I want to propose to my team to test a new(-to-us) drive base before kickoff. It would be a 6-wheel WCD with belt-in-tube. We would use 1"x2" channel with the VEXPro bearing blocks and a single stage clamp-on gearbox with 2 CIMs. The center wheel would be direct drive with belts going from an internal pulley to the front and rear wheels. Does anyone know if that’s been done before? I know teams have used chains to do this. I have two specific questions for starters:
in order to get two belts into a 1" x 2" channel, we would have to go to 9mm wide HTD belts. would those be adequate for the forces (driving, pushing, direction change) involved during an FRC match?
Could we get away with using 3/8" hex axles or should we use 1/2" hex?
I would strongly advise against 3/8th hex in a drivetrain. Especially for a live axle set up, you really need the strength of half inch hex in my experience.
I have personally never had experience with belt in tube, but I can tell you that belt set ups are very touchy. Center distances and tension are crucial. You may be worried about servicing the belt in case one snaps if it is in tube. Chain is much more robust and forgiving, so it is more common to see such a system in tube. Again, I have never tried it, but with my experience with belt, I would proceed with caution. Consider specifically what you hope to accomplish over a typical belt drive, a typical chain drive, and a typical chain in tube drive.
9mm wide HTD belt will certainly not be strong enough. With proper tensioning 9mm wide GT2 profile might be enough if the robots center of gravity is low. 971 has extensive experience with 9mm belts so they will be able to fully explain 9mm belt’s limitations. My recommendation if you want to push the sizing of belt in tube would be to use 15mm wide belt in 1.5"x2" tubing. 192 uses 15mm wide GT2 for our belt in tube drive, but HTD would probably work too if it was 15mm wide.
Sounds like it’s a bad idea then to do belt-in-tube, at least with 1" x 2" tube since it can only handle 9mm belts. Is 1.5" x 2" available in 1/16th wall thickness??
I was leery about 3/8" axles myself. We’ll stick with 1/2" Hex.
I called Harbor and they don’t carry anything in a 1/16" wall. You’d have to go online or find another supplier. If I remember right, Alro carried thin wall, but the closest location would be in GR I think.
Against #25 chain, I would say some points are less important than others.
The weight of #25 chain for a DT is about 0.7lb, versus maybe 0.2lbs for 5mm pitch 9mm width HTD belts. Adding in the weight of the pulleys (0.1lbs for 24t pulleys times 6 pulleys) versus sprockets (0.03lb for 16t times 6 sprockets) then the total for chain is about 0.9lbs and the total for pulleys is about 0.8lbs. I assumed you would use the same sprockets and pulleys everywhere, so it might be a bit different, but you will probably save little weight.
People have run direct C-C chain drives before in tube for whole seasons.
We have never had even #25 chain break on us, and there are more stories IMO about belts breaking than chain. OC if you use either the wrong way they will break or throw.
EDIT: Wait, we did have one break once. At Chezy Champs, our intake jammed against a screw head, stalling a BAG on a 25:1 reduction, and the chain actually did break. That was I think the second time we stalled out the intake; the first time it did not break the chain.
This is the main reason why I like belts, personally. The lack of noise is so nice and adds to the overall quality. This and minor efficiency gains.
Also a great reason. I really like using belts for other applications than DT due to the noise and the look of them, not to mention that you can go exactly C-C with belts, whereas with chains you might need to add a little distance to account for manufacturing tolerances (see this post: http://www.chiefdelphi.com/forums/showpost.php?p=1397554&postcount=26).
If I can throw in my two cents, kickoff is far too close to properly prototype such a system. When it comes to trying something like this you have to give yourself proper time to prototype, fail, iterate, fail again, rinse, and repeat.
Be realistic here, kickoff is in 19 days and you have two major holidays in there (in America at least). This sounds like an off season project to my ear, and the off season is over. It’s pre-season, prepare for the build season.
There’s time enough to try new ideas when your season isn’t on the line.
I can Speak to a 9mm Belt in Tube Drive, Having just completed a Offseason project with the Team that used exactly that.
We ran a WCP 2 CIM DS Gearbox, Speeds of 16.5 ft/s and 5.5 ft/s, in a 6 wheel drive with a single Belt from the Center to the Front Wheel. We used 24T 9mm HTD Pulleys from VexPro, and they have worked perfectly for our needs. We milled out a bearing hole, and used exact CC plus 5 thousandths to get the right tension.
I can say without a doubt that this method is highly effective. Thus far, we have run into no problems, other than a few hiccups in assembly. In multiple pushing matches with our 2014 bot (A very pushy robot), we stalled both bots, and did not have any breaking or ratcheting on the belts.
We would make changes if we were to do this in a competition, such as a method to access the belts without disassembling the frame, and maybe a tensioning method, as we have not run the robot for long enough to accurately judge long term wear on the belt.
I am more than happy to answer any other questions that people have.
HTD and GT2 belts won’t stretch by a noticeable amount. Once you have the right C-C spacing, you should never need to touch it again. That said, a tensioning method may make assembly and achieving proper C-C spacing a bit more forgiving.
I’d echo this as well. The WCP belt C-C calculator works well. However, we made the mistake of not being able to install it with slack and add tension. We just did exact C-C plus a couple thou, so getting the pulley and wheel assemblies on we’re a bit of a struggle.
Maybe it’s just a Muskegon thing, but we are still playing with a prototype drivetrain as well. Both us and 4004 have a pretty good student base to take care of preparation items for the upcoming season. This just feels like a perfect time to keep playing with new ideas. If our season success was determined by the work we do leading up to kickoff rather than after kickoff, I know quite a few teams that would be in trouble every season.
