D7690

D7690-11
Standard Practice for Microscopic Characterization of Particles from In-Service Lubricants by Analytical Ferrography

The objective of ferrography is to diagnose the operational condition of the machine sampled based on the quantity and type of particles observed in the oil. After break-in, normally running machines exhibit consistent particle concentration and particle types from sample to sample. An increase in particle concentration, accompanied by an increase in size and severity of particle types is indicative of initiation of a fault. This practice describes commonly found particles in in-service lubricants, but does not address methodology for quantification of particle concentration.

This practice is provided to promote improved and expanded use of ferrographic analysis with in-service lubricant analysis. It helps overcome some perceived complexity and resulting intimidation that effectively limits ferrographic analysis to the hands of a specialized and very limited number of practitioners. Standardized terminology and common reporting formats provide consistent interpretation and general understanding.

Without particulate debris analysis, in-service lubricant analysis results often fall short of concluding likely root cause or potential severity from analytical results because of missing information about the possible identification or extent of damaging mechanisms.

Ferrographic analysis, as described in this practice, provides additional particle identification capabilities beyond methods described in Guide for the following reasons:

(1) The ferrographic particle separation method is magnetic thus making it possible to readily distinguish between ferrous and nonferrous wear particles.

(2) Ferrography separates ferrous (magnetic) particles by size.

(3) Deposition is on a glass substrate so that particles may be examined using transmitted light as well as reflected light allowing particle types to be identified that cannot be identified when examination is done using only reflected light.

(4) Ferrograms may be heat treated providing important distinctions between ferrous alloy types (steel versus cast iron), further distinctions among various nonferrous alloys and distinctions between inorganic and organic particles.

Caution must be exercised when drawing conclusions from the particles found in a particular sample, especially if the sample being examined is the first from that type of machine. Some machines, during normal operation, generate wear particles that would be considered highly abnormal in other machines. For example, many gear boxes generate severe wear particles throughout their expected service life, whereas just a few severe wear particles from an aircraft gas turbine oil sample may be highly abnormal. Sound diagnostics require that a baseline, or typical wear particle signature, be established for each machine type under surveillance.

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