en 10 _diametral pitch_.

CIRCULAR PITCH.--Now the term "circular pitch" grows out of the
necessity of getting the measurement of the distance from the center of
one tooth to the center of the next, and it is measured along the pitch
line.

Supposing you wanted to know the number of teeth in a gear where the
pitch diameter and the diametral pitch are given. You would proceed as
follows: Let the diameter of the pitch circle be 10 inches, and the
diameter of the diametral pitch be 4 inches. Multiplying these together
the product is 40, thus giving the number of teeth.

[Illustration: _Fig. 123. Bevel Gears._]

It will thus be seen that if you have an idea of the diametral pitch and
circular pitch, you can pretty fairly judge of the size that the teeth
will be, and thus enable you to determine about what kind of teeth you
should order.

HOW TO ORDER A GEAR.--In proceeding to order, therefore, you may give
the pitch, or the diameter of the pitch circle, in which latter case the
manufacturer of the gear will understand how to determine the number of
the teeth. In case the intermeshing gears are of different diameters,
state the number of teeth in the gear and also in the pinion, or
indicate what the relative speed shall be.

[Illustration: _Fig. 124. Miter Gears._]

This should be followed by the diameter of the hole in the gear and also
in the pinion; the backing of both gear and pinion; the width of the
face; the diameter of the gear hub; diameter of the pinion hub; and,
finally, whether the gears are to be fastened to the shafts by key-ways
or set-screws.

Fig. 122 shows a sample pair of miter gears, with the measurements to
indicate how to make the drawings. Fig. 123 shows the bevel gears.

BEVEL AND MITER GEARS.--When two intermeshing gears are on shafts which
are at right angles to each other, they may be equal diametrically, or
of different sizes. If both are of the same diameter, they are called
bevel gears; if of different diameters, miter gears.

[Illustration: _Fig. 125. Sprocket Wheel._]

It is, in ordering gears of this character, that the novice finds it
most difficult to know just what to do. In this case it is necessary to
get the proper relation of speed between the two gears, and, for
convenience, we shall, in the drawing, make the gears in the relation of
2 to 1.

DRAWING GEARS.--Draw two lines at right angles, Fig. 124, as 1 and 2,
marking off the sizes of the two wheels at the points 3, 4.