pic: Single Speed Gearbox Iterations

[sanddrag]

Originally Posted by JVN

Whatever gear math people are doing which says 14 is better than 12... I haven't seen it, and I like to think I've played with these numbers quite a bit.

-John

The benefit could be a better running gear mesh. I believe a 12 tooth gear has an undercut at 20 pitch and 14.5 degree pressure angle, while I think a 14 tooth is out of the range requiring the undercut geometry. That said, for typical FRC applications, I'd still go with 12 tooth gears, since they fit through the hole, as others stated. And yes, I too highly recommend a close-fitting .75" hole to locate the motor.

[Garret]

Originally Posted by sanddrag

The benefit could be a better running gear mesh. I believe a 12 tooth gear has an undercut at 20 pitch and 14.5 degree pressure angle, while I think a 14 tooth is out of the range requiring the undercut geometry. That said, for typical FRC applications, I'd still go with 12 tooth gears, since they fit through the hole, as others stated. And yes, I too highly recommend a close-fitting .75" hole to locate the motor.

Would a better running mesh mean more a more efficient or quieter stage in the gearbox? And would there a mathematical method of calculating at what point there is an undercut?

[AdamHeard]

Originally Posted by Garret

Would a better running mesh mean more a more efficient or quieter stage in the gearbox? And would there a mathematical method of calculating at what point there is an undercut?

I'd have to confirm this, but an undercut shouldn't be any less efficient. It just needs the tooth to be reduced in size near the base to prevent impact of teeth; the involute surface should still be maintained.

It really is a substantial strength decrease; but as shown by numerous robots running them they are still strong enough.

[Garret]

Originally Posted by AdamHeard

I'd have to confirm this, but an undercut shouldn't be any less efficient. It just needs the tooth to be reduced in size near the base to prevent impact of teeth; the involute surface should still be maintained.

It really is a substantial strength decrease; but as shown by numerous robots running them they are still strong enough.

Thanks, I never realized the strength decrease but it makes a lot sense thinking about it now.

[PAR_WIG1350]

Originally Posted by AdamHeard

I'd have to confirm this, but an undercut shouldn't be any less efficient. It just needs the tooth to be reduced in size near the base to prevent impact of teeth; the involute surface should still be maintained.

It really is a substantial strength decrease; but as shown by numerous robots running them they are still strong enough.

In the first stage of a gearbox the force would be lower than it would be in later stages so the effect is minimized, identical gears in later stages might not fare as well. This is why the gear teeth in the fisher price gearbox get larger with each successive stage.

[AdamHeard]

Originally Posted by PAR_WIG1350

In the first stage of a gearbox the force would be lower than it would be in later stages so the effect is minimized, identical gears in later stages might not fare as well. This is why the gear teeth in the fisher price gearbox get larger with each successive stage.

I'm not sure what you're getting at with this post, this matches what I said about increasing toothsize as you go further in the stages.

EDIT: Reread, you're referring to the undercut gears not the plastic gears. Your Point is valid.

[roystur44]

Suggestion,

A couple of pressed in stainless steel guide pins would make this baby slide right in/out and align to the mating shaft and frame. Using guide pins can reduce the number of bolts to the frame and provide a accurate fit.

See how a car transmission bolts to a engine. Usually a couple of pins on the bell housing that mate to blind holes on the engine. The pins help to align the shafts and just a couple of bolts to hold the transmission on.

Roy

[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.