Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/108300
GREASES • Contamination elements (silicon, calcium, sodium, potassium and aluminum) • Additives or thickeners (magnesium, calcium, phosphorous, zinc, barium, silicon, aluminum, molybdenum and boron) Of special interest for diagnosing a bearing or grease condition is the amount of iron and chromium, which are present as wear particles from the bearing material. Non-ferrous materials like copper, lead and tin indicate corrosive or abrasive wear from the bearing cage. If dust (silicon or calcium) or sea water (sodium, potassium or magnesium) is present, this information can help determine the reason for the presence of wear metals. The amount of metallic soap elements or a comparison of the additive content in fresh and used grease can also reveal whether the recommended grease is in use. The infrared spectrum of a sample provides information regarding contamination and any changes in comparison to the reference spectrum. By a spectra subtraction of used grease with reference grease, the FTIR method indicates what kind of unknown grease is in use. In addition, a mixture of different greases in many cases is revealed. The identification of the original grease and the base oil type can be found by searching a library of reference spectra and can support the cause of a failure. Particle Quantifier The particle quantifier (PQ) index is specialized for the determination of all magnetic iron particles. An index value between O and 9,999 characterizes iron particles present in the sample independent of the particle size. Because rust particles are nonmagnetic, they are not measured. The PQ index test is based on the principle that iron (and iron wear) is magnetic and can be detected by a magnet. If a grease sample contains magnetic iron wear particles, a magnetic field is disturbed. This change in the magnetic field can be measured. Remember, the PQ index gives the total content of magnetic wear particles. Contrary to the iron wear information determined by OES, the PQ index provides information about all iron wear particles. Also, when using OES for used grease samples, only particles up to 5 microns can be detected because larger particles are not excited. Grease Condition by FTIR Fourier transform infrared (FTIR) spectroscopy identifies the type of base oil and thickener of the used grease. By comparing the unused fresh grease reference to the used grease sample, additive depletion or contamination by another grease type can be determined. In comparison to FTIR spectroscopy of oil, the measurement and interpretation of a grease spectrum are more complex. The thickener compounds especially can be very dominant within important areas of the spectrum that are normally used for the calculation of the water content or oxidation. FTIR spectroscopy is based on the principle that the molecules present in a lubricant can absorb infrared light at corresponding wavelengths depending on its typical structure. Changes in the used grease in comparison to the fresh grease reference spectrum are calculated on the typical peaks at predefined wave numbers and interpreted as oxidation, water, etc. A very small grease sample (less than O.1 gram) is applied to an attenuated total reflectance (ATR) cell. In the contact zone, the grease sample will be exposed to infrared light. An infrared spectrum showing the absorbance of the infrared light on the corresponding wave number will be recorded and interpreted. 26 January - February 2013 | www.machinerylubrication.com An FTIR spectrum can provide information regarding contamination and any changes in a grease sample. The FTIR method can also show whether synthetic or mineral base oils are used. If a mineral oil is used as the base oil, FTIR can indicate whether the base oil was oxidized because too much time passed without regreasing or because the temperature was too high. If the grease contains extreme pressure (EP) additives with zinc and phosphorus, the degradation of the additives can be seen. The water content in the grease may also be provided. Water in Used Grease by Karl Fischer Titration Besides solid contaminants, which can be identified by the OES elements silicon, calcium or aluminum, water is a type of contamination that is often the cause of corrosion. Typically, short regreasing intervals are the result of too much water. Unfortunately, determining the amount of water in grease is not as easy as in an oil sample. For water determination according to the Karl Fischer method, a small grease quantity (approximately 0.3 grams) is placed into a glass vial and sealed with a septic cap. In a small oven, the sample is heated to approximately 120 degrees C. The steamed-out water is transferred by nitrogen into a titration vessel in which an electrochemical reaction between the water and a Karl Fischer reagent takes place. A titration curve is recorded, and the water content is defined precisely. Depending on the grease type and application, the water content in the grease should not exceed the recommended values. Too much water in a grease can produce a variety of adverse effects, including corrosion on bearing metals, increased oxidation of the base oil, softening of the grease, and water washout of the grease. If the result for water content according to the Karl Fischer