Gear Speak: A Guide to Gear Language, Part 1

Those of us that have been in the manufacturing field for any length of time know that we speak our own language. Further, each segment seems to have its own dialect – either words unique to their respective industry, or in some cases, common words but with different meanings when applied to the segment. Mechanical components are certainly no exception to this rule. Today, we’re going to talk gears. We’ll take a look at the many terms used in the gear world. Let’s start with the words used to describe their anatomy:

Teeth - these are the raised, pointed portions around the edge of a gear. They mesh with the teeth of another gear to transmit motion and power.

Addendum - radial distance between the pitch diameter and the top of the gear tooth.

Dedendum - radial distance between the pitch diameter and the bottom of the gear tooth.

Pitch Diameter – is a theoretical circle that passes through the point where the two gear teeth mesh, and the contact happens. It separates the addendum and dedendum, and represents the diameter at which the correct tooth spacing is achieved.

Root Radius – is the radius of the root circle containing the bottom of each tooth surface.

Diametral Pitch – this one is pretty simple, it represents the number of teeth per inch, indicating the size of the teeth.  A higher pitch means a finer tooth, while a lower number indicates a larger, coarser one. When it comes to gear operation, a finer diametral pitch will provide much greater precision than a course one. This is normally obtained by dividing the number of teeth by the pitch diameter.

Module - conversely, for metric measurements, the term module is used to indicate the size of the tooth. It has an inverse relationship to diametral pitch by finding the number of millimeters per tooth. The larger the value, the coarser the tooth; the smaller the value, the smaller and finer the tooth.

Gear Ratio - is the numerical relationship between two mating gears based on the number of teeth and the number of rotations it takes the secondary gear (pinion) to make one full rotation in relation to the main (driven) gear.

Gear Mounting

Now that we’ve covered the basic terms related to a gear’s physical attributes, let’s look at how they are mounted. There are several approaches, which roll up into three main categories: hubbed, hubless, and split hub gears. An example of a hubbed gear is a pin hub gear, which has a central hub with an extrusion that is mounted using a set screw. A split hub gear has a basic hub with saw cuts, and requires a fastener to clamp down around it. It could be mounted onto an axle, shaft or other rotating component.

A hubless gear lacks a central hub or sometimes a bore. There are a few methods of mounting them including clamping, press fitting hubs, and adhesive bonding. A spline or keyed connection may also be used in a bore. If the gear has holes, a bolt circle mounting approach may get the job done. Finally, the gears could be welded or brazed in certain applications. With split hub gears, clamping is used to mount them.

To learn more about various hub styles, click here to see our blog on the topic.

Stay tuned for “Gear Speak, Part II”, where we’ll review the key terms relating to the interaction of 2 or more gears!