Page 29 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

10 Making Things Move

the belt. This helps the motor drive the FIGURE 1-10 Timing belt on the engine of a
belt without slipping, called positive drive, car as a closed pulley system
because the belt and the teeth on the
pulley mesh together.

You can find a similar system inside
cameras that use 35mm film. The holes
on the edge of the film actually match up
with little teeth on the pulley wheel the
film wraps around.

Closed pulley systems can also use smooth
belts and pulleys that are spaced so the
belt is tight enough not to slip on the
pulleys. This is called friction drive,
because the belt is made to fit tight
around the pulleys so the friction between
the pulleys and belt stops it from slipping.
LEGO systems use pulleys with belts that
are color-coded depending on length, as
shown in Figure 1-11.
Closed pulley systems are used to translate rotational motion between axes. There is
a mechanical advantage only if the driven, or input, pulley is smaller than the output
pulley, as shown in Figure 1-11.

Any pulleys in between the input and output are called idlers, because they don’t do
anything other than redirect the belt. Sometimes the idlers are spring-loaded, or
mounted such that they are adjustable, so the tension on the belt can be controlled.

The mechanical advantage of closed system pulleys is easier to calculate than with
levers. It’s just the ratio of the pulley diameters. If a 1 inch (in) diameter pulley is stuck
on a motor and drives a 3 in diameter pulley, the mechanical advantage is 3:1. This
means that the system can turn something that’s three times harder to turn than the
motor could by itself.