In a gear train with an idler, the idler's primary function is to

• A. Change direction of rotation
• B. Increase torque
• C. Change gear ratio
• D. Decrease noise

Answer: (A)

The main idea is that an idler gear is used to pass motion from one gear to another while reversing the direction of rotation. When you place a single idler between a driver and a driven gear, the driver turns, the idler turns in the opposite direction, and the driven gear ends up turning in the opposite direction to the driver. The overall speed ratio stays determined by the teeth counts of the driver and driven gears, so the idler doesn’t change the gear ratio. It mainly provides layout flexibility and proper meshing, with only minor effects from friction. It doesn’t inherently increase torque, and while it can influence noise or alignment indirectly, those are not its primary purpose.