Page 35 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

16 Making Things Move

A wedge is like two inclined planes set base to FIGURE 1-16 An axe uses a
base. Wedges can be found on knives, axes, and mechanical advantage to split
wood.
chisels. If you drive an axe into a piece of wood, as
shown in Figure 1-16, the mechanical advantage is
the length of the blade divided by the width of the
base. In this case, you see a 6:1 mechanical
advantage. That means that if you swing the axe
and it has a downward force of 100 lbs when you
hit the wood, the splitting force that the wood feels
coming off the axe is 600 lbs on each side.

5. Screws

A screw is really just a modification of an inclined
plane. There are two main types of screws:

1. Screws used for fastening parts together.
Fastening screws use their mechanical advantage to squish two or more pieces
of material together.

2. Screws used for lifting or linear motion (called power screws). Power screws
have a slightly different geometry thread to allow them to lift or push an
object that slides along the threads, like in the screw jack in Figure 1-17.

TRY THIS Cut a piece of 8 1/2 × 11–in paper in half along the 11-in side,
and then cut one of the remaining pieces diagonally from corner to corner.
Next, line up the shorter side of the triangle with a pencil and start wrapping
the triangle around the pencil. Notice the spiral shape? This shows how a
screw is a modification of an inclined plane—the triangle.

As with any simple machine, the mechanical advantage is the ratio of what you put in
to what you get out. One example of a power screw is a screw jack that you might
use to prop up your car before changing a tire. Let’s say the screw jack has a handle
length of 12 in, as shown in Figure 1-17. The pitch of the screw is the distance
between threads, and is the distance the screw will move up or down when turned