Re: pic: Another Belt-In-Tube Drivetrain
 

Chris, what're your thoughts on this belt in tube design? IIRC, 2791 has had success with belt in tube.

Re: pic: Another Belt-In-Tube Drivetrain
 

I mean pull them out from the front or back of the tube. It looks like the front and back are open, so one should be able to just draw them out from there.

Re: pic: Another Belt-In-Tube Drivetrain
 

The issue is the pulleys. There's only .030"-.060" of space around the pulleys, so they'd get caught on any rivets poking into the tube (like rivets do).

Re: pic: Another Belt-In-Tube Drivetrain
 

Could you pocket the tube right above the pulley such that you could pull it out that direction?

Re: pic: Another Belt-In-Tube Drivetrain
 

I think you'll be fine with the 1.5 tube and no flanges, good way to do it. I am more concerned about fitting the 24T pulleys in there, are you sure your belt thickness is correct? When I was choosing 22T for our drives I was pretty sure that was as large as could fit. Maybe i was using a different drop than you or something, but there isn't much room when we assemble, they're almost rubbing the tube on some of our drives.

I don't know when this will be but next time i get access to one of the robots i'll try and take lots of pictures and show how we do assembly and everything. We have it so assembly and disassembly are really really fast.

Re: pic: Another Belt-In-Tube Drivetrain
 

OH.
I get it now... whoops.

Maybe if you attached the bellypan using 4-40 buttonheads and tapping the tube? If you use an 1/8" bellypan, 1/4" 4-40s would be long enough. Then it wouldn't stick into the tube.

Re: pic: Another Belt-In-Tube Drivetrain
 

Hi Ty:
Can you talk about the bumper supports?
(How are they assembled?
Have they been used in the past?)

Dave

Re: pic: Another Belt-In-Tube Drivetrain
 

This might let you get the pulleys out, but there's so little space around the pulleys in the tube that you wouldn't be able to get the belts off the pulleys before taking the pulleys out of the pockets.

Excellent, thanks Joey. I'll double check the thickness of the belts in the CAD, do you know of any resources online that will tell me the outside radius of a pulley with belt on it?

These bumper supports are an evolution of a concept I copied from 118 last year. The thread I linked has a more in depth explanation, but the short of it is that single-piece bumper assemblies with reinforced corners are plenty strong enough to withstand the rigors of an FRC season and you don't need much extra structure behind the .75" plywood.

In 2016, 319 ran this bumper support setup, which is all 1"x1"x.0625" tubing, along with a single-piece bumper. We didn't have any issues with bumpers all season despite playing in over 100 matches. One thing we did notice, however, is that putting chain or belt inside your chassis rails puts a lot of limitations on where you can mount things to your chassis. We ended up mounting more than we wanted to the .0625" wall bumper supports in 2016.

For this evolution, I copied 558's concept of a single piece of 1"x2"x.125" for bumper supports on the side, and the chassis rails for bumper supports on the front. This gives us a rigid place to mount things, and means we don't have weight on the robot dedicated solely to holding bumpers. The angled 2"x1" supports were also 558's idea, but I modified ours a bit since 319 has welding capability in-house. If you remove the cutout, you can bolt the bumper rail to the top of the angled piece, and then bold the angled piece to the top of your chassis. I'll get ahold of 558 and see if they can post a few pictures of their setup.

Re: pic: Another Belt-In-Tube Drivetrain
 

Ty, thanks for the hat tip on design inspiration. You can see 558's 2016 chassis was a little different because of the angled front cuts so the front and rear rails were on top of the drive rails. We did a fully bolted together construction on the chassis and rivets on the belly pan. A version for use on the flat floor is shown here. Again, all bolt together construction. Feel free to ask any questions via PM or we can start a new thread if needed.

Re: pic: Another Belt-In-Tube Drivetrain
 

This design should work very well for you. 24 tooth pulleys and 15mm wide belts are basically bulletproof in my experience, and with your welded design you can make the tubing and frame as compact as possible. 2791 has used 24T pulleys and 15mm belts with both 4 and 6" wheels without any ratcheting or belt failure at all. Other teams in our area even used this setup for this year's game, with 8" pneumatic wheels, without any failures. This is a very robust setup and you shouldn't have any issues with it that I can see here.

The only slight design tweak I would suggest would be to switch the bearings out for ThunderHex and 1/2" round. In exchange for a little bit more lathe work on your shafts you can use a 1/2" round bearing on the inside shaft and a ThunderHex bearing on the outside. Round bearings tend to be more robust in WCD setups, and as a small bonus if you design the shaft correctly you can retain the entire shaft using a single snap ring.

Re: pic: Another Belt-In-Tube Drivetrain
 

Thanks. What size tubing do you use for your rails? is it 2" tall? If so, what kind of drop are you using and do you run into the issues that Joey mentioned above?

Agreed on the ThunderHex. we ran Thunderhex on our chain-in-tube this year and loved it. I just already had the hex bearings imported into OnShape.

Re: pic: Another Belt-In-Tube Drivetrain
 

Shaker's drives haven't been welded for the past several years, so we have used 3" tubes to allow ample rivet clearance. In flat field games like 2014, we can use 4" wheels with 3" tubes no problem. In other years we had to switch to 6" wheels for extra clearance. With the 3" tubing, 1/8" drop was just fine (if anything a little more than we needed in 2014). If we went to 2.5" tubing or smaller we would have to stagger the drop or even switch to a zero drop drive, sanding outer wheels down to a smaller diameter if we needed to add drop after the fact.

The designers on the team are looking into ways to use smaller tubes in future drivetrains, to save weight and to allow adequate ground clearance with 4" wheels. 2.5" tall tall tubing, with rivets staggered to avoid the belts and pulleys, are a possibility. Experimenting with smaller tooth count pulleys is another option, albeit a risky one. 21T pulleys have a number of teeth divisible by 3 for easy machining, and they allow JUST enough clearance for a 2" tall tube if you stagger the rivets, but without testing I can't be certain they would be robust enough.

Re: pic: Another Belt-In-Tube Drivetrain
 

Here's a 22t 5mm HTD pulley with the counterbore in it. This only leaves .0195" of wall at its thinnest point, and only .005" of clearance from the radius of the bearing.

SDP/SI does sell 23t 5mm HTD pulley stock, however.

Re: pic: Another Belt-In-Tube Drivetrain
 

Ah, I didn't think about the counterboring issue. That makes things more difficult. On 2791 we just used 1/8 washers between the bearing flange and the tube to get everything to fit. This is a bit more compact than going to 2" wide tubing but lets you use smaller pulleys.

Re: pic: Another Belt-In-Tube Drivetrain
 

What are your concerns with using smaller pulleys? I assume that the belts would slip before the pulleys or belt would fail, but is belt slippage your concern with a pulley smaller than 24t?