Simulation: constrains and Rigid Body Modes

[PinRudolf]

In the past I've been doing some simulation work in Inventor. I am trying to get a grasp on how to do the same work in Fusion 360 but I'm hitting my head against the same problem over and over: when I use constraints in a partially constraint model the 'Remove Rigid Body Modes' option drastically alters the result for the worst. 

Below sample shows a simple bracket with 2 forces on it. One force pulling up, the other acting as the reaction force; pulling down. To prevent the component from moving I've switched  Remove Rigid Body Modes on. This simulation looks good to me. The following image shows the stress.

 

Now I don't want to be adding the reaction forces manually so I'll use a constraint, a pin constraint in this case, in the bottom hole. (Remove Rigid Body Modes is still on.) The result looks very wrong; it looks like the stress is completely countered at the constraint. 

When I switch Remove Rigid Bodies Mode off the result is clearly wrong and the maximum displacement of the component is almost 10 km. (The minimum displacement is roughly 5km, I wonder how this is calculated...)

This issue does not come up when the part is fully constraint. But in this case both holes should be able to rotate.  

I haven't had this issue using the Inventor Simulation and I'm guessing there are differences I haven't noted which make me use the Nastran solver incorrectly. 

Does anyone have an idea what I'm doing wrong?

 

[PinRudolf]

Just to show what I'd expected from the second simulation I've created the Inventor one as well. Please note the stress is symmetrical and not just in the top half. I've used Inventors equivalents of  the Bearing Load, the Pin Constraint and Remove Rigid Body Modes. 

Note worthy is that Inventor does not use a global acceleration but only soft springs. In wonder how the Fusion model would work out if I could switch off the global acceleration but keep the soft springs on.

 

[John_Holtz]

Hi,

Your second example with the pin constraint is not following the requirements. See Guidelines for using Remove rigid body modes.

In order to use the pin constraint, you should add a constraint to a corner on the top of the C to prevent motion in the tangential direction. This will make the model statically stable but still allow it to flex in the proper manner.