CHAPTER 17 ■ DC MOTORS
Describing Gear Ratios
When all of the teeth of the larger gears have been added up, and all of the teeth of the smaller gears have
been added up, the comparison between them is called the gear ratio. Usually, there are a greater number
of larger teeth than smaller teeth. In that case, the gear ratio can also be called the gear reduction, since the
speed is being reduced.
Absolute Gear Ratio
Let’s say a gearbox has 1535 larger gear teeth compared to 65 smaller gear teeth. This gearbox would have
an absolute gear ratio of^1535 / 65. A motor spinning at 6000 RPM and connected to this gearbox would spin at
254 RPM with lots of additional torque.
6000 RPM × 65 teeth / 1535 teeth = about 254 RPM
Absolute gear ratios can get pretty bizarre. For example, an extremely reduced motor could have an
absolute ratio of^63950067 / 21125.
Simplified Gear Ratio
To make the ratio easier to comprehend, the fraction is usually mathematically reduced. Like^4000 / 100 could
be described as^40 / 1. This makes it easier to say to yourself, “The speed of this motor has been reduced by
40 times and the torque increased 40 times.”
Simplified gear ratios are almost always reduced to the lowest term, even if the value isn’t exactly
accurate. So,^1535 / 65 would not be reduced to^307 / 13 , but is usually stated as^24 / 1. That’s not quite right, but it is
more understandable.
■ Tip You can be sure you’re looking at a simplified ratio when one of the values is “1”, because no spur gear
is going to have only one tooth.
Figure 17-26. Centered shaft on a plain motor (left) versus the offset shaft on a gearhead motor (right)