Resonance dwell nomenclature

[floattuber]

What do the terms "resonance dwell" mean to you? Does it mean tracking dwell or fixed freqency dwell?

Or is it more of a general term that further needs to be defined by "track" or "fixed"?

 

[jimkinney]

Dwelling at a fixed frequency. Some tests will step from resonance to resonance dwelling at each for some period of time.

We did this a lot during gas turbine testing to gauge the life of the engine structure as the turbine sped up to operating speed passing through each components resonance.

Jim

Jim Kinney
Kennedy Space Center, FL

 

[Twoballcane]

In the test world this means to expose the unit to one frequency for a period of a time, but it also has to come with a “g” level, for example, 50Hz at 0.25g for 10 mins.

Tobalcane
"If you avoid failure, you also avoid success."
“Luck is where preparation meets opportunity”

 

[floattuber]

Thanks. I'm having a discussion with my tech about the definition of "Resonance Dwell".

At this site, the conventional nomenclature of "Resonance Dwell" specifically means a tracked dwell (follow any frequency shifts) while "Fixed frequency" means stay with one frequency regardless of whether there is a shift in the resonant frequency.

To me, "resonance dwell" needs the qualifer "tracked" or "fixed" to mean anything. I wanted to know if there is a standard industry definition.

 

[StrykerTECH]

It means dwelling at a resonance frequency (usually more than 1). This is common when running ElectroDynamic Shaker testing.

The resonance frequency has to be interpreted from a g/rms chart or displacement chart. It will vary depending on the part and where you are sensing during testing.

A parts resonant frequency cooresponds with its natural frequency. Generally each harmonic is a multiple of its natural frequency. Testing a "resonance dwell" is generally much more intense of a test being that you can see spikes of 10 to 20 times more g's at the natural freqency.





StrykerTECH Engineering Staff
Milwaukee, WI

http://www.stryker-tech.com/

 

[StrykerTECH]

Oh, forgot to mention. Do a sine sweep to find the resonant frequencies.
They will show up as spikes on your grms read out.

StrykerTECH Engineering Staff
Milwaukee, WI

http://www.stryker-tech.com/

 

[floattuber]

StrikerTech, but once that resonant frequency is found and you dwell at it...what if the resonant frequency changes during the course of the dwell. Say it shifts 3% higher. Do you adjust the frequency on the fly to continuously track it, or do you stay at the original frequency? That's the heart of my question.

Here we have two routes, we can stay at one frequency regardless of any shift or the computer can continuously track and adjust the frequency to maximize the response.

 

[StrykerTECH]

The short answer to that is No, you dwell on the frequency orginally determined to be the resonant frequency.

But, . . . why is it shifting? Generally that means one of two things. poor fixturing or the structure/part/assembly is yielding and through the course of the yeilding its natural frequency is changing, which is generally bad.

StrykerTECH Engineering Staff
Milwaukee, WI

http://www.stryker-tech.com/

 

[Twoballcane]

Generally you dwell at the frequency and G level that the device will be attached to (i.e. planes, ships, trains…) and not necessarily at the Fn of the unit. This will show that the device will function correctly on whatever you are putting it on. However, this method is very old. These days, it’s best to use random vibration where all frequencies are excited instead of one frequency. I must of missed it, but what is your application? If for Environment Stress Screening you may want to take the g level down by 1/3rd . If you are doing a prequalification test, you may want to take the g level up by 10 to 20% to gain margin.

Tobalcane
"If you avoid failure, you also avoid success."
“Luck is where preparation meets opportunity”

 

[lionelo]

Actually the tracking of the natural frequency is done on the phase not on the amplitude of the vibration.
First do a sine sweep to find the natural frequency of your part. Next determine the frequency where the amplitude is maximun and store the value of the phase.
Then do vibration endurance with the phase constant.