% difference natural frrequency and forcing frequency to avoid resonace

[fesoyd1213]

Hello.

If some one know of codes or standards that mention about the required difference between the natural frequency and forcing frequency (excitation). This is about evaluation of a structural member (beam) supporting a rotating object (fan).

Thanks

 

[MikeHalloran]

AFAIK, there ain't no such code or standard.

You are perfectly free to support a fan with a resonant structure.

You might even wish to do so, in order to allow the fan to rotate around some axis other than its geometric axis, in order to compensate for an accumulated unbalance, e.g. due to adherent dust, asymmetrical corrosion, damage from bird strikes, etc. That's pretty much what 'vibration isolators' do.

The more usual concern is to adjust the operating speed so as to not excite a structure with a known resonance. I think it's customary to try to place the excitation frequency 20 pct above or below the resonance in order to minimize the energy fed to the resonance. Yes, some systems have been built with excitation frequencies above a known resonance; they should be engineered to minimize the time spent sweeping through the resonance.

Another useful rule of thumb is that a beam or floor with a static deflection around span/360 is going to resonate around 4.0 Hz. If you need an exact number for an existing structure, a small stepping motor slowly ramped through that frequency, even if its load is balanced, will bring people running, looking for some huge runaway piece of machinery.



Mike Halloran
Pembroke Pines, FL, USA

 

[Tmoose]

If you can tolerate twice the vibration amplitude that you "deserve" then +/- 20% may be OK.

http://www.industrial-electronics.com/images/electric-motor_9-36.jpg

As far as "standards" that prescribe proximity of resonance or critical speed to operating speed, here is one derived from a General Motors new equipment standard from the 90s.

http://www.wbdg.org/ccb/NASA/GUIDES/rcmguide.pdf

In Table E-9.1.2. "Critical Speed Offset Requirement" it says -
ROTOR DESIGN FIRST ACTUAL CRITICAL SPEED LOCATION
Rigid Shaft - At least 25% Above Rated Motor Speed
Flexible Shaft - Maximum of 85% of Motor Speed

"9.2.4 Resonance
Natural frequencies of the completely assembled fan unit shall not be excited at the operating speed. (Running speed should be at least 25 percent removed from a natural frequency of the system.)"

 

[IRstuff]

We've used a factor of 5

TTFN
faq731-376

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[tbuelna]

I've seen requirements for specific aircraft drive systems regarding analysis/design margins between forcing frequencies and component structural modes, but I haven't seen anything similar for commercial applications.

Here's an example from FAA FAR part 29:

http://www.flightsimaviation.com/data/FARS/part_29-931.html

You should also consider whether the spec requires validation by just analysis, or by analysis plus test. Validation by analysis usually means much more conservative margins than analysis plus qual test.

 

[electricpete]

thread384-160351


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(2B)+(2B)' ?

 

[GregLocock]

The reasons for the wildly varying recommendations are many. For instance a generic target for car engine ancillaries is that they should not resonate until at least 115% of the maximum most likely powerful excitation, typically firing frequency at red line. That's a pretty small margin of 'safety', but not many cars operate at the red line for long, and typical mounting brackets are bolted together which increases their inherent damping. And of course, extensive durability testing of prototypes indicate where that margin is insufficient.

Cheers

Greg Locock


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