Understanding the Shock Pulse Method (SPM)

1. Definition: What is the Shock Pulse Method?

The Shock Pulse Method (SPM) is a specialized and proprietary condition monitoring technique used primarily for detecting and analyzing the condition of rolling-element bearings. It is a type of vibration analysis but differs from traditional spectral analysis in its methodology.

SPM focuses on measuring the high-frequency shock waves or stress waves that are generated when a rolling element in a bearing passes over a flaw (like a spall or a crack). A healthy bearing produces a clean, quiet shock pulse pattern, while a damaged bearing produces strong, distinct shock pulses that can be easily detected.

2. How SPM Works

The core of the SPM technique is a specialized accelerometer and measurement methodology:

1. Tuned Accelerometer: SPM uses a specialized accelerometer that is tuned to resonate at a very high frequency (typically around 32 kHz). This resonance acts as a mechanical amplifier, making the sensor extremely sensitive to the high-frequency, low-energy impacts generated by bearing defects.
2. Shock Pulse Detection: The instrument measures the transient shock waves generated by impacts. It is specifically designed to respond to the pressure wave of the impact itself, not the resulting vibration.
3. Signal Processing: The raw signal is processed to produce two key values:
   • Carpet Value (dBc): This represents the background level of shock pulses. It is an indicator of the overall lubrication condition. A high carpet value suggests poor lubrication, causing a continuous, rough rolling contact.

   – Maximum Value (dBm): This represents the highest shock pulse detected during the measurement period. A high maximum value is a clear indicator of a physical defect, such as a spall or crack.

4. Data Normalization: A critical part of the SPM method is that the raw decibel values are normalized based on the bearing’s size (shaft diameter) and rotational speed. This allows the system to provide a simple, color-coded condition assessment (Green, Yellow, Red) that is easy to interpret.

3. SPM vs. Envelope Analysis

SPM is conceptually similar to Envelope Analysis (or demodulation), which is another common technique for detecting bearing faults. Both methods are designed to extract the repetitive, low-energy impacts of bearing defects from the noisy background vibration of the machine.

• SPM: Uses a resonant sensor to mechanically amplify the signal and focuses on the amplitude of the shock waves (dBc/dBm).
• Envelope Analysis: Uses a standard accelerometer and applies a digital band-pass filter to the signal. It then analyzes the frequency spectrum of the enveloped signal to identify the specific bearing fault frequencies (BPFO, BPFI, etc.).

Both are highly effective techniques. Envelope analysis can often provide a more detailed diagnosis (e.g., distinguishing an inner race from an outer race fault), while SPM is often praised for its simplicity, repeatability, and effectiveness in detecting lubrication issues.

4. Applications

SPM is a powerful tool for any predictive maintenance program, particularly for:

• Early Bearing Fault Detection: It can detect bearing defects at a very early stage, providing ample time to plan and schedule a replacement.
• Condition-Based Lubrication: By monitoring the “carpet value,” technicians can be alerted when a bearing requires grease and can confirm that the lubrication was effective.
• Slow-Speed Machinery: Because it is so sensitive to impacts, SPM can be very effective for monitoring the condition of very slow-speed bearings, which can be a challenge for traditional vibration analysis.

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