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On 9/21/2017 11:45 AM, David Bowman wrote:
The spinners shown on youtube clips spinning at very high speeds and for
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
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
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