eSource 98 Wear Debris When to Perform

eSource 98 Wear Debris When to Perform

Posted 01 April 2017
Each asset has a life span. Often, it isn’t until premature failure occurs will an in depth investigation ensue. While inadequate lubrication, new component defects, the use of wrong components or materials, poor installation, or poor maintenance practices or workmanship are to blame, routine oil analysis combined with wear debris analysis might have indicated a pending failure.

It is not realistic to have wear particle debris analysis performed on every asset but it is entirely reasonable to test critical assets for wear debris on a scheduled basis. It is also recommended to perform wear debris analysis when beginning an oil analysis program. This will help establish which assets may be most vulnerable to failure. 

Lubricants have several functions; reduce friction and wear, protect against rust, corrosion, oxidation, transfer power, act as a seal, clean and flush surfaces, keep contaminants in suspension. These are all well-known functions. A lesser considered function yet not less important is to act as a conduit of information. The lubricant can provide insight into what is occurring within a system without having to disassemble and inspect. Prior to an oil sample being drawn for wear debris analysis, it is prudent to consider the following observations; overheating, high vibration, high system pressure all may indicate it is time to analyze the oil for wear debris. Once an oil sample is tested, there will be a indicators which will encourage the use of wear debris analysis. Data can only be meaningful if a there are more than one test performed. The first indication will be an increase in wear metals (Fe, Pb, Sn, Al, Cu) over a period of several tests. These metals are common in lubricated systems. Secondly if there is an increase in the Particle Quantified Index value (PQI). The PQI is a test which measures the distortion of a magnetic field applied to the sample while still in the bottle. It is used to quantify ferrous particulate. The test has limitations. It does not measure non-ferrous metals and a single large particle reads like many smaller particles. The Particle Quantified Index is useful when used in conjunction with the wear metal concentration. When PQ Index is lower than the iron (ppm) from ICP, chances are there are no particles larger than 10 microns. If the PQ Index increases dramatically while the iron (ppm) remains consistent or decreases, larger ferrous particles are being generated. The use of this increasing trend triggers ferrography or wear debris analysis.

Wear Debris Analysis

Ferrography can be Direct Read or Analytical. In Direct Read Ferrography, a magnetic sensor measures ferrous particulate up to 200 microns. The results are reported in Ds (<5 µm) and DL (>5 µm), the ratio between the two indicates severity. Direct Read Ferrography is often used as a screening tool for Analytical Ferrography which explores the shape, size morphology of the particles that are captured. Direct Read Ferrography offers is excellent for trending ferrous wear particles. The measurement of particles is expressed as unit less Wear Particle Concentration (WPC) value and a ratio of small to large particles are expressed. As the ratio of large particles (DL) to small particles (DS) increases, there is an indication of a greater generation of large particles. In Analytical Ferrography, a slide is produced with the aid of magnets to separate wear debris and arrange particles according to size. A microscope is used to identify type and generating wear mode according to the shape and size of the particle. Magnetic separation of particles using a ferrogram slide maker is used. The oil sample is diluted for improved flow and the sample flows down a ferrogram glass slide. The slide rests on a magnetic cylinder, which attracts ferrous particles. Ferrous particles align themselves; largest particles being deposited at the entry. Nonferrous particles flow downstream and are randomly deposited. Solvent is added to the remaining sample. The solvent evaporates leaving particles on slide. A Ferrogram slide is analyzed by a trained technician using a bichromatic microscope.

Analytical Ferrography also known as Microscopic Particle Examination (MPE) is one of many useful tools used to understand various wear mechanisms in an oil sample. Using ferrography proactively helps reduce or eliminate impending wear failure by examining the various wear debris particles. Different forms of wear particles can occur when a lubricated component experiences various conditions. The analysis will identify the particles by type and estimate the concentration of each particle type. Abnormal wear particles are formed a number of ways.  There are 3 main categories of wear particles; fluid or particle wear, sliding / rolling / impact wear, and chemical wear. The following table summarizes these categories and lists out the mechanisms of each:

Fluids or Particle Wear

Sliding, Rolling & Impact

Chemical

Abrasive

Adhesion

Corrosion

Erosive

Spalling / Fatigue

Electro Corrosion

Cavitation

Brinelling

Electric Discharge

Polishing

Fretting (Corrosion)


Written By:

Michael D. Holloway, MLA I, MLA II, OMA1, MLT I, MLT II, CLS, LLA I
Principle Consultant, Certified Reliability Leader
ALS Tribology

 

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