vibration g's 1

[faquir]

Hi!

I'm making a work for college which involves analyzing the vibrations of a turbofan engine rotating at 2 speeds, N1 beeing the fan speed and N2 beeing the core speed.
The data available to me is a table of the vibration displacement[mils] filtered for N1 and it's frequency, and the vibration velocity[ips] and it's frequency. I have also N1 and N2 in rpm's and time of recording in seconds. I want to calculate the g's
due to N1 and N2 in every second. I've used the normal formulas of conversion ( for example in this site:

http://www.spaceagecontrol.com/calcsinm.htm

) to the values but i'm obtaining high values of g. I've already check the calculations n times and really can't see what I did wrong, or maybe i can't use this formulas in this specific problem I don't know. I'm new in this vibration business!

Thanks for the attention!

 

[faquir]

*vibration velocity [ips] filtered for N2

 

[drawoh]

faquir,

If you have the displacement and the frequency, and the vibration is sinusoidal, you can work everything else out fairly easily...

x (t) = X sin [ω] t
v (t) = x' (t) = [ω] X cos [ω] t
a (t) = x'' (t) = - [ω][sup]2[/sup] X sin [ω] t

--
JHG

 

[faquir]

Hi drawoh!

I've used exactly those formulas and then converted from ips^2 to g's but i get values in the order of 100 g's...

 

[btrueblood]

Did you divide the value of in/s^2 by 12 (to get ft/s^2) and again by 32.17 (to get g's)?

 

[faquir]

btrueblood,

Yes I did. And I realized something i havn't realize before. The frequency given in Hz for each speed N1 and N2 is not the same
as N1[rpm]/60 and N2[rpm]/60. Is much higher! What is the meaning of this?

 

[IRstuff]

What are the numbers?

TTFN
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[btrueblood]

Is it perhaps the blade passing frequency?

 

[boo1]

N1 and N2 are the rotational speeds of those engine sections
N1 - Fan Speed (low pressure compressor or fan)
N2 - Core Speed (high pressure compressor)

 

[drawoh]

faquir,

I have analyzed reciprocating optical scanners, which generate significant forces at high frequency. I have been surprised that the optical systems to not walk across our shop floor when the scanners are running.

What was not immediately obvious to me is that the optical systems were fairly heavy relative to the scanner. The acceleration of the complete sensor was correspondingly small. a=m/F. Any gunners out there will understand this.

In your case, your turbine probably is on anti-vibration mounts. The high G-force acceleration may be at high frequency, and the mounts may be isolating it efficiently.

A good sanity check might be to calculate backwards. Assume 100g vibration at whatever frequency, and work out the displacement.

--
JHG

 

[faquir]

dawoh,

ok the engine is mounted in a support so it can be tested, maybe is absorbing maybe not.but if i assume 100 g's and calculate the displacement out of the frequency, what value should i use? N[rpm]/60 or the values provided in Hz? and is there any typical order of magnitude of displacement that i should expect

s

 

[IRstuff]

Displacements are typically due to unbalances in the moving elements, like a c.g. offset.

TTFN
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[drawoh]

dawoh,

... what value should i use? N[rpm]/60 or the values provided in Hz? and is there any typical order of magnitude of displacement that i should expect

In the equations above, [ω] is circular frequency, [ω]=2[π]f. You should be using whatever frequency you think is causing problems. Note btrueblood's comment, above.

I suspect you will be surprised at the answers you get, particularly with the higher frequencies.

--
JHG

 

[Dougt115]

I hope you are not using Hz in your equations. Either rad/sec or degrees/second. Hz are cycles/second and Sine will not compute correctly.

Sin(1Hz*1 second)does not equal sin(360 * 1second)

 

[faquir]

Hi!!

I've solve the problem i think. Apparently these strange numbers of frequency where obtained because the sensor is mounted near a phonic wheel with 30 teeth. So the frequency was ~ N*30/60. I'm using N/60 now and the g values are at most 1.
thanks for all the help and have a good day!