An intermediate gear that meshes with both the driving and driven gears. It reverses rotation or bridges a long center distance but does not change the overall ratio, which depends only on the first and last gears. Idler teeth are loaded on both flanks, one mesh pushing each way, so their roots see fully reversed bending and are rated at roughly 70 percent of the normal allowable bending stress.
Idler cracked at the roots first, every tooth gets bent both ways on an idler, so derate it or it'll fatigue before the rest of the train.
One idler reverses direction, two idlers keep it, and neither changes the ratio.
Also heard as
- idler gear
- intermediate gear
Related terms
Gear ratio
The number of teeth on the driven gear divided by those on the driver, setting speed reduction and torque gain.
Bending stress
The tensile stress in the root fillet of a loaded tooth, which decides whether teeth break off.
Tooth breakage
A gear tooth cracking off, most often by bending fatigue starting in the root fillet on the loaded side.