Round Belting - Solid vs. Hollow, Polycord vs. McMaster

[Darren Collins]

I have a question about what rollers people are using with the polycord. Specifically for the drive roller and then the idlers. I had thought pvc pipe, but I presume there are other more elegant solutions available. Thanks.

[Woodworker88]

Originally Posted by Ice Berg

Someone mentioned that the green was more rough than the other colors, it also happens to be the one that needs a larger pulley. Is this because it is more rough?

No, it needs a larger pulley diameter because it is stiffer. The fact that it is rougher is separate and not really related.

[ChuckDickerson]

Back at the Atlanta Championships in 2006 I got a sample of some yellow polycord for a team (sorry I don't remember which team) that is very rough almost like sand paper. It is 1/4" in diameter. I am not seeing something like it at McMaster but it is hard to tell. Does anyone know the exact stuff I am talking about and where to get it? Maybe a part number? McMaster says the metric stuff is "rough" but how rough? It doesn't say about the texture of the inch stuff. What polycord products have teams used successfully in the past? We can't afford to buy a bunch of this stuff to try out so need to limit the selection.

[falconmaster]

Originally Posted by dtengineering

Thanks to everyone for the helpful replies. For 1/4" belting the solid core at McMaster is $.90/ft and the hollow is $1.22/ft. It sounds like we can't go wrong by getting either of them, and it is pretty apparent that the custom welding set up won't be needed.

As for the brand-name polycord at smallparts... well... it doesn't sound like the 400% increase in price ($4.20 in 10' lengths) is worth it.

And the tips on the wide belting from econobelt are good too.

Thanks to all,

Jason

How is the polycord with the grip on the balls? Is it similar to surgical tubing?

[TD78]

Originally Posted by artdutra04

Don't waste $500 on the McMaster "welding kit"; the solid core polycord can be welded with just a lighter. Just cut the cord 10% shorter than the actual distance necessary, melt the ends with the lighter, stick them together (use the inside of aluminum angle to keep them lined up), and wait five minutes. Now use a razor and trim off all the globs of melted cord around the joint so it is smooth and wait another half hour to let it cure and achieve full strength.

But when cutting the polycord, make sure you cut at a 45 degree angle. Creates a MUCH stronger bond when melting the ends together.

Good luck!

[Ian Curtis]

Originally Posted by artdutra04

Don't waste $500 on the McMaster "welding kit"; the solid core polycord can be welded with just a lighter. Just cut the cord 10% shorter than the actual distance necessary, melt the ends with the lighter, stick them together (use the inside of aluminum angle to keep them lined up), and wait five minutes. Now use a razor and trim off all the globs of melted cord around the joint so it is smooth and wait another half hour to let it cure and achieve full strength.

We found out on Saturday that 10% is pretty tight, and you can safely get away with much less. We had round belting on 3" diameter PVC rollers with grooves laid in for the belting to follow. These rollers were mounted on 1/2 steel shafts in plywood "bearings." The plywood supports were about 25" apart. Four urethane belts at 10% did a number of the shafts. We could clearly see the shafts bowing, and our 18V Dewalts had a hard time turning the assembly. To alleviate some tension, we moved the rollers closer together. The distance around the rollers was originally measured at 55 inches (so we cut the belts to be 49.5" long. We moved the rollers a good 2" closer, and found the belts were still taut, and had no problems with belts slipping, even when we jammed in seven balls and put a board across the top opening.

In short, we found cutting the cord at 10% shorter to be pretty aggressive. At 3% shorter (instead of 10%) the balls moved just as well, and the whole assembly ran much smoother.

Art, is there any reason for the half hour cure time? After five minutes or so ours were cool to the touch, and we haven't had any fail that were joined using your welding method.

[Cory]

Originally Posted by iCurtis

We found out on Saturday that 10% is pretty tight, and you can safely get away with much less. We had round belting on 3" diameter PVC rollers with grooves laid in for the belting to follow. These rollers were mounted on 1/2 steel shafts in plywood "bearings." The plywood supports were about 25" apart. Four urethane belts at 10% did a number of the shafts. We could clearly see the shafts bowing, and our 18V Dewalts had a hard time turning the assembly. To alleviate some tension, we moved the rollers closer together. The distance around the rollers was originally measured at 55 inches (so we cut the belts to be 49.5" long. We moved the rollers a good 2" closer, and found the belts were still taut, and had no problems with belts slipping, even when we jammed in seven balls and put a board across the top opening.

In short, we found cutting the cord at 10% shorter to be pretty aggressive. At 3% shorter (instead of 10%) the balls moved just as well, and the whole assembly ran much smoother.

Art, is there any reason for the half hour cure time? After five minutes or so ours were cool to the touch, and we haven't had any fail that were joined using your welding method.

10% is a total flat out lie on McMaster's part.

In 06 we used 5/16" clear urethane belting (solid core) and ordered the recommended 10% short. It was near impossible to get them onto our rollers. The process we had to go through to do it was so jerry-rigged and dangerous that I won't even describe it here. Not to mention the fact that once it was on, the efficiency of the entire system was horrid.

I'd recommend more like 3-5% stretch for anyone else who hasn't joined their belts yet.

[artdutra04]

Originally Posted by iCurtis

In short, we found cutting the cord at 10% shorter to be pretty aggressive. At 3% shorter (instead of 10%) the balls moved just as well, and the whole assembly ran much smoother.

Art, is there any reason for the half hour cure time? After five minutes or so ours were cool to the touch, and we haven't had any fail that were joined using your welding method.

The 10% value comes from the manufacturer spec sheets. Normally, these belts are designed for conveyor-type installations, so having increased tension allows them to transfer more force through it. In our robots, we may not need the same level of tension depending on the application and polycord thickness. If you use something like 1/8" polycord, sticking to 8-10% reduction is a good idea.

As for the half hour cure time, again that's from the manufacturer recommendations. According to them, you can use the polycord after five minutes, but full strength in the joint isn't achieved until thirty minutes later.

[AdamHeard]

Remember, if you make belts 3% stretched, they can always be a made shorter... you can't lengthen belts.

[Bruceb]

Dont see why you couldnt lengthen them. Just cut them and add in a piece just like you did to make the loop.
Bruce

[Bruceb]

BTW.. I just got done making 10 belts from 1/4 round solid urethane(orange) belting 10 percent short. They are tight but running on half inch steel shafts 10 inches long and in bearings they seem fine.
Bruce

[ebarker]

I have bought poly cord in the past but we didn't put it on a 'production' robot.

During AimHigh a girl on a team from Florida showed me how to weld it. She used a lighter and heated the ends and pressed the ends together and clamped it. I cannot remember the setup hold times but if you setup a jig it should go easy.

Also the cord is designed to have a certain amount of tension so you need to figure out how long you really want it then make the cables and then mount them. Think of them as tough rubber bands.

If you know someone that has a nice sheet fed Heidleberg press you can probably some some of this stuff in action.

I'm gonna run on a little more while I'm here.

FYI - we may make a conveyer out of timing belt, probably the XL size, and use plastic pulleys scattered across a drive shaft will filler between the pulleys. the filler will be turned down the the inner pitch diameter of the pulley. plastic pulleys are cheap and you can get them with single, double, and no flange.

timing belt is made in large tubes then is sliced like a giant salami.

you can go buy any width timing belt you want if you order it from www.econobelt.com or www.sdp-si.com , for example 2 foot wide if you like. There are whole families of standard length.

and the belts don't need to be as wide as a moon rock, only wide enough to touch the crown.

and similiar to the timing belt, you can get flat belt (un-timing belt).

right after we shipped aim high robot I went to a Harley Davidson bike ralley and saw some custom bikes with 4 inch wide belts driving between the engine and wheel. It was totally cool looking and had graphics imprinted onto the belt.

does anyone know how to do the printing ?

[falconmaster]

Originally Posted by dtengineering

I was admiring some of the robots in the 2006 "Behind the Design" book, and noticed that round belting was commonly used in some of the nicer ball handling mechanisms. Team 121 specifically referred to using Polycord belting material.

Searching back through CD (yes... I do search before I post!) I found this thread as the best reference but it was a bit out of date and didn't quite answer the questions I had, so:

1) McMaster Lists both solid core (you join it by welding the ends) and hollow core (you use a barbed insert) round belting material. The hollow costs a little more, but should be easier to join. There is a welding kit for the solid stuff, but it is expensive, and it should be possible to rig something similar at home. Does anyone have sufficient experience with both the solid and hollow core belt to recommend one over the other? Is it as easy to rig up a welding set up for the solid core stuff as it looks like it should be?

2) The McMaster belt material runs about $1.00/ft (more for hollow, less for solid) in the 1/4" size. Smallparts.com lists similar sized Polycord for $4.20/ft (when buying 10 foot lengths). Has anyone experimented with this enough to compare it to McMaster's offerings?

Thanks,

Jason

Incidentally, if you're stuck for ideas on how to play this year's game, you could do worse than read the Behind the Design book. There are some great ball-handling ideas in there.

Is the grip with the polycord similar to surgical tubing?

[Billfred]

Originally Posted by falconmaster

Is the grip with the polycord similar to surgical tubing?

Having handled both lately, the polycord feels a bit grippier. I can't offer you any better insight with regard to hard figures, but there's a difference.

I've had bum experiences with hot plastics in the past, which makes me leery of using any custom heating rigs. That said, I also wouldn't want to use some bum setup with hollow-core belts. Has anyone had bad experiences with them in an FRC setting?

[Stephen Kowski]

Originally Posted by Billfred

I've had bum experiences with hot plastics in the past, which makes me leery of using any custom heating rigs.

custom heating rigs like the one you saw me successfully using friday?

if anyone wants to try this with the solid core go ahead and save yourself some money and buy "Weller®/Portasol® Super-pro Self-igniting Butane Soldering Iron Kit"....it has a chiseled tip and work like a dream, plus its like $60 as opposed to $500