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Restraints for Frequency Study

Question asked by Alec Chalmers on Feb 23, 2010
I am attempting to determine the natural frequency of a weldmment that is part of a gimbal assembly. This weldment rotates around the axis formed by the cylinder in the center of the structure (refer to attached JPEG). It is held at the top (small triangular end) by a bearing and at the large end by a torque motor. During assembly the end play is taken out by means of shims. I have tried restraining the moodel with "Roller/Slider" restraints on the mating surfaces at each end and "Fixed Hinge" restraints on the mating cylindrical faces. This returns a first mode frequency of .00037Hz and then a second mode frequency of 312Hz. When I fix the base (large end) only the first mode frequency is reported to be 714Hz. Anyone have any ideas why allowing the structure to rotate would result in such a low natural frequuency.

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