An idler gear is placed in between the input and output shafts, that was used to change the direction of rotation of the output shaft / input shaft or to fill a large gap between the input and output shafts without affecting the speed ratio.

Idler pulleys are used in car, larry, and also the truck engines to improve belt drive performance. The power generated by the crankshaft rotates the wheel, that turns the belt and enables torque, speed, and mechanical power to be transmitted.