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228 Making Things Move

Two good sources for all these kinds of pulleys and belts are McMaster and Stock
Drive Products. ServoCity is a good source for smaller sprockets and chains, especially
if you’re working with servo motors or the ServoCity DC motors.

Standard Pulleys and Belts

Standard pulleys provide a friction drive, so they are very sensitive to getting the belt
stretched just enough to transfer motion between pulleys, but not so much that the
tension causes friction or structural problems. Two pulleys connected by a belt will
rotate in the same direction. To get them to rotate in opposite directions, put a half
twist in the belt to create a figure 8.

Pulleys can be totally flat on the perimeter or have grooves that accommodate round
or V-shaped belts. Some belts are very stiff and need a lot of tension to make them
work properly, which will not bode well if you have a cardboard-and-popsicle-stick
construction. So before committing to a belt, make sure you have the rest of the
structure in place. There’s no really good way to estimate the stiffness of a belt before
you buy it, but in general, the thinner and skinnier it is, the more flexible it will be.

Timing Pulleys and Belts

Timing belts provide positive drive since the belt teeth mesh with the grooves in the
timing belt pulley. You can find these in cars (see Figure 1-10 in Chapter 1), and also
on a smaller scale in printers, copiers, and in the CupCake CNC (see Figure 7-40).
There are a dozen different series of sizes with names like MXL and HTD, but the
series name is less important than just making sure your pulley and belt are the same
series, and that your pulley is wide enough to accommodate your belt. The timing belt
pulley and belt should be the same pitch, similar to meshing gears.

Sprockets and Chains

Sprockets and chains provide a positive drive similar to gears because the sprocket
teeth and chain mesh together. Standard bicycle chain is 3/8 in, and you can find
smaller metal chains and even plastic chains with snap-together links. Figure 7-41
shows an aluminum sprocket and 14 in chain mounted to a servo motor.