Page 253 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

Chapter 7 Bearings, Couplers, Gears, Screws, and Springs 231

hand-driven and used to squish grapes in wineries before mechanical presses were
invented.

McMaster and Nook Industries (www.nookindustries.com) are two good sources for
power screws.

Springs
Springs can be very useful components in your mechanisms. They can keep lids
closed, return solenoids to their original position, create latches and ratchets, and
more. Springs can store energy, as mentioned in Chapter 5, and are often
components in the mechanical toys we’ll talk about in Chapter 8. Here, we’ll cover
the different kinds of springs and how they can be used.

Compression Springs

When most people hear “spring,” the compression spring is the type that comes to
mind. You can find tiny ones inside mechanical pens and pencils, and larger ones in
the shocks on mountain bikes. First, let’s go over some vocabulary so you’ll know
what all the words mean when you shop for springs. Figure 7-43 shows how these
terms apply to a compression spring.

• Inner diameter The diameter of the biggest rod that will fit inside
the spring.
• Outer diameter The diameter of the outer edge of the spring.
• Wire diameter The diameter of the wire that is wound to make the spring.

• Free length The length of the spring before you do anything to it.
• Solid height The height of the spring when completely squished.

• Spring rate or stiffness The k in Hooke’s law (in units of force/length),
which tells you how much the spring will squish under a given weight:

Force (F) = Stiffness (k) × Distance (x)