large mass method 1

[plzzhelp]

I have gone through few posts on this topic but my doubt is not yet clear which is..... i have to apply forced acceleration at many locations of my model, so can I use more than one largemass but i think the whole point of using large mass becomes waste when there is another large mass and the excitation would no longer be acceleration but becomes normal forced excitaion
Iam using msc/nastran but even in nastran manual this point is not clear...... please help me

 

[rob768]

The idea is to fix the model to one single large mass, I assume you want to use it to rule out rigid body moves. If you use two masses, you will genereate problems since those masses wil then dominate the calculation.

 

[Transient1]

I agree with rob768, the use of multiple large masses is problematic (think moments of inertia). What kind of problem requires different accelerations on muliple areas of a part?

Maybe, you can simplify your analysis method. Also, what is the goal of the analysis?

 

[plzzhelp]

Thanks a lot for your replies.
Actually Iam trying to give different acceleration inputs at the four wheels of a simple vehicle model. I used just one large mass at one point and fixed the other three and repeated it 4 times giving acceleration at each point and fixing the others and calculated response at a point far away from these four points and finally added these four for the net response. Is there any problem with this method?

 

[rob768]

The idea of the large mass method is that it is unmovable with regards to the studied object, which as i assume is your car body. In your case, i also assume the connection between car body and earth is by means of a series of spring/dampers. In my opinion, you can just disregard the earth, and apply your accelerations directly to the springs.

 

[Spirit]

Plzzhelp,

the Large Mass Method is the standard procedure to apply accelerations for NASTRAN previous to 2001 release; from this version, a more robust method has been implemented that does not require the large mass and allows the excitation via forced displacements, velocities or accelerations as well. You should refer to the 2001 Release manual for the details, and to the pertaining QRG for the new syntax of dynamic loads definition.

Hope this helps. Regards,

Spirit



'Ability is 10% inspiration and 90% perspiration.'

 

[plzzhelp]

Thankyou all very much for your replies they clarified many of my doubts.

thanks spirit, iam using MSCnastran2001, but was following older manuals , after looking at your reply I checked my manuals. Thanks a lot

But unlike my case whr different accelerations are to be applied at different locations,if enforced motion is to be applied at single point or same enforced motion at different locations LMM is a better option I think.