Preventing Equipment Failures with CBM Tools: The Two-Out-of-Three Rule

Preventing Equipment Failures with CBM Tools: The Two-Out-of-Three Rule

Reliability engineers are tasked with designing and sustaining robust predictive and preventive maintenance programs that cut unscheduled downtime, boost production, and elevate overall equipment reliability. Understanding how assets perform—and being able to predict and halt failures before they occur—is critical. By combining advanced monitoring technologies, engineers can present decisive evidence to management when an asset shows signs of impending failure.

Combining Oil, Vibration, and Thermographic Analysis

Integrating oil analysis, vibration analysis, and infrared thermography—often called the “two out of three technology rule”—provides a confidence level of 95 % or higher that an asset truly needs to be taken out of service for maintenance. This approach, known in reliability engineering as the Bomb Report, highlights correlated anomalies across two or more technologies, making it a powerful tool for executive decision‑making.

Key Payback Technologies

1. Oil Analysis
2. Vibration Analysis
3. Infrared Thermography

These three methods are essential for a comprehensive condition assessment. A well‑designed monitoring program integrates them to improve performance, reduce failures, and mitigate downtime.

Technology-Specific Problem Detection

Vibration Analysis

Vibration monitoring captures waveform trends and pattern recognition to detect faults such as:

1. Bearing failure or misalignment (fan, pump, motor)
2. Coupling lock‑up or failure
3. Unbalance (fan or motor)
4. Resonances (belt frequency, sheave/pulley alignment)
5. Lubrication issues
6. Electrical faults
7. Structural problems
8. Belts
9. Gears

Oil Analysis

Oil testing evaluates fluid properties, contaminants, and wear debris—akin to a “blood test” for machinery. Industry data indicates that over 70 % of failures are lubricant related. Oil analysis can detect both fluid and machine defects early, serving as the first line of defense. Typical reports include:

1. Presence of foreign fluids or destructive surface contaminants
2. Overall fluid physical and chemical condition
3. Type, amount, and morphology of machine wear materials

Infrared Thermography

Thermography identifies heat patterns and temperature differences, revealing hot spots caused by electrical resistance or excess friction. It is valuable for assessing moisture, thickness, bonding, capacitance, and friction, and is particularly useful for detecting heat loss in mechanical equipment. Common anomaly categories include:

1. Motors
2. Bearings
3. Couplings
4. Belts and sheaves
5. Steam traps
6. Heat exchangers
7. Electrical components (most frequent use)

Strengths and Weaknesses of Vibration + Oil Analysis

Technology

Correlative Method

Indication

When Used

Vibration	Time sequence	Wear particle build‑up precedes significant vibration increase in most instances	Routinely (monthly)
Thermal analysis	Time coincident	With major wear particle production (near end of bearing life); occurs as the bearings fail	When bearing degradation is suspected
Advance filtration/debris analysis	Time sequence / coincident	Major bearing damage has occurred when significant amounts of material appear in the lubricating system filters	Routinely with each filter cleaning or change

Correlation of Lubricant Wear and Vibration/Thermography

The Bomb Report

The Bomb Report is an executive‑level summary that highlights:

1. Asset location
2. Date of confirmed problem
3. Nature of the issue
4. Required corrective actions
5. Short‑ and long‑term repair options
6. Issued work orders
7. Predictive/preventive technologies used to detect the problem
8. All supporting reliability data

Presenting a Bomb Report grabs management’s attention and drives the urgency of corrective action. Relying on a single data point—such as a marginal oil spike—can lead to unnecessary downtime. The confidence threshold of 95 % or higher is achieved when two or more technologies corroborate an anomaly.

Reliability engineers must be versed in multiple monitoring methods to appreciate each technique’s strengths and limitations. Choosing the right combination—oil with vibration, or vibration with thermography—depends on the specific situation and the information each tool provides.

Example of Correlation Between Technologies

Two Out of Three Technology Rule of Thumb

1. At least one technology is used on every major piece of equipment (700 currently monitored)
2. If an anomaly is observed, confirm it with another technology
3. Two technologies confirming a production concern triggers corrective action
4. Issue work orders and track follow‑up
5. Trend results to verify PM effectiveness (MTTR, MTBF, statistical analysis)
6. No two machines are alike; treat each uniquely
7. Document successes to build business cases for upgrades (EVA, cost benefit, payback, NPV, executive summary)

Conclusion

Correlating multiple monitoring technologies dramatically improves diagnostic accuracy and enables maintenance teams to preempt outages. By following the “two out of three” rule, reliability engineers can confidently decide when to rebuild, replace, or perform targeted maintenance, ensuring sustained equipment reliability and operational excellence.