Page 41 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

22 Making Things Move

mechanical advantage. (howstuffworks.com has an excellent write-up on the topic,
which you can find at www.science.howstuffworks.com/gear7.htm.)

Gear Ratios
Gears of different sizes transmit a mechanical advantage, similar to how pulleys work.
As always, the mechanical advantage is the ratio of how much we put in to how
much we get out.
The smaller of two gears in a set is usually called a pinion, and is the one being driven.
Let’s say we have a 20-tooth pinion attached to a motor shaft. Then a 100-tooth spur
gear (of the same diametral pitch, of course) mates with the pinion to rotate an
adjacent shaft. The pinion must rotate five times to turn the output gear once, so the
mechanical advantage is 5:1.

When the gear train is being used to magnify force, the input gear will always be
smaller than the output gear. This setup is great when you have a motor and need to
multiply the work it can do by itself, or when you need to slow the motor’s output to
a speed that fits your application.
To use a gear train to magnify speed, reverse the gears so the big gear is the input
gear. The gear train is at a mechanical disadvantage in this configuration, but since
one turn of the input gear on the motor turns the mating gear five times, the speed
of the output is magnified by five.
So, take a look around you and see what kind of mechanisms you can find that have
gears in them. How about that old clock, your blender, or a can opener? The kitchen
is a great place to go looking for all sorts of useful mechanisms.

Design Constraints and Degrees of Freedom

The principle of minimum constraint design 3, 4 is one of the first things I teach my
students. It has been around for over a century, but it’s rarely taught in schools. Most
designers and engineers learn it through trial and error. That process takes time, and if
you’re reading this book, you probably don’t want to mess up designs for years to
gain that experience firsthand. So here’s the short version: Don’t constrain any design
or moving part in more ways than necessary. That’s it. Let’s examine this concept a
little more in depth.