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Using both vibration analysis and oil analysis provides a much better predictive maintenance (PdM) solution compared to relying solely on vibration or oil analysis alone. This is because mechanical issues, such as soft foot or loose mounting bolts, can be detected through vibration analysis long before wear particles show up in oil analysis.
On the other hand, while vibration analysis is effective in identifying certain markers related to equipment health, it may not capture early lubrication issues that can lead to failure. By incorporating oil analysis alongside vibration analysis, lubrication issues can be detected and addressed long — maybe years — before they escalate into more severe problems detectable by vibration.
In May 2020 at a coal-fired power plant in the Midwest, the oil was changed on a $150,000 conveyor gearbox that was supposed to be filled with an ISO 320 gear oil. However, in July 2020, an oil sample was taken and exhibited a viscosity of 183 centistokes (cSt), 148 parts per million (ppm) of iron, and a Particle Quantifier Index (PQI) of 234.
This was a clear indicator that the wrong lubricant was in the machine and damage was occurring. In October 2020, the viscosity tested at 175 cSt, with 57 ppm of iron and a PQI of 170. In July 2021, the viscosity slightly increased to 178, with 74 ppm of iron and a PQI of 171.
The February 2022 oil sample showed a higher viscosity of 212, partially due to oxidation, 84 ppm of iron, and a PQI of 78. In November of 2022, a noise was detected, and another oil sample was taken. The sample showed a viscosity of 209, an iron content of 39, a PQI of 10, and a copper content of 9 ppm, indicating bearing-cage damage was probably occurring.
Vibration sensors were installed, and an eminent failure was predicted.
These oil sample results highlight the progressive degradation of the equipment over time, with indicators of potential failure appearing in oil analysis two years ahead of the corresponding vibration abnormalities. Had the vibration sensors been installed earlier, they likely would have detected the issue sooner, but not before the damage became irreversible.
By ignoring the oil-analysis data, only 43% of the 10-year design life of the gearbox was realized, resulting in a loss of $85,000 of the usefulness of the gearbox alone. This figure did not even consider the lost production value.
If we can determine the wear rate of a particular component, we can calculate the costs of our current state of lubrication. ISO 281:2007 can give some insight into the relative life of bearings.
This may take considerable effort to determine the correlating wear rates. However, one other source may be the Life Extension Table (LET) that was published by Noria Corporation more than twenty years ago and is still a key resource for most maintenance programs.
Once we have established the wear rate, we can make some financial calculations. Here are three examples:
When elevated levels of impact and non-synchronous peaks are observed in the vibration spectrum, it raises concerns about the equipment's condition. In this case, the analyst noticed impact levels exceeding 10g at the input shaft of the gearbox, near the drive end. To investigate further, the analyst requested an oil analysis sample and a lift check on the input shaft.
To minimize downtime, the oil sample was taken as the first step. The intention was to gather information about the gearbox's health without taking the asset offline. However, the oil analysis report revealed no upward trend in wear metals or gross particle counts, indicating that the gearbox's condition was not deteriorating.
Despite the oil analysis results, the analyst proceeded with a lift check on the input shaft, which required the asset to be taken offline. This check involved measuring the clearance at the bearing. The lift check unveiled a clearance of 0.015 inches — well above the specified limit for the unit. As a result, the customer decided to procure a new gearbox, recognizing the presence of a fault.
These results clearly demonstrate the importance of understanding both vibration and oil analysis data and recognizing that they can sometimes provide conflicting results. It emphasizes the need for a comprehensive approach to fault diagnosis, where reliance on a single diagnostic technique is not recommended. Ultimately, the diagnosis should be driven by the specific fault within the asset and should not rely solely on one diagnostic technique.
A vibration analyst noticed gear mesh harmonics on a recent waveform and asked the plant if a recent oil sample was available to share with the lubrication team. The oil sample report showed that an ISO 220 was the current lubricant and in good health, aside from being a bit dirty.
The oil analyst noticed the temperature sensor on the vibration probe was at 130 degrees (Fahrenheit) which was probably cooler than the actual oil temperature. A quick inspection of the equipment tag showed that an ISO 320 was called for when temperatures were above 125 degrees F.
If only one method of condition monitoring had been employed, the problem could not have been isolated as quickly.
Utilizing vibration and oil analyses together provides you with complementary information to enhance the overall approach to maintenance — ultimately preventing costly breakdowns and improving equipment performance in the long-term. Monetary and time savings impact can be calculated more accurately, enabling cost justifications for an improved lubrication program. The harmonious interplay between vibration and oil analysis in problem diagnosis illustrates how combining both methods enables quicker and more accurate identification of issues and offers the necessary, broader blanket of protection than using only one of the two methods alone.
This article was a featured learning session at the 2023 Reliable Plant & Machinery Lubrication Conference & Exhibition. To learn more about our upcoming 2024 Reliable Plant & Machinery Lubrication Conference & Exhibition, visit the conference website.