FINA Lubricants - Ferrography

Forensic Scan Ferrography (FS)

Forensic Scan (FS)

Forensic Exam (FE) or Analytical Ferrography

Glossary description: It is a technique utilizing a precision magnetic gradient to systematically strip iron-laden and other susceptible particles from a lube for study.

How is the test performed? A small amount of sample is diluted with solvent and flowed across the length of an inclined microscope slide, the slide is flanked by specially designed permanent magnets, causing magnetic particles to be systematically stripped and deposited. Note that Ferrography DR (direct read) had the fluid running through a capillary.

A Forensic Scan (FS) is the visual examination of the slide.

If the FS is classified as POSITIVE then a FE (Forensic Examination) is performed. The all. At the time of this writing there is no single test to supplant all others.

Result/Interpretation: A ferrogram is an ordered precipitation (from large to small) of magnetic particles on a slide, and a semi-random precipitation of non-magnetic particles, as influenced by gravity. The slide is viewed under a ferroscope (a special microscope for illuminating with two differently colored light sources). An experienced technician “grades” various types of particles (see below). A pertinent photograph is usually taken as well, and a brief narrative may also accompany the photograph. The unit history report will usually have a coded interpretation of the information.

Ratings: 0 = None detected 1 = Few 2 = Moderate 3 = Significant 4 = Very dense

In addition to numerical ratings there will be a letter classification to identify particle type. These include the following letters: positive FS rating occurs if a significant amount of debris or contaminants is visually observed on the slide. However if there is no significant wear or contaminants on the slide, then the FS is classified as NEGATIVE and a FE is NOT performed.

A Forensic Exam (FE) consists of two photographs of the debris from the ferrogram slide using a combination of magnifications.

The photos will show the major constituents of the slide. The FE can have an overall condition of Normal, Monitor, Abnormal and Critical, based on the level of contamination or wear.

Technical overview: Analytical ferrography addresses particles that are larger than those detectable by rapid-process atomic emission or absorption spectrometers (which are limited to particles <10 microns). Further, the process yields morphology (shape) of particles, which in turn frequently reveals the cause of a problem. It is one of the most powerful and revealing tools used in lube analysis consulting. It is not, however, an end-all. At the time of this writing there is no single test to supplant all others.

Ratings:

0 = None detected 1 = Few 2 = Moderate 3 = Significant 4 = Very dense

In addition to numerical ratings there will be a letter classification to identify particle type. These include the following letters:

(A) Abrasives: Sand, “dirt”, usually silicon-based particles that appear translucent.

(C) Cutting wear: Curved or spiral metals signifying, e.g., a lathe or lapping-type effect from an abrasive lodged in a relatively soft bearing while cutting into the shaft.

(D) Dark metal-oxides: Indicative of severe heat from metal-to-metal contact, e.g., lube film break down.

(E) Fatigue-oriented metal: Metals with diameter >15 microns in size, indicating component distress and potential catastrophic failure.

(F) Fiber: Individual strands, often from filter media, occasionally from shop rags and other debris.

(L) Laminar: Metals having a “peeled” look (thin, long), often from roller bearings.

(N) Non-Ferrous metal: Any metal that is not primarily iron, e.g., brass, bronze, aluminum, Babbitt.

(O) Corrosive: Extremely small (<1 micron) particles found in the “tail” (last flow point) of the ferrogram, indicative of hostile environment or over-extension of lube drain.

(P) Friction Polymer: Appears as a nearly clear lacquer-like particle, often with embedded metals. Tends to indicate lube distress from “hot Spots” in the system or misapplication of lubricant, e.g., too low a viscosity, resulting in insufficient film strength.

(R) Rubbing wear: Small particles (<15 microns technically, but usually much smaller) which are expected or normal in many systems.

(S) Sphere: Nearly perfectly round and mostly small particles (<3 microns) often from fissures in roller bearing elements. A significant increase may foretell spalling of the bearing. It may also be weld/grind material.

(X) Red oxide: Usually oxides of iron (e.g., rust) are an indication of latent water contamination.

Suggested application: To support inspection decision. AF is a requirement for most predictive and proactive maintenance programs.

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