When two spur gears of different sizes
mesh together, the larger gear is called a wheel, and the smaller gear is called
a pinion. In a simple gear train of two spur gears, the input motion and force
are applied to the driver gear. The output motion and force are transmitted by
the driven gear. The driver gear rotates the driven gear without slipping.
The wheel or the pinion can be the driver
gear. It depends on the exact function the designer wishes the mechanism to
fulfill. When two spur gears are meshed the gears rotate in opposite directions,
as shown in the figure below.