ALS Tribology eSource

MicroscopeMicroscopic Particle Examination
by Greg Lewis
Technical Resource Department

The Microscopic Particle Examination (MPE) is a more detailed analysis of the wear type, mode and cause either indicated by a standard oil analysis test or suspected by the engineer. The MPE is comprised of a standard oil analysis report with the addition of a full Analytical Ferrography. It can assist in making that difficult decision of pulling the machine out of service now or waiting for a more convenient time, such as a planned plant shutdown.

Conventional spectrometric oil analysis (SOAP) measures the concentration of an element (for instance, iron) in an oil sample. The statement of wear is produced from the concentration of iron rather than the actual particles of oil. While very useful, SOAP alone does have limitations:

  • The size of the particle that is picked up by the spectrometer is limited to a maximum of about 8 microns (micrometers).
  • The spectrometer gives no indication of the morphology, or form, of the wear particles.

In a normal situation, wear particles are small (less than 8 microns) and are detected by the spectrometer. However, as normal wear changes to abnormal wear the particles tend to get larger, which are less detectable to the spectrometer. This is the reason ALS Tribology is sometimes asked, “Why am I seeing an increase in debris on the magnetic plug or filter screen but the SOAP analysis does not indicate a problem?” The particle quantification (PQ) test is an excellent screening tool for ferrous particle both large and small; however, there is no way to ascertain the form of the wear particles without performing an MPE. An MPE, combined with SOAP, gives a much clearer picture on the condition of your component.

With the added Analytical Ferrography combined with a standard condition monitoring report, MPE testing can detect:

  • Discoloration of particles, which indicates overheating.
  • Corrosion (rust) particles.
  • Black oxides, which indicates that lubrication is insufficient.
  • The type of wear mechanism occurring to help pinpoint what is causing the excessive wear.

Although MPE testing is suitable for any component, it is especially beneficial for gear and roller bearing systems. Components such as differentials, final drives and industrial gearboxes (are the most commonly analyzed units.

It is recommended to perform an MPE at regular intervals on critical equipment. Having an analysis history gives more meaning to the latest result.

The Microscopic Particle Examination is an extra tool in the maintenance engineer's toolbox. MPE testing can help detect faults before they lead to a major failure. The test also helps determine which part of the component needs attention before dismantling the equipment.

ALS Tribology offers MPE testing in Cleveland, Ohio, and Phoenix, Arizona. Contact us to learn more about this valuable service. A sample MPE report is attached. 

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