Machinery Lubrication

Machinery Lubrication Sept Oct 2014

Machinery Lubrication magazine published by Noria Corporation

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20 | September - October 2014 | By Steffen BotS, oelcheck What lubricates more efficiently, oil or grease? More than 95 percent of all components are lubricated with oil, not because it lubricates better but because it has a wider variety of properties. Grease generally is used only if oils cannot be utilized because sealing the lubrication point is problem- atic or expensive. Typically, grease is applied on rolling bearings, with nearly 80 percent of these bearings lubricated with grease. During analysis, oils and grease behave similarly. Although both contain important information, it is more difficult to interpret the results for grease. The following methods represent the most important ways to analyze lubricating grease. Atomic Emission Spectroscopy Atomic emission spectroscopy (AES) by the Rotrode method can be used to provide information about wear, impurities, thick- eners and additive levels in a grease sample. Increased iron and chromium values may indicate that a rolling bearing has been subject to wear, while copper, lead and tin show corrosion or abra- sive wear from bearing cages. Possible impurities such as silicon (dust), calcium (lime) or hard-water deposits help to identify the causes of wear. Deviations between fresh and used grease in terms of the content and composition of the additive package or the thickener reveal that another grease is being used. Particle Quantifier Index The particle quantifier (PQ) index specializes in determining magnetizable iron particles. Unlike AES, which cannot easily detect iron particles greater than 5 microns, the PQ index records all the wear particles that are magnetizable regardless of their size. The height of the PQ index is then measured in conjunction with the AES iron content. An extreme PQ index (more than 500) indicates that acute wear has taken place regardless of the AES iron values. Often there has been pitting or material fatigue. A high PQ level (more than 200) with a low AES iron value (less than 100) signifies that acute wear is occurring and causing relatively large wear particles. An increased PQ index (more than 100) in combination with a correspondingly high AES iron value is a sign of typical material fatigue during which "normal" wear occurs. A low PQ index (less than 50) accompanied by a high AES iron value (more than 100) is always a sign of corro- sion and rust formation. Rust is barely magnetizable, so it produces a low PQ index. Fourier Transform Infrared Spectroscopy The principle of Fourier transform infrared (FTIR) spectroscopy is based on there being molecules present in a lubricant that absorb infrared light to different degrees because of their chemical struc- ture. Changes to the used lubricant can be compared to the fresh grease reference spectrum and then depicted, calculated and inter- preted as typical "peaks" for certain wave numbers. In addition to identity controls, oxidation can also be proven with FTIR spectros- copy, for instance. As they age, molecular compounds alter and absorb more infrared light than fresh grease. Through the process of Fourier transformation, these values can be read and the molec- ular vibrations represented in an FTIR diagram. Depending on the molecular compounds, the peaks develop as corresponding wave numbers. Synthetic lubricants frequently contain ester-based components. Because of the oxygen molecules contained within them, they absorb infrared light in almost the same wavelength range as the double oxygen bonds that arise through oxidation. This is why oxidative changes to a synthetic oil cannot be calculated accurately using FTIR alone. The RULER test is needed for this. GREASES Best Methods for Analyzing Grease How FTIR works IR Detector Infrared light Absorbance at spectral frequency (e.g., 3400 wave numbers for water) Scattering and broadband absorbance at all frequencies (e.g., soot) Path length typically 100 microns (0.1 mm) Transmission oil

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