Spur gears are the most common type of gears. They have straight teeth and are mounted on parallel shafts as shown in the figure below.

Sometimes, many spur gears are used at once to create very large gear reductions. 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.

1. Spur Gear Materials

Gear composition is determined by the application, including the gear's service, rotation speed, accuracy and more.

• Cast iron provides durability and ease of manufacture.
• Alloy steel provides superior durability and corrosion resistance. Minerals may be added to the alloy to further harden the gear.
• Cast steel provides easier fabrication, strong working loads and vibration resistance.
• Carbon steels are inexpensive and strong but are susceptible to corrosion.
• Plastic is inexpensive, corrosion resistant, quiet operationally and can overcome missing teeth or misalignment. It is used in toy gear mechanisms.