same direction.

Moreover, idler gears can also transfer movement among distant shafts. This is especially useful when making a gear larger just to bring a distant gear together is impractical. Larger gears occupy more space, so in the event of distant gear, consider opting for a toothed belt or a roller chain stead so that torque can be transmitted over distance. On the other hand, for a shorter distance, you may consider using a train of idler gears. The number of gears, particularly in either even or odd numbers, will dictate whether or not the final output gear spins in the same direction as the input gear.

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