2000,Barrett,M.,Analytical Ferrography - Make It Work for You

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Analytical Ferrography - Make It Work

For You

Michael Barrett and Matt McMahon, Insight Services

IntroductionAnalytical ferrography is among the mostpowerful diagnostic tools in oil analysistoday. When implemented correctly itprovides a tremendous return on your oilanalysis dollars. Yet, it is frequently excludedfrom oil analysis programs because of itscomparatively high price and a generalmisunderstanding of its value.

The test procedure is lengthy and requiresthe skill of a trained analyst. As such, thereare significant costs in performing analyticalferrography not present in other oil analysistests. But, if time is taken to fully understandwhat analytical ferrography uncovers, mostagree that the benefits significantly outweighthe costs and elect to automatically incorporate it when abnormal wear is encountered.

Figure 1. Ferrogram Slide Maker

Separates Particles fr om the Oil

Principle To perform analytical ferrography the solid debris suspended in a lubricant is separated andsystematically deposited onto a glass slide. The slide is examined under a microscope todistinguish particle size, concentration, composition, morphology and surface condition of theferrous and non-ferrous wear particles.

This detailed examination, in effect, uncovers the mystery behind an abnormal wear condition bypinpointing component wear, how it was generated and often, the root cause.

FerrogramAnalytical ferrography begins with the magnetic separation of machine wear debris from thelubricating oil in which it is suspended using a ferrogram slide maker (Figure 1). The lubricating oilsample is diluted for improved particle precipitation and adhesion. The diluted sample flows downa specially designed glass slide called a ferrogram. The ferrogram rests on a magnetic cylinder,which attracts ferrous particles out of the oil (Figure 2).

Due to the magnetic fluid, the ferrous particles align themselves in chains along the length of theslide with the largest particles being deposited at the entry point. Nonferrous particles andcontaminants, unaffected by the magnetic field, travel downstream and are randomly depositedacross the length of the slide. The deposited ferrous particles serve as a dyke in the removal of nonferrous particles. The absence of ferrous particles substantially reduces the effectivenesswith which nonferrous particles are removed.

After the particles are deposited on the ferrogram, a wash is used to remove any remaining

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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.

Figure 2. Ferrogram Slide Maker

Separates Particles fr om the Oil

Particle Identifi cation

The ferrogram is examined under a polarized bichromatic microscope equipped with a digitalcamera. The microscope uses both reflected (top) and transmitted (bottom) light to distinguishthe size, shape, composition and surface condition of ferrous and nonferrous particles (Figure 4). The particles are classified to determine the type of wear and its source.

Particle composit ion is first broken down to six categories: white nonferrous, copper, babbitt,

contaminants, fibers and ferrous wear. In order to aid the identification of composition, theanalyst will heat treat the slide for two minutes at 600ºF.

White nonferrous particles, often aluminum or chromium, appear as bright white

particles both before and after heat treatment of the slide. They are deposited randomlyacross the slide surface with larger particles getting collected against the chains of ferrousparticles. 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 berandomly deposited across the slide surface with larger particles resting at the entry pointof the slide and gradually getting smaller towards the exit point of the slide.

Babbitt particles consisting of tin and lead, babbitt particles appear gray, sometimes withspeckling before the heat treatment. After heat treatment of the slide, these particles stillappear 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, thesenonferrous particles appear randomly on the slide, not in chains with ferrous particles.Contaminants are usually dirt (silica), and other particulates which do not change in

appearance after heat treatment. They can appear as white crystals and are easilyidentified by the transmitted light source, that is, they are somewhat transparent.Contaminants appear randomly on the slide and are commonly dyked 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 notchange in appearance after heat treatment. Sometimes these particles can act as a filter,collecting other particles. They can appear anywhere on the ferrogram, however they tendto be washed towards the exit end.

Detect and ConWater In Oil


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Figure 3. The Metal Alloy of the Particles

Determines Whether They Line up

On or Adjacent t o the Magnetic Field

Ferrous particles can be broken down to five different categories, high alloy, low alloy, darkmetallic oxides, cast iron and red oxides. Large ferrous particles will be deposited on the entryend of the slide and often clump on top of the other. Ferrous particles are identified using thereflected 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 alloyand white nonferrous is position on the slide. If it is white and appears in a chain, it’sdeemed to be high alloy. Otherwise, it’s considered white nonferrous The frequency of high alloy on ferrograms is rare.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 usuallyappear 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) - polarized light readily identifies red oxides. 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 theslide is generally considered to be a sign of corrosive wear. It usually appears to theanalyst as a “beach” of red sand.

After classifying the composition of particles the analyst then rates the size of the particles usinga micrometer scale on the microscope. Particles with a size of 30 microns or greater are giventhe rating of “severe” or “abnormal.” Severe wear is a definite sign of abnormal running conditionswith the equipment being studied.

Figure 4. Red and Green Filters Help

the Analyst Differentiate Wear Particles

from Organic and Tr anslucent Materials

Often, the shape of a particle is another important clue to the origin of the wear particles. Is theparticle laminar or rough? Laminar particles are signs of smashing or rolling found in bearings orareas 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 metalsurfaces 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


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machine. Is the particle spherical in shape? To the analyst, these appear as dark balls with awhite center. Spheres are generated in bearing fatigue cracks. An increase in quantity isindicative of spalling.

ConclusionAnalyzing the size, shape, color, magnetism light effects and surface detail of wear particles, askilled analyst can paint a picture above the nature, severity and root cause of abnormal wear. This information enables maintenance to implement effective corrective action.

Case Study I

Equipment : Ingersoll Rand single-stage centrifugal pump. Appli cat ion : A steel mill uses this pump to boost the water pressure it supplies to the hot mill for

descaling. Thin scale forms rapidly on the hot slab and must be removed before entering the millstands. The high pressure descaling water breaks it loose from the slab and removes it. Thepump has babbitt sleeve bearings with a Kingsbury thrust bearing on the outboard bearings. Thisis a 12,000 gpm, single inlet pump driven by a 2000 hp motor at 1780 rpm.

History: Oil samples from this pump are routinely sent for analysis and historically have exhibited

normal machine and lubricant conditions. Spectroscopic analysis and direct reading ferrographyare routinely performed in order to trend the wear of the lubricated bearings. Analyticalferrography is automatically performed when there is a machine abnormality.

Problem Sample: The sample drawn on 7/3/00 exhibited excess iron wear, abnormal silicon and

excessive total ferrous wear. The machine was given a critical condition and analyticalferrography was automatically initiated to pinpoint the source of the problem.

100X 500X

Analyt ical Ferrogr aphy

Analytical ferrography showed high amounts of dark metallic oxides and low alloy. This correlatedwith the data from the spectrometer and the direct read ferrogr aphy.

Conclusions: The maintenance department issued a work ticket to change the oil and inspect

the pump bearings. Upon inspection it was found that the thrust bearing was severely damaged. The thrust shoes were deeply scored and the thrust assembly was badly damaged. The pumpwas pulled from operation.

The routine oil analysis uncovered a problem that was easy to repair. The inspection occurred ona planned basis, thus preventing possible downtime. If the pump were left in service, it wouldhave catastrophically failed. This pump costs more than $50,000!

Case Study IIEquipment : Westinghouse 5,000 HP Motor.


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Appli cat ion : A steel mill uses this motor to drive a double reduction gearbox on a roughing mill at

an input speed of 450 rpm. The motor has babbitt sleeve bearings and is coupled to the gearboxwith a #11 Kop-Flex gear coupling.

History: Oil samples from this motor are routinely sent for analysis and historically have exhibited

normal machine and lubricant conditions. Spectroscopic analysis and direct reading ferrographyare routinely performed in order to trend the wear of the lubricated bearings. Analyticalferrography is automatically performed when there is a machine abnormality.

Problem Sample: The sample drawn on 5/18/00 exhibited excess lead wear, excess tin wear

and a high total ferrous wear count. The customer was called and submitted another sample. Thesample drawn on 5/24/00 exhibited the same abnormalities and the machine was given amarginal condition and analytical ferrography was automatically initiated to pinpoint the source of the problem.

Click Here to View Spectrometer and Direct Read Ferrography Data

500X 500X Analyt ical Ferrogr aphy

Analytical ferrography showed high amounts of low alloy steel and dark metallic oxides. Thecolored particles on the top slide are low alloy steel that changed color after heat treatment.

Conclusions: The results of the oil analysis initiated a visual inspection, which revealed that the

coupling appeared to be locked. The maintenance department issued a work order to inspect themotor bearing and coupling. Upon removal of the bearing cap, it was found that the faces of thebearing were wiped. The bearing surface that the shaft rode on was also starting to wipe. Thebearing was changed.


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Replaced Motor Bearing

Maintenance completed the repairs on a planned basis with no delay to the mill. If left unrepaired,the motor bearing would have failed catastrophically. Such a failure while the mill was runningwould have caused a four-hour delay to the mill with a cost in excess of $60,000!

Please reference this article as:

Michael Barrett and Matt McMahon, Insight Services, "Analytical Ferrography - Make It WorkFor You". Practicing Oil Analysis Magazine. October 2000

Issue Number: 200010

Practicing Oil Analysis

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2000,Barrett,M.,Analytical Ferrography - Make It Work for You