Analytical ferrography is frequently excluded from oil analysis programs because of its comparatively high price and a general misunderstanding of its value. The test procedure is lengthy and requires the skill of a trained analyst. As such, there are significant costs in performing analytical ferrography not present in other oil analysis tests.
But, if time is taken to fully understand what analytical ferrography uncovers, most agree that the benefits significantly outweigh the costs and elect to automatically incorporate it when abnormal wear is encountered.
Principle
To perform analytical ferrography, the solid debris suspended in a lubricant is separated and systematically deposited onto a glass slide. The slide is examined under a microscope to distinguish particle size, concentration, composition, morphology and surface condition of the ferrous and non-ferrous wear particles.
This detailed examination uncovers the mystery behind an abnormal wear condition by pinpointing component wear and how it was generated.
Ferrogram
Analytical ferrography begins with the magnetic separation of machine wear debris from the lubricating oil in which it is suspended. This is accomplished using a ferrogram slide maker. The lubricating oil sample is diluted for improved particle precipitation and adhesion. The diluted sample flows down a specially designed glass slide called a ferrogram. The ferrogram rests on a magnetic cylinder which attracts ferrous particles out of the oil.
Due to the magnetic field, the ferrous particles align themselves in chains along the length of the slide with the largest particles being deposited at the entry point. Nonferrous particles and contaminants, unaffected by the magnetic field, travel downstream and are randomly deposited across the length of the slide. The deposited ferrous particles serve as a dike in the removal of non-ferrous particles. The absence of ferrous particles substantially reduces the effectiveness with which nonferrous particles are removed.
After the particles are deposited on the ferrogram, a wash is used to remove any remaining lubricant. The wash quickly evaporates and the particles are permanently attached to the slide. The ferrogram is now ready for optical examination using a bichromatic microscope.
Particle Identification
The ferrogram is examined under a polarized bichromatic microscope equipped with a digital camera. The microscope uses both reflected and transmitted light to distinguish the size, shape, composition and surface condition of ferrous and nonferrous particles. The particles are classified to determine the type of wear and its source.
Particle composition is first broken down to six categories: white nonferrous, copper, babbitt, contaminants, fibers and ferrous wear. In order to aid the identification of composition, the analyst heat treats the slide for two minutes at 600ºF.
- White non-ferrous particles , often aluminum or chromium, appear as bright white particles both before and after heat treatment of the slide. They are deposited randomly across the slide surface with larger particles getting collected against the chains of ferrous particles. The chains of ferrous particles typically act as a filter, collecting contaminants, copper particles and babbitt.
- Copper particles usually appear as bright yellow particles both before and after heat treatment but the surface may change to verdigris after heat treatment. These also will be randomly deposited across the slide surface with larger particles resting at the entry point of the slide and gradually getting smaller towards the exit point of the slide.
- Babbitt particles consisting of tin and lead, appear gray, sometimes with speckling before the heat treatment. After heat treatment of the slide, these particles still appear mostly gray, but with spots of blue and red on the mottled surface of the object. Also, after heat treatment these particles tend to decrease in size. Again, these nonferrous particles appear randomly on the slide, not in chains with ferrous particles.
- Contaminants are usually dirt and other particulates which do not change in appearance after heat treatment. They can appear as white crystals and are easily identified by the transmitted light source as they are somewhat transparent. Contaminants appear randomly on the slide and are commonly collected by the chains of ferrous particles.
- Fibers , typically from filters or outside contamination, are long strings that allow the transmitted light to shine through. They can appear in a variety of colors and usually do not change in appearance after heat treatment. Sometimes these particles can collect other particles on the ferrogram.
Ferrous particles can be broken down to five different categories, high alloy, low alloy, dark metallic oxides, cast iron and red oxides. Large ferrous particles will be deposited on the entry end of the slide and often clump on top of the other. Ferrous particles are identified using the reflected light source on the microscope. Transmitted light will be totally blocked by the particle.
- High Alloy Steel - particles are found in chains on the slide and appear gray-white before and after heat treatment. The distinguishing factor in the identification between high alloy and white nonferrous is position on the slide. If it is white and appears in a chain, it’s deemed to be high alloy, otherwise, it’s considered white non-ferrous
- Low Alloy Steel - particles are also found in chains and appear gray-white before heat treatment but then change color after heat treatment. After heat treatment they usually appear as blue particles but can also be pink or red.
- Dark Metallic Oxides - deposit in chains and appear dark gray to black both before and after heat treatment. The degree of darkness is indicative of the amount of oxidation.
- Cast Iron - particles appear gray before heat treatment and a straw yellow after the heat treatment. They are incorporated in chains amongst the other ferrous particles.
- Red Oxides (Rust) – can deposit in chains and appear to be sometimes transparent red particles. Sometimes they can be found in chains with the other ferrous particles and sometimes they are randomly deposited on the slide surface. A large amount of small red oxides on the exit end of the slide is generally considered to be a sign of corrosive wear.
After classifying the composition of particles the analyst then rates the size of the particles using a micrometer scale on the microscope. Particles with a size of 30 microns or greater are given the rating of “severe" or “abnormal." Severe wear is a definite sign of abnormal running conditions with the equipment being studied.
Often, the shape of a particle is another important clue to the origin of the wear particles. Is the particle laminar or rough? Laminar particles are signs of smashing or rolling found in bearings or areas with high pressure or lateral contact. Does the particle have striations on the surface? Striations are a sign of sliding wear. Perhaps generated in an area where scraping of metal surfaces occurs. Does the particle have a curved shape, similar to drill shavings? This would be categorized as cutting wear. Cutting wear can be caused by abrasive contaminants found in the machine. Is the particle spherical in shape? Spheres are generated in bearing fatigue cracks. An increase in quantity is indicative of spalling.
By analyzing the size, shape, color, magnetism, light effects and surface detail of wear particles, a skilled analyst can paint a picture about the nature, severity and root cause of abnormal wear. This information enables the maintenance team to implement effective corrective action after reviewing the results of his analytical ferrography report.
AUTHOR BIO :
Mike has been with Insight Services for 14 years, currently managing the marketing and sales efforts for the company. Insight Services is an industrial
analytical ferrography company located in Cleveland, OH. They excel in same day turnaround and exceptional customer communication.
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