Gear Theory
Terminology for Involute Gears
See the figure below for fundamental spur gear terms.
Pitch circle
The pitch circle is the circle representing the original cylinder which transmitted motion by friction, and its diameter the pitch diameter. The pitch circles of mating gears are tangential.
Base circle
The base circle is an imaginary circle used in involute gearing to generate the involutes that form the tooth profiles.
Addendum factor
The addendum factor is the radial height of a tooth above the pitch circle; also, the radial distance between the pitch circle and the addendum circle.
Addendum circle
The addendum circle, or outside circle, is the circle that contains the tips of the teeth, and its radius is the outside radius (tip radius).
Dedendum factor
The dedendum factor is the radial depth below the pitch circle; also the radial distance between the pitch circle and the dedendum circle.
Dedendum circle
The dedendum circle, or root circle, is the circle that contains the ends of the tooth spaces, and its radius is the root radius (foot radius).
Clearance
The clearance is the difference between the addendum factor of one gear and the dedendum factor of the mating gear.
Center Distance
The center distance of a pair of meshing spur gears is the sum of their pitch circle radii. One of the advantages of the involute system is that small variations in the center distance do not affect the correct working of the gears.
Backlash
The backlash is the difference between the circle thickness of one gear and the tooth space of the mating gear.
Tooth Thickness
Circular thickness or tooth thickness is measured on the tooth at the pitch circle, that is, it is the length of an arc.
Module
The module is the pitch diameter divided by the number of teeth.
Diametral Pitch
The diametral pitch is the number of teeth per length of pitch circle diameter. A toothed gear must have an integral number of teeth. The diametral pitch is, by definition, the number of teeth divided by the pitch diameter.
Line of Action
Contact between the teeth of meshing gears takes place along a line tangential to the two base circles. This line passes through the pitch point and is called the line of action.
Pressure Angle
The pressure angle is the angle between the line of action and the common tangent to the pitch circles at the pitch point is the pressure angle.