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!

*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.

Jobs

Shock Test on Shaker Mil-STD-810G. Over Test on equipment vibration isolated

Shock Test on Shaker Mil-STD-810G. Over Test on equipment vibration isolated

(OP)
Hello,
We have an equipment mounting over a damping base which resonance frequency is 21Hz. If must futfill the following shock requirement:
MIL-STD-810G Terminal Peak Sawtooth 40Gs 11ms, after a 30Gs test we have peak in the equipment over 70Gs. we are using a vibration exciter. In the standard there is a paragrah:

d. Classical shock pulses (vibration exciter). If a vibration exciter is to be employed in the conduct of a classical shock pulse, it will be necessary to optimize the reference pulse such that the net velocity and displacements are zero. Unfortunately, the need to compensate the reference pulse distorts the temporal and spectral characteristics, resulting in two specific problems that will be illustrated through example using a terminal peak sawtooth (the same argument is relevant for any classical pulse test to be conducted on a vibration exciter). First, a typical pre-pulse compensation of around 20% of the reference pulse peak will result in a time history that is outside of the 5% pre-pulse amplitude tolerances given in Figures 516.6-10 and 11. Second, as illustrated by the pseudo-velocity SRS in Figure 516.6-12, the velocities in the low frequency portion of the SRS will be significantly reduced in amplitude. Also, there is generally an area of increased amplitude associated with the duration of the pre- and post-test compensation. Observe that the low frequency drop-off in SRS levels between the compensated and uncompensated pulse is readily identifiable and labeled f1. Likewise, the frequency at which the compensated and uncompensated pulses converge is readily identifiable and labeled f2. The drop-off at f1 is considered to be acceptable if and only if the lowest resonant frequency of the item being tested, fN, is greater than f1 by a factor of two or more (fN ≥ 2*f1). [u]The amount of gain in the region f1≤f≤f2 is directly related to the duration and magnitude of the compensation pulse. The potential for over-test in this spectral band must also be carefully considered prior to proceeding

Does anyone know if we are overexciting the equipment due to the use of a shaker? Could we avoid it using per example an impact machine or modifying the shaker control?

Regards Manuel

RE: Shock Test on Shaker Mil-STD-810G. Over Test on equipment vibration isolated

My background is aerospace electronics. It is common to shock test isolated equipment on a shaker. On isolated equipment you can expect a high shock response. The test in MIL-STD-810 (I am guessing you're doing crash safety shock, procedure V since you mentioned 40G 11ms Sawtooth) calls for a base excitation of 40G, not an equipment response of 40G. i.e. the control accelerometer should be on the plate, measuring the response in the equipment is good engineering knowledge but is not an example of an overtest so long as you shaker correctly reproduces the 40G, 11ms, sawtooth wave form.

RE: Shock Test on Shaker Mil-STD-810G. Over Test on equipment vibration isolated

Thank you, both, for your comentaries.
we are doing MIL-STD-810G functional shock method 516.6 procedure I, the image I attached to my post was obtained from the Standard. For the test, the control acceleromenter is on the plate (shaker), like you say there is one too on the equipment, in which we obtain values around 80Gs for a 30Gs 11ms test. Because of it I began to study deeper the Shock method and found the commentary about vibration exciter. I think 40Gs would be so hard for the equipment. Do you think that 40Gs 11ms is not appropiated for functional shock? and is more related with safety crash.
Regards

RE: Shock Test on Shaker Mil-STD-810G. Over Test on equipment vibration isolated

"Do you think that 40Gs 11ms is not appropiated for functional shock? and is more related with safety crash."

That depends on the specific customer, the environment, and the specific application. But, since you were levied a test called "Functional Shock," it's not a crash test, by definition, even if the levels are potentially comparable to a normal crash test. For ground vehicles, a 40-g shock might represent a ballistic shock, which must be survived.

The bottom line is that if your equipment cannot handle the amplification, your isolators must be redesigned. That said, your company also needs to determine whether the customer might be willing to waive the requirement or relax it.

Bear in mind that if this were a naval application, ballistic shock would be more like 300 g's.

TTFN
I can do absolutely anything. I'm an expert!
homework forum: //www.engineering.com/AskForum/aff/32.aspx
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers

RE: Shock Test on Shaker Mil-STD-810G. Over Test on equipment vibration isolated

I agree with the above^^ It matters what the real world shock environment is, not whether or not it is hard on the equipment.

Getting 80Gs on the equipment with a 40G base excitation isn't a surprisingly high amount. I've seen spikes on isolated equipment over 100G from that same pulse.

RE: Shock Test on Shaker Mil-STD-810G. Over Test on equipment vibration isolated

(OP)
Thank you for your help

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


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close