Page 103 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

84 Making Things Move

Lubricants and Grease
It’s usually a good idea to lubricate things that move. It keeps friction lower, which
increases efficiency by allowing more input power to transfer to the output. It also
helps keep mechanisms quiet.

A lot of bearings, motor gearboxes, and other components come with grease already
in them. For quick, multipurpose fixes, WD-40 is a good light lubricant; the company
claims it has over 2,000 uses. Another well-known brand that’s a better lubricant is
3-IN-ONE. From bike chains to squeaky scissor hinges, a drop of this stuff will do
the trick.

There are dozens of types of oils and greases available. Grease is thicker than oil and
tends to stay where it’s put. Oil can be runny. If your chosen multipurpose lubricant
doesn’t do the trick, try looking on McMaster for your application—for example,
search for “gear grease”—to find a specific recommendation. Beeswax reportedly
works well with moving wooden parts.

Free Body Diagrams and Graffiti Robots

One of the biggest problems my students have is figuring out how much motor
torque they need for a certain project. It’s impossible to size a motor accurately
without doing at least a little analysis of what you want the motor to do. Do you want
your motor to drive a small mobile robot quickly? Or maybe you want to use your
motor to lift a heavy weight or pull on a rubber band?
Being able to sketch free body diagrams is a handy skill when you’re trying to
determine how to mount something or what kind of motor torque you need, and any
other time you need to choose a component based on strength or torque. A free
body diagram is like a simplified snapshot of all the forces and moments acting on a
component. You can also use these diagrams to figure out the forces and moments
you don’t know. The body referred to is just one object or component of a system.

Here is what is included in a free body diagram:

• A sketch of the body, free from any other objects, with only as much detail as
necessary (most of the time, a dot is all that’s needed)