LEGO MINDSTORMS Robots Manual page 36

Of Geared and Ungeared Motors
There's one more topic related to gears that's important. Most electric motors turn too fast and with too little power to be useful. Gears are usually used to swap speed for power until a good
balance is achieved. This process is called gearing down or gear reduction.
The motors that come with RIS are internally geared, which means that the motor case actually contains an electric motor and some number of gears. The output shaft is already adjusted to turn at
a reasonable speed with a reasonable amount of power. This means you can attach wheels directly to these motors to drive your robot around.
The LEGO group makes four different kinds of motors that can be driven from the outputs of the RCX:
standard motor
This has been the standard motor of the LEGO TECHNIC line for many years. It is an ungeared motor, which means its output shaft rotates very rapidly, with little power, when electricity is
applied. To do any useful work with it, you'll probably have to use gears to reduce its output speed.
micro motor
This is a tiny motor with low speed and low power. You probably can't use this motor to move your robot, but it could be useful for lighter tasks. It's harder to find than the other motors.
geared motor
Two of these motors come with the MINDSTORMS RIS kit. They are internally geared so that the output shaft has enough power to drive your robot around. They are more efficient than the
standard motor. The geared motor is shown in Figure 2-14.
train motor
LEGO sells an entire line of train sets. The train motor can be controlled by your RCX; as a matter of fact, you can make an "intelligent" train by mounting the RCX in one of the cars.
For a Rainy Day
To see exactly how efficient the geared motors are, try this experiment. Use one of the
"wire bricks" to attach two motors to each other. When you turn the shaft of one motor, the
other motor's shaft will turn simultaneously. What's going on here? Just as you can supply
power to make the motor turn, turning the motor with your hand generates power. This
power is transferred to the other motor, where it's converted back to the movement of the
shaft.
Figure 2-14.
The geared motor
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