Machinery Lubrication

Machinery Lubrication Sept Oct 2013

Machinery Lubrication magazine published by Noria Corporation

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Page 41 of 67

Wear Debris Analysis LESSONS IN LUBRICATION BENNETT FITCH NORIA CORPORATION ANATOMY of Wear DEBRIS This article is the fourth part of a series of "anatomy" lessons within Machinery Lubrication. In this issue, the various modes by which wear debris is created will be examined along with the physical characteristics of each type of wear particle. In addition, microscopic analysis and similar instrumentation will be used to provide an in-depth look at these particles' unique appearances and how they are formed. Mechanical equipment deteriorates over time. Depending on the type of work and environmental conditions a machine endures, internal mechanical deterioration can occur in the form of fatigue, rubbing, sliding, abrasion and corrosion. From a macroscopic perspective, the wear debris produced from these deterioration processes may appear as insignificant specks of mass that mostly act and look the same. However, on a microscopic level, this wear debris has a unique morphology (shape and size) and surface topography (roughness, texture and surface pattern) based on the deterioration process or wear mode by which it was produced. If clearly understood, the morphology and topography can offer clues that can be used to prevent imminent machine failure. Wear debris can be defined as particles produced from the breakdown of surfaces within a machine. These particles can range from a submicron size to chunks of metal as large as can be imagined. Wear debris analysis generally focuses on the small, destructive particles, many of which are too small for the human eye to see. These particles may be less than 1 micron to 200 microns in size. If you have particles larger than 200 microns, you probably don't need anyone to tell you that there is a serious issue within the machine. Wear Debris Sampling and Analysis There are many wear debris analysis techniques (see Table 1). Some of these methods can even analyze wear debris right from the oil sample. Analytical ferrography requires 40 | September - October 2013 | wear debris to first be isolated from the collected oil sample. Photomicrograph images of the ferrograms or filtergrams are then studied using an optical microscope. The main advantage of TABLE 1. WEAR PARTICLE ANALYSIS TECHNIQUES INSTRUMENTATION ANALYSIS METHOD RESULTS Spectrometric analysis High heat vaporization of metals Parts per million (ppm) of elements Particle counting Laser light scatter or pore blockage Number and size of particles Direct image particle counting Shadow casting from laser Number and size of particles Ferrous density analysis Magnetic flux Number and size of particles Analytical ferrography Microscopic analysis Shape, size, texture, color, orientation, etc. analytical ferrography is its ability to determine particle shapes, sizes and textures as well as identify elements. Ferrography utilizes either magnetism or membrane filtration to collect particles. ISO standards, such as ISO 16232, ASTM D7670 and D7690, are used to properly prepare these samples and analyze the particles' visual characteristics. These observed characteristics shed light on where and how these particles were generated. Ferrograms As an oil sample flows down a specially designed glass slide called a ferrogram, a magnet is positioned underneath to trap the ferrous particles. The particles tend to collect in strings along the magnetic field. While many of the non-ferrous particles will flow past and not become trapped, some will be held up by gravity or by contact with the trapped ferrous particles. Both bottom and top lighting can be used to help characterize the particles' critical features. Non-ferrous particle Ferrous particles In this ferrogram, ferrous particles are aligned along magnetic fields.

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