Polycord stretchers
 

Hi guys,

In 2012, a lot of teams used polycord or strings connected between two rollers in order to lift the balls to the shooter.

So my question is... how can you keep the polycord stretched?
I thought of maybe connecting one of the rollers on a slot and then adjusting the distance between the two rollers... but I look for better solutions.

Thank you very much,
Amit.

Re: Polycord stretchers
 

We cut the polycord to roughly 90-95% of the required length (2x distance between pulley axles, plus one full pulley circumference [or rather, 2 halves]) It really depends on how taught you want your belts, by which trial and error is the best method.

It was not the easiest method, but we fused our cords while totally loose, and once cooled, stretched and installed the pulleys by hand. On our 2012 robot, we had between 5 and 8 belts on a pvc tube acting as a very wide pulley. We stuck one end into the bearing block, and used leverage to stretch all the belts at once until we could jam the other side into its block.

I would recommend not fusing the belts while under tension, as the whole process is about as frustrating as it gets. Perhaps some reasonable method exists, but none that I am aware.

The idea of putting the pulleys in a slot is a good one, provided you can fix the pulley in their stretched position.

Re: Polycord stretchers
 

Quick Connect Round Urethane Belting is quite easy to handle. It has the downside of snapping once every competition if you don't check that the quick connects are seated every match. This downside is greatly mitigated if you pay attention to the minimum pulley size.

You can't tell this from the mcmaster site, but looking up the part # that's printed on the belt led to some spec from the manufacturer in 2012 that said 3% stretch is optimal for this particular cord. We measure the belt length using string, then divide by 1.03. Following mfr's spec with this belt is key to it success in FRC - an overstretched belt will cause extra load on the motor, just like a pulley that is too small.

Re: Polycord stretchers
 

Out team is based in Israel, so we will search the stores for this item. Thanks!

Re: Polycord stretchers
 

McMaster also sells a Welding Kit for Urethane Belting which includes a unique clamp. This clamp holds both ends of the belt together while the belt is mounted on the pulleys. A modified soldering iron/plate is inserted between the ends (which melts both ends at the same time), then the iron is removed and the clamp squeezes and holds the melted ends together (note that while the belt is under tension, the joint is not while cooling/curing). After sitting sqeezed together for 5 minutes, the belt is supposed to be at partial strength and the clamp can be removed...just trim off the flash. The belt is at full strength in about an hour...or less in an emergency.

Please note that we did NOT pay the $700+ that McMaster charges...I don't recall where we got our kit, but it was about $100.

Re: Polycord stretchers
 

Like JesseK said, it is very important to use the manufacturer's data regarding belt length. Instead of using a welding kit we just melt both ends at the same time using a small butane torch and then press them together. The downsides to this method are that you won't be able to do it in the pit and it sometimes requires two people (an extra person to turn off the torch). To clean up the flashing, rolling the belt on a bench grinder works like a charm.

Re: Polycord stretchers
 

I've said it before and I'll say it again: avoid the use of open flame with urethane belts, the fumes are not good. Instead - why not use a heat gun? Works just a well, won't fume, and you can use it in the pits. Expose just the ends of the cord from behind a shield of scrap, the ends will melt like a marshmallow when they're ready to connect.

Re: Polycord stretchers
 

Thanks for the idea, I will try it out. Are there not fumes from any method that melts the urethane at the same temp?

Re: Polycord stretchers
 

Not necessarily. Flames will tend to cause chemical reactions and melting, while lower-temperature heat will mainly melt it.

A science lab hot plate (with a protective piece of metal on top) makes a good instant urethane melter for this application. I have never encountered objectionable fumes using that method.

Re: Polycord stretchers
 

I seem to remember my team making a jig, placing the two ends of the cord in the jig, melting it, then clamping the top half down. This was my freshman year back in 2012 and I wasn't working on it, so my memory is somewhat fuzzy.

As for attaching them, it was the worst part of assembly, we had to get ratchet straps and get several people to make the polycord go on and I think we did it maybe twice, because it was such a pain we tried really hard to avoid it.