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Unread 11-15-2016, 09:58 PM
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Polycord Pulleys

Hey Everyone,

Where do you source your polycord pulleys? We need some that will go onto 1/2 inch hexshaft. Do you just make your own in house? I've done some Google searches but not much luck...

Thanks
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Unread 11-15-2016, 10:09 PM
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Re: Polycord Pulleys

https://www.mcmaster.com/#round-belts/=1524smf
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Unread 11-15-2016, 10:13 PM
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Re: Polycord Pulleys

Not the polycord itself, but what it sits on.

Thanks
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Unread 11-15-2016, 10:14 PM
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Re: Polycord Pulleys

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Originally Posted by TheRozb View Post
Hey Everyone,

Where do you source your polycord pulleys? We need some that will go onto 1/2 inch hexshaft. Do you just make your own in house? I've done some Google searches but not much luck...

Thanks
2815 used polycord in 2009 and 2012. Both times, we used some ABS or Delrin rod from McMaster, cut the necessary grooves on a lathe, then broached it for 1/2" hex.
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Unread 11-15-2016, 10:19 PM
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Re: Polycord Pulleys

We used a lathe to throw some grooves onto these. http://www.andymark.com/product-p/am-1305.htm

Nowadays I'd just 3d print them
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Unread 11-15-2016, 10:20 PM
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Re: Polycord Pulleys

PLA? ABS? Is it strong enough to handle the force?
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Unread 11-15-2016, 10:21 PM
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Re: Polycord Pulleys

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Originally Posted by Billfred View Post
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?
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Unread 11-15-2016, 10:25 PM
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Re: Polycord Pulleys

Quote:
Originally Posted by TheRozb View Post
PLA? ABS? Is it strong enough to handle the force?
If you have access to a 3D Printer you could design and print them.


In 2016 we used PLA Pulleys (with a 10-15% infill) and there were practically indestructible. Through the entire season we only broke one. The one we did break was destroyed by smashing it with a hammer.
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Unread 11-15-2016, 10:37 PM
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Re: Polycord Pulleys

Quote:
Originally Posted by Chak View Post
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.
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Unread 11-15-2016, 10:58 PM
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Re: Polycord Pulleys

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

Last edited by Tom Line : 11-15-2016 at 11:01 PM.
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Unread 11-15-2016, 11:51 PM
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Re: Polycord Pulleys

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.
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Unread 11-16-2016, 12:14 AM
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Re: Polycord Pulleys

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.
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Unread 11-16-2016, 01:19 AM
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Re: Polycord Pulleys

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.
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Unread 11-16-2016, 06:08 AM
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Re: Polycord Pulleys

One very successful method is sandwiching a banebots wheel between two polycarb discs. I know 1741 used it for their intake this year.
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Unread 11-16-2016, 06:40 AM
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Re: Polycord Pulleys

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.
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