Hello everyone,

I have Solidworks Simulation standard but not the license that allows me to do modal vibration analysis.

Now I would like to estimate the natural frequency of a tubular frame with a big mass mounted on top.

So I thought If I apply a force at the location of the mass (700mm above the frame) by means of a "remote load" and I simulate the deflection of the interface at which the remote load is attached.

Then I estimate the deflection "dx" of the mass at 700mm above the interface by assuming a rigid connection between the mass and the interface.

Then I could determine some sort of stiffness "k" = F/dx

Then I calculate the frequency by considering my system as a single DOF spring-mass system with f0= 1/2pi * (k/m)^0.5

I get a result of 23Hz which is in the order of what could be expected (customer demands 25Hz minimum) but I wonder how accurate this estimation would be?

Off course I applied the remote load in the direction which results in the biggest deflection of the frame.

Can anyone tell me whether my assumption would be correct or am I doing too much simplifications here?

Lo Wie

I like your resourcefulness on this one. If it were me I would try this approach (which seems plausible) with a known solution. Like a cantilever where you have methods to compute the natural frequencies from say Roarke or other reference. If your approach has good agreement, which I suspect it will, then you need to the same calc on your structure for all the primary modes you can think of to try and ensure you know the lowest mode. If the thing is square and has qtr symmetry, then in addition to bending, calculate the torsion case, the heave case, and there are any slender braces, you want to make sure the transverse fixed/fixed bending modes are higher (it would have to quite long and slender) and any thing else you can think might apply. Let us know how you end up bounding up your uncertainties.