CHAPTER 17 ■ DC MOTORSRevealing Pairs of Smaller and Larger Gears
A spur gear is a circle with pointy teeth on the perimeter. The teeth push against the teeth of other gears to
make them move (see Figure 17-25).
On a spur gearhead motor, the DC motor shaft turns the first smaller gear against the first larger gear.
Because the smaller gear has fewer teeth, each time the smaller gear rotates once the larger gear only rotates
partially around.
For example, if the smaller gear has 12 teeth but the larger gear has 40 teeth, then the smaller gear
rotates around three and a third times (^40 / 12 ) before the larger gear has been pushed around once. In this
example, if the smaller gear were connected to a motor running at 6000 RPM, the larger gear would only be
spinning at 1800 RPM.
smaller gear RPM × smaller gear teeth / larger gear teeth = larger gear RPM
6000 RPM × 12 teeth / 40 teeth = 1800 RPM
Three and a third teeth push the same “distance” that only a single tooth previously pushed. If each little
twist comes from three and third teeth pushing, then the twisting force (torque) of the larger wheel has been
increased by 3^1 / 3 times.
This seems fair. If the motor is being fed the same voltage and the same current but only producing
1800 RPM, the missing energy must be going somewhere. It goes into increased torque. By placing a series of
smaller gears and larger gears together in the gearbox, the motor speed can be reduced more and more with
torque increased more and more.
Gearbox Shaft
A shaft is connected to the last gear. The end of this new shaft appears out the top of the gearbox for wheels
and things to be connected to it. This new shaft out of the gearbox replaces the old shaft coming out of the
motor.
When examining a motor, if the shaft doesn’t come out of the center, it’s likely that the motor is a
gearhead motor (see Figure 17-26). Standard motors naturally put the shaft in the center of magnets. But,
many gearhead motors connect the new shaft wherever the last gear fits into the gearbox. Note, however,
that the last gear could be in the center, so a centered shaft doesn’t guarantee the motor isn’t a gearhead.
Figure 17-25. Smaller spur gear with 12 teeth rotates only 12 teeth of the larger spur gear, which has
40 teeth total