Fixture FEA Guide

[yusf]

Dear All;

We all know that the resonance of the fixture creates over test of equipment being tested on shaker table.What we need to do is to desing and analyze fixture before testing.

My question is that during FEA of design of any fixture

1 .) Is it enough to do a modal analysis only for fixture itself OR

2.) an analysis should be carried out in which a point mass attached to the geometry of fixture...OR

3.) both fixture and FE model of equipment attached on it should be analyzed?

any guidelines will be appreciated.

thanks in advance.

 

[Twoballcane]

Some initial thoughts:

I would do a model analysis on both the fixture and unit under test (UUT) (separately) to make sure that both frequencies are at least 1 to 3 octaves apart. If not, adjust fixture (add/subtract mass, stiffen design) till you do. Optimally you would want your fixture’s first mode to be hire than the UUT’s.

If the UUT is a simple structure, you can use the mass moment of inertia (MMOI) to create a dummy load to attach to the fixture. Once this is done, you can simply support (let rotate but fix transition in all three axis) the fasting points of the fixture and do another modal analysis. This modal analysis will be unique to this set up. From here you can apply random, sinusoidal, or shock and calculate out (FEA) the deflections, G loads, stress…etc.




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

 

[Twoballcane]

Also, it is good practice to ramp up the vib/shock load than doing 100% (or zero dB) at the beginning, basically characterize or get a feel of what will happen before you go to the qualifying level.

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

 

[jeyaselvan]

Ideally your fixture is expected to be rigid in the range of your shaker test frequencies. I feel it is good to approximate your load as point mass and analyse the FE model of the fixture for modes. If your mass is going to induce stiffness to the fixture, then you would do a prestressed modal analysis, which accounts for the stiffness changes associated with your mass. This is a kind of doing modal analysis on the statically deflected fixture (due to your load), whose stiffness could be different.

Hope this helps

Regards
Jeyaselvan

 

[Tmoose]

I would not ignore the attachment of the fixture to the shaker table. Usually When I've assumed A totally fixed mounting surface for a boundary condition I've been wrong.

Omer Blodgett frequently admonishes providing a continuous load path in weldments. I've seen several vibration problems solved with an arc welder and a few pounds of steel when that concept was ignored or overlooked in the original design.

Dan T

 

[drmuh]

Hi,
I would model both to account for stiffness and mass introduced by the test item to the fixture. If you don't have a fixture at hand, it will be faster to optimize for min. freq (max. sweep frequency) from a design space with test item FE attached. I think if you assume a point mass for the test item, then you have to decide which type of constraints you need to use to connect the mass to the fixture. As you can not know beforehand at which frequencies you will experience a resonance, MPC type may induce problems.

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