Need help Advice on designing FIXTURE for VIBRATION test.

[JohnDV201]

Good morning everyone,

I have been tasked to design a fixture that will be mounting our display units to our vibration system. We want to simulate the vibrations that our units will go through when shipped.

I've never designed a fixture before. Does anyone have any kind of advice or articles I can be linked to to help me get started? Anything would be much appreciated.

Thanks in advance.

 

[Mechduff]

Mount and isolate them at the floor and save yourself a world of headaches.

 

[MintJulep]

Stiff Stiff Stiff.

 

[JohnDV201]

We plan on mounting the vibration table onto isolation pads. Any advice on how to design this vibration fixture? I see a lot of pictures of tables with an array of threaded holes. I assume that mounts are fastened into these threaded holes am I correct?

I heard that making the fixture out of magnesium would give a more stiffer fixture when undergoing vibrations.

 

[MintJulep]

Exotic materials for fixtures are generally a waste of money unless there is a special need - for example you need to make the fixture very light to avoid over-loading the test machine.

Stiffness is generally a property of geometry.

Understand the loads and load paths that will be transferred by the fixture during testing. Make sure the connections between the fixture and the table are sufficient to transfer these loads. Make sure that you will not have joint separation as this leads to all sorts of control problems.

Make sure that all joints within the fixture are sufficient to transfer the loads. If you go for a bolted design, then again make sure that you don't have joint separation.

If you go for a welded design then design for fatigue.

Make sure that the fixture (with TBT mounted) has no resonant frequencies anywhere within the range of frequencies that you will be testing.

 

[rb1957]

what is the point of the exhibit ? to provide a rigid support for your system to it can operate with a test vibration and then turn your system on and see it dampen the vibration ?

then as above you want a very stiff structure and you want to isolate it from the surroundings. if you biuld vibration systems, there should be people there to help.

it would be useful to know the driven frequency of your test, so you can design a structure who's natural frequency is much higher.

"build it stout, out of things you kow about" ... forget Mg ... Steel if you don't care about weight, Ti if you do and have money to spare, Al if you are cash strapped. thick plate table top, closly spaced I-beam stiffeners.

how big is this thing anyways ??

 

[KENAT]

Are you working with an external vibration test house or equivalent?

If so they may be able to give you some advice on this, else if you're doing it internally then there should be someone who knows a thing or two about it.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[IRstuff]

How does your system engineer want it tested? How much of a worst case is supposed to be tested. Once that's determined, your fixturing options ought to be fairly well constrained. Shipping vibration testing would presumably imply packaging and cushioning for normal shipping. How are the boxes stacked during shipping?

One possible worst-case is to constrain the box by only one side, i.e., fix the bottom of the box onto the shaker table and vibe horizontally. This would cause the box to sway and possibly magnify the vibration on the display.



TTFN
faq731-376

 

[JohnDV201]

Thanks guys,

I will attach an image of the system as well as the old fixture that was on there. It is only a sinusoidal controller (I know, our company is kind of small....) and a very old system from LSD.

This system was originally used at a auto parts manufacturer company.

@IRStuff
The test that is required is as you said it, for packaging and normal shipping. We want to perform these test to ensure that our product (custom displays) don't have loose and missing hardware when we ship it out.

 

[JohnDV201]

Here is the old fixture. They nicknamed it an R2 unit for a reason ^_^.

The fixture is quite heavy and is definitely made out of stainless steel from what I can see.

 

[IRstuff]

The white and red parts are the shaker itself, are they not? The cubish thing with the plate with holes in it is the actual fixture? If so, that cubish part is aluminum, I can't tell for sure whether the plate is aluminum, but that would be my guess.

For the purposes of quality assurance, the packaged unit test is not appropriate, since the packaging cushions and damps most of the external vibrations, which is what it was intended for. You should be looking at an environmental stress screening (ESS) test, which typically combines shake with temperature to make sure that screws don't fall out, boards don't break, etc. You could forego the temperature test, but the ESS vibe is what I think you ought to be doing.

TTFN
faq731-376

 

[JohnDV201]

Use, you are correct. THe R2 unit is the shaker itself and the cube + plate are the fixture.

We are not going to test the units packaged but rather mount the unit itself onto the table.

Would it be a bad idea to cut down the cubic looking fixture to lose some weight and add on a larger table top? The maximum weight from my memory is 50 lbs.

 

[MintJulep]

Just call the manufacturer and tell them that you want to buy a table for your shaker.

 

[IRstuff]

I don't think you can cut that down much. The cube looks like a force spreader, i.e., it takes the force imparted by the smaller shaker head (~4"??) and distributes over a larger area (~8"??). A direct mounting might possibly result in some flapping of the plate, which may impart undesirable stimulus to the UUT.

TTFN
faq731-376

 

[JohnDV201]

Attached is a 3D PDF file of the fixture I have decided on. It is a 3/8 THK plate and I am thinking of welding the bottom ribs all around. The holes will have Helicoils inserted in all of them due to our constant mounting and unmounting.

My concerns are:

1. Will so much heat being applied during welding cause warp to occur on the plate? My manufacturer believes that with such a thickness it should not.

2. Will the fatigue from the vibration testing be too much for the welds? I plan on welding with maximum rigidity in mind (.75 * Thickness of material) and having the weld all around.

What do you guys think?

 

[JohnDV201]

I forgot to add that sadly, my CAD program does not have the FEA to perform a vibration test where I work.

 

[MintJulep]

Just call the manufacturer and tell them that you want to buy a table for your shaker.

 

[JohnDV201]

I have tried and my shaker table is obsolete since it's very old now. Plus, any parts they have are very expensive.

 

[MintJulep]

1. Will so much heat being applied during welding cause warp to occur on the plate? My manufacturer believes that with such a thickness it should not.

Yes, in all likelihood it will warp. By the way, I'm assuming aluminum, since that is what these are typically made from.

Since flatness is important you should specify a post weld stress relief, and final machining to achieve the flatness necessary.

2. Will the fatigue from the vibration testing be too much for the welds?

Well, it's a vibration table, so the stress cycles will be really high. What is the anticipated stress range? What is the fatigue sensitivity associated with the weld joint? Again, assuming aluminum, there is no endurance limit, so it's a matter of how long until failure.

 

[JohnDV201]

For the stress test, I think I'll run an analysis if my Solidworks at home has shock and vibration.

I've looked over a lot of fixtures from this site that seem to be welded.

http://www.baughneng.com/pages/lti_fixtures.php