3.3 BELT DRIVES AND GEAR TRAINS:
A belt drive is a system of pulleys connected by belts.Each belt connects 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 ratations.
• 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,which moves faster becouse it is smaller.The size ratio between the wheels is D3/D4=2. If Wheel 3 makes 1.5 rotations,wheel 4 makes 1.5*2=3 rotations.

to calculate the ratio of 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*D3/D2*D4

3.4 CHANGES IN DIRECTION AND ROTATION:
We can use various system to change the direction of rotation or the axis of rotation in a belt drive.We can also vary the distance between the wheels.With belts,we can change the direction of rotation and the axis of rotation quites are more precise and they transmit more rotary force,or toque.

Parallel Axes:

Pendicular axes:

Crossed axes: