From Spectral Peaks to Fault Hypotheses: Unbalance, Misalignment and Looseness

From Mechanical Maintenance to Vibration Analysis - Part 5

In Part 4 - Understanding the Time Waveform and FFT Spectrum, we learned how running speed, harmonics, sidebands and acquisition settings help us read vibration data.

Now we take the next step: turning observed patterns into fault hypotheses.

A spectral peak is a measured fact. A fault name is an interpretation. Evidence must connect the two.

This distinction is essential. A strong 1X peak may support an unbalance hypothesis, but it may also be influenced by resonance, eccentricity, a bent shaft, looseness or transmitted vibration. Good analysts do not stop at pattern recognition; they test competing explanations.

Separate observation, interpretation and confirmation

Use three levels when communicating a diagnosis:

1. Observation: What does the data actually show?
2. Interpretation: Which mechanisms could reasonably produce it?
3. Confirmation: What additional evidence distinguishes the leading hypothesis?

For example:

Observation: Pump outboard radial velocity has a dominant, stable 25 Hz peak. The machine runs at 1,500 rpm.

Interpretation: The peak is 1X running speed; unbalance is one plausible cause.

Confirmation: Check radial direction, phase stability, response at both bearings, operating-speed behaviour, rotor cleanliness and evidence of resonance before recommending balancing.

Build the fault hypothesis from multiple clues

A useful hypothesis combines more than frequency. Review:

• Frequency: Is the component at 1X, 2X, a harmonic, subharmonic or another calculated frequency?
• Amplitude: Is it significant relative to the baseline, alarm criteria and nearby locations?
• Direction: Is the response mainly horizontal, vertical or axial?
• Phase: Is phase stable, and how does it compare across bearings, directions and the coupling?
• Waveform: Is the motion sinusoidal, impacted, clipped, modulated or irregular?
• Operating condition: Does the pattern change with speed, load, temperature, flow or pressure?
• Machine geometry: Where are the bearings, coupling, overhung components, supports and potential clearances?
• History: What changed after cleaning, overhaul, alignment, pipe work or process adjustment?

Hypothesis 1: Rotor unbalance

Unbalance exists when the mass centre of a rotating component does not coincide with its rotational axis. The resulting centrifugal force rotates once per revolution and increases strongly as speed increases.

Patterns that may support unbalance

• A prominent 1X running-speed component.
• Vibration is commonly stronger in radial directions than axial direction.
• The time waveform may look relatively sinusoidal when 1X dominates.
• 1X phase is often reasonably stable when speed and load are steady.
• The response may increase markedly near a structural or rotor resonance.

Possible physical causes include product buildup, erosion, a missing balance weight, an incorrectly installed component, rotor damage, casting variation or eccentric mass distribution.

Why 1X alone is not enough

A shaft rotates once per revolution, so many mechanical conditions can produce 1X. A bent shaft, eccentric sheave, resonance, misalignment, looseness, rub or external vibration may all contribute. A large 1X peak identifies a synchronous response; it does not identify the root cause by itself.

Useful confirmation checks

• Compare horizontal, vertical and axial readings at both bearings.
• Measure 1X phase with a reliable tachometer and check repeatability.
• Inspect the rotor for deposits, missing material or loose components.
• Review startup or coast-down data for resonance amplification.
• Check whether the vibration changed after cleaning or mechanical work.
• Before adding correction weight, confirm that looseness, soft foot and alignment are acceptable.

Hypothesis 2: Shaft misalignment

Misalignment means the shaft centre-lines of coupled machines are not collinear under their normal operating condition. It may be angular, offset or a combination. Thermal growth, pipe strain, soft foot, foundation movement and assembly errors can all alter the running alignment.

Patterns that may support misalignment

• Elevated 1X and/or 2X components near the coupling.
• Significant axial vibration, particularly with angular misalignment.
• Radial response on both sides of the coupling, especially with offset misalignment.
• A repeatable phase relationship across the coupling that is inconsistent with simple unbalance.
• Coupling temperature, bearing temperature or seal problems may accompany the vibration evidence.

The exact spectrum depends on coupling type, machine stiffness, bearing arrangement, load and degree of misalignment. Some misaligned machines show a strong 2X component; others are dominated by 1X. Therefore, the absence of a large 2X peak does not rule misalignment out.

Useful confirmation checks

• Compare axial and radial readings on both sides of the coupling.
• Take phase readings at consistent locations and directions across the coupling.
• Check coupling condition and temperature.
• Verify soft foot, base condition, hold-down bolts and pipe strain.
• Review cold alignment targets and expected thermal growth.
• Perform precision alignment using an approved method, then compare vibration before and after correction.

Hypothesis 3: Mechanical looseness

Mechanical looseness is excessive movement between parts that should remain fixed or move within a controlled clearance. It can occur at a machine foot, baseplate, foundation, bearing housing, bearing fit, shaft fit, coupling or structural joint.

Patterns that may support looseness

• A family of running-speed harmonics: 1X, 2X, 3X and higher.
• Sometimes subharmonics such as 0.5X, depending on contact and clearance behaviour.
• An impacted, truncated or asymmetric time waveform.
• Amplitude and phase that may be unstable between repeated measurements.
• Large differences between nearby points or directions.
• A nonlinear response: relatively small changes in load or speed produce large vibration changes.

Harmonics develop because looseness can distort an otherwise sinusoidal motion. Contact, clearance and changing stiffness create a non-sinusoidal waveform, which the FFT represents as multiple harmonics.

Useful confirmation checks

• Inspect hold-down bolts, grout, baseplate, welds and structural joints.
• Check bearing and housing fits where permitted by the maintenance procedure.
• Compare casing points immediately above and below suspected joints.
• Look for fretting, polished contact surfaces, cracked paint or movement marks.
• Perform a controlled bump or impact test if resonance or structural flexibility is suspected.
• Check whether phase is repeatable; erratic phase may support intermittent movement.

Comparison table: clues, not rigid rules

Evidence	Unbalance hypothesis	Misalignment hypothesis	Looseness hypothesis
Spectrum	Often dominant 1X	Often 1X and/or 2X	Often multiple running-speed harmonics; possible subharmonics
Direction	Usually radial emphasis	Axial and/or radial near coupling	Any direction; may differ sharply across a joint
Waveform	May be near sinusoidal	Periodic but often more complex	Impacted, clipped, asymmetric or irregular
Phase	Often stable at constant condition	Cross-coupling relationships can be diagnostic	May be unstable or inconsistent
Physical checks	Deposits, erosion, missing weight, rotor damage	Alignment, thermal targets, coupling, soft foot, pipe strain	Bolts, fits, grout, cracks, fretting and structural movement

Important: These are tendencies, not universal acceptance rules. Faults can coexist, and resonance can amplify any forcing frequency.

Worked motor-pump example

A direct-coupled motor-pump runs at 1,500 rpm, so 1X equals 25 Hz. The analyst records:

• Motor drive-end axial: strong 25 Hz and 50 Hz.
• Pump drive-end axial: strong 25 Hz and 50 Hz.
• Radial readings: moderate 25 Hz.
• Phase readings across the coupling: repeatable but significantly different.
• Coupling temperature: higher than its established baseline.
• Inspection history: pump pipe work was modified during the last shutdown.

Observation: 1X and 2X are elevated, axial vibration is significant on both sides of the coupling, cross-coupling phase is repeatable, and temperature has increased.

Leading hypothesis: Misalignment or externally imposed strain affecting alignment.

Competing explanations: Coupling defect, bent shaft, looseness, soft foot or structural amplification.

Next actions: Check pipe strain, soft foot, hold-down condition, coupling condition and hot-versus-cold alignment requirements before moving the machine. Repeat vibration and phase readings after any correction.

This conclusion is stronger than saying, “The spectrum has 2X, therefore the machine is misaligned.” It connects the spectrum with direction, phase, temperature, maintenance history and physical checks.

A practical hypothesis-testing workflow

1. Verify the data. Confirm point, direction, units, mounting, speed, load and acquisition settings.
2. State the observation. Record frequencies, amplitudes, directions, waveform features and phase behaviour without naming a fault.
3. List plausible mechanisms. Include at least one competing explanation.
4. Rank the hypotheses. Use machine design, operating condition and history.
5. Choose a discriminating test. Ask which measurement or inspection will separate the leading possibilities.
6. Correct the verified cause. Do not balance, align or tighten components merely because a pattern looks familiar.
7. Verify the result. Repeat measurements under a comparable condition and document the change.

How to write a defensible recommendation

Avoid:

The pump is unbalanced because the spectrum has 1X.

Prefer:

Radial vibration at the pump outboard bearing is dominated by a stable 1X component that has increased from its comparable-load baseline. Rotor unbalance is a leading hypothesis. Inspect the impeller for buildup or damage, check hold-down integrity and resonance response, and collect repeatable 1X phase data before deciding whether field balancing is appropriate.

Final takeaway

• Unbalance often emphasizes stable radial 1X, but 1X is not unique to unbalance.
• Misalignment may produce axial and radial 1X/2X near a coupling, but the exact pattern varies.
• Looseness may generate harmonics, impacts and unstable behaviour, but harmonics alone do not prove it.
• Direction, phase, waveform, operating condition, history and inspection turn a pattern into a testable hypothesis.
• The repair result is part of the diagnosis. Always verify the post-maintenance condition.

Coming in Part 6

Part 6: Rolling-Element Bearing Faults. We will examine bearing defect frequencies, high-frequency acceleration, enveloping, harmonics and sidebands—and why lubrication, load and installation must be considered before condemning a bearing.

Discussion question: Which fault have you found hardest to distinguish in the field—unbalance, misalignment or looseness?

References and further learning

• Fluke, Diagnosing Imbalance, Misalignment, Looseness, and Bearing Wear.
• Fluke, Mechanical Looseness: What It Is and How to Detect It.
• Fluke Reliability, Phase in Vibration Analysis: Theory, Applications, and Examples.
• Bently Nevada, Vibration and Dynamic Measurements.

Educational note: The patterns are simplified learning guidance, not universal fault rules or alarm limits. Apply approved site procedures, applicable standards, manufacturer guidance and qualified engineering judgement to real machinery.