pic: Single Speed Gearbox Iterations

[Nate Laverdure]

Originally Posted by sanddrag

I believe a 12 tooth gear has an undercut at 20 pitch and 14.5 degree pressure angle...

Both the 12-tooth and the 14-tooth pinions will be undercut, but the 12-tooth more severely so. From page 141 (PDF link):

Originally Posted by Boston Gear catalog

On 14.5 PA gears undercutting occurs where a number of teeth is less than 32 and for 20 PA less than 18.

The formula for the number of teeth (z) required to eliminate undercut is:
z = 2 / sin^2(a)

where a is the pressure angle. Interestingly, this isn't dependent on pitch.

[PAR_WIG1350]

Originally Posted by Nate Laverdure

Both the 12-tooth and the 14-tooth pinions will be undercut, but the 12-tooth more severely so. From page 141 (PDF link):

The formula for the number of teeth (z) required to eliminate undercut is:
z = 2 / sin^2(a)

where a is the pressure angle. Interestingly, this isn't dependent on pitch.

It is independent from the pitch since all gears of the same pressure angle and tooth count are geometrically similar. All 14.5 degree pressure angle gears with 24 teeth are the same profile, just at different scales.

[rcmolloy]

Hey guys I really appreciate all of the discussion and its helping out a LOT. There is a bunch of things that I've seen, like the CIM .75 shaft guide extrusion, that have really helped me out with more detailed design.

On that note, wouldn't it be possible to have a press fit of x OD and .75 ID to fit over the CIM that would allow the possibility to help guide into larger sized holes? I feel as if the trade off would only help getting a much larger reduction in a smaller packaging and ultimately a smaller finished assembly. If anyone opposes to the fact or sees it as being not beneficial then please let me know!

[Chris is me]

Originally Posted by rcmolloy

On that note, wouldn't it be possible to have a press fit of x OD and .75 ID to fit over the CIM that would allow the possibility to help guide into larger sized holes? I feel as if the trade off would only help getting a much larger reduction in a smaller packaging and ultimately a smaller finished assembly. If anyone opposes to the fact or sees it as being not beneficial then please let me know!

This is totally possible, but if you want a larger reduction in a smaller package, that's exactly what a 12T pinion would do for you!

[rcmolloy]

Ah ok! I suppose I just had my logic a bit backwards when writing the post. Also, proved it with a little bit of math too haha.

[Richard Wallace]

Originally Posted by rcmolloy

..., wouldn't it be possible to have a press fit of x OD and .75 ID to fit over the CIM that would allow the possibility to help guide into larger sized holes?

You probably have noticed plastic guides on FP motors in the FRC Kit of Parts. (shown here, for example) They are used in exactly the way you described. Of course as Chris just pointed out, the reason for using a pinion larger than the motor's metal pilot feature is not to enable a larger gear-down ratio. It is to enable a larger pinion pitch diameter. This will reduce the stress at that gear mesh, increasing the service life of the (plastic) gear which mates with the pinion.