Polycord Slippage

[scaryone]

Hi All
Trying to use orange polycord to drive two conveyor belts. Having the same problem we always face which is the belts slip on the pulleys once a load is applied. We are using 1/4" belts on pulleys made from 2" PVC Dowels with a channel cut for a groove. The groove is not "V" or "U" shaped. Are we using:
A: Wrong material for Pulley
B: Too small diameter Pulley
C: Worng shaped groove

maybe a combo thereof or something else I've not thought of.
btw (our belts are 10% smaller than the circumferance

Thanks,

[JCharlton]

You'll want to try a 'V' groove. The wedging effect of the V can generate a higher friction on the belt. Also the material of the pulley has an effect. The polycord grips onto metals better than plastic.

[jwfoss]

10% reduction in belt length may be too much tension and you are causing the rollers to flex and therefore bind. I would try to reduce the lengths to 3-5% reduction overall.

[wireties]

We are doing much the same thing but the rollers (usually) turn. Look at the rollers first. How difficult is it to rotate the rollers (with polycord-induced tension but w/o motors attached)? If you can't easily turn them with your hands (but they need not spin freely), the polycord will slip. Try grease, better alignment, change polycord from 10% to 5% - something will work because ours work just fine.

Good luck!

[MrForbes]

3% stretch is working ok for us. We are using 1-1/2" ABS (about 2" outside diameter) rollers that are about a foot long. We're using ball bearings in end caps to support the rollers. We don't have any grooves in them, instead we have a simply plywood "comb" above the lower roller to keep the cords in place.

Pictures of your robot would help us help you figure out what's wrong...

[Bruceb]

How about a picture of the "comb" Does it help rerail derailed cords?
Thanks
Bruce

[MrForbes]

Here's our comb. It doesn't allow the cords to derail, they're pretty well contained. But you have to experiment to figure out if it will work for you, it will not prevent the cords getting tangled up if for example the balls are trying real hard to push the cords sideways.

[Phyrxes]

Here is sample of ours from a past robot.

The rectangles with the diagonals are cut out to create gaps for the cords to move through. This particular one was designed to keep two bunches of cords together, not space individual cords. The final version that was use was significantly less material but this was the first iteration.

Circles are rivets to attach this to the bottom (or top) of piece spanning the run.

We usually make the comb out of hdpe or nylon.

[Chris Fultz]

In the past, we have used PVC pipe (1-1/4 size, which is 1-5/8 OD) and then used the couplings as cord spacers. With 3/16 cord, we had about 5/16+ spacing.

Using bearings in the ends of the rollers helps considerably. There is a lot of tension on the tubes from the belts, and having a solid rolling surface for the bolts / axles is important.

We have used a 90% belt size in the past with no issues. They are tight, but stay tight through the season.

[AdamHeard]

It's also possible the belts are too loose and aren't getting enough normal force to get enough traction.

A more likely issue is a sheave that is not stiff enough causing friction through deflection.

[de_]

add an adjustable belt tensioner

[Mark 3138]

We had slipping last night and cleaning the cord and pulleys helped...alot. We used mineral spirits. Alcohol would probably work also.

[scaryone]

I wonder if belt dressing would work

[WarehouseCrew]

Jim (Mr. Forbes), what motor and gearing is your team using to drive the polycord rollers? Are you using solid or hollow (tube) polycord? Does thickness play a big factor, We have 1/4" tube, but have yet to fabricated lifter (hopefully tonight). Thanks.

[MrForbes]

This post kind of describes our system.

http://www.chiefdelphi.com/forums/showpost.php?p=1128143&postcount=5

There is a 9 tooth sprocket on the transmission, and 22 tooth on the roller. We are using hollow 1/4" round urethane belting from McMaster-Carr.

We haven't experimented with different sizes, we just used what it appears that other successful teams have been using for several years.