View Full Version : polycord and yellow belt
From what I've seen most teams (including ours) are either using yellow belt or polycord for an elevator from the ground to the shooter/ launching device. What is the worst thing that has happened with your teams cord whether its snapping or something else. I just curious if teams have problems with these things.
I'd say ours is the cords walking around on the rollers.
DonRotolo
12-03-2012, 21:07
The worst thing that happened to our orange urethane belting is that students got this idea to cut and re-join it after almost every match "just in case". :eek: sheesh.
Oh Don i never told u we only did it at the most twice a day
quinxorin
12-03-2012, 21:18
We've been using polycord since 2010, and honestly, no problems. As long as we follow the instructions and cut it 11% shorter than measured, it works beautifully. It doesn't just work for applications like collecting balls; if you want to prevent a motor from stalling, use polycord on a pully designed for a toothed belt, and the polycord will start to slip before the motor stalls. You can change its tension to adjust how much force it takes to make it slip.
The belts on our grabber in 2011 would fall off every once in a while, but that was more a symptom of the design of the rollers (http://lightningrobotics.com/gallery/image/3340.JPG) than anything else. It wasn't a problem, either; there were 4 belts on each side, and it only took 1 for the grabber to work. It was also the only part of the robot that worked at all that year, but at least it gave us something to be proud of, dog toys and all!
470-RBTX
12-03-2012, 21:18
The poly belting is some of the most durable stuff I've ever seen and used. 470 used belting from our Lunacy bot (which has endured 3 FIM competitions and countless demonstrations) on this years robot, and have had no problems with it. We just cut it to dimension and and rejoin it and it keeps holding strong, in the 3 years of us using that robot it's never snapped.
Billfred
12-03-2012, 21:36
I'd say ours is the cords walking around on the rollers.
This is the problem we've had this year; we've tried to fix it by limiting the cord's ability to walk.
MrForbes
12-03-2012, 21:49
We first used it this year, it's been working fine for us, no problems. It did take us a little while to figure out how to keep it in place, we figured this out on our prototype and it's been working fine on our robot.
We're using a "comb" to keep the cords in place, at one end. They run on very simple ABS rollers. We used the connectors and hollow cord, and only about 3% stretch.
Billfred
12-03-2012, 22:19
We're using a "comb" to keep the cords in place, at one end. They run on very simple ABS rollers. We used the connectors and hollow cord, and only about 3% stretch.
Do you have a photo of this comb? (We used 5% stretch, though our belts are welded.)
MrForbes
12-03-2012, 22:21
sure....I have pics of almost everything!
http://www.selectric.org/nerds/2012robot07.jpg
Do you have a photo of this comb? (We used 5% stretch, though our belts are welded.)
Second the picture motion. Our polycord was coming off our pulleys/rollers as well, but decreasing the amount of ball compression helped a TON, and we still could increase the amount of stretch in the polycord.
Something we considered but haven't done was switching over to rapid prototyping instead of pulleys.
-Duke
We love our poly cord, we don't have a problem with walking, maybe once or twice at max (were using custom rollers with grooves cut in them) and the welds are strong as can be. We did a test weld to test its strength (put it in a vice and pulled on it) and it was amazing, after 3 students and a mentor pulled on it, all it did was stretch.
We are using Squirrel's comb. Except we made it out of 1/16" plexi. Works great.
mathking
12-03-2012, 22:41
Here is a picture of our "comb" for the belting. It will walk a little if a ball goes through, but it then walks back.
12301
Jim Wilks
12-03-2012, 22:42
We are using 3% stretch on solid "welded" polycord. All seems well. What do others see as the advantage of greater amounts of stretch?
S.P.A.M.er 17
13-03-2012, 10:20
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.
This is the problem we've had this year; we've tried to fix it by limiting the cord's ability to walk.
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.
scottandme
13-03-2012, 11:38
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.
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?fbid=860014058055&set=a.860013888395.2214743.28400614&type=1&theater
Forgive the image, the photo has a bag between the bot and camera ;)
Jared Russell
13-03-2012, 12:10
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).
http://farm8.staticflickr.com/7053/6773484286_a86a1e8a27_z.jpg
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.
were do u get these belts cause i couldn't find them on mcmaster carr
Jared Russell
13-03-2012, 12:20
We use McMaster part # 6075K15 for flat belts.
artdutra04
13-03-2012, 12:21
Oh Don i never told u we only did it at the most twice a dayThat'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.
Peter Matteson
13-03-2012, 13:20
The worst thing that happened to our orange urethane belting is that students got this idea to cut and re-join it after almost every match "just in case". :eek: sheesh.
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.
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).
http://farm8.staticflickr.com/7053/6773484286_a86a1e8a27_z.jpg
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.
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.
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.
BrendanRadabaug
13-03-2012, 16:12
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
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
You use the chain as the means of moving the ball?
BrendanRadabaug
13-03-2012, 16:48
You use the chain as the means of moving the ball?
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.
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.
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.
i curious if you have a video of your lifter/elevator and love to see it run with that chain
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.
Same here. The barbs were good for prototyping, but are no where near as strong as a welded connection. We made some special tools to help weld it and remove the flash.
The only problem we had was with the balls wanting to push the cord off a pulley under significant compression. We had to greatly reduce the compression to make it work, but I think our pulleys were also not as well machined as they might be. Some pulleys never came off and some always did. We will need to examine them when we get the time and improve our design.
RayTurner1126
13-03-2012, 18:39
we used the hollow poly cord with a barb and it hasn't come apart yet, but we were having quite the problem with it slipping right off our rollers, even though it's 8% (i believe) short. what we did was added a zip tie across it that allowed for just the right amount of increased tension in the middle and it didn't interfere with functionality at all! I recommend adding some sort of tensioner in the middle of the two ends of the loop of poly cord to avoid slippage problems, but other than that i've loved the poly cord. apparently in 2006 we used a belt that had to be sewn together and it was absolutely awful, it kept wearing down and bunching up and they had a ton of problems with it, that's why we went with poly cord this year. it's a good choice!
We are using 3/16" solid polycord and it works great. Most teams use 1/4" or larger diameter but we are on a tight budget so we went with the 3/16". We are using 1.5" pvc pipe with endcaps for the 3 rollers and the aluminum rod that came in the KOP for shafts. We use a heat gun to join the ends and 5 % stretch. All this is driven by one window motors mounted directly to the pvc endcap. To guide the cords a volunteer sliced up some pvc connectors to make rings that fit over the pipe. To hold the rings we glued the strips of clear vinyl tubing (with PVC cement). I'm pretty sure we don't need the rings and could just glue on the tubing strips. The system is student built and none of the rollers are parallel with each other. The saying around the workshop now is.."polycord is VERY forgiving".
From what I've seen most teams (including ours) are either using yellow belt or polycord for an elevator from the ground to the shooter/ launching device. What is the worst thing that has happened with your teams cord whether its snapping or something else. I just curious if teams have problems with these things.
worst that has happened is that when our holder/lifter fills up, the polycord derails... so our secondary driver has to keep a close eye on it. other than that, no problems
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