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Unread 11-16-2016, 05:28 PM
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Re: Polycord Pulleys

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.
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Unread 11-16-2016, 06:03 PM
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Re: Polycord Pulleys

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.
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Unread 11-16-2016, 09:41 PM
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Re: Polycord Pulleys

This thread may be of interest. This post describes the method 1712 had used in the past for creating our rollers/pulleys.
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Unread 11-16-2016, 09:57 PM
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Re: Polycord Pulleys

Quote:
Originally Posted by Max Boord View Post
3d printed pulleys work well depending on infill and load.
They worked decently well for us this year!
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Unread 11-16-2016, 10:09 PM
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Re: Polycord Pulleys

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.
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Unread 11-16-2016, 10:33 PM
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Re: Polycord Pulleys

Quote:
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
We primarily used aluminum tubing with hex shaft caps pressed into the ends and gum rubber wrapped around the outside. You can wrap the gum rubber around a couple times to get your desired diameter. Make sure to put a hex shaft through both caps while you press the second one in so that they are properly aligned. We used super glue for the gum rubber, and it held well throughout the season. You can also press bearings into the ends if you want a free-spinning roller. All you need machine-wise is a band saw and an arbor press or shop press.

Aluminum Tubing: https://www.mcmaster.com/#89965k741/=152mo6j
Hex Shaft End Caps: https://www.competitionrobotparts.co...oller-end-cap/
Gum Rubber: https://www.mcmaster.com/#93625k158/=152mt6p


We also use small BaneBots wheels partly because they were just lying around in our shop, but mostly because they gave us the right diameter we needed.

BaneBots Wheels: http://www.banebots.com/category/T40P.html

Here's are some pictures of our robot that show our intake and indexer. Hope this helped.

https://drive.google.com/open?id=0B7...Hp2ZFczM0p2UU0
https://drive.google.com/open?id=0B7...mt6OElubzc3X0U
https://drive.google.com/open?id=0B7...lhrUGJiekFFdWc
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Last edited by hectorcastillo : 11-16-2016 at 10:35 PM.
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Unread 11-16-2016, 10:45 PM
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Re: Polycord Pulleys

We used these on our intake: https://www.mcmaster.com/#5234k92/=152mzbr

Once we mounted the rubber tubes on the aluminum inner tubes, we chucked it up in the lathe and took a super sharp razor blade to cut grooves. It worked fairly well for us, but in the future, we'll probably use slightly larger than the 1" aluminum tube because the rubber tube sections tended to walk a bit. To drive the intake from a 0.5" hex shaft, we bought some aluminum .5" hex gears from Vex and turned the teeth off to make them fit inside the tube (we do not have an arbor press or a hex broach, so this was our poor-man's solution). We initially tried a press fit, but I think we resorted to either bolting it or using a roll pin.

Last edited by Michael Hill : 11-16-2016 at 10:49 PM.
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Unread 09-13-2017, 10:19 PM
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Re: Polycord Pulleys

For steamworks we purchased HDPE rod and polycord from McMaster Carr. We made the pulleys in house. I ground the round cutter out of HS tool steel. Bored the i.d. to 1/2" and broached them.

Note* turning plastic is tricky. It will heat up and grab the cutter. You need to be careful on speeds and feed rate. I kept a brush with a little water in contact, at all times.

Welding the polycord: we made a fixture out of two pieces, of aluminum, clamped and drilled, on the seam. And, a duct tape hinge, it was quick. Heated with a heat gun was the best heater. Bunsen burner came in second.
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Unread 09-14-2017, 09:44 AM
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Re: Polycord Pulleys

I am a fan of laser cutting disks of 1/8" ABS or even acrylic (which I normally don't like to use because cracking but for this application it works beautifully) and then stack them to make whatever pulley diameter we want.

We make disks in 1/2" steps (2", 2.5", 3",...). We make them with 1/2" hex and 1.125" center holes so that we can make pulleys that are transferring torque or not to the shaft.

We put the "standard bolt pattern" (6X 0.196" holes on a 1.875" bolt circle) so that we can stack them up using our standard fastener (in our case a #10-24 screw -- others choose other standards, I still prefer #10-24).

For larger disks, we have additional hole patterns to keep the disks from separating at the edge where the belts run.

On just to be clear if we want to make a 3" pulley we use 4 disks each 1/8" thick.
  • 3.5" disk
  • 3.0" disk
  • 3.0" disk
  • 3.5" disk

The larger disks form the walls of the pulley. The smaller ones form the portion where the belt rides.

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Unread 09-14-2017, 12:17 PM
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Re: Polycord Pulleys

I had pretty good luck with 3D printing pulleys for our robot this past year, the biggest challenge was designing them in such a way that they would be printable on my printer without supports.

Initially we started with this design:

We ran into a few issues with this, however, because of the overhangs which resulted in some poor-quality prints.

Later we revised the design to this:


The wedge profile on this version made the part a bit wider in order to accommodate the same belt radius, but it also made it much easier to print. The only issue we had with these was that the belts would constantly jump out of their tracks when interacting with balls, which resulted in the creation of these spacers:



After that, we never had any issues with the belts jumping out of their tracks. The 1in wide spacers also worked nicely because they were roughly the same width as two 2" am compliant wheels which made for easier to line up all the pulleys.




The only real drawback to these was the amount of time (and plastic) they used (due to the number of parts required). If I had to do it again I would probably use a thicker perimeter wall and lower infill.
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Last edited by cbale2000 : 09-14-2017 at 12:22 PM.
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Unread 09-14-2017, 01:32 PM
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Re: Polycord Pulleys

We made our own pulleys and used some from West Coast Products (I didn't order those .. WCP has polycord pulleys, right?).

We started with the polycord pulley from McMaster Carr (example https://www.mcmaster.com/#6284k63/=19drk4f) and we put two fender washers on the sides. Then we drove roll pins through to hold the washers on to the plastic and used a 1/2 hex broach to make a hex hole through the washers and pulley. The plastic doesn't broach fully and stretches a bit.. so it fit on the hex shaft tightly. Secured it with a shaft collar for good measure. Used it for the drive belts on the gear grabber.. Probably some nice, ready to go pieces out there.. but this worked pretty well.
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Unread 09-14-2017, 05:39 PM
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Re: Polycord Pulleys

There are plenty of answers for round polycord already, but if you're using a flat belt, you can create a crown on a shaft using an o-ring and some electrical tape. This helps it track really well if you're not going to be side-loading very much. Just make sure you wrap the tape in the correct direction so it doesn't start coming off!
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