I’ve ultimately decided to cool and join the design party over here on Chief. Over the past two days, I have been working on this gearbox during my lunch hour at work. What you can see is 3 complete iterations that I have come through to the “almost” final product at the end.
The box has a reduction of 5.72:1 returning with a top speed of 11.44 ft/s which isn’t terrible for a single speed bot. The inital reduction is a 14t to 50t and the second and final is 28t to 45t. All gears are 20dp Aluminum 7075 from both Andymark and WCP.
Sorry if the colors are terrible haha.
There will be a 3D model in a few weeks or so since I do not have Solidworks here at work!!!
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.
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%.
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.
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.
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).
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.
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.
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?
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.
Lighter, simpler, more efficient.