Modal Vibration 3D - modeling equipment base with isolators

keithmcarthur

I am attempting to model the base of a machine structure that is mounted on isolators an need some advise on modeling. The isolators are elastomeric and I have the values for deflection/deformation. I'm not sure how to model the isolators as they allow for some displacement in all directions.

To add to the complexity of the model the base is mainly fabricated using C-channel and there are several warnings using non-symmetric beam shapes. Presently I have only attempted to model this using beam elements.

Based on the vibration analysis performed on this machine I believe there are natural frequencies of the base structure in the machine operating speed. I'd like to prove this out an model possible solutions for stiffening, etc. to push the natural frequencies out of the the fan operating speed range. Attached are some sketches of the fan layout and its base.

Any advice would be appreciated.

Thank you,

Keith

Sergio

You could use SPRINGA elements to reproduce the behavieour of your rubber mount assy. As far as I understood from de CCX documentation you can enter the force/deformation table to represent your non lineal mount assembly. I'm not sure if one element can have the three directions stiffness or you should add three, one for each direction and then you can enter the different behavieur.

I have modeled and simulate a lot of engine mounts, but never replace it by springs for simulating the main behavieur of the powertrain (that part of the bussines was modeled in ADAMS entering the three curves of each mount)

I would model a simple rubber mount with his bumpers and limitations, compute the three load deflections curves, and then create a new model with springs using the computed data to compare with the first model. A very interesting problem!

keithmcarthur

Sergio, great suggestions. Thank you for your insight.

Sergio

As far as I understand probably your mounts performs lineal at least in the range of your application, so then if you have the stiffnes vales for each direction you can use three SPRING elements directly. I worked with automotive engine mounts that must work in wide range of loadings, that´s why bumpers and limitations were nedeed and important to know the load/deflection curve, but your machine looks more an stationary station that will work on a steady regimen.

Do you have more information on the issolators?

keithmcarthur

They are Trellborg cushyfoot, part number 20-00692-01.

Information can be found here:
http://www.trelleborg.com/en/anti-vibration-solutions/products--and--solutions/rail--and--mass--transit--solutions/cushyfoot--product--range.

Max Load: 1280 kg, actual load: 1260 kg
Deflection: 16 mm at maximum load.

When modeling these would you use use a spring element length equal to the max deflection?

Sergio

There is very few info on the site! It should gave you at least the static stiffness in X Y and Z direction.

I would start defining a spring by two nodes separated by the issolator unloaded height, so when you apply the load (weight) you would have the preload state. You will need to define also the other two stiffness (by deflection and max load you can define the vertical, but still lack the lateral ones).

Will try to take a look on the Trellerbog model maybe during the weekend to see if I can compute some stiffnes values for the three directions.

Regards

Sergio

Can you measure the unloaded height of the mount? I'm not sure if the height on the table is with or without preload.

keithmcarthur

The height on the mount in the table is the unloaded free height. (120 mm), thank you.

Sergio

A little late, but here are the results, hope they be usefull. In the PDF file are the load deflection curves and stiffness values for each direction, assuming a preload of half of the maximun load (8mm)






Regards

keithmcarthur

That's fantastic. Thank you very much Sergio.