While 1x RPM usually indicates imbalance, mass imbalance typically shows similar readings horizontally and vertically. The disparity suggested (specific to fans where the mass is located off the bearing span) or dynamic imbalance due to a bent shaft extension.
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Key takeaway:
In the world of reliability engineering and predictive maintenance, theory only gets you so far. You can memorize the frequency of ball pass defects or the harmonic patterns of looseness, but when you are standing in front of a screaming gearbox that is critical to production, you need practical experience. While 1x RPM usually indicates imbalance, mass imbalance