3.3 Belt drives and gear trains.

A belt drive is a system of pulleys connected by belts. Each belt connects a pair of pulleys, so they turn together.
To understand how a belt drive works, we can analyse the example above:

• Wheel 1 turns wheel 2 which moves faster because it is smaller. The size ratio between the wheels is D1 / D2 = 1.5. If wheel 1 makes 1 rotation, wheel 2 makes 1.5 rotation.
• Wheel 2 and wheel 3 are connected to the same axis, so they turn together. If wheel 2 makes 1.5 rotations, wheel 3 also makes 1.5 rotations.
• Wheel 3 turns wheel 4, wich moves faster because it is smaller. The size ratio between the wheels is D3 / D4 = 2. If wheel 3 makes 1.5 rotationsm, wheel 4 makes 1.5 x 2 = 3 rotations.

To calculate the ratio os transmission between the first wheel and the last wheel of a belt drive, we must multiply the ratios of transmission of the first pair of wheels and the second pair of wheels:

                    N4 : N1 = D1 x D3 : D2 x D4

N is teh speed of rotation and D is the diameter of the wheel.