Dedendum

Dedendum is a term used in gear technology to describe the radial distance from the pitch circle to the bottom (root) of the gear tooth. It is one part of the tooth height, the other being the addendum, which is the radial distance from the pitch circle to the tip of the tooth. The whole tooth depth is the sum of the addendum and the dedendum.

In gear design, the dedendum provides clearance between mating gears and helps prevent interference during meshing. Standard values differ by gear standard, but in metric (module) terminology a common full-depth arrangement uses addendum a equal to the module m and dedendum b around 1.25 m, giving a total tooth depth h = a + b = 2.25 m. The outer diameter Do and the root diameter Dr are related to the pitch diameter Dp by these relationships: Do = Dp + 2a and Dr = Dp − 2b. For example, with a = m and b = 1.25m, Do = Dp + 2m and Dr = Dp − 2.5m.

The dedendum influences several aspects of gear performance, including the size of the root, interference risk, and contact ratio. Different standards (ISO, AGMA, JIS) prescribe dedendum values to balance strength, manufacturability, and noise considerations. Stub gears use a smaller dedendum, while other gear types may adopt larger or smaller allowances depending on service conditions and manufacturing practices.

In summary, the dedendum is the radial distance from the pitch circle to the tooth root, defining root size and contributing to the overall tooth depth and proper meshing with a mating gear.