Gears and pulleys do useful work. An almost infinite number of uses for gears and pulleys exist, from automotive transmissions to ship rigging. Furthermore, mechanical clocks rely solely on gears and pulleys to move the hands. By understanding the strength requirements, you will gain an understanding of why only certain materials can be used to make gears and pulleys.
For marine applications, stainless steel is the preferred material to make pulleys, winches and an assortment of riggings since it will not corrode or rust. This is particularly true in saltwater environments, where ordinary steel would rust quickly. An area pulleys are found in sailboat rigging is at the top of the mast. Pulleys are used to hoist up the sails. Other areas where pulleys are used are the points at the bottom of the sails, where the ropes attach to the boat. Also, on all boats, the anchor chain runs through a pulley, which is made of stainless steel.
For car transmission gears, steel is the preferred material. Steel is strong and holds its shape well, which is critical since all the engine's output horsepower flows through the transmission. For example, one gear may have to transfer over 100 horsepower to the next gear. If the gears were made of a soft material such as aluminum, the teeth would simply shear off.
At its most basic level, a mechanical clock is a transmission. The speed of the turning shafts reduce down in speed, so one shaft makes one turn every 60 seconds. Other shafts turn even slower for the minute and the hours. Many clocks also use steel for their gearing.
Many gear mechanisms use brass for the gears. Brass is not as strong as steel, but it is strong enough for areas where only a little bit of strength is needed. Many clocks have brass gears since the stress is very small. The oldest-known gear mechanism is the Antikythera mechanism. Archaeologists believe it was made around 80 B.C. X-rays revealed its gear was made out of bronze, which is an early type of brass. Its function is not yet fully understood, but research is ongoing.
Historically, wood was used to make pulleys before metals were used. Many early sailing ships, such as the Viking or ancient Roman ships, used wooden pulleys extensively for hoisting sails with ropes. In 2010, wood is not used to make gears and pulleys since metals are lot stronger. Some hobbyists, however, make clocks with all wooden gears.