Basic Gear Frequently Asked Questions


What is a rack?

A rack is a straightened out gear, which has straight sided teeth evenly spaced which can theoretically extend to infinite length. It is the basis for most gear calculations

What is a lead?

In parallel axis gears, lead is a linear measurement of how far a helical trace must travel to make a complete rotation around the gear. Thus, a high helix gear has a short lead, a low helix gear has a long lead and a spur gear has an infinite lead.

What is an involute?
A gear tooth has a curved profile which is generated by a straight sided rack shaped tool

What is the difference between the metric and English system of gear calculation?


  • Tooth size expressed and “Diametral Pitch”.
  • Inches and decimal inches
  • Convert Diametral Pitch to Module: M=25.4/DP


  • Tooth size expressed as “Module”.
  • Centimeters, Millimeters
  • Convert Module to Diametral pitch thus: DP=25.4/M