×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

effect of mass on vibration response

effect of mass on vibration response

effect of mass on vibration response

(OP)
If I have 2 installations on a helicopter floor, similar natural frequencies but 1 weighs 5 kg the other 500 kg, would the response of each to the same random vibration input (PSD) be the same ??

RE: effect of mass on vibration response

(OP)
Do you mean because of mass damping effects ?

RE: effect of mass on vibration response

It depends where you measure your response.  If you put your accelerometer on the helicopter frame that leads up to your devices, this would be your forced frequencies (maybe peaking with the RPM of the rotor) thus your PSD in.  If you put your accel on top of the masses, I would say that your PSD out would be different from one mass to the other.   

Tobalcane
"If you avoid failure, you also avoid success."  

RE: effect of mass on vibration response

A helicopter is not a perfectly rigid object, so its vibration characteristics vary from location to location, helicopter to helicopter.  

Nonetheless, MIL-STD-810 and MIL-E-5400 use envelope curves that encompass any installation location on any of the helicopters covered by the specs.

TTFN

FAQ731-376: Eng-Tips.com Forum Policies

RE: effect of mass on vibration response

(OP)
Thanks guys - I'm using Mil-Std-810 to give my input PSD (and peaks due to rotor and BPF) - it just seems logical to me that the bigger mass would have a lower output g level, I'm just not sure how to quantify it !

RE: effect of mass on vibration response

You have to be carful and not think sinusoidal.  As in what your entire unit will do at a certain frequency.  In random all of your frequencies will be excited.  So your "box" may not see much but your board and small electronic parts like flat packs and such will see higher frequencies and may have higher G loads.   

Tobalcane
"If you avoid failure, you also avoid success."  

RE: effect of mass on vibration response

If I understand what you are saying, the answer is no.  The input stimulus is independent of your UUT.  You mount the UUT on the shaker table, and you apply the input specified in 810.  The helicopter is way more massive than your UUT, so your UUT will shake like the helicopter, not the other way around, particularly for something that's 5 kg, or even 500 kg.  A UH-60 has an unloaded mass 10x that of your largest UUT, so there will be minimal impact to the input levels or spectrum because of your UUT, particularly if there are no resonances within the spectral limits.

TTFN

FAQ731-376: Eng-Tips.com Forum Policies

RE: effect of mass on vibration response

I disagree, the modal mas of the structure at the mounting point will be much less than the total mass of the structure so in reality even quite a small mass sees 'mass damping' (horrible phrase). For instance in a car masses of the order of 5-15  kg, and tuned mass dampers of the order of 1 kg, are enough to significantly affect the vibration in the structure, which has a mass of the order of hundreds of kg.

This does not apply on a test rig type qualification test of course.  

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies  http://eng-tips.com/market.cfm?

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources