If a machine with an overhung rotor goes out of balance, how will the vibrations behave?

When a machine with an overhung rotor goes out of balance, the vibrations primarily manifest as an increase in the 1X amplitude across multiple directions—vertical, horizontal, and axial. This behavior is attributed to the nature of overhung rotors which are inherently more susceptible to dynamic imbalances because of their design.

In an overhung rotor arrangement, the rotor is positioned away from its support bearing, which means that when the rotor is unbalanced due to forces acting unevenly upon it, these forces will cause the rotor to vibrate in a response that aligns with the imbalance.

The 1X vibration component corresponds to the rotational frequency of the rotor, which means any imbalance will generally produce a fundamental frequency vibration that can appear in all planes of vibration—vertical, horizontal, and axial. Thus, as the unbalanced force acts upon the rotor, it causes movement in all three directions, leading to a notable increase in the 1X amplitude across each of these axes. This comprehensive response to imbalance is characteristic of how overhung rotors behave under unbalanced conditions, making the option identifying the increase in vertical, horizontal, and axial vibrations the most accurate.