Does somebody know what is this type of belt/polycord?

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Looks like McMaster-Carr, cut to length and with the ends heat-melted together. Nicknamed “polybelt”, also available in round form.

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Our build blog has how 111 welds polybelt, which may be helpful.

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In our very specific case (5895) it’s 1-1/2" Wide Polyurethane Belt, McM: 6075K17 - McMaster-Carr

For welding, this year we invested in the (relatively expensive) urethane welding kit from McMaster, 100% worth the money if you can spare it and need to make a lot of belts. (we had to make something like 40ish belts across a few different integrations) - there are a couple of other alternative methods out there that we’ve used in the past, but after using the “Real” kit, cannot recommend it enough.

Rollers / Pulleys are 3D Printed, just a tapered profile where the center third (~5") of the roller measures ~1.625" and then smoothly tapers down at either end, want to say the minimum diameter is something like 1.375".

There’s also a timing belt connecting each of the front rollers to the rear roller, so the flat belt is not used for power transmission which let us get away with running them a bit looser than if they were also for power transmission - anecdotally this seemed to improve efficiency and/or minimize bending of the roller shafts due to tension.

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Just in case it wasn’t clear, that’s a picture of 5895’s robot. The above post is straight from the source.

For anyone looking for a cheaper alternative, we picked up a belt welder from Practical Parts (company by two alums of 2169 KING TeC) for Houston this year after two harbor freight hot knives died in rapid succession, and it gave us the best weld we’ve ever made (granted, it was on a round polycord, not flat, but no reason to believe it wouldn’t work on flat). Since we just got it I can’t vouch for longevity, but in terms of weld quality and customer service, we are in love.

I’d also recommend picking up a clamp (the one from Practical Parts looks fine, we already had one so didn’t pick up theirs) because it improves the weld quality significantly, and it is SO MUCH SAFER than whatever other method you could think up on your own.

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Mildly amusing history surrounding the origins of that product: Our 2016 bot’s polycord loops gave us some trouble throughout the season and we didn’t have anything good to hold the polycord together while welding a new connection, and removing the intake drums were arduous, so I had to build one of our own. Pictured here:

The pain endured by this became the inspiration for Practical Parts to design something a little more sophisticated. Junior-in-high-school me would have loved having the clamps. For that reason I fully endorse the Practical Parts polycord clamps, though that may be a conflict of interest because I am good friends with the owners.

(I didn’t think I make a Chief Delphi post again but just as I was lurking on CD for the first time in a while I find a topic I am very passionate about)

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Somewhere deep in the history someone on CD created a belt welding clamp that we used as the basis for our own clamp. Our first use of it was in 2017, if that gives you any clues, and we still use it today. Maybe someone else can dig through and find it.
If our shop wasn’t being torn down at the moment for remodeling I would go take a few picks.

My team used them in our 2021 robot. After that we decided to never use them again ever because of all of the pain it gave us

You don’t need a tool to do the welding. We used these belts last year. To weld them together, we just built a jig. Got a small piece of plywood, screwed two parallel pieces of 2x2 an inch apart (using 2x1 aluminum as a spacer) butted the two ends together with a tiny bit of overlap, pointed a heat gun at the joint and then, after they were good and hot, pressed down with that piece of 2x1 aluminum for 30 seconds or so.

I’d strongly recommend not writing off polybelt/polycord because of one bad experience. It is a very common and very useful part in FRC, it just requires some practice and the right tools to use effectively.

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Got to the shop today. Here are a couple pics of our home-made belt clamp.


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That is super nice - we made a jig out of an aluminum block with a milled recess as wide as the belt and a little shallower than the belt thickness with a “trench” right where the belts meet (wow, that’s a description for sure) but that’s really nice.

I might make one just because it’s so cool.

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