pic: 2016 Offseason Drive Gearbox

When figuring out details for a possible 2016 offseason robot I came to the realization that it is significantly cheaper for us to make our own gearboxes. From there I used as many parts that we own already as possible. All we actually have to buy for this gearbox is the 24t gear and the 2 11t gears. Meaning this gearbox costs $23 for us to make…

11.99 ft/s free speed running a 6wd with 8" pneumatic wheels

The lightening seems just a bit too much…

I recommend material going from the upper mounting holes back to the outside of the plate and then from the bearing back down to the bottom of the plate.

It’s nice to have an identical front and back plate, but the top of your front plate will be weaker than the back plate because it doesn’t have the added strength of the CIM. I don’t know whether or not it will be strong enough, but I’ve learned the hard way that removing a motor removes a lot of strength. Plus, you could probably even still use just one part file and just CAM it twice, if that’s a concern.

Comments on the lightening being too aggressive are bang on. Add some webs from the lower mounting holes to the outside of the plate, and a web or two on the bottom, at the very least. You’re also missing a few fillets on your corners on the bottom.

The other comment I would add is that custom gearboxes are cheaper in terms of dollars, but the cost comes out of other things. If you are outsourcing parts to a sponsor, be very careful that you do not ask too much of your sponsor. They are donating machine time and operator time to your team - despite what they say there is a finite amount of this they may be willing to give, so be careful not to outsource more parts than necessary to generous sponsors. If it is a waterjet / laser sponsor, it’s probably not much more time, but gearbox plates could be a few more hours on the CNC mill. For an offseason robot maybe skip the lightening to cut down on machine time?

If you’re doing this in-house, none of that really applies. In either case, get it tested early and make sure you’ve worked out all the bugs of your custom gearbox. They can be a little tricky sometimes.

It’s great to see you learning how to design robot parts - keep doing this. Keep posting your results, and be sure to work on your CAD in such a way that it allows for easy future iteration. Adam Heard’s 973 RAMP videos are invaluable for this sort of thing.

Thanks everyone I’ll make sure to relighten it when I get home and post the results.

With the gearboxes getting cut it actually works out well. We recieved a new cnc mill sponsorship and they have some interesting size constraints due to their 2 day turn around times. The thinnest parts they will do are 1/4 and The largest width and length is 7.5x7.5 so basically it’s perfect for gearbox plates. The way it’s working out is they added credit to my account and I can use that to ‘purchase’ the gearbox plates. I have enough credits in my account to outlast my time on the team so I’m thinking custom gearboxes are the way to go. Thanks everyone for the help I’ll post the updated render later.

It’s interesting you were able to get the price down that low with the parts on-hand.

Is this meant to mount to a sheet metal frame or a 2x1 (or similar) tube frame?

I think I see a snap ring right in the middle of your output shaft. This snap ring will be close to, if not entirely, impossible to install.

Other options:

E-Clip - Ok in this application, but not recommended. I don’t like transferring torque through any sort of groove, especially the deep grove required for this application

Shaft Collar - Not the best, but could work in this spot

Change up your shaft retention locations. Review other team’s CAD’s to see their arrangement of shaft retention on a typical output shaft.

Please post results.


I didn’t even notice this when I first commented. This is a huge problem because a snap ring groove between loads (between gears and direct drive wheel) is a stress riser. With some bad luck you could fail your shaft here.

If you are looking to prevent the shaft from sliding out in that direction, you can turn the far end of the shaft to 1/2" round (or 3/8" round) and use a 1/2" round (or 3/8" round) bearing on the inner plate. That, combined with a snap ring groove on the outside of the inner plate, will fully retain your output shaft.

Oh yea I don’t even need that snap ring. I was not thinking when I put it there. It’s been removed but I didn’t feel like re rendering. The shaft is turned down on the back to 1/2 round and it’s tapped so it wont fall off.

Here is the ‘finished’ version

Here is how the output shaft is actually held in. (spacer, bearing, and plate hidden) https://gyazo.com/7331d2fcb0f597dad4bb6db74bdbd5d2

and here is it on the WCD itself which should answer the question about what its mounting to

The WCD is a WIP. I still have to model the belts and shafts on the outer wheels and figure out how I want to layout the frame with the shooter. Before anyone says there is not enough support with the corner gussets I should mention there will be a belly pan.

Even building the rest of that WCD will be really cheap. All we have to buy is the Versablocks and CAMs.

So… yes we are building an entire drive train for $166. The only parts I am not 100% sure on is hardware but in the end that’s pretty negligible and I am pretty happy what I was able to make with our stockpile of parts.

Also, I may be attempting a flipped CIM gearbox soon with the same goals. Due to the crazy thickness of the pulley there is room. If that works out we will probably build that.

The corner brackets might be robust enough, but I’m mostly concerned about how you’re going to mount a belly pan with pulleys that are bigger than the tube?

The pulleys are sized well for this drive, I’m not saying go smaller on those, just that you’ll have challenges mounting a belly pan without going through belt loops.

The same criticisms about the lightening pattern still apply.

What changed from the original? I can’t seem to find any differences.

Yea the belly pan won’t be that fun but it’s almost a necessity with me only having 2inches where I can mount gussets. If I got rid of the versa blocks I could get away without it but I don’t think I want to do that. I am exploring the possibility of custom versablocks and if I did that I could size them such that I could get a gusset underneath

I changed up the lightening pattern on the bottom and I am still learning how to get the other one to work. Chak made it look good (I gave him the plate file) but when I tried to do what he did I broke Solidworks and he was sleeping.

Extra support at the bottom fixed all the fillets and deleted the circlip.


I would, again, recommend a web going to the “6 o’clock” directly down from the output bearing or something a bit less extreme on the angles to the corners. The pocket directly below the bearing is concerning to me.

Also, a web going to the “3 o’clock” and “9 o’clock” from what appears to be the top mounting holes to the outside of the plate.

Compare these gussets to the length of the VersaChassis gussets. They aren’t that different. I’m not saying you dont’ need a belly pan or other cross support, but having to take apart your frame to change a belt (not that you should ever have to do that) is gonna be a nightmare.

Could you post a photo of the gearbox mounted to the 2x1 rail, without the direct-drive wheel?

Curious what the bearing situation is there, and if there are tips that may help with machining the rails w.r.t. the direct drive wheel’s shaft alignment.