An unexpected bearing failure in an Atlas Copco GA series machine is more than an inconvenience; it often means a catastrophic air-end failure, immediate production halts, and significant financial losses. This is a critical aspect of the monitoring of machines. While many maintenance teams own a shock pulse meter, they often misinterpret the data, missing the critical window between an early warning and a complete breakdown. Effective condition monitoring is essential.
This guide provides an expert, field-tested process for using the Shock Pulse Method (SPM) correctly, a key pillar of modern condition monitoring techniques. We will move beyond the basic "green, yellow, red" condition scale and give you the actionable intelligence needed to protect your GA compressors, diagnose issues accurately, and shift from reactive repairs to a predictive maintenance strategy. The Shock Pulse Method is a patented technique for monitoring rolling element bearings.
Foundational Understanding: SPM, Shock Pulse Method and Vibration
To use SPM effectively, you must understand what this measurement technique captures and why it's different from standard vibration analysis. While a vibration sensor, or accelerometer, measures the overall movement of the equipment housing, a shock pulse transducer is a high-frequency specialist. This is a primary difference between shock pulse and vibration.
What SPM Truly Measures
The Shock Pulse Method is engineered to detect the microscopic, high-frequency stress waves generated by impacts within a rolling element bearing. This pressure wave is a result of collisions. Think of it as listening for the "crunch" of a tiny piece of debris being crushed between a rolling element and the raceway at the rolling interface—an event that generates a faint shock pulse signal completely invisible to a standard vibration sensor until significant damage has occurred. These stress waves travel through the metal, and the SPM transducer, which is tuned to its resonance frequency of around 32 kHz, picks them up. The instrument performs signal processing on these electric pulses to provide a clear bearing condition assessment.
Why It's Ideal for GA Compressors (and other Screw Compressors)
The bearings in a GA screw compressor operate under immense and constantly changing loads. SPM provides the earliest possible warning of subsurface fatigue, spall formation, raceway damage, or lubricant film breakdown—long before these issues create enough energy to be seen as a low-frequency vibration. Understanding both vibration and shock is key.
The Key Metrics Explained
Interpreting shock pulse readings depends on understanding three core values. Getting this right is the difference between a good call and a missed failure.
dBi (Initial Value): This is the theoretical "zero point" or baseline for a new, perfectly installed, and lubricated bearing of a specific size where the shaft will rotate at a specific speed. Its purpose is to normalize all subsequent readings, allowing you to compare measurements over time fairly. This absolute value is calculated automatically by the instrument after you input the input data for shaft speed and bearing diameter. The formula is:
dBi=20⋅log(n)+12.5⋅log(d)–67.5
Where n is the shaft speed in RPM and d is the bearing bore diameter in millimeters.
dBc (Carpet Value): This value represents the level of the weaker, more numerous shock pulses. Consider the decibel carpet value your direct indicator of lubrication quality and oil film thickness. A low, stable dBc means a healthy, thick oil film is separating the metal surfaces, preventing metal-to-metal contact. A rising dBc indicates the oil film is thinning, leading to increased friction.
dBm (Maximum Value): This value captures the strongest, peak-level shock pulses. The max value is your direct indicator of physical bearing damage. These high-energy impacts are caused by rolling elements passing over existing damage like pits, spalls, or cracks. The peak amplitude of the shock provides this data.
Delta (dBm - dBc): The difference between these two amplitude levels provides critical diagnostic context, which we will explore in the next section.
Calculation of data not mentioned on the data sheet.
For VSD’s, data for other element speeds can be interpolated to obtain an “indicative” value. Also, for other pressures, interpolation can be used to get an “indicative” value
SPM measurements for VSD compressors
Rpm1, 2: Given motor speed on the data sheet.
Rpm: at which motor speed the dB-value must be calculated, this value should lie between Rpm1 and Rpm2
dBx1,2: the dB-value at the respective motor speed (dBm or dBc)
dBx: the wanted dB-value at a certain motor speed, Rpm.
SPM measurements for VSD compressors for RPM
Pressure1,2: Given pressure on the data sheet, Pressure: at which pressure the dB-value must be calculated, this value should be lying between Pressure1 and Pressure2(*)
Note:
The relationship between speed or pressure and dB values is not linear and therefore using the above equation will only give approximate values. Therefore, it is advised to measure SPM at full load operation as stated before and for VSD’s at full speed. Especially for VSD’s, if customer installation does not allow it to operate at full speed, it is advised to consult us.
Early Warning Signs & Symptoms (Interpreting the Data)
A single shock pulse measurement is a snapshot; a trend is a story. By analyzing how dBm and dBc evolve, you can diagnose the root cause of a problem with high confidence and assess the overall bearing condition.
Real-World Scenarios We See in the Field
Here are the four most common patterns our engineers encounter on Atlas Copco units and what they mean:
Scenario 1: High dBc, Low and Stable dBm
Diagnosis: This is a classic lubrication condition problem. The bearing is "running dry," meaning the lubricant film is compromised, but significant physical damage has not yet occurred.
Causes: Incorrect lubricant viscosity, low oil level, oil contamination (e.g., with water or particles), or a degraded, overdue lubricant.
Action: This is your early warning. Immediately investigate the lubrication system. Take an oil sample, check levels, and confirm the correct lubricant is in use. Resolving this can prevent bearing damage entirely.
Scenario 2: Rising dBm, Low and Stable dBc
Diagnosis: This indicates developing mechanical damage. The lubrication is adequate, but the bearing surfaces themselves are beginning to fail (e.g., micro-spalling, pitting).
Causes: Natural fatigue near the end of the bearing's L10 life, minor installation defects, or contamination causing localized damage.
Action: The bearing is damaged and will not heal. Increase the measurement frequency and begin planning for an air-end replacement or overhaul.
Scenario 3: Both dBm and dBc are High and Rising
Diagnosis: This is a severe condition and a recipe for imminent failure. The bearing is physically damaged, and the lubrication is compromised. The damage creates debris, which further contaminates the oil, accelerating the failure process.
Action: This machine is at high risk of a catastrophic failure. The air-end should be scheduled for replacement as soon as possible to avoid damaged bearings from halting production.
Scenario 4: Spiky, Erratic dBm Readings
Diagnosis: This often points to loose contamination. A piece of debris is intermittently passing through the bearing's load zone, creating a random, high-energy shock. It could also indicate electrical discharge machining (EDM) in units powered by a VFD without proper motor shaft grounding.
Action: Take several readings to confirm the pattern. If it persists, an oil flush and analysis are warranted. For VFD applications, verify the grounding of the motor shaft.
Step-by-Step Diagnostic Process and Measurement Techniques
Consistency is the key to trustworthy SPM data. Inconsistent measurement techniques will produce worthless data. Follow this process rigorously for effective shock pulse monitoring.
Preparation is Key
Achieve Stable State: Always take readings when the unit is at full load and normal operating condition. Data taken on an unloaded or cold machine is not comparable.
VSD Units - Max Speed: For Variable Speed Drive (VSD) units, any vibration measurement or shock pulse reading must be taken at 100% motor speed to be comparable. The rolling velocity is a key factor. If the application prevents running at full speed, choose the highest possible speed that can be reliably reproduced for every future measurement.
Use Adapters: Use permanently installed SPM measurement adapters (studs). Hand-placing the probe introduces variables in pressure and angle that corrupt the data. Ensure the adapter and transducer tip are clean.
Setting the Initial Value (dBi)
Correctly input the GA unit's speed (shaft diameter and rpm) and the drive-end/non-drive-end bearing bore diameter (shaft diameter) into your instrument.
Crucial Note on Errors: If you realize a measurement was taken with the wrong dBi value, the reading is not lost. You can correct it manually on the decibel scale:
Correct dBn = Measured Value + Wrong dBi - Correct dBi
Example: Measured 30 dBn with dBi=10. The correct dBi is 20. The corrected value is 30 + 10 - 20 = 20 dBn.
Taking the Measurement
Firmly connect the shock pulse transducer to the adapter.
Allow the reading to stabilize on the instrument's screen.
Carefully record both the dBm and carpet value.
Log the date, time, running hours, and operating conditions (load %, speed).
Establishing a Baseline
Never trust a single measurement on a new air-end. A new good bearing goes through a running-in period where readings can be temporarily elevated.
For a new or overhauled air-end, establish a reliable baseline by taking readings at approximately 100, 500, and 1,000 operating hours. The average of these stable readings becomes your benchmark.
Applying Alarm Thresholds (The Turbo Airtech Rule of Thumb)
These are general guidelines based on the ISO standard; your baseline is the true reference point. The amplitude of the shock is measured in decibels.
+15dB Above Baseline (Attention): This is a statistically significant change. Increase measurement frequency to every 500 hours or monthly. This is the time to investigate lubrication and perform analysis and shock pulse trend evaluation.
+25dB Above Baseline (Alert): This level strongly indicates developed, likely visible wear on the rolling bearings. Begin the process of planning the air-end overhaul and ordering all necessary parts and seals.
+30 to +35dB Above Baseline (ALARM): Catastrophic failure is a real and imminent possibility. The risk of the air-end seizing, causing damage to the drive motor and gearbox, is extremely high. Schedule an immediate shutdown and replacement.
Common Causes & Prevention Strategies for Bearing Damage
Causes of High dBc (Poor Lubrication Condition)
Root Causes: Incorrect oil (bearing type may require a specific lubricant), oil degradation past its service life, contamination with water/particles, or simply a low oil level causing metal-to-metal contact.
Prevention: The solution is procedural. Implement strict adherence to Copco's specified lubrication type and change intervals. Complement this with a proactive oil analysis program to catch contamination and degradation early.
Causes of High dBm (Bearing Damage)
Root Causes: The primary cause is prolonged poor lubrication. Other major causes include improper bearing installation (excessive preload from an out-of-spec housing), contamination entering the air-end during service, and VSD-induced electrical discharge machining (EDM) that creates microscopic pits in the bearing surfaces.
Prevention: Demand meticulous rebuild procedures from your service provider, ensuring all tolerances and the material stiffness are met. For VFD-driven equipment, ensure proper motor shaft grounding rings are installed and functional to prevent EDM damage to the rolling element bearings.
Nomenclature
· MOBH Male outlet bearing housing.
· FOBH Female outlet bearing housing.
· MIBH Male inlet bearing housing.
· FIBH Female inlet bearing housing.
· OBH Outlet bearing housing.
· MDS Motor drive side
· MNDS Motor non-drive side
Nomenclature for SPM Measurement
Key Takeaways
SPM for GA units is about trending data over time, not reacting to a single shock pulse reading. Always establish a firm baseline for every new air-end.
Remember the core difference: dBc is your lubrication health meter, and dBm is your bearing damage meter.
Consistency is paramount. Always measure at full load and, for VSDs, maximum repeatable speed (RPM).
A +15dB increase from your established baseline is the first signal to take action. A reading +30dB above baseline means the bearing is on borrowed time.
Properly interpreting SPM data transforms your maintenance strategy from a reactive, costly fire-drill into a predictive, planned, and cost-saving operation.
The Turbo Airtech Advantage
Interpreting SPM data has nuances, especially in equipment with variable loads, VSDs, or recurring issues that defy simple analysis. If your readings are ambiguous, if alarm levels are reached unexpectedly, or if you are fighting repetitive bearing failures across your Atlas fleet, our experts can provide a deeper, data-driven condition assessment.
We look beyond the numbers to diagnose the true root cause—whether it’s operational stress, a systemic lubrication issue, or a subtle installation error. Contact Turbo Airtech for an expert consultation to fortify your SPM program and protect your critical assets.
Disclaimer: Turbo Airtech is an independent, OEM-neutral parts and service provider. The content provided is for educational and informational purposes, leveraging our deep experience with the equipment mentioned. All brand names, including Atlas Copco and SPM Instrument, are the trademarks of their respective owners and are used for reference purposes only.
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