Single Speed, Single Reduction Gearbox in Tube

My team doesn’t have a CNC machine in our shop (and AFAIK isn’t planning on purchasing one for the 2020 season) and our sponsors do mostly laser cutting and small CNC jobs for us. Therefore, although we really want to, we can’t really make our own transmissions in-house.
I saw this thread yesterday, and I instantly had an idea - what if, since the 9T pinions make it easier to have a single reduction gearbox that’s well suited for a drivetrain, I could design a “SS, SR” gearbox inside of a 50x20 tube (which is our standard for drivertrains since 2017)? Then we could make it in our shop in our hand-operated mill.

And so, I did (conceptually, at least, mostly just for checking the gear packaging now). Here are some photos:

Initially I went with a 9:54 gear ratio but then after looking in JVN I decided to bump it up to a 9:60 for higher acceleration. The 60T 20DP VEXpro gear has a 3.1" outer diameter, which means that with a 4" wheel there isn’t a lot of ground clearance (and even less clearance inside the tube, which makes a center drop pretty impossible in this tube size). While this is planned for a 2017 all-flat field game, I’ll admit I didn’t check the ground clearance yet with the 2018/2019 ramps, so there’s a chance it will actually fit even without a center drop.

The wall thickness of the tube is 2mm = ~1/13in, which makes it somewhat impractical IRL, although this is meant to be a part of a tank drive and not a WCD and so the outer tube will have a 3mm wall thickness which takes impacts pretty good (at least in my experience from the last season).

I’ll upload the CAD model once I finish designing the entire drivetrain. I would love to hear your feedback about this concept and the implementation!

The only problem I see with this, assume you use it on a surface like the 2018 game, you could get caught on the gear on the angled part of the platform

Can you assemble this?

You probably want more surface area against the tube where the motors bolt on, maybe a laser cut “O” or really big washer or two, with clearance holes for the bolts?

Have you used bearings set directly in 2mm Al with no additional support before? I definitely wouldn’t expect this to hold up in WCD but with an outer rail so there’s no cantilever I can see it working on flat fields like this year.

I think your wheel well can get narrower. You’ve got pretty wide clearance on both sides of what I assume is #25 sprockets.

In general, this is a fantastic concept - the ground clearance isn’t great but it is acceptable. Driver practice & path planning can deal with most floors (see 254 choosing to keep ~1" clearance last two years and just not drive onto compound corners of ramps).

Is there a reason that your sponsor can’t waterjet cut thin gearbox plates for you to make a more traditional gearbox? 2-3mm aluminum is definitely waterjet cut-able, and unpocketed with proper standoffs it should be plenty strong for a gearbox. That should let you do a more traditional design that will be easier to assemble and service.

Great question, and one I am still thinking about. I am still not sure about this, as the main problem is the fact that there are only two bolts connecting the motors to the tube that are accessible.

I haven’t (not horizontally at least), and I’m still looking into it. I plan on tapping into the ends of the hex shaft and hold them in place with bolts and washers, and I hope that it’ll provide some way of better support for it.

Yes - AFAIK, they do not have a waterjet.

Sorry, meant to say laser cut.

If they’re a laser sponsor they should be able to cut 3/16" aluminum, depending on how powerful it is. You can also study up on sheetmetal gearbox design from folks like 971, 192, and 148 and learn how to artfully bend thinner (.09, .08, .06 etc) 5052 laser cut aluminum into robust gearboxes that don’t rely heavily on tight tolerances.

They are able to do 5mm (I’m from Israel, so working in metric…), but I’m worried that since the laser isn’t actually cutting the plane perpendicularly and can be inconsistent from time to time, the tolerances in a thicker material will keep the gearboxes’ bearings from being concentric and would cause a small twist in the shafts, which after a season like we had in 2018 (62+ matches and shipping a robot to the US) would hurt the gearbox.

I had thought about this as well but every time I get nervous about removing that much material from the tubing, especially right in the middle of the frame.

Piggybacking on this idea, with the 9t pinions you can get some pretty awesome reduction in a tight package. Someone could waterjet a single 3 motor plate out for the kit of parts frame, teams could use a bearing to align the plate with the kit frame, transfer drill the holes using the plate, and boom - you’ve got a really small single stage 3 motor gearbox mounted. I’ve never used purely motor mount holes to mount a gearbox, wonder if anyone has tried that?


We recently tried sending some of our own custom gearboxes to laser cutting at a sponsor, the bearing holes were too tight but the c-c distance was alright as far as we could tell (they were cut from 5mm aluminium). We expanded them with a de-burring tool so the bearings would fit, but you could use the Vexpro 1.126" reamer with a press drill for that

Yeah I highly recommend cleaning these up with a reamer instead of a deburring tool.

We don’t have one of those :sweat_smile:
we just had to improvise a solution there

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Apex Robotics 5803 used this style of drivetrain in 2017 and 2018 with no issues. We did 3 motor stations and 8wd, but the concept is the same. In both seasons the drivetrain saw over 100 matches.

A few points to note:

  • I would not recommend running bearings directly in 2mm wall tube (about 0.078") In 2017 and 2018 we ran the bearings directly in 0.1" wall Versaframe, and both times the bearing holes were noticeably looser by the end of the season. Not enough to cause issue, but enough for us to decide to use 0.125" wall thickness for 2019 for going off HAB2.

  • You need to machine the bearing bore in both sides of the tube in on setup to ensure concentricity. The first time we attempted bearings pressed directly in the tube, we did not do that and we had to hand-rework the bearing holes (not good) to get the shafts to fit through.

  • Low gear clearance in the shadow of a wheel is absolutely fine on a flat field or with shallow ramps like 2018/2019. In 2017 we ran 64T gears on 3.5" colsons, or about 0.1" clearance between the gear teeth and the carpet and had no issues. Going at it again I’d use the 9T pinions and get some extra clearance for sure, but I wouldn’t be concerned with what you have.

  • If you have a belly pan around the tube, the cutouts in the middle aren’t a problem. Bumpers help a lot in terms of distributing the load from lateral hits into other parts of the frame.

  • I would propose trying to get wider box tubing and flipping the bearings so the flanges are on the outside. Then you only have to shim out the motors by 2mm to clear the flange. I’d see if your sponsor can laser-cut a shim plate that goes under both motors rather than using standoffs, you want the face of the motor to help maintain perpendicularity.

  • If you can source WCP products in addition to Vex products, I highly recommend their Chain-in-Tube #25 sprockets (16T, 17T and 18T) even for non CIT applications. It will reduce your cantilever.

Our 2016 robot had a sheet metal drive gearbox (not for a Westcoast style drive though). Not a perfect design, pretty tough to run maintenance on.

@jonathano if you want thicker plates you could probably come by our shop during the season with a plate and we’ll CNC route it for you guys, it would probably be at around week 4 though, as we’ll be fabricating in week 3 for ourselves.

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We made an offseason outreach robot with a similar style of transmission, but it was scaled down for use with 775s and 6 tooth pinions, (it’s crazy fast, but it worked) I don’t have good pictures of it but ill take some on Monday. I manufactured it with a hand mill and mostly hand drillilng the holes, so meshing the gears properly took some fine tuning and bodging, but if you had this sent off to be done on a proper CNC router/mill, I dont see any reason why it wouldn’t work fairly well. Though our experience wasn’t from any real match play so take it with a grain of salt.


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