Balance & Unbalance Diagnostics
From Concepts to a Specific Diagnosis
Modules 4.1 and 4.2 built the vocabulary and tools — frequency content, phase angle, Bode plots. This module applies them to the single most common vibration diagnosis: mass unbalance. Understanding what actually confirms unbalance (not just suspects it) is the foundation every other vibration diagnosis in this track builds on by contrast.
The Heavy Spot Mechanism
A heavy spot is a point on the rotor where mass is unevenly distributed relative to the true rotational centerline. Every revolution, that heavy spot generates a centrifugal force pulling outward at its specific angular position — because this happens exactly once per revolution, it produces vibration at exactly 1X frequency, the signature introduced in Module 4.1.
Two Tests That Confirm Unbalance
1X frequency alone isn't enough to confirm unbalance — other mechanisms can also produce 1X content. Two additional checks distinguish real unbalance from other possibilities. First, unbalance-driven vibration amplitude scales with the square of rotational speed, since centrifugal force itself scales that way. If actual vibration amplitude doesn't follow this predictable speed-squared relationship as speed or load changes, that's a signal something other than pure unbalance may be contributing.
Second, pure mass unbalance produces a stable phase angle that stays essentially constant over time at a given steady speed — the heavy spot's angular position doesn't change, so its phase relationship to the Keyphasor reference (Module 4.2) stays fixed too. A phase angle that drifts or changes over time at constant speed suggests something other than simple static unbalance — perhaps a developing condition like a loosening component or thermal bow, rather than a fixed heavy spot.
1X frequency + amplitude following speed² + stable phase angle, together, confirm unbalance. Any one of these alone is suggestive but not conclusive — real diagnosis checks all three.
What Actually Creates Unbalance in Service
Unlike the deliberate manufacturing balancing process from Module 2.1, in-service unbalance usually develops from an identifiable cause: blade deposits creating uneven fouling around the circumference (connecting to Module 1.4's fouling discussion), erosion or corrosion removing material unevenly, a lost or shifted original balance weight, or blade damage. A rising 1X trend often prompts investigating what recently changed — a water induction event, a period of heavy fouling — before jumping straight to a correction.
Confirming unbalance through frequency, amplitude-vs-speed², and phase stability matters because misdiagnosing another problem as unbalance — and field balancing to compensate for it — can mask the actual underlying issue rather than fixing it, potentially letting a real problem continue developing.
Field Balancing — The Trial Weight Method
When unbalance is confirmed and correction is warranted, field balancing corrects it without full disassembly. The process: measure baseline vibration amplitude and phase, add a known trial weight at an accessible location, remeasure the response, and use the change between baseline and trial-weight readings to calculate where a permanent correction weight should go and how large it should be. This works because the rotor's response to a known trial weight reveals its sensitivity — both magnitude and phase direction — to weight added at that specific location, allowing the correction to be calculated mathematically from a single trial run rather than through repeated guess-and-check adjustment.