Gear Ratio on 67:1 Appears to be 70:1

[ddegn]

I have several of these really nice gear motors (thanks to Black Friday Sales).

Pololu - 70:1 Metal Gearmotor 37Dx54L mm 12V with 64 CPR Encoder (No End Cap)

My RPM figures kept coming out wrong when I used the listed 4331 count per revolution so I finally opened up one of the gearboxes and counted the teeth on the gears.

Here’s the gear count and math I came up with (from comments in my encoder code):

’ Gears in Pololu motor: 10:25,10:30,12:28,10:40
’ reduction = 25/10 * 30/10 * 28/12 * 40/10 = 5/2 * 3 * 7/3 * 4 = 572 = 70
’ 70 * 64 = 4,480 transitions per revolution

The 70:1 figure is easier to work with than the 67.67:1 figured claimed on the product page and as and added bonus, the 70:1 figure is correct.

I’ve counted the teeth on each gear at least twice. I’m pretty darn sure the 70:1 figure is correct. My RPM calculations work correctly when using this gear ratio.

It would be really nice if Pololu would correct the gear ratio listed on the product page so people can correctly compute speed and distance using the encoder feedback.

BTW, I really like these motors. I’ve recommended them to other robot builders several times.

(Sorry if this has been previously mentioned, I attempted to search the forum but “67:1 gear ratio” don’t make good search parameters.)

[grant]

Hello.

Your math is correct. The gear ratio for those gearmotors is actually 70:1. Thank you for pointing out the error on the motor’s product page. We just changed the listed gear ratio and name on the product page to reflect the correct ratio. We have also added a fraction that shows the exact gear ratio to the product pages of all of our 37D mm gearmotors, and we will similarly be adding exact ratios to the pages of most of our other metal gearmotors soon.

- Grant