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

[ ... ]

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

I have a very hard time accepting that the air at the outlet of a
blowgun supplied with shop air is traveling at close to Mach2.

It could simply be giving a little kick to each ball as it
rotates past -- picking a different part of the sloping trailing edge of
the balls depending on speed, so the actual speed of the bearings could
exceed the airflow velocity.

I think it much more likely that air bouncing back off the balls and
retainer interferes with the airstream from the nozzle. I'd be surprised
if the balls reached anywhere near the velocity of the air, and I doubt
the airsteam approaches the speed of sound.


[ ... ]

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.

Not if the race fractures and the pieces fly off, which is apparently
what happened.

You mean the outer race only, I presume.

Initially, yes. In his last post Eric says the inner race also came off
his finger after the bearing disintegrated, which would further explain
why his finger was not more seriously injured.


[ ... ]

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),

That assumes that there's no slip between the inner race, the balls, and
the outer race, which is not the case even at rest. If you hold the
races of an open bearing stationary and push on the ball and cage
assembly it'll slide around easily. That's the reason that a ball
bearing is not happy operating with a load that's very small compared to
its capacity. As the bearing's outer race spins faster the clearance
between the various parts will only increase.

It will slip -- but it can still give a little drive. Try
holding only the inner race and pushing on the bearing cage while the
outer race is unconstrained and see what happens.

But a little drive isn't going to get the race to 180 KRPM (to use my
revised number).


[ ... ]

so a six-ball race at ultrasonic could indeed have the
outer race spinning fast enough to fail explosively.

I don't buy it, at least not yet. A reference to a common blowgun
producing supersonic flow would be a start.

Huh, look what a google on - supersonic air flow blowgun - turned up...
http://yarchive.net/metal/ball_bearing_spinup.html

Interesting, I get a:

================================================== ====================
Not Found

The requested URL /metal/ball_bearing_spinup.htm was not
found on this server.
================================================== ====================

when I try that URL. What is it -- this discussion?


In a strange sense, yes. But 13 years ago on this very group. This is
the thread on google...
http://tinyurl.com/loxlq

Ned Simmons