Re: [Phys-L] fidget spinner: more data, more analysis, timestamps, force-law plot

[brian whatcott <betwys1@sbcglobal.net>]

On 9/21/2017 11:45 AM, David Bowman wrote:
> It seems to me that the shape of the spinner probably has the biggest effect on whether the deceleration force is dominated by a linear or quadratic term in speed (assuming comparable balance and friction in the bearing, etc). /snip/ I suspect that there is some sort of kinetic dry friction coefficient effect in the bearing contacts that may explain (or at least model) it.
>
> Both of these hypotheses are testable.  BC's photos show he has both an axially symmetric and a lobed spinner whose spin-down data can be contrasted.  Also, if a spinner shows a constant drag force term perhaps a tiny drop of very light lubricating oil on the bearings may make that contribution greatly reduced, if it is indeed caused by a dry kinetic friction mechanism.
>
> David Bowman
The spinners shown on youtube clips spinning at very high speeds and for 
long periods  are indeed symmetrical in the rotation axis - often a nest 
of four or five ball bearing races.
Commercial ball bearing races are usually provided with grease seals to 
hold in the grease, with which they are all provided. The aficionados 
remove these seals and wash out the lubricant grease. There IS one 
design detail where dry friction may then still occur - and that is the 
cage holding the balls in their radial positions.
It seems the practical choice is to run dry in order to avoid the ball 
to race viscous  work, which supposes that the ball to cage friction is 
very low -  being dependent on the bearing load (which tends to  affect 
the highest loaded balls'  radial spacing)  -  so low in this application.
 There is still a frictional element in balls rolling between inner and 
outer grooved races which are sometimes provided with vee-profile rather 
than circular grooves to minimize this differential friction effect.

Brian W