Page 249 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

Chapter 7 Bearings, Couplers, Gears, Screws, and Springs 227

Pulleys and Sprockets, Belts and Chains
Belt or chain drives are often preferred over gears when torque needs to be
transferred over long distances. Imagine how funny a bicycle would look with a bunch
of gears between the pedals and the back wheel. They are also more forgiving about
misalignment than gear systems are.

Sprockets, like the ones on your bicycle, are used with chains. Pulleys are used with
belts, and can be flat or V-shaped with matching belts or grooved pulleys with
matching toothed belts. We covered the latter type, called a timing belt pulley system,
in Chapter 1. The pulleys and sprockets that come with hubs and set screws are
mounted on shafts and motors to do the work. Remember that you have a mechanical
advantage only if the input pulley is smaller than the output pulley, and the advantage
is just the ratio of their sizes. For example, if your input pulley is half the diameter of
the output, your mechanical advantage is 2:1.

It’s common to include one or more
tensioners in a pulley system (see FIGURE 7-40 MakerBot timing belt pulley
Figure 7-40). Tensioner is the system with tensioners (image used with
permission from MakerBot Industries)
common name for a pulley that’s
spring-loaded and/or adjustably
mounted in a slot to keep the belt
tight while the mechanism runs. Idler
Tensioners are often tightened after
Pulleys
the belt is installed, which makes
installation much easier than needing
to stretch the belt over pulleys that
are already in position. Tensioners are
similar to idler gears in that they
don’t change the mechanical
advantage of the system; they just
alter the behavior. In fact, they’re
often called idler pulleys, and
commonly have bearings or bushings
as hubs to allow for smooth rotation.