Vibration Test Fixtures Design
Vibration Test Fixtures Design
(OP)
Hello,
This is the first time I design a fixture for a vibration test, so I guess my question might be basic to most of you.
I have an assembly that should be carried by a certain platform.
The Vibration Spectrum of the carrying platform was measured and is given as part of the requirements.
As part of the assembly design an engineer who is specialized in dynamic analysis made sure that the natural frequencies of the assembly are far enough (higher) and out of the input spectrum.
Now I have to plan a vibration test, and shake the assembly through a fixture that is connected to the shaker table. I am attaching a schematic description of the test:

I read that the guideline of fixtures design is making them as light and stiff as possible for achieving a natural frequency as high as possible, so no resonance may occur in the test due to the fixture.
Now I wonder how should I check by analysis the fixture I design.
I initially thought that if we have a separate analysis of the tested assembly we only have to check separately the test fixture alone (and make sure its natural frequency is high enough) as described below:

But I was told that this isn't right and that I have to include the Tested Assembly mass in its C.G without taking into account its stiffness etc. as described below:

Remark: I am sure that the most accurate way is an analysis of the entire assembly, but I would like to know what is the right way of design when you don't have an access to the tested assembly, and there is a necessity to simplify.
I will be happy if you refer me to a simplified example in which there is a calculation that illustrate the importance of each factor in the calculation.
Thank you
This is the first time I design a fixture for a vibration test, so I guess my question might be basic to most of you.
I have an assembly that should be carried by a certain platform.
The Vibration Spectrum of the carrying platform was measured and is given as part of the requirements.
As part of the assembly design an engineer who is specialized in dynamic analysis made sure that the natural frequencies of the assembly are far enough (higher) and out of the input spectrum.
Now I have to plan a vibration test, and shake the assembly through a fixture that is connected to the shaker table. I am attaching a schematic description of the test:

I read that the guideline of fixtures design is making them as light and stiff as possible for achieving a natural frequency as high as possible, so no resonance may occur in the test due to the fixture.
Now I wonder how should I check by analysis the fixture I design.
I initially thought that if we have a separate analysis of the tested assembly we only have to check separately the test fixture alone (and make sure its natural frequency is high enough) as described below:

But I was told that this isn't right and that I have to include the Tested Assembly mass in its C.G without taking into account its stiffness etc. as described below:

Remark: I am sure that the most accurate way is an analysis of the entire assembly, but I would like to know what is the right way of design when you don't have an access to the tested assembly, and there is a necessity to simplify.
I will be happy if you refer me to a simplified example in which there is a calculation that illustrate the importance of each factor in the calculation.
Thank you
RE: Vibration Test Fixtures Design
TTFN (ta ta for now)
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RE: Vibration Test Fixtures Design
RE: Vibration Test Fixtures Design
RE: Vibration Test Fixtures Design
RE: Vibration Test Fixtures Design
What is your specimen? I'll bet some of the people here have some experience with similar items. Another consideration is if the specimen has its own natural frequencies. If so then some of its mass will decouple above those frequencies helping you out a bit.
Another approach would be to look at what is driving your shaker table. It will have a certain natural frequency depending on your fixture mass and the specimen mass. Just design you fixturing so its natural frequency is above the table/drive natural frequency.
RE: Vibration Test Fixtures Design
You already have instruction from someone to analyze the fixture with the UUT mass but not with any structural contribution.
That is the right way.
Why not compromise and do both?
RE: Vibration Test Fixtures Design
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm
RE: Vibration Test Fixtures Design
As for the question why I don't include the true tested body in the analysis, there are 2 reasons:
1. The tested body is still being designed by another group in the company I work in, and for making the design phase shorter- there is a decision of working in parallel.
2. I am not familiar at all with dynamic simulations, and I am afraid of garbage in garbage out when dealing with big assemblies, on the contrary to simulation of one single part in which the chance to have a mistake is significantly smaller.
RE: Vibration Test Fixtures Design
The shaker table engineers should be telling you what to do to get useful results.
RE: Vibration Test Fixtures Design
RE: Vibration Test Fixtures Design
Cheers
Greg Locock
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RE: Vibration Test Fixtures Design