F-P gears and Diametral Pitch

Technical Motors
1

Do any of you know what Diametral Pitch each of the gears in the F-P transmissions are? If not, is there a way to calculate the DP of a gear? I want to find this out so I can use that information to calculate the gear spacing in a custom gearbox.
Thanks
Bruce

2

You can calculate diametral pitch with:

Pitch Diameter/Number of Teeth

3

Actually, that would be:
number of teeth/pitch diameter
(e.g. 32 teeth/1inch=32DP)

But since pitch diameter is hard to find you can use:
(number of teeth + 2)/Outside diameter
but that is less accurate so a 32DP gear could yield 29.99, or 34. It really works well for some gears, but for others the result will leave you scratching your head.

4

I highly recommend you measure the shaft spacing in the Fisher Price gearbox and use that in your custom design. I recently counted up the teeth and calculated the pitch diameter of the first two gears (they are 32DP by the way) and made a gearbox based on those numbers. The gears were way too tight! After double checking my numbers and not finding any mistakes, I measured the plastic gearbox, and found the spacing was much larger than expected. I suspect the motor pinion has been profile shifted, which directly changes the center distance.
I’m happy to share my design if you want. Send me a PM.

5

It’s always a good idea to design loose, inefficient gear boxes are much more useful than seized gearboxes.

6

Tonight at robot building we were staring at the caliper that was measuring the plastic ring on the FP motor…I was wondering how we were gonna make that funky sized hole with the limited equipment we have available. Then I realized that the orange plastic ring is approximately .040" smaller than 7/8". Since we have to drill a 7/8" hole for the bearings to hold the output shaft, we could drill a 7/8" hole for the motor too, and shim it with a strip of .020" thick aluminum (which we have). This will also let us adjust the gear spacing, by stacking two shims on one side, if we get the spacing wrong.

If this all works, I’ll be sure to post some pictures. We’re planning to use Lexan plates about 3/16" thick to make the gearbox, and 3/4" long standoffs to hold the sides together.