Xasin
1
Hi guys!
I’ve been working on my robotics project for the RoboCup Secondary for quite a while now, and one problem I have been running into is a constant and rather annoying lack of traction.
The robot, picture attached, uses two 2cm-wide belts printed out of NinjaFlex.
Although this is enough for most obstacles, the contest also features 25° incline slopes, which pose quite a problem. Additionally, the ground of the contest area is a pretty flat and smooth, so it is rather hard to get a grip good enough to climb these inclines.
I’ve already tried a couple of things, from anti-slip varnish and PlastiDip, to coating it with silicone from the hardware store.
Silicone has proven itself to work quite well, however with the downside of very quickly rubbing off, and actually having too much traction to work reliably. The varnish and PlastiDip however do not provide quite enough, and also slowly wear off (although the wear is tolerable, as the coatings only need to persist throughout the 1-week contest time).
So I want to ask you guys!
What do you think could I try too? Is there any way to make the silicone coating more persistent, in your opinion? Or do you know of some other way that will give the belts more grip?
The contest is in one week, so any quick answers would be greatly appreciated! 
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Print a more textured pattern… look at tires and commercial treads. they are not completely smooth, it by printing a texture it will alow allow any material you use as a coating to stay on better and hot wear off
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Xasin
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Alright, I’ll be looking into that in a bit!
NinjaFlex is a tricky material to print with, but I do still have some time to complete them, so … We’ll see.
If you could find some rubber bands of the right size that might do the trick?
3 Likes
Create
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Can you increase the weight of the robot at all? That’s the quickest way of getting more friction force. Or increase the surface area of the tracks? (Sorry for conflicting advice but…) I’d actually suggest that smooth tracks are the best on a smooth floor, if you look at racing cars and bikes you’ll see slick tyres unless the terrain is bumpy or wet. Generic car tyres are bumpy just because most of us don’t have a pit team to change them when it rains!
My vote would be to chop up rubber bands and cement the pieces onto the existing belts. Maybe in bands across the belts?
(1) it is fast for you to do
(2) rubber is real sticky
(3) the added roughness will help you negotiate changes in slope and any small variances in surface
(4) its cheap
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I’d hazard that racing vehicles require slick tyres to maximise friction to counteract the centrpetal forces while cornering and inertia when acclerating at extraordinary rates. Neither case is likely in the scenario outlined by the OP.
Also, while more weight may increase friction, it will also increase the force pulling the vehicle back down the 25 degree slope - probably cancelling out any benefit for the situation the OP is interested in dealing with.
Having said that - I like the fact that you have widened the scope into other factors 
Try to lower the robot’s center of gravity. see if there is a battery on top that can be moved to the bottom or that type of thing. That will keep more of the weight on the tracks when its on a steep incline.
The tracks will always bear all the weight of the vehicle. Whether it is advantageous to have the weight centred or shifted to the front or back depends on the relative coefficients of friction of the ground surface and and slope surface, though I doubt shifting the weight will have much impact here. It would, however, have a big impact on the OP in terms of work to do… 
Create
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As you say it’s to maximise friction, whether the force is from going round a corner, or up a slope the goal is the same, so i stand my by comment. The equation for friction force is f=uR where u (actually symbol mew) is the coefficient of friction, a constant and R is the reaction force. Added weight increases this force therefore increases friction. Sounds counter intuitive but easy to test with a simple experiment using a plank, shoe and some weights to go in it. I actually suggested this because i made a stair climbing robot and this was part of the analysis I did. For steep inclines you are right, but for what were are taking about here, ie less than 45deg, it should work
Xasin
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I would, but the robot is actually really compact, and moving anything around would be a bit hard.
In my next design I’ll keep the stepper motors lower though, they definitely are the heaviest elements.
Xasin
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Rubber bands seem to be suggested quite frequently. I’ll definitely have a look, and aside from it requiring a lot of glue (which I do have), your idea should probably work really well …
Now, time to look for a pack of fresh, disposable rubber bands 
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Yes - good point. I have not made a stair climber, but I imagine you want to shift the weight very deliberately from step to step? Would love to see a photo!
Adding weight at this stage though (for the OP) may be tricky given the form factor and other tasks the robot may need to carry out. Also, additional weight puts extra strain on the motors and battery. Too much weight and even if you sort the friction problem, the robot won’t be able to lift itself up any slope at all! 
ALSO - I learned that it does not matter how much surface is in contact - the friction will remain the same - Amontons Second Law of Friction - which makes you wonder about those slick tyres… Friction (and contact patch size)
what am I looking at here?
Xasin
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Battery life is no problem - the robot needs to run for max. 8 minutes, and the stepper motors have some power left to give.
A 4-Cell LiPo supplies plenty of power.
Also, is that really true?
I would have expected more surface area to provide more friction … But I am guessing this is only true for flat surfaces? For anything bumpy, or even (like tires) flexible and soft, there would be other causes of friction after all.
Besides, the slick tires probably waste less energy as they are smaller, and thusly deform less. And you can’t run them on normal bikes because they would most likely wear a lot faster, too.
Still, very interesting to see that!
Yeah, it’s weird! I guess as surface gets bigger, the force per square cm gets smaller and so it makes no difference. Knobbly tyres would have less contact area, but higher force. Just a quick read makes it obvious that friction is complex and often solved empirically though.
If you only need 8 minutes of run time and want super sticky, you could coat the belts in double sided tape
Xasin
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Heh, been there, tried that. Double sided tape would work great!! Unless you have a robot like mine where the outer edges of the belt rubssideways quite a bit when it attempts to turn. I’ll try out the rubber bands first
What have you tried for tread patterns, as well as a tread depth that could wear and still last long enough to finish the competition?
It won’t work at all, friction for visco-elastic rubbers decreases as you increase surface pressure. That’s why racing cars have wide tyres.
