Natural Frequencies on machines at rest vs. operating speed
Natural Frequencies on machines at rest vs. operating speed
(OP)
On a typical motor driven fan mounted on rolling element bearngs, does the natural frequency change when the machine reaches its operating speed?





RE: Natural Frequencies on machines at rest vs. operating speed
Cheers
Greg Locock
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RE: Natural Frequencies on machines at rest vs. operating speed
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Natural Frequencies on machines at rest vs. operating speed
When rotating, the gyroscopic effect increases natural frequency (compared to lump mass at location of the disk).
When stationary (for example lateral bump test), the rotational inertia decreases the natural frequency (compared to lumped mass at location of the disk).
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(2B)+(2B)' ?
RE: Natural Frequencies on machines at rest vs. operating speed
In this case there is no difference for 1st mode, but 2nd mode is 162hz during stationary bump test mode vs 244hz 2nd mode while rotating.
Slide 1: Graphical Rotor Geometry (steel)
Slide 2: Numerical Rotor Description (SI Units)
Slide 3: Bump Test while stationary incl tilting disk effects: F2 = 162hz
Slide 4: Treat disk as lumped mass with no disk effect. F2 = 193HZ
Slide 5: Rotating Critical Speed. F2 = 244hz
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(2B)+(2B)' ?
RE: Natural Frequencies on machines at rest vs. operating speed
See roman numeral VII at the bottom here:
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(2B)+(2B)' ?
RE: Natural Frequencies on machines at rest vs. operating speed
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Dr Michael F Platten
RE: Natural Frequencies on machines at rest vs. operating speed
The 12 Dec 10 20:05 posted between bearings example was done in 5 minutes to show the principle, but if it would make you feel better I can surely change the parameters to lower the frequency of the 2nd critical near 60hz (let me know if you want to see that).
The linked overhung fan 12 Dec 10 20:14 is model of a real industrial fan.
Attached is an article of a well-studied overhung fan. On the bottom of page 39: you'll see:
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(2B)+(2B)' ?
RE: Natural Frequencies on machines at rest vs. operating speed
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(2B)+(2B)' ?
RE: Natural Frequencies on machines at rest vs. operating speed
I would have thought that the gyroscopic forces played a larger role in increasing the damping coefficient. Moreover, as the gyroscopic effect effectively changes the reaction axis to the mode shape, would not the mode shape itself change?
Fe
RE: Natural Frequencies on machines at rest vs. operating speed
In rotating equipment there are a number of scenario's where skew-symmetric components are added to the K and C matrix and the role of these skew symmetric terms is reversed (skew symmetric terms in the K matrix represent non-conservative/damping forces and skew symmetric terms in the C matrix represent conservative forces).
An example of skew symmetric terms in the K matrix would be in a fluid film bearing with cross-coupled stiffness term kxy = -kyx. This can add or remove energy from depending on direction of rotation of the mode compared to the machine.
An example of skew symmetric term in the C matrix would be the gyroscopic terms +/-w*Ip. It represents conservative force because grysoscopic effect represents stored kinetic energy only and therefore cannot possibly add or remove energy from the system. Therefore it is not damping.
Yes, the mode shape changes. I never said otherwise... in fact showed the changing modeshape in my attachment.
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(2B)+(2B)' ?
RE: Natural Frequencies on machines at rest vs. operating speed
I should have opened the attachment
Also, I like you explanation. I now see why the damping is not effected.
Gyroscopic coupled vibration would make for some very interesting experiments. Could be used in vibration isolation to actively change the resonant frequencies ect.
Fe
RE: Natural Frequencies on machines at rest vs. operating speed
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(2B)+(2B)' ?
RE: Natural Frequencies on machines at rest vs. operating speed
http://www.machinerywatch.com