Letv

=

velocity in revolutions per minute.

a = centre distance.

b= number of teeth in both wheels.

t = width of tooth, or space, or pitch-line.

D" = working depth of tooth.

f =

= amount added to working depth for clearance. D" + f = whole depth of tooth.

In France "module" pitch is almost universally used, and is gradually coming into use in this country. This is a modification of diametral pitch, and has the advantage of only requiring measurements in millimetres and such fractions thereof as 0.25, 0.5, and 0.75. Diametral pitches call for unusual divisions of the inch, such as sevenths, ninths, elevenths, etc., which are apt to be confusing to both the designer and the workman. The module corresponds to the height of the tooth above the pitch-line, and is the pitch diameter in millimetres divided by the number of teeth. Conversely the pitch diameter is equal to the module multiplied by the number of teeth. The following formulæ will enable the designer to calculate all necessary dimensions of gears of module pitch, and the table, No. 10, will facilitate the work. Fig. 35 shows at a glance the comparative sizes of module-pitch teeth from Mod. 1 to Mod. 12.

Formulæ for gears of module pitch

Let M = module in millimetres.

D' pitch diameter in millimetres.

=

f

ƒ = amount added to working depth for clearance. C = circular pitch in millimetres.

To design a change-speed gear for a car the data required are (a) normal speed of the motor, (b) number of speeds required, (c) value of these speeds in miles per hour, and (d) diameter of the driving wheels. By the aid of table No. 11, p. 95, the revolutions per minute of the driving wheels for any given number of miles per hour can be readily obtained by multiplying the number in the