pic: Introducing BarBox: My First Attempt at Gearbox Design



After 3 years in FRC, it’s about time I design a gearbox too. This is my first ever gearbox. Soon to be incorporated in my first ever WCD (which won’t be a robot and/or FRC legal).

Specs:

Name: BarBox
Type: 2 stage single speed
Input: Single CIM motor
Ratio: 8.33:1
Stages: First stage 12T to 50T, 12T being the gear on the CIM. Second stage 24T to 48T, 48T being the gear that drives the output shaft.
Size: 3.5"X6.5"
Weight: about 2.7lbs without CIM

Output shaft is hex and nearly 3" outside of the gearbox.

Gears are represented by cylinders, whose diameters fit the gears’ pitch diameters.

Next stages:
-Replacing cylinders with actual gears.
-Making gearbox lighter.
-Adding mount for encoder.
-Open profile version.

Last but not least, thanks to everyone who gave me tips and suggestions: R.C., tim-tim, Akash Rastogi and Andrew Lawrence. (Said tips and suggestions.)

I like it so far especially for a first gearbox I just wanted to add that you could aquire CAD files for vertualy any standard gear at rushgears.com and a good practice also is to go to some place like mc master carr’s web site, VEX pro, or granger find stock gears to base your designs off of. Also I had a question about what your reduction was and how heavy this box is?

Thanks for the tip, I’ll look into it when I get the time for alterations. The gearbox is 2.7lbs, I’ll try and make it lighter.

Trading out the middle portion of the gearbox frame for a polycarbonate version might save a bit of weight.

Honestly I never understood why so many gearboxes (like the toughbox) use metal enclosures between the plates considering its probably the most structurally insignificant part of the whole thing.

While I don’t know which gears you’re using specifically and thus their bore sizes, maybe try a smaller gear size on the second stage. You could obtain the same reduction using AM 14 and 28 tooths etc and make the box more compact and a tad lighter.

Additionally replacing the enclosure with some standoffs would retain structure but save considerable weight.

I’d recommend replacing the metal enclosure with some standoffs, then if you need to, put thin lexan shields around it. Also, the CIM motor shaft isn’t supported on both ends, so you don’t really need the outer plate to be that big, and you could make it about half size. Also, you could probably get away with making some holes in both plates, depending on their thickness.

I can’t tell from the image, but it looks like the CIM is held on by two screws. It’s a better idea to make the top of the CIM flush with the plate, and cut a 0.75" hole in the plate for the top of the CIM to sit in.

From an interchangeable part standpoint, why not have the hole for the CIM shaft/boss in the side plate be on both side plates? This way, you can make 2 of the same part per gearbox and have more flexibility if you need to replace anything.

How are you making the metal piece in between the plates? Machining all of that from solid seems like way overkill for what a couple of standoffs could do.

If you want an enclosed design easily, you could take a page from the Toughbox Nano and build this gearbox out of tube stock.

I couldn’t find a gear combination that can use smaller gears without altering the ratio too much. Unfortunately 14 to 28 can’t work, both because in that case the output shaft will intersect the 50T gear on the first stage, and because there’s no 28T with 0.5" hex bore. There is a 29T one, but still too small.

The hole exists. You can’t see it in the picture though.

The output shaft sits on two different bearing sizes (0.875" in the back and 1.125" in the front), so the front and back plates have different holes for the bearings too.

Most of you recommended standoffs, I’m going to try that. I’ll try and make holes in the plates too. Christ- ehh… Open profile version will be coming early.

Made some changes, introducing BarBox 2.0:

New weight: 1.75lbs

http://img10.imageshack.us/img10/8269/jfhf.png

Thanks for the tips everyone!

One other thing to think about when designing your gearbox is how you would mount it.

It’s a bit similar to how you’ll mount a toughbox. You attach the bottom four screws to a channel with fitting holes, and have the shaft go through another hole in the channel. There are other ways to do it but that’s the way I wanted to mount the gearbox so that’s what I designed for.

What machining capabilities do you have available to make your parts?

I don’t, but this is not supposed to be manufactured, it’s just for practice and fun.

I’m liking your progress so far! It reminds me a lot of one of West Coast Products’ gearboxes.

Small nitpicky thing on your latest version-- the edges are looking rather sharp. Whether you’re planning on manufacturing it or not, it’s best practice to design for manufacture, and nearly any manufacturing method that you would use in competitive robotics is going to need to have some sort of radius where you’re cutting. I personally design most of my practice stuff to be manufactured with a 1/2" endmill because our primary machining sponsor uses CNC mills. Also for sharing screenshots, you might want to use shading with edges (most CAD programs have different rendering modes-- wireframe, shading, shading with edges, etc), which would make it a bit easier to pick out the finer points of your design.

Just something to consider for your designs.
(Also, I love the color scheme-- red and black looks very snappy!)

Another benefit of designing for an endmill radius, is it will get rid of some of the stress concentrators in your design. The round rings and cross member meeting spots appear to be a pretty sharp angle which could be prone to failure.

Why is this necessary?

And a note on standoffs: It’s very hard to find .375 OD tube with #10 clearance ID. Rather than buying .375 rod and drilling it out I prefer to buy .0625 wall with .375 OD and put .0625 deep counterbores in the plates to capture the standoff. Makes things much lighter and simpler to make.

Ditto,

If your turning down the back guy currently to .375", you might as well save yourself time and just lathe it down to .500".

I know it might be lighter to go that smaller bearing but honestly, its much easier programming/making 2 of the same part then it is 2 different parts.

Adrian,

We do about the same thing, buy .375" OD Tube with a .235" ID. 1658T41 - McMaster. We go parting crazy and you can make a lot of spacers in no time, especially on a bar feed :P. We use this material for the gearboxes we make. Do you really need the cbores? We’ve noticed its incredibly harder to assemble gearboxes that way.

-RC

I agree that counterbores are harder to assemble with and unnecessary. What we’ve been considering for next year is buying aluminum 6063 .375" OD tube with .145" ID and still machining it out. While we will have an extra step, the ID will be closer, the hole will be centered throughout, and our drills will not gum up because we are taking out quite a bit less material. However I’m worried about the 6063 aluminum. I’ve heard it is a bit less strong than 6061 and more gummy. I’m assuming it will be fine but if anybody else does this I would love to know.

That’s not a bad idea. You shouldn’t have issues with 6063, it is weaker:

6061: http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA6061t6

6063: http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA6063T6

But I can’t see you having an issue with loading them as “spacers”. As we’ve been using 6063 for all our spacers since its so darn cheap/easy. We also use a handful of plastic spacers on gearboxes as well. No issues with either or.

6063 can gum up pretty badly when machining but your taking off so little that it shouldn’t be an issue. We sometimes drill out the current spacers for 1/4" bolts.

-RC