CHAPTER 17 ■ DC MOTORSOdd Gear Ratios
Gear ratios rarely reduce exactly to the lowest term. This isn’t bad luck; it’s by design.
A 40-tooth gear and a 10-tooth gear always have the same pairs of teeth rubbing against each other.
If either gear has a slightly flawed tooth, the other gear’s matching tooth is going to get rubbed down or
damaged more quickly since it must mesh with the bad tooth every time.
On the other hand, a 40-tooth gear and an 11-tooth gear would rotate through all pairs of meshing
teeth. The wear would be equally distributed across all teeth, causing the gearhead to last longer. Tooth #1
on the 11-tooth gear visits the following teeth on the 40-tooth gear: #1, #12, #23, #34, #5, #16, #27, #38, #9,
#20, #31, #2, #13, #24, #35, #6, #17, #28, #39, #10, #21, #32, #3, #14, #25, #36, #7, #18, #29, #40, #11, #22, #33,
#4, #15, #26, #37, #8, #19, #30, and then back to #1.
Indicating Both Absolute and Simplified Gear Ratios
Simplified gear ratios are easier to read and comprehend, but absolute gear ratios reveal how many teeth it
actually took to reach that value. This gives a sense of how much friction is involved, how heavy the gearhead
may be, and how likely the gearhead is to break.
The more teeth involved in reducing the RPM, the more friction there is. However, each tooth is having
to push less, and therefore may be able to endure more.
Using a Colon
To make gear ratios easier to read, they’re often written with a colon between the values. Instead of^40 / 1 , the
gear ratio is usually written as 40:1.
Looking at Real Gears
Earlier, I used an example of 1535/65. I suppose it is possible to have 1535 actual teeth and 65 actual teeth,
but that's usually not the case. Let’s look at the actual number of gears and teeth inside a real motor
(see Figure 17-27).
Figure 17-27. Count of teeth for actual gears inside of a Hsiang Neng motor’s gearhead