Should You Use Ferrography In Your Sampling Program?

By BRETT WINBERG and LESLIE MOROVEK

Ferrography is a research method for qualitative determination of contaminant particles in lubricating oils and greases .

Ferrography, or wear-particle analysis, is the identification of all particles suspended in the lubricating fluids of any oil-wetted machinery. This technology was developed by the U.S. Navy in the 1970s. Today, it is available worldwide through commercial laboratories.

Ferrography provides a non-invasive look at historic, current and future conditions of a machine's lubricated components. This is all accomplished without the time and expense of physical examination.

In ferrography, a microscopic sample of the contaminant particles is prepared from the lubricant by magnetic separation and gravity. The amount, size, shape and type of the particles are determined by optical microscopy from the sample. Ferrography is suitable for all conventional mineral oils and synthetic lubricating oils and greases.

The cleanliness rating as well as the relative amounts of different contaminant and wear particles are reported. The report also gives a short summary of factors relevant to condition monitoring such as the contamination level of the lubricant and the condition of the lubricated system in question. A typical ferrography sample is viewed trough optical Microscopy.

Analytical methods identifying the size, shape, composition and concentration of particles is the core of ferrography. Once a trained analyst determines these factors, an association between the wear particles and the specific component of origin can be determined. This is done through direct examination of the particles.

Glass substrate, or ferrogram analysis, is one common method of particle identification. Predict/DLI of Cleveland developed a method of particle distribution that uses a magnetic gradient field. A combination of incline, sample preparation and a magnetic field ensure all particles present in the lubricant sample are deposited on the substrate for examination. Particles ranging in size from less than 1 micron to greater than 2,000 microns are released on the substrate.

Non-ferrous wear particles are released in a random manner throughout the length of the substrate, often appearing between the strings of ferrous particles. Contaminants, such as sand and dirt, fibers and friction polymers also are distributed in an irregular fashion throughout the length of the substrate.

Chemicals fix the particles to the slide and aid dispersal of the lubricant. Reduction of sample surface tension through the use of diluents, increased sample temperatures and mechanical means further aids in the release of particles from the sample.

This method of ferrographic examination provides a complete picture of the internal components of a piece of machinery. An analyst can identify all particles-from ferrous wear particles to contaminants such as insect parts-and evaluate the effect of their presence.

Cumulatively, the particles present in a sample carry with them the story, or fingerprint of the internal workings of an individual piece of equipment. Identifying these particles and the wear mechanisms that generated them can effectively demonstrate the equipment's operating history and current state of performance, as well as generate alarms to future wear conditions.

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