There are certain terms relating to gears with which the mechanic should be familiar. Some of the most important of these are explained below. (See Fig. 155.)

Spur. - Spur originally meant a projection or tooth, but is now used to distinguish spur gears from other varieties of gears, such as bevel gears and worm gears.

Pitch Circle. - The pitch circle of a gear is the distance around the teeth and is the same size as the friction rollers or cylinders would be if no teeth were present: i.e., when two spur gears roll together their pitch circles are considered to be constantly in contact.

Pitch Diameter. - The pitch diameter of a gear is the diameter of the pitch circle.

Fig. 154.   Train of Gears.

Fig. 154. - Train of Gears.

Circular Pitch. - The circular pitch of a gear is the distance measured along the pitch circle from the center line of one tooth to the center line of the next.

341 Gearing Terms 271

Fig. 155.

Diametral Pitch. - The diametral pitch of a gear is the number of teeth per inch of pitch diameter. (For example, if a gear has 30 teeth and its pitch diameter is 3 in., the diametral pitch is 30 / 3 or 10.)

Addendum. - The addendum of a gear is the height of the top part of the tooth, i.e., the distance from the pitch line to the point of the tooth.

Dedendum. - The dedendum of a gear is the working depth of the tooth below the pitch line. It is always equal to the addendum.

Working Depth. - The working depth of the teeth of a following gear is the depth to which the teeth in the meshing gear center into the spaces between the teeth of the first or driving gear.

Clearance. - The clearance of a gear is the amount that the tooth space is cut deeper than the working depth. (The working depth of a tooth equals the sum of the addendum and the dedendum, while its total depth equals the sum of the addendum, dedendum, and clearance.)