Custom Gearboxes

My team is considering to start making our own gearboxes. I was wondering if anyone could recommend materials for the plates, materials for the standoffs, and fasteners. I have recently figured out that we can design them in a reasonable time. I would just like to know what are the does and don’ts.

So in general, aluminum for the plates and standoffs are fine. 6061 is a good general material for both, but the standoffs will generally just be whatever aluminum your supplier uses. You could do the plates out of 7075 for extra strength but probably not necessary. The fasteners can just be your standard stock 10-32, 1/4-20, etc. (I generally use 10-32 for gearboxes). I would be curious to know if anyone has tried aluminum fasteners and if that was even remotely worth doing.

I also know people have done plastic plates before, like acrylic/polycarbonate for example. I haven’t done it personally though so I can’t speak much to that; however it seems as though usually those are lightened less than an aluminum plate would.

Most teams generally machine their gearbox plates with either a waterjet or a CNC. This obviously comes down to what your teams resources are.

That’s some basic info. Post any more specific questions if you have them!

Do you tap one plate or go all the way threw and secure it with a nut? Also for the standoffs, do you buy aluminum shafts and machine it or tubing?
Thanks for the reply!

On our gearboxes we generally speaking tap the standoffs on either end when they are anything more than about an 1.5in-2in. Shorter than that and we do a through and through on either plate and secure it with a nylock. I agree with Nick about the 10-32’s for holding gearboxes together, depending on the application we will use 1/4-20’s to secure it to whatever it is being fastened too for extra strength.

+1 on everything Nick said, although I think 7075 would be a waste of money compared to just using 6061.
For the standoffs, I design with either Vexpro tube axle (which is a bit large for 10-32) or I buy something like these tubings from McMaster:

The latter has a slightly smaller ID, so it’s better suited for clearance, but it’s also steel which is harder to machine.
It’s diable to just buy aluminum shafts and drill them, but I find that’s more hassle than cutting tubes to length on a lathe.

I design for tapping because I think it’s easier, and a bit of loctite blue or just the construction of the chassis secures it. But nuts save you the hassle of tapping and you can make the front and back plates of your gearbox the same. However, if your gearbox is super long, it makes more sense just to use a hexagonal standoff (or thunderhex/hex shaft) and tap both ends for a screw.

I would not make my own custom gearboxes, especially for drivetrain use, unless I have a CNC mill/router or a waterjet. If you only have a manual mill it makes a lot more sense to just buy your gearboxes, unless you want to do something like a flipped CIM gearbox or save money.

We have access to a waterjet for curtain and will probably gain access to a CNC mill (hoping). Thanks for all of the information.

No problem. If you waterjet the plates, make sure you design the holes to be a bit undersize, then ream them out at home.

Yeah we learned that this year with some of our parts.

Just about any aluminum plate or thick polycarb will do. This year 4592 did them out of 5052 H32 which is far from ideal but even with heavy pocketing worked well as the ratio we wanted was higher than anything available commercially. If you have access to waterjet making the plates should be easy. I prefer 1/4 inch plates as it lets you take advantage of the full .25 inch bore of an R8 Bearing (.5 ID 1.125 OD) Do you guys have access to a lathe? making custom gearbox spacers are nice but ones from McMaster work too. We used plastic ones at the start and switched to aluminum for the 2nd version of our gearboxes even though the plastic ones would probably have worked fine. for spacers PVC works well enough although delrin ones from vexpro are nice as they do not require sanding to an exact length. for bolts i recommend 10-32s as they can be used to hold the gearbox together and also be used to hold the cims to the gearbox. You can make clearance holes on the front plate ant tap the rear plate so the whole gearbox dosnt fall apart before it is installed although I generally find using long screws to attach motors through the front plate to be good enough retention.

When it comes to designing custom gearboxes, try to use a mounting pattern that is identical to a COTS gearbox for your first few. That way in the worst case you can drop in an off the shelf replacement should your custom gearbox fail. For gear spacing use normal pitch diameters +.003 to account for manufacturing slop and wear and tear on the plates. make sure to include 1 or 2 spots where you can change ratios if the final speed is undesirable. Pinions are a common place for this adjustment but changing spur gears is also an option, although this will more than likely require disassembly of the whole gearbox as opposed to just removing the motors. For shafts its nice to be able to do snap rings and bearing rounds as the combination can be used with a round bearing to retain the shaft when the outer plate is removed as well as opening up the back hole for a sensor.

Depending on your machining resources, don’t forget about all the little stuff. The plates are the hard-to-machine part, but all the little bits (spacers, standoffs, shafts) can really add up time. With a bit of forethought, it’s possible to do most of these COTS from AM/VEX/McM.

Thanks, and we do have access to a lathe.

All of our past gearboxes have been made with waterjetted aluminum plates. When using 1/8" thick material, we’ve just dimensioned the holes to match the bearing size and pressed the bearings in. When we used 1/4" material, the taper left by the waterjet was substantial enough that we had to ream out the bearing holes (we used this step drill bit, works for 0.875" and 1.125" bearing holes)

We also add .003" between meshing gears, as Max recommended. If you download some CAD models from AndyMark you’ll see that they do the same. Works good!

This is a good standoff material, as there is a pre-existing hole that works with 10 machine screws. You can face off lengths in a lathe or milling machine and just run a long bolt through each standoff with a nut on the end. You could just use a solid bar and drill out a hole on your lathe, but if you are making long standoffs this gets to be pretty tedious.

+1.

In all my gearboxes I generally keep the custom parts to only the gearbox plate(s), the drive tube (if WCD Style) or sheet metal chassis, and maybe an encoder mounting plate of some kind. Otherwise the only machining I’m looking at doing is cutting shafts to length and tapping them for retention.

You can just buy standoffs from McMaster, etc. if you want instead of making your own. If your economical and don’t mind spending the time making your own can be worth it, but the ones from McMaster aren’t too pricey and are well made if you only need 8 or so for two gearboxes.

One thing I do tend to do that is a little extra machining is a little trick for mounting a gearbox to a drive tube. Sometimes I’ll take the standoffs that would be mounting the gearbox to the inside wall of the tube, and instead I turn part of the standoffs down on a lathe to .249" diameter. Then on my drive tube I put a .251" thru hole on both sides. What this does is allows the standoffs to slip fit through those holes in the tube, and you put your bolt into the end of the standoff on the outside face of the tube. What this does is mounts the gearbox to the tube by essentially trapping the standoff onto the tube. This is a stronger way to mount then traditional methods. You can bolt the standoffs directly to the inner face, but it technically isn’t as strong and depending on the design it could be difficult to get the screws in the tube. You could run the bolt through both sides of the tube instead of inside the tube, but you then run the risk of crushing the tube some if over-tensioned.

That said, I wouldn’t suggest doing this for most teams that have limited time or access for machining as its a relatively minor gain in strength and you have to spend some lathe time to do it.

I’m not sure I understand what you mean. Do you have a picture of the setup?

Sorry for the large image.

So in the picture you can see where the standoffs are round. The 2x1 tubing goes along where those standoffs are rounded. The tube is then trapped with the face of the hexagon portion of the standoff on one side and a washer and bolt on the other side.

This isn’t the best example as I accidentally used 5/16" hex standoffs when I should have used 3/8" or larger. This would help create more surface contact to help spread the load over a bigger area.

http://i.imgur.com/R2vN13i.jpg