Gear Terminology

Important Gear Terminology in this picture :

       Center distance

       Length of path of contact (Line of action)

       Backlash

       Interference point

       Pitch point

       Reference pitch

       Center line

       Tip circle

       Base circle

       Root circle

       Tip diameter

       Reference diameter

       Base diameter

       Root diameter

       Pressure angle

       Face width

       Tooth depth

       Addendum

       Dedendum

       Tooth thickness

Gear Module

1. Reference diameter
2. The number of teeth

Module” is the unit of size that indicates how big or small a gear is. It is the ratio of the reference diameter of the gear divided by the number of teeth. Thus m = d/z (Module = Reference diameter/Number of teeth)

The mutual relation between the module and the reference diameter, etc. is as follows :
Reference diameter d = mz (Reference diameter = Module × Number of teeth)
Number of teeth z = d/m (Number of teeth = Reference diameter / Module)
Reference pitch p = πm (Reference pitch = π x module)

Then, what is the reference pitch ?
It is equal to the circumference divided by the number of teeth.
Reference pitch = Circumference (πd) / Number of teeth (z)

1. Circumference
2. Reference pitch
3. Diameter

Then, what is the reference circle ?
Assume that there are two friction pulleys in contact whose diameters are equal to the reference diameters. As the surfaces are smooth, the rotation will not go properly when great force is applied. This problem will be solved if there are teeth on the periphery of the friction pulley. And this is the concept of gearing.

1. Friction pulley (Reference circle)
2. Both reference circles come in contact

Summary
(1) The module describes the size of a gear.
(2) A pair of gears can only mesh correctly if and when the base pitch is the same.

Practicing What You’ve Learned

Spur Gear
Module m = 3
Pinion z1 = 15
Gear z2 = 55

1. Reference diameter
   Number of teeth x Module =
   z1… ? x ? = ??
   z2… ? x ? = ??
2. Tip diameter
   Reference diameter + 2 x module =
   z1… ? +2 x ? = ??
   z2… ? + 2 x ? = ??
3. Center distance
   Add reference diameters then divide by two… ? + ? / ? = ??

Helical Gear
Module m = 3
Pinion z1 = 15
Gear z2 = 55
Helix angle β = 16º15′ / then cosβ = 0.96

1. Reference diameter
   z1… ? x ? / ? = ??
   z2… ? x ? / ? = ??
2. Tip diameter
   z1… ? + 2 x ? = ??
   z2… ? + 2 x ? = ??
3. Center distance… ? + ? / ? = ??