INFLUENCE OF AXIAL FORCE ON THE DYNAMIC CHARACTERISTICS OF A BEAM SUPPORTING A ROTATING MACHINE

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.