Gear Terminology:

Before considering kinematics of gears we will define the terms utilized for describing the size, shape and geometry of a gear tooth. Here given definitions are with respect to a straight spur gear.

Pitch Circle or Pitch Curve

This is the theoretical curve along which the gear rolls out without slipping on the equivalent pitch curve of other gear for transmitting corresponding motion.

Pitch Point

This is the point of contact of two pitch circles.

Pinion

This is the smaller of the two mating gears. Usually it is the driving gear.

 Rack

 This is part of the gear that has infinite pitch circle diameter.