Vibration of rotating shaft instability

[Salmon Egg]

When a shaft is turned at a sufficiently high rate, the critical speed,
the shaft becomes unstable. That is, any microscopic unbalance or
eccentricity increases that eccentricity because of centrifugal force,
to the point that the shaft fails. It turns out that that speed is the
same as the vibration frequency of the shaft as a transverse vibrator.

The question I have is: Does the transverse vibration prevent such
instability by counteracting the centrifugal force if the shaft speed
can be rapidly increased above the critical speed before failure occurs?
That is, is failure of a shaft a result of a mechanical resonance at the
critical speed, while at a higher speed, the resonance will not be
excited?

Bill

[DD_BobK]

Post by Salmon Egg
When a shaft is turned at a sufficiently high rate, the critical speed,
the shaft becomes unstable. That is, any microscopic unbalance or
eccentricity increases that eccentricity because of centrifugal force,
to the point that the shaft fails. It turns out that that speed is the
same as the vibration frequency of the shaft as a transverse vibrator.
The question I have is: Does the transverse vibration prevent such
instability by counteracting the centrifugal force if the shaft speed
can be rapidly increased above the critical speed before failure occurs?
That is, is failure of a shaft a result of a mechanical resonance at the
critical speed, while at a higher speed, the resonance will not be
excited?
Bill
--
An old man would be better off never having been born.

The vibration increases greatly at the critical speed but the shaft
does not necessarily "fail", the damping may be low but it is not
zero.

You "run" through the critical speed before the shaft can begin to
vibrate, so, yes...... being (far enough) above the critical speed,
the resonance will not be excited.

cheers
Bob

[mark krawczuk]

hi, put some hose clamps on the shaft to balance it.

Post by Salmon Egg
When a shaft is turned at a sufficiently high rate, the critical speed,
the shaft becomes unstable. That is, any microscopic unbalance or
eccentricity increases that eccentricity because of centrifugal force,
to the point that the shaft fails. It turns out that that speed is the
same as the vibration frequency of the shaft as a transverse vibrator.
The question I have is: Does the transverse vibration prevent such
instability by counteracting the centrifugal force if the shaft speed
can be rapidly increased above the critical speed before failure occurs?
That is, is failure of a shaft a result of a mechanical resonance at the
critical speed, while at a higher speed, the resonance will not be
excited?
Bill
--
An old man would be better off never having been born.

[Salmon Egg]

Post by mark krawczuk
hi, put some hose clamps on the shaft to balance it.

Post by Salmon Egg
When a shaft is turned at a sufficiently high rate, the critical speed,
the shaft becomes unstable. That is, any microscopic unbalance or
eccentricity increases that eccentricity because of centrifugal force,
to the point that the shaft fails. It turns out that that speed is the
same as the vibration frequency of the shaft as a transverse vibrator.
The question I have is: Does the transverse vibration prevent such
instability by counteracting the centrifugal force if the shaft speed
can be rapidly increased above the critical speed before failure occurs?
That is, is failure of a shaft a result of a mechanical resonance at the
critical speed, while at a higher speed, the resonance will not be
excited?
Bill
--
An old man would be better off never having been born.

That may be the case, butI have not investigated it. The higher speed
opens up the possibility of other bending modes becoming unstable.

There is a toy called a Levitron that is fundamentally unstable. It
tries to levitate a magnet in a magnetic field. Earnshaw's theorem
points out the instability. Nevertheless, by spinning the toy at a
suitable angular velocity, it becomes dynamically stable.

Bill

[DT]

Post by Salmon Egg
When a shaft is turned at a sufficiently high rate, the critical speed,
the shaft becomes unstable. That is, any microscopic unbalance or
eccentricity increases that eccentricity because of centrifugal force,
to the point that the shaft fails. It turns out that that speed is the
same as the vibration frequency of the shaft as a transverse vibrator.
The question I have is: Does the transverse vibration prevent such
instability by counteracting the centrifugal force if the shaft speed
can be rapidly increased above the critical speed before failure occurs?
That is, is failure of a shaft a result of a mechanical resonance at the
critical speed, while at a higher speed, the resonance will not be
excited?

This can be true. When testing protoype turbines, we often had a list of
speeds to avoid. We accelerated through each critical speed to the next
data point. It's important to start below the critical speed and shoot
through. If you go too slowly it can hang up and not have enough power
to accelerate through.