Anyone have a source or two for polycord or polycord pullies. The one I have found is on back order at thw quantity we want…
http://superiorbands.com/polycord.html has a great selection of poly cord in different diameters and you can buy up to 500ft. or more. The expedite shipping also. As for McMaster Carr they should have pullies ready to order? We just ordered a few.
This. I believe we bought 3/16" IIRC, solid core, clear (which was mostly a favor to me because orange has connotations where I’m from.)
We also bought UHMW rods, originally 2" but we found 1.75" was closer to what we needed. One of our mentors has a lathe and has been working on cutting them into 3" pulleys with two grooves apiece. Once bored to the correct round size, I take them to work and broach them for 1/2" hex on the arbor press there. We’re then sliding them onto 1/2" churro, as the finished pulleys are a little easier to get on and off compared to regular hex stock; we may switch for the final production robot.
(Full disclosure, I work at AndyMark that sells the 1/2" hex broach.)
(And before anyone goes “but where are the kids in all this?”: We put them to work as much as possible, but this is a case where the necessary machines and tools aren’t in the same town or even the same county as the school. The kids will be getting very good at polycord welding once the rollers are done!)
Polycord welding is an art form and takes time (and alot of trial and error) to get right.
Agreed, though I’ve done it on other teams in 2009 and 2012 so we should have more trial and less error. 
In my experience, getting both ends hot by sticking them in front of a heat gun until they get a little melty and then smooshing them together & holding them together until they cool down has been sufficient. What are you doing that requires lots of trial and error? 
See Art Dutra’s method from 2009 that has always served me well.* That was just a few years ago, right? :ahh:
*Don’t do 10% shorter. 3-5% is sufficient and much easier.
We use hex shaft and cut the disks for the poly cord out of 1/8 polycarbonate. We cut them on the waterjet but you could probably find a different way to cut them and then use a hex broach for the hole. (These are for driven pulleys or idler pulleys on a live axle) We are presently using 3 - 1/8 inch center pieces and two 1/8 inch outer pieces with a difference of 3/16 so that the round drive belt is 1/16 proud. This gives us a 3/8 spacing in the middle of the pulley with the two side plates…
Are you using a dead axle for them or a live axle? idlers can just spin on a dead round axle (with or without bearings if you use something slippery like UHMW for the pulleys… )
In the past we have also used a piece of PVC pipe and glued on more PVC pipe cut from connectors as spacers between the belts. We put a hub inside the pipe on both ends and ran an axle through it. This worked OK but was not optimal although it was pretty cheap to make. The belts all drive together but you will have to put a sprocket on one of them connected to the motor (with chain) to put power to the whole assembly.
Back in 2012 we used 1.5" OD x 1/16 wall round aluminum tubing with strips of roughtop tread as spacers. We riveted in endcaps made of nylon and tapped them for 1/4-20 bolts, which served as the axles. The powered rollers had a hex broach on one side instead. I want to say we gave 6% stretch on the polycord. We used the green belts, which are supposed not to catch and come off track because of their rough surface, and in my experience that’s pretty much true. That system worked like a charm forever.
if you have access to a lathe nothing beats solid delrin for polycord. they work great with dead and live axles. delrin lubricates itself. if you need to drive it its also super easy to hex broach. machine it to the desired diameter then notch it for the poly cord to ride in.
4063 3D prints all of our polycord pullies.
In my limited experience I used a hot knife to weld the polycord. Heat up the hot knife, press both ends of the polycord on opposite sides of the knife to get them nice and gooey, and then slide the ends off the hot knife and into each other. Works splendidly if you don’t overheat the polycord
That is exactly how we do it also. As in rephrasing my prior post, back when I was a student there in 2005-2008 there wasn’t much documentation (or we just didn’t have the knowledge to look it up) therefore having way more trial and error. Now it is smooth sailing.
Art’s method is sound, and how 2815 did their polycord runs. He did omit the last step: after it’s cooled for that half-hour, stretch it as hard as you can. If it breaks, you didn’t do it right and need to do it again.
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