Fig. 6.
An annular or internal gear-wheel is one in which the faces of the teeth are within and the flanks without, or outside the pitch-circle, as in [Fig. 6]; hence the pinion p operates within the wheel.
When the teeth of a wheel are inserted in mortises or slots provided in the wheel-rim, it is termed a mortised-wheel, or a cogged-wheel, and the teeth are termed cogs.
Fig. 7.
When the teeth are arranged along a plane surface or straight line, as in [Fig. 7], the toothed plane is termed a rack, and the wheel is termed a pinion.
Fig. 8.