Plexiglass help.

I just set up the ramp for my NZ design, and it seems as though the barrels and balls aren’t sliding down the ramp at the rapid pace I’ve seen (or not sliding at all).

The question is this:

The plexiglass is brand new, so does this have something to do with it? I thought about going with my gut feeling, and using sandpaper to scratch it up a bit, but I don’t want to make the problem worse. Or is the angle just not steep enough to allow the objects to drop?

Did you take the plastic covering on the polycarbonate off?

Also, I’m quite sure sanding it down is going to make it coarser which doesn’t help in your case…

Try mounting the ramp at a steeper angle.

Or you can mount the six bar / four bar offset so it gradually changes the angle of the ramp (steeper as it goes up) so you can collect while the ramp is nearly flat and score elements without having the issue of them not sliding off.

Yes, I took the covering off. The thought behind the sanding was that some of the adhesive may still be on the plexiglass (I honestly don’t know much about plexiglass because this is the first time I’ve used it). Heck, there may not be any adhesive at all.

First, you should be using polycarbonate sheet (Lexan, Makralon) not acrylic (Plexiglas). Acrylic sheet is not legal for the VEX Robotics Competition.

Second, once you peel the film off, you can easily tell if there is any adhesive by touching it. I’ve never had adhesive residue on polycarbonate, so I don’t think it’s likely.

Sanding will only make the surface rougher and likely increase the friction (there are some weird things that can happen with a very smooth surface and the barrels, but for the most part roughing up the surface will not reduce friction).

The most likely problem is that your sheet is not tilted at a sufficiently high angle.

Thank you very much for your help. I am using lexan, not plexiglass, and I apologize for the confusion. I’ll try using a steeper angle.

After wanting to figure out how to reduce friction ourselves, our team found the coefficients of kinetic friction of barrels on lexan and aluminum. We found that new, unaffected lexan, had a much higher coefficient than aluminum. However, after sanding the new lexan with 200-grit sandpaper thoroughly, we found that the coefficient of the sanded lexan dropped below that of aluminum. This is most likely only because the adhesive surface was removed due to the sanding.

And by adhesive surface, I mean the slightly sticky surface new lexan tends to have when the peel is taken off, not the actual peel itself. So basically, when lexan is sanded with 200-grit sandpaper, it doesn’t have nearly as much kinetic friction with objects as new, unaffected lexan does.

the surface profile that the sanding provides creates less contact surface area. the relationship between surface area and friction is now changed. (less surface area) less overall friction = faster moving game elements.

Indeed. I believe that’s why it worked for me (i did sand the lexan after some testing).

That is not true.

Since kinetic friction is equal to [Normal Force] x [Coefficient of Friction], the amount of surface area has no effect. Since in this example, the objects remain the same, the normal force is a constant. Thus, the coefficient of friction must be changing.

I stand corrected.
Can you please explain how the coefficient of friction is changing noting that the objects remain the same.

From a math perspective, xc=y where c is a constant. if x is increased, then y increases. x and y are proportional. This can be applied to the above equation [Normal Force][Coefficient of Kinetic Friction] = Kinetic Friction.

My guess as to why the coefficient of friction is decreased is fresh polycarbonate has a suction-like effect on game objects. On my team’s NZ style robot, we noticed improvement over time. I personally would not sand the intake down, I would just use the robot a lot during practice. Over time, scratches will begin to appear and the slide will function better. However, if is important to realize that if it doesn’t work at all on fresh polycarbonate, it likely will not work on worn polycarbonate.

Yeah, the removal of the slightly adhesive surface of fresh lexan contributes to the significant drop in the coefficient of friction. However, the only way in which surface area contributes is if patches of adhesive surface still remain on the lexan. Though surface area doesn’t factor into frictional equations, an increasing amount of contact area also increases the possibility of hitting a patch of slightly adhesive lexan surface that hasn’t been removed; therefore increasing total kinetic friction. So yes, worn down lexan won’t help a significant amount, because the adhesive surface hasn’t been removed completely. This is why we sanded ours down thoroughly.

This depends on what kind of sand paper you use. If you use traditional sand paper yes it will make the surface more coarse. If you use wet/dry sand paper it will have the opposite effect. Try getting some 800 and 1500 grit. What I did was spray my Lexan with glass cleaner and sanded with 800. The glass cleaner should start to turn black. Once you sand the entire surface with 800, wipe off the surface and spray it with some more cleaner. Then sand the rest with the 1500. Doing this will make your Lexan extremely smooth and it is competition legal. Hope that helped.

Yes, it did. My robot’s almost finished and autonomous programming is about half finished, thanks to everybody that’s helped me out in the past week or so.