Polycord Pulleys

[Chak]

Originally Posted by Billfred

cut the necessary grooves on a lathe

Are the "necessary grooves" just a semicircle of the correct diameter, or is there more of a trick to it?

[Billfred]

Originally Posted by Chak

Are the "necessary grooves" just a semicircle of the correct diameter, or is there more of a trick to it?

As I remember it, we went a little deeper in to ensure enough contact between the drive axle and the cord--but that might be just voodoo for us. You can always wrap a little wedgetop or roughtop around the outside for extra grabbing, if necessary.

I'd definitely keep the width at or below the width of the belt itself, to keep them from walking or having more room to deform and let something by.

[Tom Line]

We've made a lot of these. It needs to be a deep channel - polycord is sticky and likes walking out if you put any side load on it.

However, consider using flat polycord belting. Round polycord slips even when it's very tight. Flat belting doesn't. And with the belting, you can just build up a high spot on the roller using anything you like - tape, rubber o-rings, etc. and the belt will keep itself straight.

Disclaimer: belt can be much harder to weld than cord.

Mcmaster sells round belt pulleys.
https://www.mcmaster.com/#round-belt-pulleys/=1525gp9

[Max Boord]

3d printed pulleys work well depending on infill and load. 179 used them to transfer load between the middle to front and back rollers and had to wrap them in duck tape to get enough friction to transfer the load effectively. on 4592 we ran 2 775pros on our intake with quite a bit of compression and found a need to replace a pulley or 2 every match due to cracking however this should not be an issue in most situations as most of ours where broken by hitting other robots and the middle CDF plank. The best solution we found was to run loads of them (i think we ran 7 to 11 depending on the revision of the intake) and be ok with running with a few missing/ broken.

[AlexanderLuke]

We have been 3d printing custom designed pulleys for prototyping with polycord when the time arises. I can attest to the need for making the grooves deep. The belts will run off the pulley or slip off with side loads if the groove is not deep enough.

The information other people have already posted is a great resource for you as well.

[CalTran]

Back in 2012 when my team used polycord belting, we just lathed up some delrin and it worked fine.
I would imagine 3D printing would work just as well if you can; the pullies aren't going to be under particularly high stress.

Haven't seen it mentioned yet, but be aware that over the season, the polycord will stretch and you'll have to have some extra made for replacement. Or have a system to de-tension them between matches to help the lifespan.

[carpedav000]

One very successful method is sandwiching a banebots wheel between two polycarb discs. I know 1741 used it for their intake this year.

[Ari423]

Team 423 used VexPro 3.25" VersaWheels one next to each other as an intake last year. The gap between the treaded parts of the wheels is .370" which is the perfect size to hold in 3/8" poly cord (I think it was 3/8 but it may have been one size smaller). They occasionally would walk into the next groove over if they got a lot of side force but for the most part they worked like a charm. On the other end, we used a piece of PVC tube with slices of a larger PVC tube screwed into it to create grooves. If you don't have access to a lathe or 3D printer, PVC tubing is very versatile and you can find it in almost any hardware store.

[Cothron Theiss]

If you check out team 188's intake from this year, I believe they put small Colson's on a lathe and cut out a pretty deep groove for the polycord. It looks nice and very effective, though it's probably a bit of an expensive solution if you have access to a lathe or a printer.

[George C]

3 or 4 years ago my former team needed quite a few of them about dia 3". We had access to a lot of scrap pieces of 1" thick sheet UHMW. We rough sawed them round on a band saw, drilled a dia 1/2" hole, broached it to hex and then mounted the resulting blank on a piece of 1/2 hex shaft chucked up in a lathe. A hex shaft collar held it in place against the chuck jaws. As the UHMW is so soft, we were able to turn them to diameter in one (maybe two) pass. I ground a HSS cutter to profile - radius plus to match the dia of the polycord plus a taper back on each side to encourage the polycord to stay in its slot. Again, because UHMW is so soft, we were able to feed straight in to depth. If I recall, we used 1/4" polycord and went in about 1/2".

Some had to go on a round shaft so we left the hole round, broached a keyway and used a piece of keyed shaft for the mandrel. Some needed a set screw so we left a hub on the pulley and drilled and tapped for a grub screw.

It was cheaper than buying round bar and parting off pieces.

[Lil' Lavery]

This thread may be of interest. This post describes the method 1712 had used in the past for creating our rollers/pulleys.

[BrendanB]

We made custom pulleys using some acetal we found at a local plastics company. With several lathes and willing students it took some time but the learning experience was valuable. Not the most efficient process for making polycord pulleys but we liked them. Walking out is an issue but we wrapped some cable in between our pulleys & wheels to fill the space so if they walked out they would still function.

I was a fan of how 188 did theirs. It looks like they just cut a groove into some of the colson wheels from vexpro.

[gerthworm]

Originally Posted by Max Boord

3d printed pulleys work well depending on infill and load.

They worked decently well for us this year!