Photo courtesy Emerson Power Transmission Corp.
Spur gears are the most common type of gears. They have straight teeth, and are mounted on parallel shafts. Sometimes, many spur gears are used at once to create very large gear reductions.
Spur gears are used in many devices that you can see all over HowStuffWorks, like the electric screwdriver, dancing monster, oscillating sprinkler, windup alarm clock, washing machine and clothes dryer. But you won't find many in your car.
This is because the spur gear can be really loud. Each time a gear tooth engages a tooth on the other gear, the teeth collide, and this impact makes a noise. It also increases the stress on the gear teeth.
To reduce the noise and stress in the gears, most of the gears in your car are helical.