For a shaft or a system with a single load (disk, blade, etc.) they're not useful.
If you have a gas turbine engine with lots of disks, each with a different number of blades they're very useful. Trying to navigate shaft speed through multiple blade passing frequencies can be a pain.
Jim
Jim...
The 119p supplement to Bruhn is available on Archive.org
https://archive.org/details/BruhnSupplement/mode/2up
McCombs was also one of the authors of AFFDL-TR-67-184 Analytical Design Methods for Aircraft Structural Joints while at Vought.
Jim
Jim Kinney
Kennedy Space Center, FL
Usually you want the response of the structure to be outside of your vibration frequency of interest. Then you don't usually have to worry about the structure responding the vibration. This only works if you know exactly what you frequency of interest is.
For space applications, random...
Typically we test 20-2000Hz at 1-4 octaves per minute. So 100 to 400 sec * 2000 Hz = 200000 to 800000 cycles at peak stress.
Usually our loads are low enough that we don't worry about fatigue, but we have to check the box that we looked at it.
If it were critical, then you are correct about...
I have so many copies of parts of textbooks, manuals, analyses from coworkers, etc. from over the years, that I wish I had the originals for. Some aren't complete, some are illegible, and some may have had other useful stuff that I didn't copy. Live and learn.
That may be an SAE manual, you...
You could calculate the actual number of cycles at each frequency fairly easily from the sweep time, but for fatigue calculations we conservatively use the highest frequency and assume the entire test was at this frequency. It's quicker too.
Jim
Jim Kinney
Kennedy Space Center, FL
You can't readily go from SRS to PSD. You can reverse Miles' equation, but there are limitations on the shape of the PSD.
The easiest method might be to synthesize a PSD, convert it to a Vibration Response Spectra (VRS) and compare it to your SRS. This would be an iterative process, and steps...
Tom Irvine has a Python Blog and a wiki on his Vibration Data website that might be helpful.
http://vibrationdata.wordpress.com/category/python/
http://vibrationdata.com/python-wiki/index.php?title=Main_Page
Good Luck,
Jim
Jim Kinney
Kennedy Space Center, FL
Didn't design it, but live in one. Based on other housed built after mine, it has re-bar not WWF. Single pour, so no cold joints, but there are no control joints. Settling cracks pop up all around so have to keep after it with caulk to keep sealed. Being in hurricane country this is a...
I don't have it handy, but Steinberg covers random vibration fatigue in "Vibration Analysis for Electronic Equipment"
Jim
Jim Kinney
Kennedy Space Center, FL
If you don't have the data from the control accels mounted to the shaker head (or test fixture), or response accels mounted to the DUT, it doesn't count.
Inputs to the shaker rarely matches what the DUT actually sees.
Jim
Jim Kinney
Kennedy Space Center, FL
What about a Panoramic Annular Lens? Not quite scattered to prismatic, but closer.
Jim
Jim Kinney
Kennedy Space Center, FLhttp://files.engineering.com/getfile.aspx?folder=d5918002-b83b-41e6-9d84-7b122339ae56&file=sem.org-2007-s29p03.pdf
...acceleration of the whole spectrum.
If you are only interested in a specific frequency, then you need to use Miles' Equation: Grms = sqrt( pi/2 * f * Q * ASD ). ASD is the PSD value at the specific frequency, and Q is typically (but not always) 10, it's based on the damping ratio if you...
I recently acquired a pair of accelerometer calibrators. An APS Dynamics Model 129 low frequency, and a Bouche Laboratories Model 1000 high frequency. Both were filthy, but appear to be nearly unused once cleaned up. There are no electronics for either.
My question, is there somebody that...
If they are locking inserts, there is even less re-usable life. The deformable thread type are usually good for 6-12 uses then they won't lock anymore. The threads are ok, but they won't stay locked. Locking life is even less if they are over-torqued.
Jim Kinney
Kennedy Space Center, FL
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...
Part VI is NACA TN-3786 and is here:
http://ntrs.nasa.gov/search.jsp?R=209038&id=9&as=false&or=false&qs=Ntt%3DHandbook%2Bof%2BStructural%2BStability%26Ntk%3Dall%26Ntx%3Dmode%2Bmatchall%26Ns%3DHarvestDate%257c1%26N%3D0
Jim
Jim Kinney
Kennedy Space Center, FL
That's a good question, and so far the only answer that I can come up with is that we have always done it that way, which isn't always a good answer. It's in MIL-1540C as you mentioned, but it's also in NASA-STD-7003, IEST-RP-DTE032.2 and even ESA standards. I haven't confirmed it yet, but I...
