CHAPTER 17 ■ DC MOTORS
The gear attached to the motor shaft has 12 teeth and it meshes with a larger gear that has 28 teeth. That
is attached to a gear that has 10 teeth, which meshes with a larger gear that has 28 teeth. That is attached to a
gear that has 10 teeth, which meshes with the final output gear (right side of picture) that has 40 teeth.
The gear ratios of each stage are therefore:
(^28) /
12 x
(^28) /
10 x
(^40) /
10 =
(^31360) /
1200
The absolute gear ratio would be described as 31360:1200, even though there really aren't that many
actual teeth. That fraction could be reduced to 392:15.
The simplified gear ratio is calculated as follows:
(^31360) /
1200 = 26.13333
The simplified gear ratio would be described as 26:1. The datasheet claims the gearhead is 30:1, which
(according to my calculations) is not really the case.
Imperfect Speed to Torque Conversion
All of the gears meshing together and pushing on each other involves a lot of friction. As such, gearboxes are
usually from 40% to 90% efficient.
The lack of complete efficiency means that gearboxes aren’t able to convert all of the reduced speed
into torque. You learned earlier that a gearbox with 2:1 gear ratio converts a 4000-RPM motor to 2000 RPM,
with 2 times the torque. However, if the gearbox is only 90% efficient, the torque is closer to 1.8 times.
That being said, gearhead motors convert motor speed more effectively than lowering voltage, with the
additional benefit of a lot of increased torque.
Disadvantages of Gearheads
Some disadvantages of gearboxes are:
- increased overall noise
- increased overall mass
- increased overall length
- increased no-load current
None of those factors should dissuade you from the overriding advantages of gearhead motors.Comparing Planetary Versus Spur Gearhead Motors
There are actually two major types of gearhead motors. Spur gearboxes are the most common, but planetary
gearboxes are also available. Planetary gearboxes contain the same smaller spur gears, except now the
smaller gears rotate within the insides of the larger gears (see Figure 17-28).