Gears are direct-contact bodies , operating in pairs, that transmit motion and force from one rotating shaft to another , or from a shaft to a slide (rack), by means of successively engaging projections called teeth . The four main types of gears ( spur , helical, worm ,and bevel) and a rack and pinon are shown in Figure 7.
Tooth profiles. The contacting surfaces of gear teeth must be aligned in such a way that the drive is positive ; the load transmitted must not depend on frictional contact . As shown in the treatment of direct-contact bodies , this requires that the common normal to the surfaces must not passs through the follower .
Most gears are also required to hae tooth proflies of such a shape that the velocity ratio of the gears remains constant (unless otherwise noted ,this article will deal with such gears only ).thid requires that the common nomarl must cut the line between the pivots at a fixed point.