In the previous article, we introduced common types of gears, including spur gears, helical gears, internal gears, bevel gears, and worm gears. However, even when the gear type is the same, the actual material used may be completely different. For example, some gears are made of plastic, some are made of carbon steel or alloy steel, and others use stainless steel or even high-performance engineering plastics. Why can gears be made from such different materials even though the