Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/490438
28 | March - April 2015 | www.machinerylubrication.com LESSONS IN LUBRICATION exploration is needed with proven diagnostic or prognostic capa- bilities. The exploratory actions to be taken once a fault has been detected should also be specified and understood. Otherwise, the ability to remedy the problem effectively will be severely dimin- ished. Unfortunately, this is a missed opportunity for many facilities today that have failed to include wear particle character- ization in their oil analysis program. Exploratory Analysis At this stage, an investigation into the potential tribological issue is essentially underway. Those in charge of this investigation are like detectives, and good detectives will use all available resources to develop and support their conclusions. These resources include all data from the fault detection technologies in addition to any new approaches, e.g., further laboratory oil analysis testing. This is where value is gained from studying an oil sample under a microscope using methods such as analytical ferrography (with preparation of a ferro- gram), patch ferrography (with preparation of a filtergram), optical imaging methods and scanning electron microscopy (SEM). While determining the existence and amount of wear particles in an oil sample is essential, understanding the particles' size, shape and composition is equally important if the goal is to identify where and how those particles were produced. Standard particle counters can provide an average number of particles by size groups, but those results do not distinguish between environmental contaminants and generated wear particles. Elemental spectroscopy can offer details about certain metallic elements increasing in concentration, but these results will be skewed in cases with rising large particle counts, since large particles are not generally measured by this method. This is a key diagnostic limitation, because the population density of larger wear particles is directly related to severe wear modes such as adhesion, surface fatigue and abrasive wear. This limitation is what opens the door to technologies like analytical ferrography and other microscopic wear particle characterization methods. Analytical Ferrography When a ferrogram is prepared and analyzed under a microscope using polarized bi-chromatic techniques, it helps the analyst differ- entiate between wear particles and those of organic and translucent materials, as well as helps identify particle composition. Heat- treating techniques also improve the analyst's ability to extricate particle composition details. 68% of oil analysis programs employ analytical ferrography to analyze wear debris, according to a recent poll at machinerylubrication.com Using a Ferrogram The oil sample (presumed to contain wear debris) is initially diluted for improved particle precipitation and adhesion. The diluted sample flows down a specially designed glass slide called a ferrogram. The ferrogram rests on a magnetic cylinder, which pulls ferrous particles out of the oil and pins them to the glass slide. Due to the magnetic field, the ferrous particles align themselves in chains along the length of the slide, with the largest particles being deposited at the entry point. nonferrous particles and contaminants, which are unaffected by the magnetic field, travel downstream and are randomly deposited across the length of the slide. The deposited ferrous particles serve as a barricade in the removal of nonferrous particles, which otherwise would tumble off the end of the slide. Oil Flow Slide Magnet Large par ticles deposit at entry point where the magnetic pull is the weakest. Smaller par ticles deposit along the slide as the magnetic pull strengthens. While determining the existence and amount of wear particles in an oil sample is essential, understanding the particles' size, shape and composition is equally important.