Machinery Lubrication

Machinery Lubrication March April 2018

Machinery Lubrication magazine published by Noria Corporation

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12 | March - April 2018 | www . if there has been an increase in the particle quantifier index (PQI). The PQI is a test that measures the distortion of a magnetic field applied to the sample while still in the bottle. Used to quantify ferrous particulate, it is beneficial when utilized in conjunction with the wear metal concentration. e test has limitations, as it does not measure nonferrous metals, and a single large particle reads like many smaller particles. When the PQI is lower than the iron levels, chances are there are no particles larger than 10 microns. If the PQI increases dramatically while the iron levels remain consistent or decrease, larger ferrous particles are being generated. is increasing trend should trigger ferrography or wear debris analysis. Wear Debris Analysis Ferrography can be direct reading or analytical. In direct-read ferrog- raphy, a magnetic sensor measures ferrous particulate up to 200 microns. e results are reported in DS (less than 5 microns) and DL (greater than 5 microns). e ratio between the two indicates the severity. Direct-read ferrography is excel- lent for trending ferrous wear particles and is often used as a screening tool for analytical ferrography, which explores the shape and size morphology of the captured parti- cles. e measurement of particles is expressed as a unitless wear particle concentration (WPC) value and a ratio of small to large particles. If the ratio of large particles (DL) to small particles (DS) increases, this suggests 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 micro- scope is used to identify the type of generating wear mode based on the particle's shape and size. e oil sample is diluted for improved flow down the glass slide. e slide rests on a magnetic cylinder, which attracts ferrous particles. e ferrous particles align themselves, with the largest particles deposited at the entry point. Nonferrous particles flow downstream and are randomly deposited. Solvent is added to the remaining sample. After the solvent evaporates, only the particles are left on the slide. e slide is then analyzed by a trained technician. ere are three main categories of wear particles: fluid or particle wear, sliding/rolling/impact wear, and chemical wear. e table above summarizes these categories and lists the mechanisms of each. Abrasive Wear Abrasive wear particles are typi- cally formed from the cutting of a hard, sharp particle or from severe sliding. ey can also form as a result of a bearing or gear misalignment. Two-body abrasive wear may be due to a misalignment or an asperity of a harder metal component, which allows gouging of the opposite rotating softer metal. ree-body abrasive wear is often caused by foreign particles in the oil. e harder dirt particle imbeds itself into a softer metal and gouges away metal from the rotating component. ese wear particles are referred to as cutting, gouging, ploughing or scratching particles. Abrasive contamination can generate cutting wear particles that are long and curly, with length-to- width ratios ranging from 5-to-1 to 50-to-1. A common abrasive particle would be sand or dirt. Sand is made of silicon dioxide, while dirt is generally ferrous or aluminum oxides with organic material. ese sand and dirt particles usually are induced through a compromised breather element or through seals, O-rings, gaskets, etc. ey may be transparent, translucent, opaque, crystalline or a birefringent material. Erosive Wear Erosive wear particles are gener- ated by the loss of material due to the repeated impact of hard particles at a high velocity. Often the extent of erosion varies by the velocity, angle of inclination, particle type and concentration. is occurs in pumps, valves, nozzles and even elbows. Cavitation Cavitation involves the removal of material by repeated implosion of bubbles near or on the metal surface. e bubbles produce violent shock- waves that fatigue the surface until material is removed. is is common in pumps and journal bearings. Polishing Polishing is the continuous removal of material on one or more surfaces by very fine abrasive particles. It creates a shiny, mirror-like surface. e width of the groove (or scratch) generated by the abrasive is generally 5 to 10 percent of the grit diameter. 3 Categories of Wear Particles Fluid or Particle Wear Sliding, Rolling and Impact Wear Chemical Wear Abrasive Adhesion Corrosion Erosive Spalling/Fatigue Electro Corrosion Cavitation Brinelling Electric Discharge Polishing Fretting (Corrosion) COVER STORY

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