My, What Big Teeth You Have!

When I get a call from a new client, one of the first things I ask is “How big are the gears?”

One person’s “big” parts may be laughably tiny for someone else, but the great thing about gears is that everything is proportional. Understanding those proportions is an important step for anyone who needs to work on a variety of products.

Most modern geared products use teeth of “standard” proportions; those of you who deal with older designs may encounter other tooth profiles which complicate the explanation a bit. We’ll cover the standard stuff first and get to the legacy stuff later.

A cryptic description of a gear will start with the number of teeth, “pitch,” face width, helix angle, and tooth form. This is often enough information to provide a cost estimate or begin a sketch of the item. Pitch is a measure of the distance between the teeth, a carryover from the days when many large gears were cast to finish dimensions and the pattern makers laid them out like floor joists.

Originally those teeth were “circular pitch,” as in “x” inches or millimeters from one tooth flank to the adjacent flank. Over time, “circular pitch” (CP in gear lingo) was displaced by “diametrical pitch” (DP) or “module” — the metric equivalent of DP. The change was made to make the calculations easier; not that the beginner would notice in this time of calculators, computers, and spreadsheets.

To give you an idea of how these “pitch designation systems” relate to each other, let’s consider a 36-tooth gear. A standard 36-tooth 1 DP spur gear would be 38 inches in outside diameter (OD). If it were 1 module, that OD would only be 1.496′; a 1-inch CP version’s OD would be 12.096′.

Among the first things you will want to put in your notebook is a chart showing the “pitches” you routinely deal with. Most computer programs are quite specific on the input they require, so you will want to master the conversion formulas needed to translate any drawings into your preferred format.