after seeing some of the designs at our regionals, we would like to redesign our ball - lifting mechanism using the polyurethane tubing. Can anyone make a recommendation on sizing and/or type? We were told to get it from McMaster-Carr.
thanks,
elaine
Hope this helps:
You most likely want round urethane solid core belting.
Team 234 uses McMaster P/N 59725K803. It is kind of expensive, but has lasted two comps with no problems.
In the future, searching may get you a quicker response.
We’re using the yellow 1/4 inch polyurethane shingle routhane tubing from McMaster. Make sure you buy the barbs that hold them together. Don’t pay for the $300 welding kit or try to weld them yourself because you’ll have belt failures and it’s EXPENSIVE.
We have them cut about 10% shorter per foot to make up for stretch. We also have spans of no more than 1.5 feet between rollers.
We’ve had 0 belt breaks, so they work 
.
The kit is not necessary to weld the belts, it can be accomplished with a lighter, soldering iron hotplate etc, and when done with some care will not break on you
see also the god of polycord:
http://www.chiefdelphi.com/media/photos/33403
all welded i believe, i’ll let Adam chime in on that
yeah…lol its good stuff
We’ve had a few teams come to us and say that they’ve tried the lighter method and had belts break on them.
Pay a few bucks for the barbs and you’ll save yourself time in the build process and stress later.
I second what HighLife said - The hollow core 1/4" McMaster-Carr Polyurethane Belting (6567K57) and the associated connectors (6567K22) are the way to go in terms of belting. Quick, easy, and relatively cheap.
(That bit about keeping the spans between rollers short is crucial to note as well - We had a 40" run and the bands flexed too much, even with cutting them shorter)
We have spans of about 4 feet (80 total inches of belt) running moon roks from the floor to our top “hopper”. On our practice bot, we did the “welding” ourselves. This is with solid core belting. Through hours of practice, we have had no problem with belts breaking. You just have to take your time and give the weld time to set. It can be done.
We used the 3/8 inch, clear, hollow core tubing and joined it with the barbs. We have had the barbs pop out, so we added duct tape after that incident, and haven’t had any problems since. We are also using 3" diameter plastic pulleys for the 3/8" belting.
I had to manually create a key/set screw hole in 2 of our 4 pulleys on the drive shaft, but it was pretty easy to do, even with a hand drill.
mcmaster carr has several smaller sizes in stock and they ship in a day or 2. custom made lengths take about a week.
they recommend an 8 - 10% length reduction from actual dimension to belt, and there is a limitation on belt diameter and roller diameter.
we have been using both styles (our fab and custom made).
We used the green belting from McMaster and did the welds with a heat gun. The ones that failed failed long before we put them on the bot. However, knowing what I know now, I’d get the hollow tubing and barbs. It can be installed to replace a broken belt without disassembling your robot. Our runs this year were pretty long and because we wanted to make sure the bands did not flex much, we shortened the band by the recommended 8%. Our bands ended up too tight and placed way too much load on the motor. We probably destroyed a CIM that way.
We use an industrial heat gun to weld the tubing together and put it in water to get it to set a little bit faster. You have to let it sit at room temp for 5-10 minutes and then the weld should be as strong as the tubing itself. It’s incredible stuff, but I would reccomend experimenting with welding it together.
Thanks for listing the part numbers and sorry, forgot to tell you about the span issues. 
We welded them with a 120 watt soldering gun, with a small flat plate bolted to the tip. All welds were ‘student destructive tested’… the students could not pull them apart.
No failures at the welds.
We used the Mcmaster-carr hollow 1/4" belting that was mentioned before. It worked well for us and none of the barbs pulled out. I think it was worth it to use the hollow belting because you can run the belts while the rollers are mounted on the robot. With the welded ones, you have to remove all the rollers if one belt snaps for what ever reason. Also, I saw other teams who had solid cord on their 'bots and unless the team used a welding kit or similar, the welds would be blackish/discolored and the heat would deform the belts slightly causing them to bump around and not be as aestetically pleasing. Pretty much the hollow cords look cleaner when done quickly and are easier to adjust. My $.02
I would recommend the 1/4" solid core McMaster-Carr Polyurethane Belting. We used it on our robot this year for the first time.
http://www.chiefdelphi.com/media/photos/33401
For all of the people that have had problems with their welded joints failing, I have a feeling that you are not melting the Polyurethane Belting enough. We tried welding with a hobby hot knife and soldering Iron with flat tipped blade, but found that it was only melting the edges of the belting. For the belts on our robot, we used the the blue flame of a candle (the orange flame will char the belt leaving ugly black marks). We then took the hot knife and smoothed the polyurethane that squeezed out of the joint (My mom calls this frosting the joint).
As others have said there is no need to buy a jig. We made our own jig by putting a 1/4" slot in a piece of 1/2" thick aluminum stock. The aluminum acts as a big heat sink too.
We bought a considerable amount of extra belting because we had heard of failures. Fortunately we have had no joints or belts break.
Ryan
We used a simple fusing technique and have never had any belts break on us despite attempting very hard to do so. We created a simple mold by drilling a hole through a delrin block then cutting it into two halfves down the center of the hole. We then heated the two ends of the cord with either a lighter or soldering iron, placed it in the mold and applied presurre for about a minute.
Welding the solid core takes a little practice to perfect, but once you get it down it will not break. We’ve found that once you melt the ends together and wait five minutes, if you pull on the joint really hard and it doesn’t break, it won’t break on your robot.
We welded the solid orange polychord with a standard propane torch.
Just melt both ends that you want to connect
Place them in a piece of Angle Aluminum, with the two ends in contact.
Hold for 5-10 minutes till cool.
This resulted in a weld that was strong enough that two people doing tug-of-war could not rip it.
The welds might not be the prettiest, but they work amazingly, and don’t jump off of our pulley’s
If I remember correctly, the polychord on our bot had a tension of somewhere more than 30%. (I think I remember it being 35%, but I might be wrong.)
We used the orange solid core from McMaster, heated both ends up with and clamped it with an old ribbon cable crimper that had some small angle added to it. to keep things lined up.