Where gears are on parallel axes, either spur or helical gears are generally employed. Either type can be used as external or internal drives. The herringbone gear is similar to two helical gears having reversed directions of spiral, placed side by side so that the teeth come together to form a chevron pattern. The rack and pinion, used to convert rotary motion to reciprocating, generally uses a spur gear.
For intersecting axes either straight bevel or spiral bevel gears are used as a rule. The latter type may be used on angle drives where the shafts do not intersect at full 90 degrees. The contact of the teeth in such gears gives a rolling motion. With non intersecting and nonparallel axes the types of gears used are crossed helical, single enveloping worm, double enveloping worm, or hypoid. Here the contact of the teeth gives a sliding as well as a rolling motion. In most cases a gear set will be used to change speed, and in such cases the smaller gear is designated as the pinion. Both the number of teeth on a pinion and the ratio of the teeth on the driving and driven member may vary , but with bevel gears there is seldom less than 12 teeth to a pinion.
While some spur and straight bevel gears are still made of cast iron, the tendency in all types of gearing is for the use of steel. Exceptions will be found to such practice, for example, in the use of bronze for one member of worm gears. Some small gears and even larger pinions are made of plastics, such as ‘Delrin,’’ ‘Nylon.’’ ‘Teflon,’’ etc. Pinions have been and may still be made of rawhide, pressed paper, etc, but our concern is primarily with lubrication of metal gears.
