Re: polycord and yellow belt
 

Our robot used the hollow polycord, joined with a barb at first. The barbs would snap out over repeated use, and finally we decided to weld them together. They have not broken since, and we have already gone through a regional. I would definitely suggest welding them for anyone considering to put polycord on their robot.

Re: polycord and yellow belt
 

Actually for us, we figured out that as long as 2 cords are touching the ball on our tower it doesn't matter where on the roller they are. So as we may move them back into their specified grooves on our rollers, it isn't a big concern if they move out of place during the match.

Re: polycord and yellow belt
 

For teams with a lathe, cutting channels for the polycord works well. We used mcmaster #8749K17 PVC pipe along with andymark hubs for 1/2" keyed shaft (drive) and 1/2" ball bearing hubs (idler). The hub OD is 2.25, so it was a nice match for the 2.25 OD, 1.125 ID pvc pipe. We used clear 1/4" polycord and fused the ends.

If you have a mill, just use a 1/4" ball end mill to cut a 1/8" deep channel in two blocks of aluminum, then you can heat the ends resting in one channel, push them together, and then cap it with the other piece of aluminum so that the fuse is nice and round.

We used way more bands than are necessary, 1 or 2 would be plenty.

The other method is to use the flat urethane belting and just crown a pulley with tape to keep it centered. Urethane v-belt would also work fine, we used that to turn our turret.

Re: polycord and yellow belt
 

We used solid poly cord in '09 it worked well. Welding it was a bit of pain so we decided to try hollow cord this year. It has been working great. We use 5% strech, it works fine. To keep it in place we simply drilled 2 .250 holes in our Aluminum pipe roller on either side of the cords. We cut a short length of cord and stuffed on end in one hole, wrapped it around the pipe and stuffed the other end in the other hole. It works great, the poly cord stays in place and the short lengths have not slipped out at all.

http://www.facebook.com/photo.php?fb...type=1&theater

Forgive the image, the photo has a bag between the bot and camera ;)

Re: polycord and yellow belt
 

After using flat urethane belting for the first time this year, we will probably never use round polycord again (except if we need a crossed belt somewhere).



Easy to weld (overlap, hit with a heat gun, and press with something flat to square the weld. Turn over and repeat). Doesn't require grooves (just build up a crown out of tape and the belt self centers, see: http://www.lhup.edu/~dsimanek/scenario/crowning.htm).

Our rollers are just aluminum tubes with bearings pressed in each end.

Re: polycord and yellow belt
 

were do u get these belts cause i couldn't find them on mcmaster carr

Re: polycord and yellow belt
 

We use McMaster part # 6075K15 for flat belts.

Re: polycord and yellow belt
 

That's probably doing more harm than good. If you melt the ends together and then immediately install it and put it under tension, the joint will not be as strong and that may lead to failures.

Properly welded polycord should last for years without failure (tons of factories and warehouses use polycord in their conveyor systems), but the only way to properly weld it is to let the welded joint fully cure for 30-60 minutes under zero tension.

If you do polycord right the first time on a FRC robot, you should never have to redo it.

Re: polycord and yellow belt
 

That is a bad idea. I make our students wait at least 15min before putting any load on the cord after welding because the weld needs to cool and solidify. Changing it that quick sounds like a bad idea.

We have green polycord from 2004 and orange from 2006 that still work fine.

We used those last year on our roller claw, and I liked them a lot, but for what we did with our intake I needed to use round for cross over and wanted to keep it consistent.

Also for keeping our belts tracking we cut rings of 1/8" that we welded to the .049" wall Al tube that forms our rollers. Not the most efficient or fastest way to make belt guides but it's signifigantly lighter than anything else we've tried.

Re: polycord and yellow belt
 

The 8-10% number is a general number that comes from the people who make the poly cord installed in its intended application. It gives tension required to allow the rollers to transmit torque. When used as a conveyor it gives reasonable deflection when pushed in a reasonable way. Little a first robot does is reasonable.

If you are able to get the results you want with a lower stretch, go for it. Higher stretch than recommend will eventually result in failure. Maybe long after the season is over.

Re: polycord and yellow belt
 

after using pollycord in 09' we decided 25 pitch chain on our "elevator" system with a piece of delrin on the back to help reduce friction. it works very well with enough friction to get the ball to the shooter with no slipping. picture is not the best but shows the chain (only one, the other got taken off before bagging) above our intake system. the flat urethane system looks very intriguing though

Re: polycord and yellow belt
 

You use the chain as the means of moving the ball?

Re: polycord and yellow belt
 

yup, after lots of prototype testing we found that the chain does not hurt the balls in any way other than getting some grease on them. testing included running chain at a moderate speed and pushing a ball against it as hard as possible for about a minute. that being said we only compress the balls about 1 inch on the robot itself. the motor/gear reduction is a banebot 550 with a 5:1 which gets the ball from the floor to the shooter in about 1.6 seconds.

Re: polycord and yellow belt
 

Really? We tested our tower with polycord and powered it with chain and when the ball walked over to the chain it always left a big black line and some thin slices.

Re: polycord and yellow belt
 

i curious if you have a video of your lifter/elevator and love to see it run with that chain