pic: Single Speed Gearbox Iterations
 

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Very nice gearbox. Is it a direct drive? Just curious - what was its intended purpose? Was it just for fun, or designed for future use?

Keep up the awesome work! I can't wait to see the 3D model!

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What motors is this for? It looks like it's some sort of CIM or two BaneBots setup...

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What size wheel does it drive to get that speed?

- Andrew

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I'm guessing a 4" wheel, directly driven by the gearbox output, at full (normal) load CIM speed (4320 RPM according to the CIM data sheet) -- so ( 4320 / 5.74 / 60 ) * pi()/3 is about 13 ft/sec. Free speed would be faster, about 16 ft/sec.

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Richard pretty much clarified everything up from a technical standpoint. I still can't believe he grabbed that much from the crowded drawing. Thanks for clarifying up the math. I felt like I was a bit off somewhere haha.

It's a 2 cim box if you can make out the mounting holes that are perpendicular to the plane that the intermediate shaft and pinions are sitting on. I'm sure the cims will stay in that orientation as of right now but that may change with another iteration down the line.

The gearbox was created because of bordom and my want to increase my design ability. AdamHeard and I are really working close on honing in my design skills this year and I just decided to start throwing stuff at him to check out. This box wasn't terribly difficult to make but it still helps me since this is the first 2D layout that I have pretty much completed up to 90%.

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Nice job. :)

Keep throwing stuff at Adam Heard, you will get really good at this.

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You can get that entire reduction in 1 stage. :) Lighter, simpler, more efficient.

12:72 will work with a 4" wheel, and actually gets more reduction than you have right now. (12:69 would be right about where you are currently.)

I always use a 12t or smaller on my CIMs because it allows us to put the gear on the CIM before installing it through the CIM 0.75" pilot hole.

Instead of spending money on COTS gears, you could go find a local shop who can make you some custom aluminum 72-toothers for cheap (or free). You're not going to need a whole lot of gear, should be nice and light.

-John

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Quick question from this; because we like to be able to put the gear on the CIM before we put it in the gearbox, I bored the CIM mounting holes to about .9 so a 14th would fit though it; so does it really matter that you use a .75 pilot hole?

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The idea behind using a .75" pilot hole is to help align/center the CIM, which has a protrusion at the base of the output shaft that is also .75" in diameter.

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I understand this but is it really necessary?

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Mark,

I highly suggest it. The thing with the .75 hole is that it supports and locates the CIM. We've had older robots have the CIMs come loose and sometimes even break bolts because the bolts took all the load. John is spot on with the .75 hole.

We use the 14T gear still. We just slide the CIM in with the spacer and put the gear on and add the push nut/retaining clip. Its not that much time (30 seconds more).

-RC

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No, but your gearbox will perform much better (with less effort) if you do.

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As said by others... it is not a good idea to locate the motor using the motor mounting screws. That little .75 pilot hub is great because it helps ensure good center-center distance.

Aside:
I don't see any reason to use a 14t over a 12t. 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

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Cool, thanks for the tips.
definitely use the .75 CIM hole on my next boxes.

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Here is our prototype from earlier in the summer:

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What I like;
-Combing the upper standoffs with the CIM bolts! So many advantages here.
-Overall profile looks clean and logical.
-Pocketing is a solid start, that's the hardest thing to do right... and everyone has a different opinion of what looks good.
-Combining the lower standoffs with your frame mounting.
-Using the same plate for both plates.

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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.

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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?

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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.

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

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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.

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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.

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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

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You're spot on with our thinking, but note that the plates are mostly the same because the CIM-side plate uses a 7/8" OD (FR6) bearing on the output shaft rather than the 1.125" (FR8) bearing on the output plate side. Otherwise, there are no differences between the plates, although we had considered tapping the screw hole in the CIM-side plate to elimnate the nuts.

The "design" is really more or less a copy of what we've observed in the Cheesy Poofs' gearbox designs. We wanted to start simple with a single speed gearbox, then move forward at a comfortable pace to determine what works best for us.

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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.

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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.

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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!

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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!

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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.

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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.