Like gears, belts and pulleys are used to change the speed of rotating axles. There are some major differences, however, between belts and pulleys and gears.

The first difference is the direction of rotation. With a pair of spur gears, the input axle and the output axle rotate in opposite directions. With a belt and pulley, both the input and output axles rotate in the same direction.

 

The second major difference is belts cannot transfer as much force as gears. The pulleys move because of the friction between the belt and the pulley. If the output axle were to get stuck, then there would be too much force for the belt friction to overcome, and the belt will slip. You should use a belt and pulley instead of a gear if there is a chance that the output axle, like the robot's wheels, might get stuck.

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Like spur gears, belts and pulleys do have a "gear ratio." This is the ratio of the diameter of the output pulley to the diameter of the input pulley. The image on the left shows the diameters for the 4 types of Lego pulleys.
The "gear ratio" for this pair of pulleys would be 11 to 2. Just like a gear ratio, that means the input axle must make 11 revolutions for the output axle to make 2 revolutions.

What are the "gear ratios" for these combinations of belts and pulleys? Notice that the last one is a combination of pulleys, so it is just like a compound gear ratio.

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See also: Gears, Compound gear ratios