Learning Forum

[eric.soring]

Hi Everyone, this is my first post, and I'm new to FEA so please have some patience with my elementary questions :) 

I am trying to replicate an as built condition for a fixture we have. I have a 4 piece assembly thats bolted to a flat plane with a significant amount of fasteners. I am using a frictionless constraint, paired with cylindrical constraints on each of my bolt locations. I believe the frictionless constraint will eliminate 1 translation, and 2 rotational degrees of freedom. My fasteners (since i have more than 1) eliminate 2 more translational, and one rotational dof. 

However when i run a modal analysis, my responses are 0,0,0 and then basically zero and my mode shapes show rigid body motion.

Is there a better way to approach this?

Thank you,

e 

[peteroznewman]

Assume the fasteners have a significant torque that creates a large preload in the bolt shank and clamps the flange of the part being fastened to the base plane with a non-zero friction coefficient.

Assume the base plane is much stiffer than the parts in your fixture as would be the case if you are bolting your fixture to a shaker table.

A simple way to fix these parts to the ground plane of the shaker table that is not modelled is with a Fixed Joint to Ground.

Hole pairs that fasten adjacent parts of your fixture to each other can be connected with a Fixed Joint scoped to the adjacent bolt holes.

Create a new Fixed Joint for each hole pair.  This should allow all the parts to support each other and be correctly supported by the bolt holes to the base plane which is ground.

In that way, the first mode in the Modal analysis should be non-zero.

There are more sophisticated methods to include beam elements to represent the axial stiffness of the bolt shaft and include a bolt head with contact to the top of the flange to include the axial stiffness of the flange, but it is best to start with a simple model and add further details as needed.

[eric.soring]

this is good advice, i will try this! thank you