Page 261 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

Chapter 8 Combining Simple Machines for Work and Fun 239

NOTE 3D sketching is the practice of using cheap materials and components
you have around to build simple, temporary, and not necessarily functional
models to wrap your brain around an idea. It’s a good idea to keep some
cardboard, duct tape, LEGOs, and spare parts around for such sketches. If you
need more flexibility, you can try your hand at the 3D modeling programs
we’ll cover in Chapter 9.

Cranks

A crank is basically a lever attached to a rotating shaft. You can use a crank as a
handle to turn a shaft, as in the old bucket-raising cranks on top of a well (see the
crank on the party sheep project shown later in Figure 8-17). In this case, you’re using
the crank as a simple wheel-and-axle machine, similar to the steering wheel in a car,
as discussed in Chapter 1. If you flip this configuration, the rotating shaft can drive
the crank itself. In this case, the crank can be used to convert rotary motion to
reciprocating, or back-and-forth, motion. The throw of the crank is the diameter of
the path it travels.

You can make cranks easily by sticking a wooden dowel into a hole in a piece of
wood, or with a pair of pliers and a coat hanger or other thick wire. The crank on the
left in Figure 8-1 is friction drive, since it just relies on friction and gravity to return to
the original state. The crank on the right in Figure 8-1 is connected to the output
shaft or lever so is not just relying on gravity.

FIGURE 8-1 Friction-drive crank that relies on gravity (left) and positive-drive oscillating
crank (right)