A Rotary machine is a significant component of many mechanical systems. It is important to clarify the dynamic characteristics in several conditions. This study deals with nonlinear dynamics of a horizontally supported Jeffcott rotor. The equations of motion are derived by considering the effects of gravity and the cubic nonlinearity of restoring force by the support condition. These effects produce the difference between the linear natural frequencies in the vertical and horizontal directions and make the stiffness in the vertical direction unsymmetric. It is theoretically and experimentally shown that due to such effects, the 1/2-order subharmonic resonances are produced in the cases when the rotational speed is in the neighborhood of twice the natural frequencies in the horizontal and vertical directions, and the frequency response curve of the resonance near twice the horizontal natural frequency is hardening-type, while near twice the vertical natural frequency is softening-type.

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