Vibration analysis is a key technique for diagnosing the technical condition of machines. Different machine faults produce characteristic patterns in the vibration frequency spectrum. By examining the frequency spectrum of machine vibrations (typically via FFT analysis), one can identify specific defect types. Below, common defect categories (imbalance, misalignment, looseness, bearing defects, gear faults) are presented in tables. Each table outlines sub-types of the fault, describing their typical vibration spectrum, the spectral components observed, key identifying features, and an illustrative spectrum graph (embedded as SVG). All frequency references use multiples of the running speed (e.g., “1×” = once-per-revolution frequency).
Imbalance
| Defect Type | Spectrum Description | Brief Description of Spectral Components | Key Feature | SVG Graph |
|---|---|---|---|---|
| Static Imbalance (One-plane) | Spectrum is dominated by a single peak at the fundamental running speed (1× RPM):. The vibration is sinusoidal, with minimal energy at other frequencies. | Primarily a strong 1× rotational frequency component. Little to no higher harmonics (a pure 1× tone):. | Large 1× amplitude in all radial directions: vibration at both bearings is in phase (no phase difference between the two ends):. Approximately 90° phase shift is often observed between horizontal and vertical measurements at the same bearing:. | |
| Dynamic Imbalance (Two-plane/Couple) | Spectrum also shows a dominant once-per-revolution frequency (1×) peak:, similar to static imbalance. Vibration is at the rotation speed, with no significant higher-frequency content if imbalance is the only issue. | Dominant 1× RPM component (often with a “sway” or wobble of the rotor):. Higher harmonics are generally absent unless other faults are present. | 1× vibration at each bearing is out of phase – there is about a 180° phase difference between vibration at the two ends of the rotor: (indicating a couple imbalance). The strong 1× peak with this phase relationship is the signature of dynamic imbalance. |
Misalignment
| Defect Type | Spectrum Description | Brief Description of Spectral Components | Key Feature | SVG Graph |
|---|---|---|---|---|
| Parallel Misalignment (Offset Shafts) | The vibration spectrum exhibits elevated energy at the fundamental (1×) and its harmonics 2× and 3× running speed, especially in the radial direction. Typically, the 1× component is dominant with misalignment present, accompanied by a notable 2× component. | Contains significant peaks at 1×, 2×, and 3× shaft rotational frequencies. These appear predominantly in radial vibration measurements (perpendicular to shaft):. | High 1× and 2× vibration in the radial direction are indicative. A 180° phase difference between radial vibration measurements on opposite sides of the coupling is often observed:, distinguishing it from pure imbalance. | |
| Angular Misalignment (Inclined Shafts) | The frequency spectrum shows strong harmonics of the shaft speed, notably a prominent 2× running speed component in addition to the 1×: Vibration at 1×, 2× (and often 3×) appears, with axial (along-shaft) vibration being significant. | Notable peaks at 1× and 2× (and sometimes 3×) of running speed:The 2× component is often as large as or larger than the 1×. These frequencies are pronounced in the axial vibration spectrum (along the machine’s axis): | Relatively high second harmonic (2×) amplitude compared to 1×, combined with strong axial vibration. Axial measurements on either side of the coupling are 180° out of phase, a hallmark of angular misalignment. |
Looseness
| Defect Type | Spectrum Description | Brief Description of Spectral Components | Key Feature | SVG Graph |
|---|---|---|---|---|
| Mechanical Looseness (Component Looseness) | The spectrum is rich in harmonics of the running speed. A wide range of integer multiples of 1× (from 1× up to high orders such as ~10×) appear with significant amplitudes. In advanced cases, sub-harmonic frequencies (e.g. 0.5×) may also emerge | Multiple running speed harmonics dominate (1×, 2×, 3× … up to around 10×). Occasionally, fractional (half-order) frequency components at 1/2×, 3/2×, etc., can be present due to repetitive impacting. | A distinctive “harmonic series” of peaks in the spectrum – numerous equally spaced peaks at integer multiples of the rotation frequency. This indicates loose or improperly fitted parts causing repeated impacts. The presence of many harmonics (and possibly half-order sub-harmonics) is a key signature. | |
| Structural Looseness (Base/Mounting Looseness) | The vibration spectrum is often dominated by one or two times the running speed. Commonly, a peak at 1× RPM and/or a peak at 2× RPM will appear in the spectrum. Higher harmonics beyond 2× are usually much lower in amplitude compared to these fundamentals. | Primarily shows frequency components at 1× and 2× the shaft speed. Other harmonics (3×, 4×, etc.) are generally absent or minor. The 1× or 2× component may predominate depending on the nature of the looseness (e.g., one impact per revolution or two impacts per revolution). | A notably high 1× or 2× peak (or both) relative to the rest of the spectrum, indicating looseness of supports or structure. Often the vibration is stronger in the vertical direction if the machine is loosely mounted. One or two dominant low-order peaks with few higher harmonics is characteristic of structural or foundation looseness. |
Bearing Defects
| Defect Type | Spectrum Description | Brief Description of Spectral Components | Key Feature | SVG Graph |
|---|---|---|---|---|
| Outer Race Defect | The vibration spectrum exhibits a series of peaks corresponding to the outer race defect frequency and its harmonics. These peaks are usually at higher frequencies (not integer multiples of shaft rotation) and indicate each time a rolling element passes over the outer race flaw. | Multiple harmonics of the outer race ball-pass frequency (BPFO) are present. Typically, 8–10 harmonics of BPFO can be observed in the spectrum for a pronounced outer race fault. The spacing between these peaks is equal to the BPFO (a characteristic frequency determined by bearing geometry and speed). | A distinct train of peaks at the BPFO and its successive harmonics is the signature. The presence of numerous evenly spaced high-frequency peaks (BPFO, 2×BPFO, 3×BPFO, …) clearly points to an outer race bearing defect. | |
| Inner Race Defect | The spectrum for an inner race fault shows several prominent peaks at the inner race fault frequency and its harmonics. In addition, each of these fault frequency peaks is typically accompanied by sideband peaks spaced at the running speed (1×) frequency. | Contains multiple harmonics of the inner race ball-pass frequency (BPFI), often on the order of 8–10 harmonics. Characteristically, these BPFI peaks are modulated by sidebands at ±1× RPM – meaning beside each BPFI harmonic, smaller side peaks appear, separated from the main peak by an amount equal to the shaft rotation frequency. | The telltale sign is the presence of the inner race defect frequency (BPFI) harmonics with a sideband pattern. The sidebands spaced at the shaft speed around the BPFI harmonics indicate that the inner race defect is being loaded once per revolution, confirming an inner race issue rather than outer race. | |
| Rolling Element Defect (Ball/Roller) | A rolling element (ball or roller) defect produces vibration at the rolling element spin frequency and its harmonics. The spectrum will show a series of peaks that are not integer multiples of shaft speed, but rather multiples of the ball/roller spin frequency (BSF). One of these harmonic peaks is often significantly larger than the others, reflecting how many rolling elements are damaged. | Peaks at the fundamental rolling element defect frequency (BSF) and its harmonics. For example, BSF, 2×BSF, 3×BSF, etc., will appear. Notably, the amplitude pattern of these peaks can indicate the number of damaged elements – e.g. if the second harmonic is largest, it might suggest two balls/rollers have spalls. Often, some vibration at the race fault frequencies accompanies this, as rolling element damage commonly leads to race damage as well. | The presence of a series of peaks spaced by the BSF (bearing element spin frequency) rather than by the shaft rotation frequency identifies a rolling element defect. A particularly high amplitude of the Nth harmonic of BSF often implies N elements are damaged (e.g., a very high 2×BSF peak might indicate two balls with defects). | |
| Cage Defect (Bearing Cage / FTF) | A cage (separator) defect in a rolling bearing yields vibration at the cage rotational frequency – the Fundamental Train Frequency (FTF) – and its harmonics. These frequencies are usually sub-synchronous (below the shaft speed). The spectrum will show peaks at FTF, 2×FTF, 3×FTF, etc., and often some interaction with other bearing frequencies due to modulation. | Low-frequency peaks corresponding to the cage’s rotational frequency (FTF) and integer multiples of it. For instance, if FTF ≈ 0.4× shaft speed, you may see peaks at ~0.4×, ~0.8×, ~1.2× etc. In many cases, a cage defect coexists with race defects, so the FTF may modulate race defect signals, producing sum/difference frequencies (sidebands around race frequencies). | One or more sub-harmonic peaks (below 1×) that align with the bearing cage rotation rate (FTF) are indicative of a cage problem. This often appears alongside other bearing fault indications. The key signature is the presence of FTF and its harmonics in the spectrum, which is otherwise uncommon unless the cage is failing. |
Gear Faults
| Defect Type | Spectrum Description | Brief Description of Spectral Components | Key Feature | SVG Graph |
|---|---|---|---|---|
| Gear Eccentricity / Bent Shaft | This fault causes modulation of the gear mesh vibration. In the spectrum, the gear mesh frequency (GMF) peak is surrounded by sideband peaks spaced at the gear’s shaft rotational frequency (1× gear RPM). Often, the gear’s own 1× running speed vibration is also elevated due to the imbalance-like effect of eccentricity. | Notable increase in amplitude at the gear mesh frequency and its lower harmonics (e.g., 1×, 2×, 3× GMF). Clear sidebands appear around the GMF (and sometimes around its harmonics) at intervals equal to 1× the rotation rate of the affected gear. The presence of these sidebands indicates amplitude modulation of the mesh frequency by the gear’s rotation. | Gear mesh frequency with pronounced sidebands at 1× gear frequency is the signature feature. This sideband pattern (peaks equally spaced around GMF by the running speed) strongly indicates gear eccentricity or a bent gear shaft. Additionally, the gear’s fundamental (1×) vibration may be higher than normal. | |
| Gear Tooth Wear or Damage | Gear tooth faults (such as worn or broken teeth) produce an increase in vibration at the gear mesh frequency and its harmonics. The spectrum often shows multiple GMF peaks (1×GMF, 2×GMF, etc.) of high amplitude. Additionally, numerous sideband frequencies appear around these GMF peaks, spaced by the shaft rotational frequency. In some cases, the excitation of gear natural frequencies (resonances) with sidebands can also be observed. | Elevated peaks at the gear mesh frequency (tooth-meshing frequency) and its harmonics (for example, 2×GMF). Around each major GMF harmonic, there are sideband peaks separated by 1× running speed. The number and size of sidebands around the 1×, 2×, 3× GMF components tend to increase with the severity of tooth damage. In severe cases, additional peaks corresponding to the gear’s resonance frequencies (with their own sidebands) may appear. | Multiple high-amplitude gear mesh frequency harmonics accompanied by dense sideband patterns are the hallmark. This indicates irregular tooth passing due to wear or a broken tooth. A heavily worn or damaged gear will show extensive sidebands (at 1× gear speed intervals) around the mesh frequency peaks, distinguishing it from a healthy gear (which would have a cleaner spectrum concentrated at GMF). |
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