Contact set up

[RobboFlea]

I'm just starting to use cosmos and I have some doubts about setting up the right constraints for the analisys I have to do. I hope someone could help me.
I have to simulate an hydraulic lift table as the one in the link below.

http://img.directindustry.com/images_di/photo-g/lift-table-11896.jpg

As you can see the legs of the elevator have one pivot on one side and one wheel on another.
For what regards the pivot I was thinking to simply set the contact option btw the support and the basement (or the top of the lift) to "bonded" to establish the right constraint.
But for what concerns the wheel I don't know how to say that the wheel could slide on the surface receiving from it only a vertical reaction.
Should I use simply a "no penetration" option for the contact?

thanks in advance for your help

Robbo

 

[kellnerp]

You don't need contact.

You do a hand analysis of the forces on the members and then analyze each member separately with prescribed displacements and forces.

If you must do FEA on the links, then use symmetry to model half the structure. Use a vertical displacement constraint on the lower roller. Use a prescribed radial constraint on the pivot. Use prescribed constraints on the top roller and pivot. Only use contact at the center pivot between the two frames. Apply prescribed force to the actuator arm and an equal and opposite force to the lower frame.

When you load it you put the maximum cylinder force on the actuator and increment the prescribed displacement on the top throughout the range of travel. You measure the reaction on the table top constraints and scale your stresses to your design load.


TOP
CSWP
BSSE

http://www.engtran.com

http://www.niswug.org

"Node news is good news."

 

[RobboFlea]

thanks for the reply but i'm not sure to have understood well what I have to do.
I have already made a hand analysis of the structure in order to dimensioning the whole structure but now I have to simulate the entire structure.
To do so you suggest to load with the actuator force (that I've found) the structure and impose that the upper surface of the platform should not have vertical displacements?
And why in this case I should not need to establish a contact between the wheel and the platform? If I don't do so who ensures me that the wheel and the platform (or the basement) will continue to keep tangent during deformation?
Please forgive me if it is a stupid question but I'm quire a newbie in using cosmos and fea in general.

thanks in advance

Robbo

 

[kellnerp]

The wheel is a slider. Constraining the axle of the wheel to NOT move in the vertical direction is far more efficient than modeling the wheel and using contact. Contact is expensive and won't give you any more information than using a constraint.

For the top constraints are used to set the height of the platform. Then you apply forces where the cylinder attaches.

You slice the table in half and only model half of it. Again this is a cost/benefit issue.

When you use the finite element method one of your objectives is to build models that run as quickly as possible and still give acceptable results. To do this an analyst will frequently simply the problem if it doesn't affect the results. This is not an aerospace project and there is a large factor of safety. So extreme detail is not necessary.

Someone new to finite element will usually treat it as a magic box that gives answers if only enough elements are thrown at the problem. A seasoned analyst has as a goal getting good answers in the least amount of time. This takes understanding and thought as well as good software.

TOP
CSWP
BSSE

http://www.engtran.com

http://www.niswug.org

"Node news is good news."