Lowest friction string material
November 28, 2011 9:49 AM   Subscribe

Mefi Physicists / Material Science Engineers: What type of string material, when looped around and suspending an axle, will allow the aforementioned axle to rotate most efficiently?

Forgive my nomenclature, I am talking outside of my field here. I am working on a project which requires an axle to rotate freely in a loop of string, while also suspending said axle above the ground. The axle will be made out of ceramic, with a slick glaze.

What will best serve this purpose? Polyester, nylon, polypropolene, something else entirely? The string does not have to be super strong, as what the axle supports will be most likely under a pound. However, this axle will rotate quite frequently so durable string would be ideal.
posted by satori_movement to Science & Nature (17 answers total) 1 user marked this as a favorite
Best answer: Basically, this is a yo yo.

Cotton thread will work. Teflon-coated thread will work better.
posted by spikeleemajortomdickandharryconnickjrmints at 9:53 AM on November 28, 2011

Is there a reason for the string to be in direct contact with the rotating axle? If not, then housing the axle within a length of tubing to create a bushing for the axle, then looping the string around the tubing sounds much more effective.
posted by No Shmoobles at 9:58 AM on November 28, 2011 [1 favorite]

NS is right. One way or another, you need a bearing. Otherwise the loop will wear away and the axle (and whatever it's supporting) will fall.
posted by Chocolate Pickle at 10:01 AM on November 28, 2011 [1 favorite]

Response by poster: Spike, ;-)

I have tried cotton strings, and polyester strings, and feel polyester is superior.

I have never seen teflon coated threads, and I can only imagine how well it might work. However, I am a little hesitant. I really do not like handling teflon products(I know it is very inert, but it squicks me out, also, I think any 'coated' string would eventually have its coating rubbed off by the axle).

The site you linked also has kevlar thread, how might kevlar perform?

No Shmoobles, since the jig is up, I may as well reveal I am working on a specially designed fixed axle yoyo, so direct contact with the rotating axle is necessary. I know this is inefficient, but a person has to follow their yo!
posted by satori_movement at 10:03 AM on November 28, 2011

How about threading that axle through some abec 11 bearings?
posted by spikeleemajortomdickandharryconnickjrmints at 10:04 AM on November 28, 2011

Even more efficiently, can the string be suspended from a pulley? If the string can spin along with the axle it won't wear out.
posted by justsomebodythatyouusedtoknow at 10:04 AM on November 28, 2011 [1 favorite]

How fast is 'frequently'? Is it spinning at 1Hz? 100Hz? 1MHz?

Will the axle be changing direction, or always spinning the same direction?

Will the string always sit in the same place along the axle, or will it travel along some length of the axle as it rotates?

Will the axle be resting in a loop of string (so the string is in contact with only bottom & sides, and top of axle is free) or will the string be wrapped around the axle?
posted by jpeacock at 10:13 AM on November 28, 2011

Best answer: Ah... graphite powder might help the string out a bit. It'll be a bit messy, though.
Maybe dipping the loop of string that suspends the axle in some material to make it
harder would work? Enamel or epoxy? It would take some shaping...I don't know if your vision is to take this to production or not so that might be too much work.
What if upon assembling your yo-yo, the axle had a loose washer placed around it? The washer could have a notch in it that the string could fasten to. Might wind up grinding down your ceramic axle, though.
posted by No Shmoobles at 10:14 AM on November 28, 2011

Best answer: Ah, a yo-yo (just saw the response :)

You don't want too little friction, b/c then it won't re-wrap itself.

Part of the experience of a yo-yo is the string wearing out, then you get a new one and break it in. The only solution I've seen to this is the clutched yo-yos, but that's cheating...

Get a braided line so there's no twist. Otherwise a polyester line will work pretty well - at first there will be some friction, then the inside of the string loop will get polished and reduce friction. That will last until one day it wears through, then you get another string ;)
posted by jpeacock at 10:17 AM on November 28, 2011

Best answer: What if instead of a ceramic axle, you used a teflon-coated axle? If you're squicky about it, take comfort in the fact that you're not touching the axle, so there's no chance of getting PTFE in your system.
posted by spikeleemajortomdickandharryconnickjrmints at 10:18 AM on November 28, 2011

Response by poster: To clarify for those who missed the earlier clarification, this is for a yoyo. The idea of using a pulley is interesting, but unfortunately not applicable here.

Jpeacock, the axle will be changing direction depending on the direction of the toss, the string will move freely around the axle. The axle will be resting as well. Maximum spinning velocity is probably a few hundred / thousand rotations a minute if I had to guess, though obviously the yoyo will not be spinning constantly.

NS, super interesting ideas, I think Jpeacock kind of nails this though. I need some friction, the question is how can I achieve the perfect balance between sleep time and being able to recover the yo?

Spike, a teflon coated axle is BRILLIANT. However, then I would need a metallic axle. When this project is done, I will post the results to mefi projects, so I can elaborate further on my motivations there, but for the time being I am going to stick to the ceramic axle.

Through ideas found in this thread, I have stumbled upon pure nylon yoyo strings, pure polyamide strings, and unique multi-material strings including kevlar / polyester hybrids, and a propriety nylon / polyester / slick polyester blend.

I will post back later with experimental details on a Duncan Imperial
posted by satori_movement at 11:09 AM on November 28, 2011

If you make any money off my idea, please buy me a beer at a meetup.
posted by spikeleemajortomdickandharryconnickjrmints at 11:57 AM on November 28, 2011

Best answer: I think you could get away with almost zero friction between string and axle by having an extremely short exposed axle (three or four times the width of the string, say) and making the sides of the yo-yo conical and rough enough to grab the string.

That way, when you gave the sleeping yo-yo the little jerk that wakes it up, the string would come into contact with the sides and friction there would wind it up.

I suspect a teflon coated string would not wind up at all, so I think you need to make the axle very slick and the string rough-- and the axle should probably be of small diameter and metal.

If you are absolutely wedded to a more conventional yo-yo configuration, I suggest trying a coating for the string that has a much, much lower dynamic than static coefficient of friction, such as a thixotropic wax that becomes slick under shear and pressure (such as exists between axle and string in sleep), but is otherwise grabby.

There are highly fluorinated graphites which are very light gray, by the way, and are reputed to be very, very slick, but I've never been able to get anybody to send me a sample of one.
posted by jamjam at 12:24 PM on November 28, 2011

Response by poster: Roger, spike! I should say, my goal is not profit, but science.

Jamjam, excellent post, and food for thought. Other yoyo companies have done similar things to improve the responsiveness of their yoyos(employing rougher surfaces around the axle).

Would thixotropic wax actually allow the string to catch after the jerk?
posted by satori_movement at 1:36 PM on November 28, 2011

I believe it would, but I'm not sure.

According to this patent, lip glosses are thixotropic, but I think they might not be stiff enough.

I'd give them a try, though, and if the results were at all encouraging, I'd add some beeswax for a stiffer consistency.
posted by jamjam at 3:03 PM on November 28, 2011

Response by poster: Just in case anyone stumbles upon this thread later, the answer is the string must have some friction or recall of the yoyo is impossible.

I bought some strings made from a material known only as "slick polyester," and found recall extremely difficult. The string MUST be able to catch onto itself to recall the yoyo, so a little friction is necessary.

So the answer is, for the purposes of a yoyo, a little bit of friction is necessary
posted by satori_movement at 11:31 AM on December 9, 2011

Response by poster: A little more humility, and I learned a deeper lesson here. Think things through thoroughly, avoid being different or difficult solely for the sake of.

The reason most people yo with cotton strings is because they have an excellent balance of sleep time to response. Cotton can sleep quite a while when expert technique is applied.

These slick polyester strings sleep indefinitely, sure, but then a bind is required to recall the yo.

Anyway, these things come down to a person's preference. Thank you for all of the advice and ideas.
posted by satori_movement at 6:16 PM on December 9, 2011

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