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

[JesseK]

Originally Posted by D 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.

We have a couple of igus rods from previous years (3/4" diameter iirc), so we'll add that to this year's rod and use those for our rollers. If you can mill a keyway in the rod, it mates perfectly with the black plastic hubs that attach directly to the window motors. If not, then perhaps you may get away with drilling & bolting through the hub and rod.

This simplicity is our only reason for using these rollers.

[DeMentor]

I just wanted to say "thanks!!!" to everyone posting on this thread for all the useful ideas. You saved us a lot of time. Will try some of the polycord and see how it compares to surgical tubing for our intended design in a quick mock-up.

[Gdeaver]

Grainger has good prices on 50' roll of 1/4" round belting, If teams haven't already ordered.

[Kims Robot]

Slightly different topic, but has anyone compared V Belt to the Polycord? Has anyone USED V Belt on a robot before? We found tons of examples of teams that used polycord, and have even used it ourselves, but have been experimenting with V Belt in our initial prototypes. It seems to work well and slip out less, but one of our mechanical engineers was thinking that as its used it has more potential to come out of the pulley...

Any thoughts?

[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

[Chris Fultz]

Originally Posted by Kims Robot

Slightly different topic, but has anyone compared V Belt to the Polycord? Has anyone USED V Belt on a robot before? We found tons of examples of teams that used polycord, and have even used it ourselves, but have been experimenting with V Belt in our initial prototypes. It seems to work well and slip out less, but one of our mechanical engineers was thinking that as its used it has more potential to come out of the pulley...

Any thoughts?

We have used synchronous belts - they work well, but you need to design around a standard size and alsignment is critical or they will walk aout of the pulleys. V belts would be more tolerant, as they are continually being 'forced' into the v-groove. However, v-belts 'seem' to stretch more, because as they wear they get seated deeper into the pulley.


Regarding poly-cord like belting - the recommendation we received is 8%, and that is working. The belts are stretched to install but are holding up well (so far).

Also, both McMaster Carr and Durabelt provide minimum pulley diameters for each belt size. For 1/4" cord, MMC recommends a minimum of 2", Durabelt recommends 1-1/2 - 2", depending on the load being carried.

[purduephotog]

Any Durabelt distributors that you can point me to? I struck out- ours in Rochester moved.

What's the cost per foot (100 ft min is what I'm told).

Now to find cheap pulleys...

[artdutra04]

Originally Posted by Gdeaver

Grainger has good prices on 50' roll of 1/4" round belting, If teams haven't already ordered.

As advice to any teams looking to save money by purchasing round belting from Grainger right now: avoid it. 228 purchased 100 feet of 3/16" orange round belting from them almost a week ago, and despite being listed as "in stock" on their website we just received an email that it won't be shipped out until the first week of February.

Since this will be too late to really help us (except for replacement/stockpile parts), last night we ordered another 150 feet of it from McMaster and it'll probably be in sometime today or tomorrow.

[Gdeaver]

I believe the problem is the 3/16 as to 1/4". They have plenty instock of the 1/4". It comes out of Chicago. I had no problem getting 1/4" . Used will call at our local Grainger to avoid shipping.

[Rosiebotboss]

For clarification purposes, the 'round belting" we are talking about as actually called what?

In the McMaster online catalog I found this, EPDM Rubber Cord and Polyurethane Cord. Is this the 'right stuff'?