In recent years the speed at which gearing is run has been greatly increased. A striking instance is that of a pair of cast-iron helical wheels, 6 ft. 3 in. diameter, 12 in. wide, making 220 revolutions per minute, the speed of the pitch line being 4,319 feet per minute; these wheels are running continuously and with little noise. There is also a cut gear in a mill in Massachusetts, 30 feet in diameter, and the speed of pitch line is 4,670 feet per minute.
Fig. 272.
An internal or annular gear wheel is one in which the faces of the teeth are within and the flank without the pitch circle, hence the pinion operates within the wheel. See [fig. 272].
In internal geared wheels there is almost an entire absence of friction and consequent wear of the teeth, as compared to ordinary spur gearing.
[Fig. 273] shows a crown-wheel which has pin teeth which are fixed by one end only, on its side face and gear into a trundle wheel.
Fig. 273.
A trundle wheel has no teeth, properly speaking. Instead of teeth, it has pins as shown on illustration, [fig. 273], arranged like the rungs of a ladder between two walls. See [page 201].
Trains of Gears.—When two wheels mesh—that is, engage with each other—as in [fig. 263], one axle revolves in the opposite direction to the other; but when internal gears mesh as shown in [fig. 272], the shafts revolve in the same direction; three or more gears running together are often called a train of gears.