worm gear addendum and dedendum, clearance, center distance 1

Snowman

The machinery handbook is not really a good source for designing these types of gears.

First of all, if the tooth heights are equal, and if the center distance is the standard center distance, and if the tooth thicknesses are also equal, things will bind up all over the place.

Gearing in North America is usually designed using the concept of a Basic Rack. There are several different types of Basic Rack forms that can be used. Each of these forms are set up so that gears made of the same basic rack form will usually have conjugate action.

Even as you vary tooth thickness, if the basic rack definition remains the same, you usually will end up with conjugate acction.

As you vary tooth thickness (ie. a worm with a thinner tooth than the worm gear), the addendum and dedendum values of each will shift however the whole depth will remain constant.


The .002" extra distance in the housing is probably for extra backlash, but not knowing your full design, this is just a guess.

You may find the following software source much more useful in your gear design problems.

Mixing design calculations is ok in
that if it is good in Europe, it
should be good in the States. Your
formula for addendum and dedendum
is correct in that the dedendums
equal the addendum plus clearance.
If you have very small modules the
clearance may want to be greater than
the .25 times the module ratio.
Because you cannot cut a perfect gear
or worm, you do need backlash to
overcome the manufacturing errors
and/or alignment errors.

Try the UTS computer package (Rockford, IL) for worm gear design. It allows for modified sections, such as thick-thin, modified addendum, various clearances, modulus differences, etc.