Page 30 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

Chapter 1 Introduction to Mechanisms and Machines 11

FIGURE 1-11 LEGO motor using a friction drive pulley system. The large pulley is
1 7/8 inches and the small one is 3/8 inch, which creates a 5:1 mechanical advantage.

Open Systems
Open systems are what most people think of when pulleys come to mind, but will be
less useful to you when making projects like the ones in this book. In an open system,
one end of the rope or belt is open or loose. A good example of this is a flag hoist. A
flag hoist is just a pulley attached to the top of a long flag pole with a rope going
around it, so you can stand on the ground and pull down on the rope to raise the flag.
One pulley fixed in place like this does not magnify force or give you a mechanical
advantage. The rope moves the same distance that the flag does when pulled.
However, it does allow you to change the direction of movement.
On the other hand, one unfixed pulley does magnify force. Unfortunately, as with
levers, we don’t get something for nothing. The ability to decrease the effort we put
in comes at the expense of needing to pull the rope or belt on the pulley a longer