pic: Single Speed Gearbox Iterations

[Mk.32]

Originally Posted by JVN

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

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?

[IanW]

Originally Posted by Mk.32

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?

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.

[Mk.32]

Originally Posted by IanW

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.

I understand this but is it really necessary?

[R.C.]

Originally Posted by Mk.32

I understand this but is it really necessary?

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

[Cory]

Originally Posted by Mk.32

I understand this but is it really necessary?

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

[JVN]

Originally Posted by Mk.32

I understand this but is it really necessary?

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

[Mk.32]

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

[ToddF]

Here is our prototype from earlier in the summer:

[AdamHeard]

Originally Posted by ToddF

Here is our prototype from earlier in the summer:

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.

[mwilbur]

Originally Posted by AdamHeard

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.

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.

[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.