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On 14 Apr 2006 20:04:38 GMT, (DoN. Nichols)
wrote:

According to Ned Simmons :
In article ,
says...

[ ... ]

What alloy are bearing races made of? What's the tensile strength?

By far the most common material is AISI 52100, UTS is around 300,000
psi. I don't believe that centrifugal force alone is enough to burst the
bearing race. I figure a peripheral speed of around 1700 FPS would be
required to reach 300 ksi stress. That's close to 2x the speed of sound,
almost 20 miles/second, or about 250,000 RPM for a common 6203 (17x40mm)
bearing.

Hmm ... assuming eight balls in the race, that would get you to
33.3 KHz frequency from the balls modulating the airflow.

And the bearing which failed on somebody's finger was 5/8" ID
(15.875mm) -- slightly smaller than the one which you postulated. But
we don't know how many balls it had. No doubt, no fewer than 6, which
would get us to 25 KHz. And we don't know exactly when his hearing
dropped out at that time in his life, nor how much more speed the
bearing got in the time since it passed his hearing range.

(Those numbers may be a bit high, depending on how chunky the
bearing is, i.e., how far it deviates from a "thin cylinder.")

I suspect that lubrication failure leading to galling of the balls and
consequently jamming is the big culprit. Perhaps the race expands enough
from the centrifugal force to rattle around and hasten the failure,

That might hasten the failure of the surface of the balls, but
if they *really* failed, all of that energy stored in the spinning outer
race would be transferred to the finger on which the bearing was
mounted, perhaps twisting it off.

Hmm ... the story did not describe whether the bearing's inner
race remained on his finger -- nor what happened to all of the balls.

but
it seems unlikely to me that it's possible to get the bearing spinning
fast enough for this to be much of a factor. The strain in the
hypothetical 6203 race would be about .15% at 100 KRPM - in other words
the bearing might loosen up a thousandth or two at that speed.

I would expect the outer race to be spinning faster than the
ball cage (think of it as a planetary gear setup with the planet gears
being driven), so a six-ball race at ultrasonic could indeed have the
outer race spinning fast enough to fail explosively.

Enjoy,
DoN.
Greetings DoN,
It was me who posted about the exploding bearing. The inner race came
off my finger but it was intact. I'm not sure if it came off because
the bearing came apart or because when the bearing did its thing I
jerked my hand back. The rollers disappeared into the insulation on
the walls. The walls didn't have sheetrock on them where the pieces
struck. I always thought the reason the thing came apart is because
something jammed. A friend's father worked at a lab where things were
spun up in centrifuges until the spinning objects failed. They were
spinning at tremendous speeds. This was when I was about 16 so the 34
year old details are sketchy. But his dad told me how much energy the
small centrifuges used and it required lots of horsepower to spin
small items to the bursting point. Way more than the air coming out of
my nozzle.
ERS