A spur wheel has teeth on its circumference which run parallel to its shaft; wheels as shown in [fig. 271] are termed helical wheels; these are similar to spur wheels except their teeth are arranged upon different angles to the shaft.
A bevel is a slant or inclination of a surface from a right line, hence a bevel wheel is one whose teeth stand beveling or at an oblique angle to the shaft, or towards the center; see [fig. 267].
Miter wheels are bevel wheels of the same size, working at right angles with one another; see [fig. 268].
The diameter of both spur and bevel wheels is measured and calculated neither from the outside nor from the bottom of the teeth, but on the pitch circle. When we speak of the diameter of a spur or bevel wheel, we mean the diameter of the pitch circle, without any reference to the form of tooth.
The addendum circle of a toothed wheel is as shown in illustration, [fig. 264]; addendum means “something added,” and, as shown in the figure, it is the part added beyond the pitch “line” or circle.
Fig. 264.
The pitch line is the most important one in gearing; the “pitch line” or “pitch circle” is supposed to be the working circle. This is shown in P—P in [fig. 274].
The periphery of a wheel is the extreme circumference, as N in [fig. 274].
All parts of gear-wheels consist of portions, to which have been given generally accepted names. [Fig. 264] shows the “addendum circle” and the “pitch line” as marked. The teeth and rim are shown in white, and the other portions are indicated by the names.