Vibration and Shock Testing

Sometimes no matter how well your vibration fixture is designed, you absolutely MUST use two or more control accelerometers to get all the way through your test spectrum. Now I’m NOT talking about using multiple control accelerometers to make a bad fixture work, there’s no excuse for that, I’m talking about a large, complex fixture that is designed optimally, but you can’t get around the laws of physics.

If you decide to control on the average of the accelerometers, fine, but understand what is really happening. When you look at that beautiful post test plot that shows you ran the test right in the middle of the spec, it’s an average of what really happening. Now look at the individual plots of each accelerometer. That’s the truth, and what it will show is that at some frequencies in some locations you have over-tested, and at some frequencies in some locations you have under-tested.

If you are concerned about your test specimen breaking due to the over test, then go ahead and don’t average, but instead control off of the peak values. You won’t over-test, but certainly when you look at the individual control plots you will find that there are places where you under-tested. Conversely, if you are concerned about under-test and not qualifying to the specified environment, then go ahead and control off of minimum values. Now your individual plots will show that you tested to the specified amplitudes everywhere, but will also show where you over-tested.

In summary, the “averaged” post test plot or the plot showing the combined peak or minimum, will look nice, but you REALLY need to look at the individual plots to see what ACTUALLY happened.

That’s right, the one you use to verify the torque on your fixture bolts. The one you send out periodically to get calibrated. Think of how much money you will save on calibration!

You see the thing is that that torque wrench is probably not doing you any good.

When we are securing a fixture to a shaker, we really don’t care about torque. We care about clamping force. Since we don’t have a good way to directly measure clamping force, we use torque as an indirect method. Torque is proportional to clamping force WHEN THE THREADS ARE PERFECT!

Do you look carefully at each bolt before you put it into the fixture? Do you look again after you remove it?

When you remove a bolt do you place it carefully in a wood or foam bolt holder to protect those threads? Or do you just toss it into the drawer until next time?

Go down to your vib lab right now and look at your bolts. Hold them up to a light so you can see how the threads look. If you see threads that are nicked, flattened or curled, just put it carefully back in the drawer, and toss your torque wrench in to the dumpster on your way back to your office.

You can thank me later.

There’s an old saying “If the only tool you have is a hammer, then all of your problems start to look like nails.”

In the old days of shock testing (writer spits into spittoon) all we had was drop table test machines. 1/2 sine shock pulses were easy to perform (just use rubber under the table), they looked good in the report (when you filtered the heck out of them), and they were absolutely repeatable.

If there is anything an engineer likes better then accuracy in Half Sine Shock Pulse Testing, it’s repeatability.

But what is our excuse today? If we are really interested in finding out how our designs will stand up to real world shocks, why would we use a shock pulse for our testing which never appears in the real world?

Passing a 1/2 sine shock test will NOT indicate that your product will survive the shocks encountered in use and in transportation, nor does failing the test tell you very much useful either, yet these tests are still performed routinely around the world.

It looks like another case of “We’ve always done it this way.”

We live in the era of “Test Tailoring”. It is NOT a big issue to instrument a package, and (gasp) take  measurements! Use those measurements to determine a realistic reliability test requirement. Then you are performing useful testing.

Hi,

I was referring to some of papers available on internet for HALT test guidelines. For Vibration tests, the paper says that chamber requirements should be -> Vibration output without load from 1 Grms to a minimum of 35 Grms.

But for test equipment, accelerometer range is +/- 500g.

I am confused what is the difference between g and Grms. How to select proper accelerometer?

Please help.

Thanks for your help in advance.

Regards,
Samir

(see Vibration and Shock Blog post for response to this)

Hi – we look forward to questions about vibration and shock testing – everything is fair game:

Test equipment

Test specs

Test parameters (selection OR measurement)

Fixture design

Data analysis

Or just let us know that you have an interest.

Steve Brenner, Consultant and V&S Blogmeister