Learning Forum

[Corey Badger]

Hello,

I have modelled a simple pinned joint configuration using no separation contacts to simulate pinned joint. I am getting strange results where the parts that has the applied loads enlarges radially outwards. See below a screenshot of the issue. The deformation in the plot are enlarged to highlight the issue. The load is applied vertically downwards using both, the gravity and force applied to the bottom hole.

[william Lucking]

If the model is unstable (pins can rotate freely, or any part) then the rotation of the pins will be very large and they will show up as radially enlarged in the deformation.

[Corey Badger]

Thanks for the answer. I am not sure what to think about it. The top pinns are fixed to the supports so they cannot rotate. The bottom ones are attached to the link arms so they can rotate with the link arms but not on their own.

What would the alternative be to simulate this problem correctly? Would the use of the friction contact address the issue of instability by providing kind off weak springs effect? 

[peteroznewman]

Corey, this is a mechanism, not a structure. Have you heard the term 4-bar mechanism?  That is what this is, two long links, one short link (the square) and the fourth link is ground. Mechanisms move. Static Structural doesn't like mechanisms. Mechanisms have large rotations. You can force Static Structural to deal with large rotations under Analysis Settings, turn on Large Deflection, then solve and see if you get a different result.

[william Lucking]

Peter is right. If this is free to move at all (aka unstable) then its going to show strange behavior (i.e. huge rotations as in your picture) for a static analysis. You could try turning on inertia relief. Weak springs can help, but not much. But I really don't understand what problem you are trying to solve. What is the loading on this? Seems you are taking the wrong approach.