Experimental nonlinear dynamic analysis of a machine supporting structure

Resumo

We present an experimental study of the effects of geometric nonlinearities on vibrations of rotating
machines support structures. Dynamic characteristics of structures depend on their stiffness, damping and mass.
The initial stiffness of a structure, computed in its unloaded state, is affected by the applied forces, the so-called
geometric stiffness. Compressive forces reduce the stiffness and the frequencies and may lead to buckling, for zero
frequencies. In bases of machines excited by the supported equipment, vibrations may affect the structures but, in
general, they may generate damage to the suspended equipment and the quality of the production. Although
machine support structures are, as a rule, very bulky, little affected by geometric stiffness considerations, the
tendency of modern structural engineering, especially in aerospace applications, is towards slender members, due
to more efficient materials and powerful analysis tools. Here we study these effects via experimental methods
designed to evaluate previous mathematical models. Our model is a metal beam under compression supporting a
DC motor. We suppose the original design provided natural frequencies away from the excitation frequency.
Nevertheless, the presence of large axial compressive force will reduce the beam stiffness and natural frequencies
leading to unexpected, potentially dangerous resonance states. Experimental imperfections led to observation of
interesting phenomena not predicted in our previous theoretical and numerical studies. We also observe, as
expected, occurrence of the so called Sommerfeld Effect, when underpowered excitation sources get their rotation
regime stuck at resonances.