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EVERYTHING YOU ALWAYS WANTED TO KNOW ABOUT GEARS 283

Other shaft types you may encounter include the hex and square, so called because of their
hexagonal or square shape. They’re used with wheels, gears, and other parts that have
matching- shaped hubs.

Everything You Always Wanted to Know
about Gears

We’ve already discussed the fact that the normal running speed of motors is far too fast for
most robotics applications. Locomotion systems need motors with running speeds of 75 to
200 RPM. Any faster than this, and the robot will skim too quickly across the floor. Arms,
gripper mechanisms, and most other mechanical subsystems need even slower motors. The
motor for positioning the shoulder joint of an arm needs to have a speed of less than 20 RPM;
5 to 10 RPM is even better.
There are two general ways to decrease motor speed significantly: build a bigger motor
(impractical) or add gear reduction. Gear reduction is used in your car, on your bicycle, in the
washing machine and dryer, and in countless other motor- operated mechanisms.

GEARS 101
Gears have two main applications:

• To transfer power or motion from one mechanism to another.
• To reduce or increase the speed of the motion between two linked mechanisms.

The simplest gear systems use just two gears: a drive gear, and a driven (or output) gear.
More sophisticated gear systems, referred to as gear trains, gearboxes, or transmissions,
may contain several or even dozens of gears. Motors with attached gearboxes are said to be
gearbox motors.
We use these gearbox motors a lot in robotics. R/C servos have their own gearbox built in, and
G most of the DC motors we use to power wheels and tracks of bots have a gearbox of some type.
Though these already have a gearbox for speed reduction, there are many applications for
adding external gears, such as making one motor drive two wheels at the same time.

GEARS ARE LEVERS IN THE ROUND

In a way, gears are round levers, and it may help to explain how gears function by first exam-
ining the basic mechanical lever.
Here goes: Place a lever on a fulcrum so that the majority of the lever is to one side. Push
up on the long end, and the short side moves in proportion. Although you may move the lever
several feet, the short end is moved only a few inches. But note that the force available on the
short end is proportionately larger than the force applied on the long end.
Now back to gears. Attach a small gear to a large gear, as shown in Figure 24- 17. The
small gear is directly driven by a motor. For each revolution of the small gear, the large
gear turns one half a revolution. Expressed another way, if the motor and small gear turn at
1000 RPM, the large gear turns at 500 RPM. The gear ratio is said to be 2:1.

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