Robot Structural Analysis

## Robot Structural Analysis

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Posts: 47
Registered: ‎05-14-2012
Message 1 of 5 (430 Views)

# forced vibration for a steel beam cantilever

430 Views, 4 Replies
05-14-2012 08:26 AM

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.

Valued Mentor
Posts: 608
Registered: ‎09-07-2011
Message 2 of 5 (417 Views)

# Re: forced vibration for a steel beam cantilever

05-14-2012 09:15 PM in reply to: alb2

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.

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

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Posts: 47
Registered: ‎05-14-2012
Message 3 of 5 (404 Views)

# Re: forced vibration for a steel beam cantilever

05-15-2012 12:30 AM in reply to: tony.ridley

Thank you very much.

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Posts: 424
Registered: ‎06-23-2008
Message 4 of 5 (394 Views)

# Re: forced vibration for a steel beam cantilever

05-15-2012 01:17 AM in reply to: alb2

Hi alb2, additional information to info provided by Tony:

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:

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.

---------------------------------------------

Regards,

Pawel Pulak
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Posts: 47
Registered: ‎05-14-2012
Message 5 of 5 (380 Views)

# Re: forced vibration for a steel beam cantilever

05-15-2012 04:14 AM in reply to: pp2008

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