Friction gear-wheels are those which communicate motion one to the other by the simple contact of their surfaces.
In frictional gearing the wheels are toothless and one wheel drives the other by means of the friction between the two surfaces which are pressed together.
Grooved friction wheels are used to give greater cohesion than can be obtained by the plain surface.
[Fig. 263] shows a pair of spur-wheels in gear. The dotted circles which meet are the rolling circles, called the “pitch line” or “pitch circle.”
Fig. 266.
A spur mortise wheel is similarly shown in [fig. 266]; it is very like in appearance to a spur wheel; it differs essentially in that the teeth are separate cogs, fixed in singly to the rim; see also [fig. 265], [page 201].
Note.—The teeth of spur wheels cast from a pattern must of necessity be larger at one side than at the other, because the teeth must have taper to permit the extraction of the pattern from the mould; therefore, in fixing wheels to gear, the large side of one should meet the smaller side of the other; should the two large sides come together the teeth will meet only at the large side, and the teeth will probably break away from the excessive strain on that point.
Skew gearing are bevel wheels working out of center; the teeth do not form radial lines from the wheel center.
[Fig. 267] shows a pair of bevel wheels in gear as described on [page 199]. A bevel mortise wheel, i. e., one having cogs inserted in its rim instead of teeth.