Belt tensioning solution
 

We undersized the small spocket on our shoulder joint drive and suffered from belt slipping. A quick solution that worked well through our practice rounds was to use Andy Mark's chain tensioner. Just fits over the 15mm Gates belt. We tried these tensioners in 2010 on our drive chains, but they didn't work. They work like a charm on the belt however. We have our fingers crossed that they'll hold up in competition.

Re: Belt tensioning solution
 

I would use two simple rollers instead.

Re: Belt tensioning solution
 

You could also slot the whole motor mount, so when you need tension you loosen the bolts and pull the whole motor in the direction needed. Then just tighten the bolts again and whala! a nicely tensioned chain or belt. Everyone of my team's wheel, gearbox, and cable mounts is like this and they work perfectly.

Re: Belt tensioning solution
 

We've never had that work very well because the tension + vibrations will inevitably cause the bolts to slide back into the non-tensioned position. That's been our experience with that style anyways. Works for a few matches then we have to do it all over again, turns into a big pain.

I would like to ask the OP on how well the back of those belts slide against that plastic. Is the friction pretty much negligible?

Re: Belt tensioning solution
 

The plastic is acetal (Delrin) so it's pretty slippery. In this application the belt's not moving that fast or that often so I think the tensioner will hold up OK. I took it apart after a day of practice and did not see any noticeable wear on the tensioner or belt.

By the way, we have slots in our shaft mounts, but we were not able to get enough tension in both sides of this long belt that way.

Re: Belt tensioning solution
 

You could try a floating idler as well. See: http://www.team228.org/media/pictures/view/5117

I've never seen them on a timing belt before, but I suspect they would work as long as you get enough tooth engagement to make sure they don't fall out.

Re: Belt tensioning solution
 

We tried that too and it looked like it would work, but the big sprocket that we had didn't have flanges on it so we were worried about it coming out. We could have cut out thin circular plates and attached them to the sprocket to act as flanges. We'll keep that in our back pocket in case we run into trouble with the plastic chain tensioner.