A horizontal 15,000 hp motor drove compressors from both shaft ends and developed concentrated axial vibration at the brush-end bearing pedestal. The measured velocity reached 1.4 in/s near twice running speed, while the opposite pedestal remained quiet. The successful correction was to detune a local pedestal resonance with added mass: during the controlled trial, axial velocity fell from 0.9 in/s to 0.3 in/s.
Measurement Basis and Acceptance Point
The term axial vibration here means motion measured parallel to the shaft axis at the bearing pedestal. Preserve that direction, sensor location, mounting method, analyzer settings, and amplitude convention throughout diagnosis. The records do not identify the readings as peak or RMS, so retain the instrument's original convention rather than converting them.
The plant became concerned as vibration approached 0.5 in/s. Treat that value as the installation's action threshold, not as a universal motor limit. The reported 1.4 in/s was 2.8 times that threshold. Establish separate axial readings at both pedestals and radial readings where available; a single local high reading has a different fault path from comparable motion across the machine train.
| Observation | Diagnostic significance | Required check |
|---|---|---|
| Up to 1.4 in/s axially at the brush-end pedestal | Focuses the investigation on axial forcing and the local pedestal response | Repeat at the identical point and direction |
| No high reading at the other pedestal | Supports a localized mode or local force path rather than uniform train motion | Compare simultaneous or closely timed readings |
| One dominant component near twice running speed | Requires exact separation of orders from electrical frequencies | Record shaft speed and supply frequency with the spectrum |
| Vibration stops after power removal | Moves attention toward energized or torque-dependent excitation | Capture the shutdown transition as a time waveform or coastdown spectrum |
Check 1: Expect the repeat baseline to reproduce a high axial component only at the brush-end pedestal, using the same measurement convention and operating condition.
Frequency Classification
Do not label a peak “2×” until its frequency has been calculated. Twice running speed is 2 × shaft rotational frequency; twice line frequency is 2 × electrical supply frequency. These can lie close enough to be confused on a spectrum with inadequate resolution.
- Read running speed from a tachometer or another validated speed reference.
- Convert speed to rotational frequency using
running frequency (Hz) = speed (r/min) / 60. - Multiply that result by two and compare it with the cursor frequency of the vibration peak.
- Read the actual supply frequency and calculate twice line frequency independently.
- Increase spectral resolution if the two calculated frequencies occupy the same displayed bin.
For a 60 Hz supply, twice line frequency is 120 Hz. That is a conditional example, not the stated frequency of this installation. Electrical air-gap forces, supply unbalance, and supply waveform distortion can produce a twice-line-frequency response. Mechanical misalignment often raises twice-running-speed content, particularly in the axial direction. Frequency identity therefore controls the next diagnostic branch.
Check 2: Expect the dominant cursor frequency to match either calculated twice running speed or calculated twice line frequency within the analyzer's resolution; do not proceed with an order label based only on the plot annotation.
Operating-State and Spatial Localization
A shutdown observation separates speed-dependent vibration from excitation that depends on electrical energization or transmitted torque. Mechanical unbalance ordinarily continues during coastdown and changes with speed. Electromagnetic force disappears when the field collapses, while coupling forces can also change sharply when torque is removed. The shutdown response alone does not distinguish those two mechanisms.
Record the powered-to-coast transition rather than relying on a before-and-after reading. Compare peak frequency, amplitude, and phase immediately before power removal and throughout decreasing speed. If the component disappears as power is removed while shaft speed remains initially close to the running value, the forcing depends on energization or torque. If it decays or peaks again at a lower speed, a speed-related forcing or resonance crossing remains active.
The quiet opposite pedestal is equally important. A train-wide axial force can exist while one pedestal responds much more strongly because its local dynamic stiffness is low near the forcing frequency. High vibration at one point is therefore not proof that the force originates at that point.
Check 3: Expect the shutdown record to show whether the component vanishes with loss of power or follows shaft speed during coastdown, while the opposite pedestal remains comparatively low.
Mechanical and Electrical Exclusions
Alignment, soft foot, axial position, and rotor condition were checked before modifying the support. Coupling and compressor alignment showed no problem, and no soft foot was found. The motor operated on magnetic center with adequate thrust-bearing clearance. An MCE test reported no air-gap or rotor-bar problem.
These checks narrow the fault path but must correspond to the operating state. Cold alignment alone can miss thermal movement, so compare the applied alignment targets with the train's thermal-growth compensation. Verify that the shaft remains on magnetic center under the load condition that produces the vibration. Check thrust clearance by the established machine procedure and preserve the actual measured values in the commissioning record.
If the spectral component proves to be twice line frequency, measure supply phase quantities at a suitable test point and compare them. Review voltage balance and waveform quality rather than assuming a healthy supply from average voltage alone. If the component is twice running speed, revisit hot alignment, coupling behavior under torque, and vibration phase before changing electrical components.
| Candidate cause | Finding in this installation | Remaining distinction |
|---|---|---|
| Coupling or compressor misalignment | Alignment check found no problem | Confirm thermal-growth compensation and loaded behavior |
| Soft foot | No soft foot found | Keep pedestal fasteners and support condition controlled during testing |
| Axial thrust contact | Motor ran on magnetic center with adequate clearance | Confirm position under the high-vibration load |
| Air-gap or rotor-bar defect | MCE test found no problem | Classify the frequency before closing the electrical branch |
| Local pedestal resonance | Dynamic spectrum indicated a natural frequency | Confirm by controlled mass perturbation |
Check 4: Expect documented alignment, soft-foot, magnetic-center, thrust-clearance, and MCE results to remain acceptable at the relevant operating condition before modifying pedestal dynamics.
Pedestal Resonance Identification
A structure amplifies vibration when forcing frequency approaches one of its natural frequencies. In a single-mode approximation, f_n ≈ (1 / 2π) × √(k / m), where k is modal stiffness and m is modal mass. The local mode can be predominantly axial, explaining why the brush-end pedestal moved strongly while the other pedestal did not.
Dynamic spectrum data collected with a 21/20 CSI Spectrum Analyzer showed a feature identified as a natural frequency of the vibrating pedestal. A spectrum alone can reveal frequency coincidence, but the strongest field confirmation is a predictable change after altering mass or stiffness. Also compare vibration phase across the pedestal and nearby stationary structure. A localized phase pattern and steep amplitude change around the suspect frequency help distinguish structural amplification from uniform rigid-body motion.
- Collect axial spectra at the high point, the opposite pedestal, and adjacent support locations without changing operating load.
- Overlay the forcing frequency and the suspected natural frequency at adequate resolution.
- Record phase at repeatable reference locations.
- Prepare a controlled temporary mass change at the vibrating pedestal.
Check 5: Expect a localized axial response at the brush-end pedestal, with frequency coincidence or a phase pattern that identifies the pedestal mode selected for the mass trial.
Controlled Mass Detuning
Adding mass lowers the natural frequency when modal stiffness remains approximately constant. The objective is detuning: move the pedestal mode away from the forcing frequency. Added mass does not remove the underlying excitation, so the test must demonstrate a lower response without creating another resonance or unacceptable motion elsewhere.
Use temporary weights whose attachment cannot loosen, shift, or interfere with rotating parts. Add mass incrementally and record each configuration. Keep speed, load, sensor position, direction, analyzer setup, and amplitude convention unchanged. In this installation, added weights reduced the axial reading from 0.9 in/s to 0.3 in/s. That is a threefold reduction, or approximately 67%, and places the trial reading 0.2 in/s below the 0.5 in/s concern threshold.
Do not compare the earlier maximum of 1.4 in/s directly with the 0.3 in/s result unless operating state and measurement setup were identical. Use the paired 0.9 and 0.3 in/s readings for the demonstrated trial effect.
- Record the immediate pre-change spectrum and overall axial velocity.
- Install the first controlled mass increment.
- Return the machine to the same operating condition.
- Repeat the spectrum, overall level, and phase measurements.
- Continue only while the response moves predictably downward and no new high response appears.
Check 6: Expect the same brush-end measurement point to fall from the 0.9 in/s trial baseline toward 0.3 in/s, with the dominant spectral component reduced and no compensating increase at the other pedestal.
Final Configuration and Trending
A temporary weight proves the dynamic mechanism; it is not automatically a permanent correction. Convert the successful trial into an engineered attachment suitable for the pedestal environment. Document mass, location, attachment, clearance, and the final natural-frequency relationship. Review the attachment for static load, dynamic load, fatigue, and retention before continuous service.
Fine-tune mass in small increments while watching both pedestal locations. Excess mass can move the natural frequency into another operating region, and a changed attachment can alter stiffness as well as mass. Repeat the test after any fastener, base, piping, coupling, or compressor work that changes the structural boundary conditions.
Trend overall axial velocity and the amplitude at the identified forcing frequency. Use the same point, direction, operating load, and measurement definition. An overall value below 0.5 in/s can still hide a changing component, so preserve both the spectrum and the scalar trend.
Check 7: Expect the permanent configuration to reproduce approximately 0.3 in/s at the controlled operating point, remain below the 0.5 in/s concern threshold, and leave the opposite pedestal without a new high component.
End-to-End Verification
- Frequency check: Expect the dominant peak to be explicitly classified against calculated twice running speed and calculated twice line frequency.
- Operating-state check: Expect the powered and coastdown records to show when the excitation disappears and how it relates to speed.
- Mechanical check: Expect acceptable alignment, thermal-growth compensation, soft-foot, magnetic-center, and thrust-clearance records.
- Electrical check: If the peak is twice line frequency, expect phase-supply and waveform measurements to identify or clear supply-related excitation; retain the MCE air-gap and rotor-bar result.
- Localization check: Expect the brush-end axial point to dominate while the opposite pedestal stays comparatively low.
- Detuning check: Expect the controlled added-mass test to reproduce the reduction from 0.9 in/s to 0.3 in/s under matched conditions.
- Service check: Expect the final attachment to remain secure and the axial trend to stay below the installation's 0.5 in/s concern threshold without a new spectral peak elsewhere.
Frequently Asked Questions
How do I distinguish twice running speed from twice line frequency?
Calculate 2 × speed (r/min) / 60 and compare it with 2 × supply frequency. For a 60 Hz supply, twice line frequency is 120 Hz; use sufficient spectral resolution to separate close peaks.
How do I tell whether high axial motor vibration is electrical?
Capture a time waveform or spectrum through power removal. A component that disappears while speed is still near its operating value points toward energized or torque-dependent forcing, so compare supply phase quantities, waveform quality, and coupling behavior.
How do I confirm a bearing-pedestal resonance?
Map axial amplitude and phase across the pedestal and adjacent structure, then make a controlled mass change. A predictable frequency or amplitude shift after adding mass confirms that local structural dynamics govern the response.
How do I evaluate the added-mass trial result?
Compare readings taken at the same point, load, speed, and analyzer setup. The controlled trial reduced axial velocity from 0.9 in/s to 0.3 in/s, approximately 67%, without using the separately reported 1.4 in/s maximum as the baseline.
How do I complete the final motor vibration verification?
Repeat the matched-condition spectrum at both pedestals after installing the engineered mass attachment. Expect approximately 0.3 in/s at the brush-end axial point, no new high component at the opposite pedestal, and a stable reading below the 0.5 in/s concern threshold.