CHAPTER X[ToC]
ON GEARING AND HOW ORDERED
The technical name for gears, the manner of measuring them, their pitch and like terms, are most confusing to the novice. As an aid to the understanding on this subject, the wheels are illustrated, showing the application of these terms.
Spur and Pinion.—When a gear is ordered a specification is necessary. The manufacturer will know what you mean if you use the proper terms, and you should learn the distinctions between spur and pinion, and why a bevel differs from a miter gear.
If the gears on two parallel shafts mesh with each other, they both may be of the same diameter, or one may be larger than the other. In the latter case, the small one is the pinion, and the larger one the spur wheel.
Some manufacturers use the word "gear" for "pinion," so that, in ordering, they call them gear and pinion, in speaking of the large and small wheels.
Measuring a Gear.—The first thing to specify would be the diameter. Now a spur gear, as well as a pinion, has three diameters; one measure[p. 122] across the outer extremities of the teeth; one measure across the wheel from the base of the teeth; and the distance across the wheel at a point midway between the base and end of the teeth.
These three measurements are called, respectively, "outside diameter," "inside diameter," and "pitch diameter." When the word diameter is used, as applied to a gear wheel, it is always understood to mean the "pitch diameter."