forced vibration for a steel beam cantilever

[Anonymous]

Hi,

Im trying to find the internal forces of a steel HEA 240 supported as in the figure above.

the electric motor has a weight of 500 kg, imbalance 0,6 kgm. Damping 10%, the self og the beam in neglected (considered 1 kg/m3). The engine can work 800/ 1000/ 1200 rot/min. Meaning that the frequencies respectively f800=13,33 hz ; f1000=16,667hz; f1200=20hz.

In the modal analysis is possible to find the frequency of the structure 14.15 hz.

I need help finding the internal forces acting on the beam and deflections, due to forced vibration ( static forces multiplied by the dynamic magnification).

Which analysis will better suited in this case?

I calculated the forces in another software (scia engineer) but i cant get the results in robot.

Any help will be appriciated.

Thank you in advance

[tony.ridley · Accepted solution]

You need to set up a modal case then follow it with a time history by the sounds of it.  You can set up forcing functions and then interrogate the results for internal forces etc. 

See this thread

http://forums.autodesk.com/t5/Autodesk-Robot-Structural/Time-History-Analysis-for-a-rotating-motor/m...

There is an example file in there too.

Tony

[Anonymous]

Thank you very much.

[Pawel.Pulak · Accepted solution]

Hi alb2, additional information to info provided by Tony:

Your document suggests that:

1/ The eccentricity of motor's position  in relation to longitudinal axis of beam can be neglected.

 In such case, because the longitudinal (tension/compression) stiffness of beam is much higher that its vertical flexural stiffness, it is enough to consider only the vertical component of imbalance. So in such case the rotating imbalance force is replaced by the vertically oscillating imbalance force.

2/ You are interested in steady-state conditions and not in transient states (starting or braking)

In above case you can use simpler harmonic analysis instead of time history.

So in such case you should define:

a/ dynamic modal load case

b/ static load case corresponding to the amplitude of vertical imbalance

c/ change above static load case into harmonic analysis for single frequency or for some range of frequencies - see the screen capture below:

It may be useful to define 2 identical load cases in step b/ and leave one of them as static one for comparison with the second one (changed to harmonic in step c/).

Do not forget about defining the weight of engine as load converted to mass or as added mass.

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Regards,

[Anonymous]

Thank you for your advice, it works great.