In all calculations for the speed of toothed gears the estimates are based upon the pitch line, the latter standing in the same place as the circumference of a pulley.
To find the diameter of a gear-wheel multiply the number of teeth by the pitch, divide by 3.1416.
To find the pitch of a gear-wheel multiply the diameter by 3.1416 and divide by the number of teeth.
To find the number of teeth in a gear-wheel multiply the diameter by 3.1416 and divide by the pitch.
The breadth of wheels, where practicable, should be at least three times the pitch.
Fig. 276.
[Fig. 276] shows a scale for proportions of teeth; it is divided into tenths and used thus:
Say wheel is 2″ pitch, then from pitch circle to addendum will be 31⁄2 tenths, and from pitch circle to root of tooth will be 4 tenths measured at the 2″ line on scale, and so on.
The decimal proportions already given in example, page 210, are adopted in many workshops. Many others use the proportions approved of by Sir William Fairbairn, which are: