Types
Spur, helical, bevel and worm
The shafts can be parallel or not, and they can meet or miss each other. For each arrangement there is a gear that fits.
The shafts that gears connect must be relatively close, but they may be parallel or nonparallel, and intersecting or nonintersecting. Britannica Scholars: gear
Spur
Parallel shafts can be connected by spur gears, with teeth that are straight lengthwise and parallel to the shaft axes. Britannica Scholars: gearHelical
Parallel shafts can also be connected by helical gears, with twisted, screwlike teeth. Britannica Scholars: gearBevel
Intersecting shafts are connected by bevel gears, with tapered teeth arranged on truncated cones. Britannica Scholars: gearWorm
Nonparallel, nonintersecting shafts are usually connected by a worm and gear. Britannica Scholars: gear The worm resembles a screw, and the gear resembles a quarter section of a long nut that has been bent around a cylinder. Britannica Scholars: gear
The commonest angle between nonparallel shafts, either intersecting or nonintersecting, is a right angle (90°). Britannica Scholars: gear
Because it is basically a screw, a worm gear may have only one thread (tooth). Britannica Scholars: gear To maintain continuous contact with parallel-shaft gears (spur and helical), the pinion must have at least five teeth. Britannica Scholars: gear
Five teeth, or one thread
A worm may have only one thread (tooth). The “at least five teeth” rule is for the pinion of a spur or helical pair, so that it keeps continuous contact. The two figures answer different questions.
For a large speed ratio in a single gear pair, a worm and gear are well suited. Britannica Scholars: gear
If the shafts must stay parallel and you still need a large ratio, you use a train.