Gas station without pumps

2017 June 25

Fidget spinners revisited

Filed under: Uncategorized — gasstationwithoutpumps @ 17:55
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In Fidget spinners, I wrote about measuring and modeling the acceleration of two fidget spinners, 5-spoke spinner that cost $6.90 made from plastic and brass and a 3-bladed spinner that cost $8.90 milled out of brass:

The 5-spoked wheel spinner weighs 32.88±0.03g, and the 3-spoke brass spinner weighs 61.14±0.02g.

The previous post looked only at the fidget spinners spinning vertically (that is, with a horizontal axis), but I had noticed in playing with the spinners that they seemed to have different drag in different orientations, so I redid the measurements with the spinners horizontal (that is, with a vertical axis). I had a somewhat harder time spinning the spinners fast with them horizontally mounted, as my makeshift support for the photointerrupter was a bit precarious.

The 5-spoke wheel seemed to run smoothly , but the fit suggests more dry friction and less fluid friction.

The 3-spoke spinner really does not like to spin horizontally.

To visualize the physics better, I tried making acceleration vs. velocity plots for the fitted models:

When holding the wheel horizontally, there seems to be mainly dry friction, almost independent of the speed of the spin.

The 3-spoke spinner has much worse drag at all speeds when held horizontally rather than vertically. The fluid drag seems to be about the same as before, but there is much larger dry friction component (possibly from brass-on-brass contact between the spinner and the axle caps).

As expected from fidgeting with the spinners, the 3-blade spinner has much more drag than the wheel, both horizontally and vertically. The change from mainly wet friction to mainly dry friction for the wheel was unexpected, though.

Update 2017 Jun 25 21:15:  My wife just pointed me to a Wired article: https://www.wired.com/2017/05/the-phyiscs-of-fidget-spinners/ which does a poorer job of the same thing I did. They sampled at a fixed rate, rather than recording time stamps on each rising edge, so they had much poorer time resolution, and they assumed constant acceleration (dry friction), which is only appropriate for low-quality bearings.

2017 June 20

Fidget spinners

Filed under: Uncategorized — gasstationwithoutpumps @ 17:34
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I recently bought two fidget spinners from Elecrow:

The 5-spoked wheel spinner weighs 32.88±0.03g, and the 3-spoke brass spinner weighs 61.14±0.02g.

The heavier 3-bladed spinner cost $8.90 and is milled out of brass (though the site claims “pure copper”, the material looks like brass and is slightly magnetic, so I’m sure it is brass).The lighter 5-spoke spinner cost $6.90.

The lighter spinner is easier to get to high speed, spins longer, has more gyroscopic effect, and has a dimple for balancing it on a pencil point, so makes the better fidget spinner in many ways.

I was curious whether I could characterize the fidget spinners electronically. I have a photointerrupter (an aligned LED and photodetector) from Sparkfun with a 1cm gap that the spinners just fit in.

Here is the 3-spoke spinner mounted in the Panavise Jr, with the photointerrupter counting 6 ticks per revolution.

Here is the 5-spoke spinner with the photointerrupter counting 5 ticks per revolution.

I set up PteroDAQ to record a timestamp on every rising edge of the photodetector, which counts 5 uniformly spaced ticks per revolution for the 5-spoke wheel, but 6 ticks (in 3 pair of closely spaced ones) for the 3-bladed spinner. I can then plot the angular position of the spinner as a function of time in gnuplot:

plot '3-spoke-spin-down-ticks.txt' u 1:($0/6.)

I tried fitting the spin-down using constant deceleration (a quadratic), using deceleration proportional to velocity (exponential decay), and using a model that has both terms: v_{0}\tau(1-e^{-t/\tau})+a t^2 /2.  I expressed position as number of turns (that being simpler to interpret than radians), and so the initial velocity v_{0}  is in turns/sec, acceleration a is in turns per second per second, and the decay time \tau is in seconds.  I got terrible fits with the constant deceleration, decent fits until the spinning got slow with the exponential decay, and quite a good fit with the combined model:

The longer spin time for the wheel is partly due to higher initial velocity, but the time constant for the decay is also much longer for the wheel, indicating better bearings.

I’m not quite sure how to interpret the slightly higher contact friction term for the 5-spoke wheel.

2017 June 16

Futuristic Lights new Kickstarter: fidget spinners

Filed under: Uncategorized — gasstationwithoutpumps @ 11:17
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I just got the following announcement from Futuristic Lights about their newest product: a fidget spinner with three LED lights.  They are doing this one as a Kickstarter, because the market is a bit different from the glover market that they have been selling to so far, and they want to make sure they have a big enough market before incurring tooling costs.  They’re looking for $18,000 to do the tooling and manufacturing run for injection molding the spinners.  As they say on the Kickstarter site, they’ve released electronics products before (3 different microlights: the Kinetic, the Aether, and the Atom), and done a previous injection-molded product (soft cases for the Atom, not available separately), but this is the most complicated mechanical design they have done, and there is a possibility that they will have to get the tooling done twice, which would delay delivery  (but the $18,000 target allows for that possibility).

The $25/spinner price is higher than for spinners that don’t light up (which seem to run around $6–$15), but is fairly low compared to gloving lights (there are three microlights per spinner, comparable to $7 microlights like their competitor’s Chroma lights, so one could view this as only $4 for the spinner, though they don’t plan to sell the spinner without the lights).

Introducing

The Galaxy LED Spinner!


We are excited to announce the Galaxy LED Fidget Spinner! The Galaxy is the highest quality LED spinner in the world.

Here’s some feature bullet points in no particular order:

  • Three minute plus spin time
  • 3 High-quality micro-lights
  • New Design
  • 14 Different Flashing Patterns
  • 4x Beautiful Color Sets for each Flashing Pattern
  • Demo mode that switches the pattern every 12 seconds
  • Battery lock so you don’t worry about it turning on in your pocket
  • The light is equally visible from both sides
  • High quality hybrid ceramic bearings
  • Easy battery replacement
  • 15 hours of battery life
  • Ability to flip lights in opposite directions for different light trail effects
  • Held together by strong neodymium magnets for easy and repeatable assembly
  • And more…

Learn more on our Kickstarter!

Go To Kickstarter!

Update 2017 July 16:  The Kickstarter campaign did not make its goal, getting only $3,146 pledged of  the $18,000 goal.  I think that they missed the market by about 6 months—a long time for fad products.  Because they did not get sufficient interest, they will not be making and selling the Galaxy fidget spinners, but will move on to other products where they expect a steadier market.

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