Many technicians and supervisors expect a bearing flagged for replacement in a CBM report to show clear physical degradation—spalling, pitting, discoloration, or wear. When it doesn’t, it can lead to skepticism about the advisory, doubts about the analyst’s judgment, or even reluctance to act on future recommendations.
But here’s the reality: CBM is about detecting failure before it becomes visible.
Every mechanical failure gives us clues before it happens — if we know how to interpret them. The P-F interval (Potential Failure to Functional Failure) represents the time between when a defect can first be detected and when the equipment can no longer perform its intended function.
Let’s take the example of a rolling element bearing:
- Potential Failure (Point 1):
Microscopic Defect Formation (Point 1 on the P-F Curve)- What’s happening:
A microscopic defect begins to form on the raceway or rolling element. This could be a subsurface crack, material fatigue, or a lubrication-related anomaly. - Functionality:
The bearing still operates normally. There are no visible signs of damage, and it may pass visual inspection. - High-frequency acceleration levels begin to rise—often the first detectable sign.
- This is typically picked up in the acceleration domain of vibration analysis.
- Envelope detection or peak acceleration trending may reveal subtle changes.
- Time to Point 2: Variable Progression
- The time from initial defect to functional failure can range from a few hours to several months, depending on:
- Load and speed conditions
- Lubrication quality
- Environmental factors
- Asset criticality
- Technician Actions at Points 1/2
- Even though the bearing appears intact, this is the ideal time to plan a replacement. Recommended actions include:
- Schedule bearing change for next planned shutdown—even if 12+ months away (with flexibility to bring forward if defect progression accelerates)
- Increase monitoring frequency to track defect growth
- Confirm availability of replacement bearings and lip seals
- Document operating conditions and any recent maintenance
- Communicate findings to planners and analysts for review
- What’s happening:
- Progressive Deterioration (Point 2):
As the defect grows, it generates repetitive impacts each time the rolling element passes over it. The acceleration trend continues upward, and distinct bearing fault frequencies start to appear in the vibration spectrum. This is the ideal window for predictive maintenance — the issue is visible, but the asset is still operational. The bearing should be getting changed out, the closer to point 2 in the timescale, the better. - Approaching Functional Failure (Point 3):
The defect expands rapidly. Friction and looseness increase and lubrication effectiveness drops. The bearing begins to degrade mechanically, and velocity readings rise sharply as vibration energy shifts to lower frequencies. The closer the bearing condition gets to point 3, the greater the risk of secondary damage to the asset. At this stage, physical defects will be present when the bearing is removed:- Spalling, pitting, or scoring on raceways
- Cage deformation or discoloration
- Evidence of heat damage or lubricant breakdown
- Functional Failure (Red Hatched Area):
The bearing can no longer support the shaft correctly. This results in severe vibration, often accompanied by noise, heat, and shaft instability. This stage manifests as a rapid rise in overall vibration velocity due to loss of bearing support — signaling imminent failure or collapse.
“WE DO NOT WANT TO BE CHANGING THE BEARING AT THIS STAGE IN THE FAILURE PROGRESSION“