Diagnosing Gear Defects

1. The Vibration Signature of Gears

Gears are a fundamental component for power transmission in industrial machinery. The meshing of gear teeth is an inherently noisy and vibrant process. Healthy gears produce a very clear and stable vibration signature, and deviations from this signature are a strong indicator of wear or damage.

Vibration analysis is a highly effective method for detecting gear defects at a very early stage, long before they can lead to a catastrophic failure of a gearbox.

2. Gear Mesh Frequency (GMF)

The most important frequency in gearbox analysis is the Gear Mesh Frequency (GMF). This is the rate at which the teeth of the gears engage with each other.

GMF = Number of Teeth on a Gear × Rotational Speed of that Gear

In a healthy gearbox, the FFT spectrum will show a clear peak at the GMF, often with a few small harmonics (2xGMF, 3xGMF). The amplitude of this GMF peak is an indicator of the load on the gears. A high GMF peak by itself is not necessarily a sign of a fault, but rather heavy load. The key to diagnosis is looking at the frequencies *around* the GMF peak.

3. Using Sidebands to Diagnose Faults

Sidebands are the most powerful tool for diagnosing specific gear problems. These are small peaks that appear on either side of the GMF and its harmonics. The spacing of the sidebands indicates which shaft is experiencing the problem.

• Worn or Eccentric Gear: If a gear is worn, eccentric, or has a manufacturing flaw, it will modulate the GMF at its rotational speed. This will create sidebands around the GMF peak that are spaced at the running speed (1X) of that specific gear’s shaft. For example, if the GMF peak has sidebands spaced at the input shaft speed, the fault is on the input gear.
• Tooth Wear: General tooth wear is often indicated by a rise in the amplitude of the GMF and its harmonics, accompanied by 1X sidebands from the corresponding gear.
• Cracked or Broken Tooth: A single cracked or broken tooth will generate a strong vibration peak at the 1X running speed of that gear, often with many harmonics. It will also produce sidebands around the GMF spaced at the gear’s running speed. The time waveform is also very useful here, as it will show a distinct, periodic impact each time the broken tooth attempts to mesh.
• Misalignment of Gears: Gear misalignment often shows a high 2X GMF harmonic, sometimes larger than the primary GMF peak. It will also be accompanied by sidebands spaced at the running speed.

4. Specialized Analysis Techniques

Because gear vibration can be complex, specialized techniques are often used:

• Time Waveform Analysis: Essential for detecting broken teeth, which show up as repeating, sharp impacts.
• Cepstrum Analysis: A technique used to more clearly identify families of sidebands that might be difficult to see in a standard FFT spectrum.

5. Stages of Gear Failure

Vibration analysis can track the progression of gear failure through several stages:

1. Stage 1 (Early): Small sidebands appear around the GMF. The overall vibration level may not change.
2. Stage 2 (Moderate): The amplitude of the sidebands increases, and harmonics of the GMF may begin to appear with their own sidebands.
3. Stage 3 (Serious): The GMF and its harmonics may have many large sidebands. The 1X frequency of the problem gear may start to rise. The noise floor of the spectrum increases.
4. Stage 4 (Catastrophic): The GMF may disappear and be replaced by a noisy, random vibration signature as the teeth are severely damaged or destroyed.

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