Speaking as someone who was on a team which had little trouble with the solid core, I think the hollow core with connectors is the way to go.
Thanks everyone for the info concerning the “polycord”. I will be placing an order for the 1/4" hollow Urethane Round Belting and barbed connectors.
Two more questions:
Our team has used hollow poly cord a couple of times.
You can do either. In the past we have generally stacked pulleys (either COTS from McMaster/SDP-SI or homemade), but a single roller with multiple grooves can be a more elegant solution. I have seen various plastics (UHMW and ABS mostly) turned into roller/pulleys on a lathe, and that works well for many.
We have only ever had problems with barbed connections when people got lazy and made poor quality cuts. As long as you make sharp, flush cuts and fully insert the barbs, you will have no issues.
we used the custom made ones. There are about 30 on our Lunacy robot (4 different lengths). we still use the robot for demos. none have broken.
I think we used either the 1/4 or 5/16th diameter.
We’re seriously considering using the round belting for a mechanism this year. One of the ideas I have in mind I either saw or heard about in the past, is to use something like 1.5" nominal ABS sewer pipe, and put short sections of 2" pipe, split lengthwise to make it just a little smaller, over the 1.5" pipe, leaving a short gap wherever a bolt goes. The outer pipe sections could be screwed to the inner one. The inner pipe would be supported at the ends by caps with bearing holes bored in the center, and a sprocket would be screwed to one end of the pipe.
Our lunacy bot used in house constructed PVC rollers (schedule 80) where the endcaps were drilled out to accept bearings and then it was glued together. To act as guides we sliced sections of PVC that had an inner diameter the same size as the outer diameter of the roller and glued them in place.
Last years roller claw used COTS aluminum rollers that someone found on McMaster.
Do be mindful of the span of your plastic rollers if you use them and how much tension they are under.
The coolest solution we ever came up with was to take a piece of thin wall aluminum tubing and mount four roller bearings on the robot and sit the tube on top of them. You can do it upside down too, the polycord pulls the tube against the bearings. We used the lexan “fingers” you can see to hole the belting in place, we never had a problem and in retrospect they could’ve been much smaller.
That’s a neat idea! You could use just about any kind of tubing with that design, as long as you can figure out how to drive it. Would another section of poly cord work as the drive system from the motor?
Our team was considering using surgical tubing instead of polycord to pick up balls. Would this work?
This is what we did here. PVC center, larger diameter PVC outer. Caps in the ends with bearings inserted.
We did essentially the same thing as 234 in 2009, but with a different size PVC. Here’s the best picture I could find where you can really see what’s happening.
We essentially did the same as 234 and what Sean posted.
I would look at thin wall aluminum piping in your application for rigidity. The amount of tension on those belts tends to bend PVC over longer lengths. We put in a plug that sat in the middle of our PVC pipes to help keep the pipe itself from bending. It mitigated the problem, but if I were to do it again, I’d go aluminum.
-Brando
Yeah I think it will. We used surgical tubing on our 2009 robot. The great thing about surgical tubing is its ability to stretch a lot which makes it pretty easy to work with because you don’t have to cut it very precisely but it is tough to join the ends together. We used a plastic version of the McMaster connector and added some super glue. The bad thing about surgical tubing is that it did seem to wear out quite a bit and I think we had to replace it once at each event we attended.
We are going to try out some solid 1/4" Polycord this year. It looks like great stuff. I just hope that it lives up to its acclaim.
Polycord is GREAT never wears out and if you join it properly it will stay together forever.
Trick
Cut the cord 5% under length and cut the ends at a long angle. Then. Take a piece of aluminum and clamp it in a vice. Have someone hold a propane torch against one side and press the ends of the cord against the other side until you see it melt a good bit. Remove them from the heat and press them together. Hold them together until the joint cools. You get probably 90 percent of the joint strength in 10 minutes but for best results let them sit overnight before streching.
Bruce
Can verify all of this, but for a 2009 ball intake system, with very long belts, stretch was much more of an issue than with solid systems. Retension belts about every event or so.
Did you use a drive pulley manufactured for poly cord belts, or make one? It looks like some of the drive belts are made out of a piece of pvc pipe with spacers or something similar. It looks like it could get pricy quick to buy multiple drive pulleys.
We found that this worked even better when you took a block of metal and drilled a hole to match the cord diameter, cut it in half and used it to clamp the polycord at the joint. Then you don’t end up with misshapen joints.
Urethane belting was a fantastic find. We’ve used it every year since 2008 for various mechanisms with no failures, solid core mostly 1/4". You can easily make your own pulleys cutting a ‘V’ groove on a lathe (no need to make a round groove, a V works great.)
One important hint: You should NOT USE A FLAME as the fumes from burning urethane are toxic. Use a heat gun (expose only the ends to the heat) or soldering iron. The ends will get molten, push them together in the corner of an ‘L’ angle and wait for it to cool. Start long, you can trim a bit out later if needed.
