pic: Another Belt-In-Tube Drivetrain

[Ty Tremblay]

Originally Posted by Steven Smith

Hopefully not de-railing the conversation, but if anyone could humor me and take this discussion up a level, or link the relevant threads if it has been adequately discussed before, I'd certainly appreciate it.

#1: What are the pro/con of going to belt in tube if you are currently running chain in tube?
#2: Do you believe the differences are significant? And why?

snip

What advantages might we be missing out on by continuing to run chain in tube?

The things I can think of:
- We have run 3" high tubing two years in a row, to accommodate extra center drop as well as a slightly larger sprocket to reduce chain loads/sprocket wear/etc. I have justified this in my head by saying the extra profile yields a stronger frame (torsional), but is it needlessly stronger? Would running belts make it easier to go to 2" profile and maintain the high safety factor I would like, even at large diameter wheels?
- Are belts (when properly tensioned) more accurate with less slop when it comes to measuring distance traveled (for auton)? Or is the difference to chain not worth mentioning?
- Is the system more efficient? More robust? Overall lighter (I know the belts are, but the hubs look heavier than an equivalent plate sprocket).
- Other?

Any rate, we will probably do another iteration of our drivetrain before build season next year, and the discussions on the belt in tube have me intrigued. Thanks for any input!

In no particular order:

• There really isn't a weight savings with this specific design. The pulleys are about .5lbs heavier than the sprockets would be, and the belts are about .5lbs lighter than the chain would be.
• While you should always strive to do it right, chain can be a little more forgiving in terms of tension, proper spacing, and alignment, in my opinion.
• You can run chain-in-tube with 1.5"x2" rails like this design. There are some great posts and videos by 2363 in the forums.
• I would say yes, 3" tall tube is needlessly strong. If you've got a welded chassis, 2"x1" is plenty strong even if the front and rear tubes are .0625" wall. A lot of the strength of your chassis comes from a properly designed and attached belly pan.
• When properly tensioned, belts are no more or less accurate than chain in terms of odometry.
• Chain wears in (which some people perceive as stretching) over time. This can often lead to less tension in the chain later in the season, and can sometimes be a cause of issues depending on chain load. Belts wear too, but I don't think they wear as fast and the effects of belt wear aren't as apparent in an FRC robot.
• My main reason for wanting to go with belt-in-tube over chain-in-tube is for pulley wear. In FRC, a chain drive will wear both the chain and the sprocket because most of our sprockets are made from AL. In a belt drive, the AL pulley is much harder than the rubber belt and won't wear nearly as fast (if at all).

[pchild]

Originally Posted by Ty Tremblay

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?

FYI, the OnShape model linked in the original post has 23 tooth pulleys, not 24 tooth. Maybe this is the discrepency?

I compared your belt thickness to the Gates GT3 Design Manual, specifically this image from page 9 and everything looked fine.

[Ty Tremblay]

Originally Posted by pchild

FYI, the OnShape model linked in the original post has 23 tooth pulleys, not 24 tooth. Maybe this is the discrepency?

I compared your belt thickness to the Gates GT3 Design Manual, specifically this image from page 9 and everything looked fine.

Sorry about that. I had realized that my belt model wasn't thick enough so I changed it and then realized that I had about .003" of space between the belt and the "roof" of the tube. I found 23T GT3 pulley stock on SDP/SI and decided to make that change since it gives me enough roof clearance (.043") and still gives me room for the bearing counterbore.

[nuclearnerd]

Originally Posted by Ty Tremblay

Thanks. I'm a little bit concerned about the lack of flanges on the pulleys, but with less than .035" of space on either side I just couldn't fit them in. That said, the .035" is small enough to let the tube wall be the "flange" if anything goes wrong and the belt tries to walk.

I'm also concerned about not having rotating flanges on at least the center pulley. My old team used the Vex belt upgrade on the AM14U chassis in 2014. It worked for the season, but the belts showed some pretty hairy wear, as the sides of the two belts rubbed past each other on the common, center pulley.

[Chris is me]

Originally Posted by Ty Tremblay

Sorry about that. I had realized that my belt model wasn't thick enough so I changed it and then realized that I had about .003" of space between the belt and the "roof" of the tube. I found 23T GT3 pulley stock on SDP/SI and decided to make that change since it gives me enough roof clearance (.043") and still gives me room for the bearing counterbore.

You have nothing to worry about with flangeless pulleys. 2791 has never had flanges on the pulleys and has never had a problem at all. This is one of the benefits of belt-in-tube.

As for GT vs HTD belts - the biggest thing to keep in mind is availability of particular belt lengths. Sourcing GT2 and GT3 belts can be difficult, but it is extremely easy to get HTD belting. I have heard that the HTD's deeper tooth profile is supposed to be better for reversing loads / ratcheting prevention as well, but I have no data to back this up.

Originally Posted by nuclearnerd

I'm also concerned about not having rotating flanges on at least the center pulley. My old team used the Vex belt upgrade on the AM14U chassis in 2014. It worked for the season, but the belts showed some pretty hairy wear, as the sides of the two belts rubbed past each other on the common, center pulley.

If belts are properly aligned and tensioned, this just shouldn't happen. When the belts are on the pulley, they aren't moving relative to each other at all. I've never run into significant problems with a setup like this.

[Ty Tremblay]

Originally Posted by Chris is me

You have nothing to worry about with flangeless pulleys. 2791 has never had flanges on the pulleys and has never had a problem at all. This is one of the benefits of belt-in-tube.

As for GT vs HTD belts - the biggest thing to keep in mind is availability of particular belt lengths. Sourcing GT2 and GT3 belts can be difficult, but it is extremely easy to get HTD belting. I have heard that the HTD's deeper tooth profile is supposed to be better for reversing loads / ratcheting prevention as well, but I have no data to back this up.



If belts are properly aligned and tensioned, this just shouldn't happen. When the belts are on the pulley, they aren't moving relative to each other at all. I've never run into significant problems with a setup like this.

Does 2791 add any additional spacing between centers in their drivetrains?

[GeeTwo]

Originally Posted by Chris is me

If belts are properly aligned and tensioned, this just shouldn't happen. When the belts are on the pulley, they aren't moving relative to each other at all. I've never run into significant problems with a setup like this.

This looks to be like a quite tight definition of "properly". If the holes aren't bored a lot closer to square than many teams can probably manage, the belts on the drive shaft will migrate one way or the other, and shall end up rubbing on the bearings (not TOO bad, but not optimal), or each other (as described), or both (one goes to a bearing, the other is jammed against the first belt, with the same results). The 'both' case actually seems rather likely as I work through the variations.

[Chris is me]

Originally Posted by GeeTwo

This looks to be like a quite tight definition of "properly". If the holes aren't bored a lot closer to square than many teams can probably manage, the belts on the drive shaft will migrate one way or the other, and shall end up rubbing on the bearings (not TOO bad, but not optimal), or each other (as described), or both (one goes to a bearing, the other is jammed against the first belt, with the same results). The 'both' case actually seems rather likely as I work through the variations.

A quality live axle drivetrain should be striving to get those bearing holes as concentric as possible for maximum efficiency and best performance anyway. This is significantly easier in a tube drivetrain, where both "sides" of the drive are actually the same piece, than it is in a drivetrain with multiple side plates. These efficiency gains are one of the subtle benefits of a "west coast" tube style drivetrain, and it absolutely isn't out of reach of teams who have some access to machining resources and put an emphasis on quality.

Originally Posted by Ty Tremblay

Does 2791 add any additional spacing between centers in their drivetrains?

I've heard adding .005-.010 can help, but 2791 has always run them exact center with no additional length. The belts initially seem a little looser than a brand new chain, but that's just how belts like to run. One of the pitfalls of manually tensioning belts is that it is somewhat easy to over-tension a belt, weakening the whole system, because it seems like belts should be tighter than they actually need to be.