What would you suspect if you saw that the 1X through 4X peaks were surrounded by sidebands with very little separation?

When observing that the 1X through 4X peaks are surrounded by sidebands with very little separation, it indicates a potential problem with the rotor components in a rotating machinery system. This specific vibration pattern is commonly associated with issues such as broken rotor bars or a cracked end-ring.

When rotor bars or end rings are damaged, the normal magnetic field interaction during operation gets disrupted, leading to variations in the speed and force acting on the rotor. This disruption manifests in the vibration spectrum as closely spaced sidebands around the fundamental frequency and its harmonics, as the fault intermittently influences the harmonics of the rotational frequency.

This pattern suggests that the irregularities in the rotor are causing consistent cyclic variations that result in the observed sidebands. The lack of significant separation implies that the faults are likely generating harmonics at a consistent rate relative to the rotational speed, which typically points towards mechanical issues in the rotor.