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In article ,
(DoN. Nichols) wrote:

According to Joseph Gwinn :
In article ,
Eric R Snow wrote:

[ ... ]

Years ago I was blowing out some bearings with air. Goofing off, with
the bearing on my finger, I spun one up and listened to the pitch. As
it spun the pitch got higher until I couldn't hear it. Just as it
passed into my ultrasonic range it exploded with a bang.

Ultrasonic! Wow. That implies a ball-passing frequency exceeding
20,000 per second or so. If the ball cage has ten balls in it, the cage
is rotating at 2000 rps, or 120,000 rpm, and the outer race a factor
faster. No wonder it exploded.

Do you recall the dimensions of the deceased ball bearing?

Since it was on his finger, a guess says that the ID of the
inner race was probably on the order of 5/8". Scale the rest to that.

The bearing
axis was perpendicular to my body so that the bearing parts were
embedded into the wall and not me. My finger hurt like hell. I think
the bearing must have exploded pretty equally because otherwise that
finger would have probably broken instead.

[ ... ]

I don't spin up bearings
any more. Not even a little.

It sounds like there is actually quite a wide safe range here. Just
stay in the sonic range?

That depends on the size of the bearing -- and your personal
hearing range. :-)

A large bearing (say 1-1/2" ID on the inner race) could probably
get to dangerous speeds while the tone remained in the audible range.

Yes. I'm tempted to figure out the rough limit. I would guess that it
will be a ratio on the max allowed rpm in service.

Right away we know that the service speed is safe for all but bearings
sold as designed to work only while pressed into a recess. So, if a
bearing is rated to 12,000 rpm, the balls travel at half that, or 6,000
rpm, and there are 10 balls (just count them), then the max tone
frequency is (6000/60)*10= 1,000 Hz.

That said, I don't have perfect pitch (few do), so I would keep it far
lower, a few hundred Hertz - a low hum, rather than a whine or a clear
tone.

And -- it is affected in part by the number of balls in the race. Some
(for heavy loads) have completely full races, others have fewer balls,
with some mechanism to space them out. I've seen ribbons of steel
formed into cups around both sides riveted together. I've seen bakelite
machined to slip in from one side once the bearings are properly spaced.
I've even seen bearings with little coil springs set between the balls
to keep them spaced out. This was in a vary slow, and very low
operating force setup -- like a gimbal gyro cage.

I've seen all these variants except the one with the little springs.
What are the springs made of?

I don't know that I would be tempted to spin such a bearing at all - the
little springs would get pulled under the balls, and the whole affair
would jam up, or the balls would end up all to one side, allowing the
bearing to disassemble itself long before the outer race could explode.

Joe Gwinn