What causes a fidget spinner to explode? (video included)

[Elite Engineer]

(AHH the dreaded fidget spinner!)

 

So in the video, a guy is rotating a fidget spinner held in a vice grip, with an air compressor. The spinner spins so fast that it eventually explodes/ tears apart.

 

My questions is, what is the most probable mechanism to cause it to explode? Fidget spinners are mostly metal with a plastic casing holding the ball bearings in place.

 

I have 2 thoughts on the cause of the explosion. Either the plastic over-heated and just split apart, or it was stressed from expansion of the spinner rotating too fast.

 

Your thoughts?

 

Pic of fidget spinner

 

 

 

Video

Edited May 27, 2017 by Elite Engineer

[John Cuthber]

I think the mythical centrifugal force is responsible.

[Elite Engineer]

I think the mythical centrifugal force is responsible.

I'm certain that's what dictated the direction of the projectile. But what specific stress made the spinner separate? Are bearing known to explode at that high a speed?

[swansont]

Nothing exploded.

 

The spinner has to exert an inward radial force of mv^2/r. If the material can't do that, it will fail.

[Endy0816]

Blows me away that ball bearings have become so popular a toy

[pzkpfw]

Reminds of the thing a few years back; a ball with another ball inside. It could count how often the inner ball went around, and I had friends who spent ages and ages getting very excited by how high they could get that count.

 

I just couldn't see the point. There was an exercise component, but not much.

[imatfaal]

Reminds of the thing a few years back; a ball with another ball inside. It could count how often the inner ball went around, and I had friends who spent ages and ages getting very excited by how high they could get that count.

 

I just couldn't see the point. There was an exercise component, but not much.

 

I would quibble on the lack of exercise component. Once you learned the technique to get the ball moving quickly you could generate enough angular momentum in the weighted inner ball that a fair amount of torque was required to hold and twist the ball in any other direction than around axis. I knew several sportsmen and women who used them to improve grip - and to provide exercise as part of rehab from injury.

 

It was a hell of a lot less passive than the fidget spinners

---

 

That's presuming we are talking about the same sort of things; I was referring to a Powerball (and no - I wouldn't have known the name without looking either) like this

 

 

I dug mine out of the cupboard of junk - a minute or so later I think my forearm would testify that the exercise component was not null

 

 

 

[Elite Engineer]

@Swansont

 

So the inward force from the circular motion caused the plastic cap/ring to compress and eventually break apart?

 

(Sorry for the incessant analysis, I find it fascinating that my previous and obvious causation wad wrong, and that there is more to it I didn't know previously)

[swansont]

@Swansont

So the inward force from the circular motion caused the plastic cap/ring to compress and eventually break apart?

(Sorry for the incessant analysis, I find it fascinating that my previous and obvious causation wad wrong, and that there is more to it I didn't know previously)

An inward force is required. If the arms can't supply it, it fails and flies apart.

[mistermack]

It's in the nature of lots of materials, that they have a yield point.

 

You can deform a spring again and again, so long as you don't get close to that point. But you only have to pass it once, and the spring breaks or permanently deforms.

 

It's probably a similar case with the materials here. You can use it up to certain speeds again and again. But once you go past a certain speed, something passes it's yield point, and breaks, and then the whole thing self-destructs.

[John Cuthber]

Something like this

https://en.wikipedia.org/wiki/Flywheel_energy_storage#Tensile_strength_and_failure_modes

[Elite Engineer]

Something like this

https://en.wikipedia.org/wiki/Flywheel_energy_storage#Tensile_strength_and_failure_modes

Wow, this was really detailed, very nice!

 

However, the article doesn't mention anything about an inward force acting on the binding materials to cause it to warp and then separate.

I don't believe the "inward force" mechanism explains why the spinner suddenly breaks apart. I say this after watching a slow-mo video

of a fidget spinner breaking apart on a rotor at 30,000+ rpm' (screen shot provided below).

In the screen shot of the video, you see that the spinner ruptures at the center, but also at the bearing housing, where the bearing are forced out of their plastic casing.

I would think the center bearing would break apart because that's where most of the inward force in directed. But perhaps not based on material being steel.

 

My one theory, however, is that the 3 bearings in the plastic arms are being forced inwardly, with such high magnitude that they break/crack the plastic arms, causing

the entire apparatus to separate at the weakest points. As the spinner separates, the pieces do not fly out in a straight direction, because the arms are still initially

spinning once breaking apart, and then eventually fly off in a straight direction.

Here's the screen shots with explanations

Edited May 30, 2017 by Elite Engineer

[imatfaal]

I would draw a little diagram of a simplified spinner if I were you and consider the forces required to keep it together; think of the accelerations / forces involved.

 

Remember:

1. Things will travel in a straight line in the absence of anything else

2. The outside bearings (*) are travelling in a circle - thus they are being accelerated inwards all the time (otherwise the straight line thingie above)

3. The acceleration inwards requires a force directed inwards - the centripetal force

4. The force means that the arms are under tension - work out roughly how much.

5. If one of the arms fails under the load - there is no longer any inward acceleration; so what happens?

 

* Why do these things have bearings on the outside arms?

[swansont]

Wow, this was really detailed, very nice!

 

However, the article doesn't mention anything about an inward force acting on the binding materials to cause it to warp and then separate.

I don't believe the "inward force" mechanism explains why the spinner suddenly breaks apart. I say this after watching a slow-mo video

of a fidget spinner breaking apart on a rotor at 30,000+ rpm' (screen shot provided below).

 

The inward force is not external to the spinner. It is not acting on the binding meterials, it is provided by them. The binding fails when they cannot provide the necessary force to cause the motion to be circular.

[Elite Engineer]

Ok, so the when the material breaks from extreme stress (i.e. the plastic arm)caused by the inward force pulling it in, there is no available inward force because the arm is detached, causing the perpendicular force to take over.

[John Cuthber]

When the plastic breaks the bits fly off and then fall under gravity. Since they are moving rather fast they fly off in pretty nearly straight lines- In accordance with Newton's Law.

[Roger Dynamic Motion]

(AHH the dreaded fidget spinner!)

 

So in the video, a guy is rotating a fidget spinner held in a vice grip, with an air compressor. The spinner spins so fast that it eventually explodes/ tears apart.

 

My questions is, what is the most probable mechanism to cause it to explode? Fidget spinners are mostly metal with a plastic casing holding the ball bearings in place.

 

I have 2 thoughts on the cause of the explosion. Either the plastic over-heated and just split apart, or it was stressed from expansion of the spinner rotating too fast.

 

Your thoughts?

 

Pic of fidget spinner

FS diagram.jpg

 

 

 

Video

you"ll find out if you investigate the debris .