Gear Terminology

1-3 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

Reference diameter
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)

reference pitch

Circumference
Reference pitch
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.

reference circle

Friction pulley (Reference circle)
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

Reference diameter
Number of teeth x Module =
z1… ? x ? = ??
z2… ? x ? = ??
Tip diameter
Reference diameter + 2 x module =
z1… ? +2 x ? = ??
z2… ? + 2 x ? = ??
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

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