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




On 2017/Sep/21, at 14:03, brian whatcott <betwys1@sbcglobal.net> wrote:

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


I think the grease is useful (necessary) when the axial froce is great.

For example the Vernier RMS is dry, as the force is usually < one kg.

bc