It was in Medieval times when a toothed wheel used in machinery came to be known as a gear. This was the very beginning, when they were made of wood, and very large compared the the fine workings of a pocket watch.
The technical function of gears is to transmit torque. In the example of the classic Mill the system of gears changed the linear movement of the water, which spun the large vertical wheel, converting it into a slower, but higher torque, rotation of a horizontal grinding stone.
The mechanical advantage is gained based on the difference in size, and number of teeth, between two connecting gears. For example, let’s look at the gears on your bicycle. Let’s see… looking… looking… hey! There are no gears on your bicycle!
In fact those things you see by the pedals, and on the rear wheel, aren’t actually gears at all. If they were meshed with each other, they would be gears, but since they are connected by a chain, we call them sprockets. A rose by any other name, however, still follows the same laws of physics.
So with your bicycle sprockets, a mechanical advantage is gained based on the size of the two sprockets the chain is connecting. If the rear wheel sprocket is the same size as the pedal sprocket, it would be a 1 to 1 ratio. No mechanical advantage. This is the “low gear” position that is easiest for starting off, or going up long hills. As the chain moves to smaller sprockets, more mechanical advantage is gained and, if you were to maintain your pedaling, the speed the rear wheel would move faster and faster with each successively smaller sprocket.
Now if gears can become sprockets simply by adding a chain, it’s also very easy for sprockets to become pulleys. The only difference between the two is the cogs, which is what you call the teeth on a gear or a sprocket. In gears it is the cogs that mesh together. Cogs on a sprocket slip into the links of a chain. The benefit of this is the lack of slipping. Pulleys have no cogs, and rely on friction to move the chain (or belt). The benefit of pulleys and belts, as used in automobile engines, is that they will not get snagged or hung up at high speeds as easily as a chain might catch on a cog.
- Source: Gear – Wikipedia