Fig. 267.
A bevel wheel and pinion must be made to suit one another by both having teeth forming together an angle of 90°, therefore they are pairs, or proportioned in the number of teeth one to the other. Any other proportion used would not exactly gear and would be termed a “bastard” gear.
Fig. 268.
[Fig. 268] represents a pair of miter wheels in gear; it will be noted that the shafts, when connected, will be at right angles to each other, the wheels being in all particulars of the same dimensions; the figure answers the purpose of a much longer description, if given in words.
A miter-wheel can easily be known by putting a square upon the face of the teeth, which are always at an angle of 45° with one another, irrespective of size.
A miter-wheel is a particular kind of bevel-wheel, the bevel being limited to an angle of 45° in each wheel.
The curve of the teeth in bevel-gears, when correctly formed, changes constantly from one end of the tooth to the other, therefore bevel-gears whose teeth are produced with a forced cutter are not theoretically correct.
[Fig. 269] represents a rack and pinion: the teeth in this form of gear are shaped similarly to those in the spur wheel, shown on [page 198], with the difference that the teeth of one are on a circle and on the rack are made on a straight line.