Learning Forum

[turgut.ataseven]

Hi.

When performing a modal analysis of a shaker fixture, should holes that will be connected to the threads of the shaker be fixed with Fixed Support, or is there a better approach? Would including a Point Mass with the mass and inertia input of the assembly that will be mounted to the fixture make the analysis much safer and yield incorrect results? How can we simulate effect of the shaker without adding Compression Only Support?

[peteroznewman]

Using Fixed Support on the washer faces where bolt heads fasten the fixture to the shaker table is a good approach. This approach assumes the shaker table is like a rigid body which is a fair assumption.

A Modal analysis is a linear analysis but a Compression Only Support is a nonlinear contact so cannot be used in a Modal Analysis.  Design the fixture so that contact with the shaker table is on a small washer face on the bottom of the fixture at each bolt location.  These faces can be included with the Fixed Support.

You should insert a Remote Mass selecting the washer faces on the fixture that the Device Under Test (DUT) bolts to. Mechanical will automatically create the location at the centroid of the selected faces. Overwrite the X, Y and Z Global coordinates of the remote mass with the location of the CG of the DUT and type in the Mass and Mass Moment of Inertia in X, Y and Z.  It helps if you have both the fixture and the DUT in the same assembly in SpaceClaim so you can create a coordinate system at the CG of the DUT and then create that local csys in Mechanical. Then you can use that local csys in the Remote Mass Coordinate System and the values are (0,0,0).  The default Behavior for the Remote Mass is Deformable which is adequate for screening the design of the fixture.

A minimum acceptable first mode of the fixture with DUT point mass is > 2x the highest test frequency. A separation > 3x is preferred. Redesign the fixture if it does not meet this minimum requirement.