Ted Vasiliw / Wind Aftermarket Technical Manager / Castrol Industrial / www.castrol.com
A quality used-oil-analysis program is an excellent method to determine the condition of the oil within the gearbox. We all know the importance of a lubricant’s ability to reduce friction and wear in gearbox components, thereby improving the reliability of the entire wind turbine. However, most drivetrain failures are not lubrication related. Oil analysis can also be useful in determining the condition, or health of the gearbox itself. Using oil analysis to determine wear conditions in the gears or bearings, corrective actions can be taken to prevent premature failures. The different analysis tools include spectrographic analysis, PQ Index, and wear particle analysis. Each tells a little something different.
Wear metals in oil analysis
Author Ted Vasiliw
These magnified particles are generated by normal gear-rubbing wear as seen using Ferrography.
Spectrometric analysis methods can determine the elemental content of oil samples, including wear debris. Typical wear metals reported may include Iron (Fe), Chromium (Cr), Nickel (Ni), Aluminum (Al), Lead (Pb), and Copper (Cu).
Spectrographic Analysis shows typical wear metals in an oil analysis report. Fe has higher counts because most everything in a gearbox is made of steel. The values are in parts per million (PPM).
Wind turbine gear-oil analysis usually requires close monitoring of iron and copper because these metals are most common. Sources of iron include bearings, shafts, and gears while copper wear usually originates from bronze alloy bearing cages.
When wear metal levels approach or exceed established limits, additional actions are required. Such actions can range from inspecting the gearbox and filter, to re-sampling to verify the results. One sample report provides limited data, so the trending of historical data is best practice for a more complete picture. Typical wind turbine gearbox wear metal limits for used oil are <70 ppm for Fe and <55 ppm for Cu (with brass bearing cages).
PQ Index
Wear metal particles detected by spectroscopy are typically less than 5 to 10 microns in size. These small particles can be generated by rubbing wear or false brinelling (fretting corrosion). Larger particles are generated by more severe wear modes such as fatigue wear, pitting and spalling, contamination wear, and indentations. These larger ferrous particles present in the used oil sample can be detected by using the Particle Quantifying, or PQ method. PQ exposes the sample to a magnetic field and is reported as a “PQ Wear Index.” The PQ Index gives us a better understanding of any wear that may be occurring in the gearbox.
The PQ Index is an arbitrary number which can be compared to the Fe ppm from the Spectro Analysis. If the PQ Index is smaller than the Fe ppm, then it is unlikely that particles larger than 5 microns are present. Alternately, if the PQ Index increases significantly while the Fe ppm remains consistent, then larger ferrous particles are being generated and further diagnostics should be performed.
Wear particle analysis – Ferrography
Direct-Reading (DR) Ferrography gives a direct measure of wear metals. It also separates large and small ferrous particles. DR is useful for trending and identifying when accelerated, abnormal wear is occurring.
Analytical Ferrography visually examines ferrous and non-ferrous solid debris present in the oil sample. Under magnification, particle size, shape, color, and surface texture are observed and categorized. Particle composition is broken into categories including: ferrous wear, white non-ferrous, copper, and fibers. Further, ferrous particles can be identified as steel, cast iron, dark oxides, or red oxides (rust). A skilled analyst can also determine if metallic wear particles are caused by cutting wear versus rolling or sliding wear. WPE
Filed Under: Gearboxes, News, O&M
