Page 272 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

Chapter 8 Combining Simple Machines for Work and Fun 249

• Bell crank Can be used to create different FIGURE 8-11 A bell crank
motion conversions and transformations
depending on which part is driven and which
part is doing the driving (see Figure 8-11). This
is why you see it in many of the boxes
in Table 8-1.

• Geneva stop Used to create intermittent
output from a constant rotary input. In the
version shown in Figure 8-12, it takes four
turns of the lower circular shape to turn the
star-shaped piece once. It’s difficult to get the
geometry just right, but you can get off-the-
shelf versions from WMBerg or make your own
with downloaded plans from Thingiverse (a 2D
version for laser cutters from www.thingiverse
.com/thing:1616 or a version for 3D printers at
www.thingiverse.com/thing:1642).
• Quick return Used to create oscillating motion from a continuous rotary
input. In the example in Figure 8-13, since the peg that drives the arm back
and forth is closer to the bottom pivot point during part of the waving
motion, the wave in one direction will be faster than in the other.

FIGURE 8-12 A Geneva stop FIGURE 8-13 A quick return
mechanism