Keeping polycord on track

For our collector, we are using a polycord based system on PVC rollers. Because the PVC is not sticky enough, we are using silicon caulk in between our pulleys. Our system relies on bent aluminium rails to push the ball to the center and the section that rises it to the shooter. However, when the ball enters off center, it pushes the polycord off, as seen in the picture. We tried adding some flat belting, but it only exaggerated the problem, getting more pushed off than the polycord. We don’t have a terrible lot of experience with polycord, so I would be very appreciative if someone could point us in the right direction.

Are your grooves deep enough?
Ours doesnt have that issue. But then again, our balls are guided by polycarbonate sheets on the other 3 sides.

I would agree, make sure grooves are deep enough.

We tried having guides move the balls sideways in our conveyor, on our prototype. It didn’t work, as you see. We ended up putting the ball centering guides below the conveyor, on the bottom of the robot, so the ball moves sideways as it’s in contact with the lower roller, then the balls go straight up. Works good.

Kinda late for this info…

Are the polycords skipping as you drive the robot?
I could see it doing this if the ball is moving in a different direction in relation to the ground.

In other words, are you able to add something to your robot to limit ball movement perpendicular to the cords?

We’ve got a bottom plate that the ball gets pulled on, so the movement of the ball is independent of that of the robot. It is made of perforated aluminum, so that may be causing some problems. Maybe coating it in something slippery might help? (And by that I don’t mean oil, I mean something like teflon)

is your polycord tight enough? we have each of our strands of polycord under about 20-25 lbs of pressure and we don’t have any of these sorts of issues

Three things:

  1. It really helps to have a reaction surface just behind the belt on longer belt runs. This stops the belts from distorting to the point of hopping belt guides and such.

  2. Flat belts do not run well on a flat roller. You need a PVC ring or something for it to center itself on. It’s counter intuitive I know. However, because the larger diameter PVC ring’s surface velocity is higher then the slightly smaller diameter roller the flat belt will center itself so long as it is touching the PVC ring. Alternitively, the way you have it, the flat belts will always want to ride up the round belts guides.

  3. I suggest not mixing flat and round belts so the above won’t happen. It can be done. However, there usually isn’t any real advantage to it.

Regards, Bryan

You can also consider making a “comb” or guide to mount along the run to constrain the polycord, traditionally we have placed one near the motor if we are using a polycord band off a pulley to drive the rollers. Just make sure you smooth the edges so it doesn’t shred the cord.

Yeah, we’re getting rid of the flat belt, it was just a test to see if it would help. The idea of a reaction surface is an interesting one, it might be worth making one up.

We had a similar issue until yesterday. As the ball reached the end of our intake track, it would push the polycord out of its groove. We determined that this was happening because the curvature of the ball was pushing the cord to one side or the other, which caused it to hit the edge of a groove at an angle when the ball reached the inner roller and pull out of place. Judging from your picture, it looks like the middle loop is being significantly pulled to one side, which may be one of the causes of this issue.

Somewhat counterintuitively, loosening the troublesome loop relieved the problem. We ended up completely removing it in the end, since the intake worked just as well with two cords rather than three, but I do not know how this would affect your design.

Also, the process of pulling the ball against an angled rail could be pushing your cord. In my experience, the balls have significant grip against many surfaces, so you could try putting a slicker surface where the ball would contact. We have used teflon tape in similar applications with positive results.

Let us know if you have any further issues, and have a most excellent final build week.

We’re using a similar pick-up system, and had the same problem. Even made our rollers the same way you did. The ball needs to be able to roll against your side guides.

Put some PVC “rollers” over your guide rails, like you see in the cad model below. That simple fix made ours work unbelievably well!

Use a belt length calculator
You’ll be surprised about how tight they are supposed to be. The “normal” stretch factor is between 8% – 12%.

we were having the same problem last night with poly cord.
Is the tension on the bottom alot or just enough and i saw u said u used pref rated aluminum have u tried lexan because thats working well for us and reduced the tension

It either looks like you might have too much squeeze on the ball as it travels up your elevator or that your cords are spaced to far apart.

I would try placing some sort of backing behind the cords or I’ve seen teams use a strip of lexan as a backing with zip tie loops (not tight, just loosely placed) to keep the cords in place.

Your problem might be caused by having so much polycord. The tension of it could be causing the pvc to bend in towards the middle. This would cause lots of jumping. We experienced a similar issue in 09 so we stuck a piece of aluminum L the size of the inner diameter of our pvc through it to straighten it out.

That’s a neat idea!

But I think we’re doing something wrong on ours, we only have one set of polycord belts, and two rollers, to do the whole harvest/lift/load-into-the-hopper thing.

But I think we’re doing something wrong on ours, we only have one set of polycord belts, and two rollers, to do the whole harvest/lift/load-into-the-hopper thing.

I very seriously doubt if you’re doing anything “wrong” :slight_smile: , just make sure that multiple balls don’t bind up on each other as they roll against each other.

The balls generate a lot of friction when they try to roll against each other. We’ve had bad experiences with single-feed systems with multiple balls binding up which led to our “dual lift” design with cords on front and back where the balls don’t roll against each other.

Your system probably works better than ours if it’s engineered correctly!

No, we do have the problem with multiple balls being able to easily jam in the intake. It’s neat to see how you solved that problem.

The main reason we don’t have much stuff on our robot, is because we’re lazy…we’ll spend hours figuring out how to avoid making more parts!

In FRC applications, you only need 3-4% stretch.