Diagnosing Bearing Defects

Devices

Portable balancer & Vibration analyzer Balanset-1A

€1,975.00 + VAT (if applicable)

Parts

Vibration sensor

€90.00 + VAT (if applicable)

Parts

Optical Sensor (Laser Tachometer)

€124.00 + VAT (if applicable)

Devices

Balanset-4

€6,803.00 + VAT (if applicable)

Parts

Magnetic Stand Insize-60-kgf

€46.00 + VAT (if applicable)

Parts

Reflective tape

€10.00 + VAT (if applicable)

Devices

Dynamic balancer “Balanset-1A” OEM

€1,735.00 + VAT (if applicable)

Bearing defects are microscopic or macroscopic flaws — cracks, spalls or pits — on the working surfaces of a rolling-element bearing. Because rolling bearings are fundamental to most rotating machinery and a frequent point of failure, detecting these flaws early is one of the highest-value jobs in vibration analysis. A defect produces a repetitive, periodic impact each time a rolling element rolls over it, and that periodicity is exactly what makes the fault visible in the spectrum long before the bearing overheats or becomes audible.

1. The Nature of Bearing Defects

A typical rolling-element bearing consists of four parts: an outer race, an inner race, a set of balls or rollers, and a cage that keeps the elements evenly spaced. A defect is a flaw on any of these surfaces. As a rolling element passes over it, the contact generates a small, sharp, high-frequency impact — a “click.” A single click carries very little energy, but the impacts recur on every pass, building a strongly periodic signal. Vibration analysis is exceptionally good at picking out this kind of repetitive impacting, which is why a deteriorating bearing can be caught months in advance rather than at the point of seizure.

2. The Four Fundamental Fault Frequencies

The cornerstone of bearing diagnostics is that, for a given bearing geometry and shaft speed, the impacts occur at very specific, predictable rates. These bearing fault frequencies are:

• BPFO (Ball Pass Frequency, Outer race): the rate at which rolling elements pass a single point on the stationary outer race. It is the most commonly observed bearing defect frequency.
• BPFI (Ball Pass Frequency, Inner race): the rate at which elements pass a point on the inner race. Because the inner race rotates with the shaft, BPFI is higher than BPFO.
• BSF (Ball Spin Frequency): the frequency at which a rolling element spins about its own axis. A BSF defect often shows energy at twice this rate, because the flaw strikes both races on each revolution of the element.
• FTF (Fundamental Train Frequency): the rotational frequency of the cage, or “train.” It is a very low frequency, typically less than 0.5X the running speed.

These rates depend on the bearing’s geometry — pitch diameter, rolling-element diameter, contact angle and the number of elements — together with shaft speed. Vibration software usually carries a large bearing database and computes them automatically, and they can be worked out directly with a bearing defect frequency calculator when the bearing part number or dimensions are known.

3. How Bearing Defects Appear in the Spectrum

A developing defect leaves a characteristic pattern in the FFT spectrum:

• High-frequency peaks: the fault frequency itself (for example BPFO) appears as a peak well up the frequency range, away from the low-order rotational peaks.
• Harmonics: the sharp, impulsive nature of the impacts usually generates several harmonics — exact multiples — of the fault frequency, and a long string of them indicates a well-developed flaw.
• Sidebands: this is the critical diagnostic marker. The fault-frequency peak is typically flanked by sidebands spaced at 1X running speed. A BPFO peak with 1X sidebands is a classic outer-race signature, while an inner-race defect (BPFI) almost always carries 1X sidebands because the rotating flaw moves in and out of the bearing’s load zone once per revolution, modulating the impact strength.

In the earliest stages these peaks are small and easily buried in the noise floor of the spectrum, which is why a specialised detection technique is normally applied.

4. Envelope Analysis for Early Detection

Envelope analysis, also called demodulation, is the most powerful method for catching early-stage bearing defects. It is a signal-processing technique that band-pass filters out the low-frequency, high-energy vibration from sources such as unbalance and misalignment, then focuses solely on the high-frequency, low-energy impacts the flaw produces. The repetitive impacts ring the structure’s natural frequencies, and envelope processing extracts the repetition rate of that ringing.

The resulting envelope spectrum is remarkably “clean,” displaying the bearing fault frequencies and their harmonics clearly against a low background. This allows detection months — sometimes years — before the bearing would otherwise fail, providing the lead time that makes planned replacement possible instead of an emergency breakdown.

5. Confirming the Diagnosis in the Field

A robust bearing call rests on matching the measured peaks to the calculated fault frequencies and confirming the expected sideband pattern, ideally backed by an envelope spectrum and a clear upward trend over successive measurements. A portable two-channel instrument such as the Balanset-1A lets an engineer capture the spectrum on the machine in its own bearings at operating speed, so a suspected bearing defect can be checked on site against its predicted frequencies. It is also worth ruling out look-alikes: structural looseness and rolling-element faults can both raise broadband energy, but only a true bearing defect lines up with the BPFO, BPFI, BSF or FTF families.

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