Misalignment & Bent Shaft Signatures
A Different Mechanism, A Different Signature
Module 4.3 established how to confirm mass unbalance. This module covers the next most common vibration diagnosis — misalignment — and a related condition, bent shaft, showing how their signatures differ from unbalance in ways that actually matter for correctly identifying the root cause.
Parallel vs. Angular Misalignment
Parallel (offset) misalignment occurs when two coupled shaft centerlines are parallel to each other but offset — not sharing the same line. This forces the flexible coupling (Module 2.6) to accommodate that lateral offset every revolution, tending to produce strong radial vibration at 2X frequency — Module 4.1's second-harmonic signature — as the coupling flexes through a repeating bending pattern twice per revolution.
Angular misalignment occurs when the coupled shaft centerlines intersect at an angle rather than running parallel — the shafts point in slightly different directions rather than simply being offset. This tends to produce a stronger axial (along-shaft) vibration component than parallel misalignment, since the angled coupling creates genuine push-pull motion along the shaft axis as it rotates, not just radial bending.
Parallel misalignment favors radial 2X vibration; angular misalignment favors axial vibration. Real-world misalignment is often some combination of both, producing a mixed signature.
Two Diagnostic Tests Specific to Misalignment
Checking the axial vibration component relative to radial vibration is a key step: unbalance produces almost entirely radial vibration (its centrifugal force mechanism has no natural axial component), while misalignment — especially angular — produces meaningful axial content too. Significant axial vibration alongside radial vibration should immediately shift suspicion away from pure unbalance and toward misalignment.
Comparing phase angle across the coupling — readings taken on either side of the coupling itself — is a classic misalignment-specific technique: misalignment often produces a roughly 180-degree phase difference across the coupling, with the two shaft ends effectively moving in opposite directions, while unbalance on a single rotor typically shows consistent phase relationships within that rotor. This cross-coupling comparison is unique to misalignment troubleshooting; a single-rotor unbalance diagnosis (Module 4.3) never needs to consider it.
Bent Shaft — A Related but Distinct Condition
A bent (bowed) shaft is physically different from misalignment — the rotor itself has a permanent or temporary curvature, most commonly from the rotor bow mechanism covered in Module 2.6 (uneven cooling of a hot, stationary rotor without adequate turning gear operation). A bent shaft produces 1X vibration that can look similar to unbalance at first glance.
The distinguishing test is the slow roll check: measuring vibration at very low rotational speed, where centrifugal force from any true unbalance is negligible. A bent shaft shows elevated vibration even during slow roll, because the physical bend exists regardless of speed — but true unbalance shows minimal vibration at slow roll and only becomes significant as speed (and therefore centrifugal force) increases.
Confusing a bent shaft with unbalance and attempting a field balance correction (Module 4.3) won't fix the underlying problem — a bent shaft needs to be addressed as a bend, not compensated for with balance weights. The slow roll check is exactly what prevents this misdiagnosis.