The following sections describe each monitored fault mode, its appearance in InsightCM data views, and the recommended response.
Description
Imbalance occurs when the mass distribution of a rotating component is not symmetric about its rotational axis, causing a centrifugal force that loads the bearings once per revolution. In the vibration spectrum, imbalance is characterized by a dominant peak at 1× running speed. On the time waveform it produces a clean sinusoidal signal at running speed. In the full spectrum, imbalance shows as a dominant forward-direction component at 1×. Vibration amplitude increases with the square of running speed, so imbalance becomes more significant at higher speeds.
Recommended Action
Trend the 1× amplitude and phase over successive acquisitions. A rising 1× amplitude not accompanied by changes at harmonics or other frequencies points to increasing imbalance—which can be caused by material buildup, erosion, or a missing balance weight. If the 1× amplitude increases suddenly rather than gradually, check for loss of a balance weight or mechanical damage to the rotor. Plan a balance correction at the next available outage.
Description
Misalignment occurs when the centerlines of coupled shafts are not collinear—either offset (parallel misalignment), angularly misaligned, or both. The classic vibration signature is an elevated 2× running speed component, often comparable to or larger than the 1× component, frequently accompanied by elevated 1×. Angular misalignment tends to produce strong axial vibration at 1× and 2×; parallel misalignment tends to produce radial vibration primarily at 2×. A 180-degree phase difference across the coupling in the axial direction is a strong indicator of angular misalignment.
Recommended Action
Trend the 1× and 2× amplitudes and their ratio over time. A rising 2× component, particularly if axial vibration is also elevated, warrants a coupling and alignment inspection at the next maintenance opportunity. Verify alignment using laser or dial-indicator methods. Inspect the coupling for wear, cracking, or element failure. Correlate with recent maintenance work—misalignment often appears or worsens after equipment is reassembled or a baseplate is disturbed.
Description
Rolling-element bearing defects—spalling or fatigue damage to the outer race, inner race, rolling elements, or cage—generate repetitive impacts at characteristic frequencies: BPFO (ball pass frequency, outer race), BPFI (ball pass frequency, inner race), BSF (ball spin frequency), and FTF (fundamental train frequency). Early-stage bearing damage is best detected in the envelope spectrum, where these frequencies appear as discrete peaks before the defect is severe enough to show in the standard spectrum.
Recommended Action
Calculate bearing defect frequencies for each monitored bearing using bearing geometry data and current running speed, and enter these into InsightCM to enable feature tracking. Monitor the envelope spectrum and trend the amplitude at each defect frequency. A rising trend at BPFO or BPFI, particularly with sidebands spaced at running speed, indicates a progressing defect. Corroborate with bearing temperature data—a rising temperature alongside growing defect frequency amplitude indicates accelerating degradation and should shorten the replacement planning horizon.
Description
Mechanical looseness encompasses conditions in which components that should be rigidly connected have excessive clearance, including loose foundation bolts, loose bearing housing fits, loose rotor components, and loose or cracked structural elements. The classic signature is a series of running-speed harmonics (1×, 2×, 3×, and higher) with a relatively elevated noise floor. Sub-harmonic content at ½× or ⅓× running speed is also characteristic of certain looseness conditions. The time waveform often shows a clipped or truncated appearance.
Recommended Action
Inspect and re-torque all foundation bolts, hold-down hardware, and bearing housing fasteners. Check bearing housing fits for fretting corrosion or wear. Verify structural connections—grouting, base plate, sole plates—are intact. Re-evaluate the spectrum after tightening to confirm harmonic content has subsided. Looseness that persists after tightening may indicate a structural crack or worn bearing housing requiring repair.
Description
Gear faults, including tooth wear, pitting, cracking, and eccentricity, produce vibration at gear mesh frequency (GMF), which is the product of the number of teeth on a gear and its rotational speed. Healthy gears produce a peak at GMF with small sidebands. As gear condition degrades, the GMF amplitude increases and sideband structure becomes more complex. Eccentricity specifically produces prominent sidebands at GMF ± 1× shaft speed of the eccentric gear.
Recommended Action
Calculate the gear mesh frequency for each gear stage and configure these as reference frequencies in InsightCM. Trend the GMF amplitude and its sidebands over time. A growing GMF amplitude or expanding sideband structure indicates degrading gear condition. If GMF amplitude increases suddenly, inspect for lubrication issues, contamination, or overloading. Oil debris analysis is a valuable complementary tool for confirming gear wear before opening the gearbox.
Description
Cavitation occurs in pumps and hydraulic machinery when local fluid pressure drops below vapor pressure, causing vapor bubbles to form and collapse violently. In the vibration spectrum, cavitation appears as a broad elevation of the noise floor at high frequencies, often without distinct discrete peaks, yet blade pass frequencies may be present. Cavitation is distinguished from other high-frequency sources by its strong correlation with process conditions: it intensifies when suction pressure decreases, flow rate moves away from the pump’s best efficiency point (BEP), or NPSH margin is reduced.
Recommended Action
Confirm the diagnosis by correlating vibration data with process parameters—suction pressure, flow rate, and NPSH available. Address the root cause at the process level: increase suction pressure, reduce flow restriction on the suction side, bring the operating point closer to BEP, or reduce fluid temperature to increase NPSH margin. Cavitation causes rapid erosion of impeller surfaces and should be corrected promptly. If process conditions cannot be changed, inspect the impeller for pitting damage at the next opportunity.
Description
Resonance occurs when an excitation frequency—such as running speed, a harmonic, or a blade/vane pass frequency—coincides with a natural frequency of the machine or supporting structure, amplifying the vibration response. In the spectrum, resonance appears as an abnormally high amplitude at or near a structural natural frequency; the peak does not shift with speed and is often broad. On a Bode plot, resonance appears as a distinct amplitude peak accompanied by a phase shift of approximately 180 degrees as the machine passes through the critical speed.
Recommended Action
Use a Bode plot or variable-speed sweep to determine whether the high-amplitude frequency shifts with speed (suggesting a running-speed harmonic) or remains fixed (suggesting a structural natural frequency). If resonance is confirmed, options include changing the excitation frequency, changing the natural frequency of the structure (by adding mass or stiffness), or adding damping. Consult a structural dynamics or rotor dynamics specialist before making structural modifications. As an interim measure, avoid operating continuously at a speed where excitation and natural frequency coincide.