Page 317 - Robot Builder's Bonanza

286 MOUNTING MOTORS AND WHEELS

USING MOTORS WITH GEAR REDUCTION
It’s always easiest to use motors that already have a gear reduction box built onto them. When
selecting gear motors, you’ll be most interested in the output speed of the gearbox, not the
actual running speed of the DC motor itself. Note also that the running and stall torque of the
motor will be greatly increased at the output of the gearbox.
When using motors without built- in gear reduction, you’ll need to add reduction boxes or
make your own. Although it is possible to do this yourself, there are many pitfalls:
• Shaft diameters of motors and ready- made gearboxes may differ, so you must be sure that
the motor and gearbox mate.
• Separate gear reduction boxes are hard to find. Most must be cannibalized from salvage
motors. Old AC motors are one source of surplus boxes.
• Machining the gearbox requires precision, since even a small error can cause the gears to
mesh improperly.

WHERE TO FIND GEARS
Gears can be expensive, especially the metal ones that are machined from a piece of solid
metal. Online sources like Boston Gear, Small Parts, W.M. Berg, and Stock Drive offer these
and most any other gear imaginable, but at costs that make the average robot builder blanch.
As long as your requirements aren’t too unusual, you may be able to locate the gears you
want from other products and sources.

• Hobby and specialty retailers. Next time you’re at a hobby store, look for replacement
gear sets for servos and drive motors for R/C cars and airplanes. Some are plastic; others
are metal (usually either aluminum or brass).
• Toy construction sets. Don’t laugh! Toys like LEGO, Erector, and Inventa come with gears
you can use in your robotics projects. Most are on the large side and are made of plastic.
• Surplus catalogs. New gears can be expensive; surplus gears can be quite affordable. You
can often find new gears, plastic or metal, for about 10 cents on the dollar, compared to
the cost of the same gear new. The only problem: Selection can be limited, and it can be
hard to match gear sizes and pitches even when buying gears from the same outlet.
• Rechargeable electric screwdrivers. Inside are numerous gears, typically in a “planetary”
configuration, used to produce their very high speed reductions. Before raiding the screw-
driver for just the gears, consider using the motor, too.
• Hacked toys Discarded and discounted toys make for good gear sources. These include
friction- and battery- powered toy cars, “dozer” toys, even some action figures. Tear the toy
apart for the treasure inside. These gears tend to be small and made of plastic.
• Old kitchen appliances. Go to resale stores and garage sales and look for old food mixers,
electric knives, even electric can openers. Unlike toys, kitchen appliances commonly use
metal gears— or at the least, very strong plastic gears.

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