Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[Aerobie]

I've been experimenting with a method which doubles the length of string I can pull.

Instead of pulling on the string, I loop the string through a pulley and tie the end to an anchor (such as my left wrist).  Then I pull on the pulley and each increment of motion of the pulley produces twice that movement on the string which is wrapped around the top.  So I can use twice the string length.

So far I'm learning the art of this pull.  But I think it has potential.

Alan

[Iacopo]

That's a quite an effect, Iacopo! Clearly, dissipative forces don't need large contact areas or long lever arms to generate significant braking torques on spinning tops.

It surprised me when I saw it, because I didn't expect this strong effect.
Then, after some thinking, I considered that, after all, oil has a density and a viscosity that are really much higher than those of air.  If air influences the spin time, let's figure out oil.  About 60 - 100  mm^2 of surface in contact of oil  at the center of the top, (low leverage), reduced the spin time to one third, from 27 to 9 minutes.

Using mm 1 of oil in the base, I guess it would still slow down this top, but much less dramatically, (since only the tip would be in contact with the oil), with spin times of maybe 26 minutes instead of 27.  In the past I was used to use about 1/2 to 1 mm of oil in the base.  After this test, I became acquainted to use less oil in the base, using often just a thin layer of it.   

[ta0]

I've been experimenting with a method which doubles the length of string I can pull.

Instead of pulling on the string, I loop the string through a pulley and tie the end to an anchor (such as my left wrist).  Then I pull on the pulley and each increment of motion of the pulley produces twice that movement on the string which is wrapped around the top.  So I can use twice the string length.

So far I'm learning the art of this pull.  But I think it has potential.

Alan

I believe it was Jeffs who first posted trying this, but it was on the previous forum. I did find the photos I posted when I tried it myself (dated March 2008).

With a Mexican top that has a launcher (my left hand holds both the launcher and the string):


With a top that has dual bearing tips:




I believe I just held the tips between my fingers and pulled but on the photos it looks like I may have thrown it. I don't recall! 

Joe Mauk is also using this system on the current version of the world's largest top.

[Aerobie]

To date, I've not beaten the starting RPM of conventional launches.  But, as I wrote, I think it has potential.  I may make a custom pulley which has deep narrow groove for string.

How did it work for you?

Alan

[ta0]

How did it work for you?

I did not do any careful measurements or comparisons. It was just a proof of principle so if need arose in the future it could be in my bag of tricks.
The case presented itself with Joe's humongous top where the length of the pull is restricted by the length the ramp: it worked great to get efficient energy coupling.

[Jeremy McCreary]

Just received this "optical glass" biconcave lens from Amazon (https://www.amazon.com/SEOH-Double-Concave-2000mm-Length/dp/B0088AQXHI/ref=sr_1_3?ie=UTF8&qid=1471825462&sr=8-3&keywords=seoh+concave+lens) to test as a spinning surface. It was $10.86 before shipping.




d == rim diameter = 0.075 m (given)
f == focal length = 2.00 m (given)
h == rim height at center = 0.00031 m (calculated, looks right)
R == lens radius of curvature at center = 2.20 m (calculated)
r == tip radius of curvature = 0.0015 m (measured)

The minimal surface curvature is enough to attract test tops to the center in still air, albeit very slowly. However, it's not enough to hold test tops on the lens in a gentle breeze. Hence, you'll need a deeper one for outdoor demonstrations.

With r << R, I don't expect much of a tip friction penalty due to lens curvature, but my ABS plastic tips see more tip friction with some kinds of glass than others. No idea yet where this glass stands in that regard.

This is the least curved biconcave lens I could find on Amazon at a reasonable price. I'll be trying lenses with greater curvatures over the next few weeks.

[Aerobie]

Oops!  I confess to fuzzy thinking.  After several pulley launches which were about equal in RPM to conventional string launches, I analyzed the pulley launch. 

Although the string lengthens twice as much as the motion of the pulley, the lengthening is divided equally between the portion wound around the top and the portion which is anchored.  So it's equal to a conventional pull at the top.

Alan

[ta0]

Mm, I think you were right the first time. Although the length is divided equally between the two sides, it still all has to come out of the winding of the top (if it remains stationary).

Perhaps you are pulling slower because of the extra resistance. The real goal is to have the correct gear ratio for your arm power (what an EE would call impedance matching).

[ta0]

I wanted to measure the focal point of the Foreverspin "diamond" base by focusing the sun on a piece of paper (and then doubling the value to get the radius of curvature). Unfortunately, it has been raining almost continuously for more than a week. So I used a laser pointer instead. But with a small spot, the determination of best focusing is not very precise. The value I got was about 23 cm for the Foreverspin base, while a nominal 5X bathroom mirror gave me 30 cm. Doubling these values, the radios of curvature for the base and the mirror are respectively 46 cm and 60 cm: the base is more curved than the 5x mirror. As discussed above, this is probably not good for long spins.

I did check the results by imaging an object. I moved a pen between me and the mirrors until I saw an upside-down image of the pen with exactly the same width as the original (magnification equal to 1). This happens when the object is at the center of the mirror.
By the way, if you look at your own image and move the mirror, the image will invert close to the center of the mirror, not the focal point.

[Aerobie]

Mm, I think you were right the first time. Although the length is divided equally between the two sides, it still all has to come out of the winding of the top (if it remains stationary).

Perhaps you are pulling slower because of the extra resistance. The real goal is to have the correct gear ratio for your arm power (what an EE would call impedance matching).

Your point is well taken.  I'll noodle on this a bit more.

I tried another approach to lengthening my string.  I tied it to a foot of PVC pipe, making my arm about a foot longer.  My first two attempts (with a 210g top) were short of my prior best.  But the third try was about 5% faster.  That's only enough to add about 30 seconds to the run.

Alan

[Aerobie]

Quicken Lubricant:

I bought some Quicken brand lubricant.  They make great claims for this lube, which has "novel spherical NanoDiamonds" suspended in the lube.

I tried various amounts of it, spread thin on my mirror.  Decay rates were identical to forehead oil or ordinary watch/clock oil.

Alan

[Jeremy McCreary]

Quicken Lubricant:
I bought some Quicken brand lubricant.... I tried various amounts of it, spread thin on my mirror.  Decay rates were identical to forehead oil or ordinary watch/clock oil.

It tickles me no end that fancy oils can't beat forehead oil, but I have to wonder: Could there be better oils from other body parts? And do some people have better oils than others? If so, I bet the oils found on Donald Trump are just FANTASTIC!  ;^}

[Aerobie]

Mm, I think you were right the first time. Although the length is divided equally between the two sides, it still all has to come out of the winding of the top (if it remains stationary).

Perhaps you are pulling slower because of the extra resistance. The real goal is to have the correct gear ratio for your arm power (what an EE would call impedance matching).

Try this line of thought:

The pulley moves one inch away from the top and the anchor.  We know that the anchored strand got one inch longer, this only leaves one more inch for the strand wound around the top.  So there is no multiplication.

Alan

[Jeremy McCreary]

Try this line of thought:

The pulley moves one inch away from the top and the anchor.  We know that the anchored strand got one inch longer, this only leaves one more inch for the strand wound around the top.  So there is no multiplication.

ta0's right, and so were you the first time around. For every inch of outward pulley travel, the top (being the only source of string in the system) has to reel out 2 inches of line. Hence, double the top speed for the same hand translation.

Sailors call this block and tackle arrangement a "runner". If the load is attached to the block (pulley), the sailor has to pull or let out 2 m of rope for every meter that he moves the load. His mechanical advantage over the load is 2:1, but he has to process twice as much rope to get the job done. In our case, the sailor (corresponding to the top) is paying out line.