Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/1399684
44 | July - August 2021 | www . machinerylubrication.com OIL ANALYSIS Oxidation Oxidation Description: "Signals the deterioration of the oil due to thermal breakdown and aging causing a physical change in the oil." Reported in absorbance per centimeter (Abs/cm), oxidation prevents additives from performing properly, promotes the formation of acids and increases viscosity. Interpretation: While oxidation is a slow process that gradually degrades lubricating oils, it is known that the following factors influence the acceleration of oxidation: • Temperature. • e amount of oxygen in the environment. • Water contamination. • Metals (particularly copper and iron, which are usually wear metals) and organic and mineral acids that promote oxidation. 3 Water content and wear metals directly affect oxidation, and can also be tested during used oil analysis, so it is helpful to relate these three tests while analyzing used oil results. In general, "for each 10 °C (18 °F) rise in temperature, the oxidation rate will double." at being said, for high ambient tempera- ture applications, applications that involve corrosive gases and high humidity and dusty environments, it is important to look at the rate at which oil oxidizes — these applications can be considered aggressive for lubricating oils. is would help schedule oil changes in predictive maintenance. Acid Number (AN) Acid Number (AN) Description: "Measures the oil's acidity repre- senting a change in the oil. Indicates degradation of the oil in service leading to deposit formation." "Reported in milligrams of potassium hydroxide per gram of test oil neutralized (mg KOH/g)." Interpretation: In the baseline oil analysis result, it is likely to have a high initial AN value due to additives being slightly acidic. e AN will decline during operation as the additives are being depleted. But once the AN of used oil is higher than the baseline oil value, oil degrada- tion has occurred. An AN above 4.0 means that the oil is "highly corrosive, risking an attack on metal surfaces." Also, testing for oxidation may be difficult for some synthetic lubricants because of infrared band interferences in some components; in these Table 2: Common elements reported during used oil analysis. Classification: W = Wear, C = Contaminant, A = Additive Category Element Class Comments Wear Metals Iron W,C Usually found on reduction components and bearings. Chromium W Mostly found in bearings but can also be found on gearboxes with Cycloidal reduction components. Care should be taken when iron increases at a similar ratio as Chromium. Nickel W Found in reduction components in very small quantities. Aluminum W,C Found in greases as a thickener. Mostly considered a contaminant. Copper W,C,A Usually found on bearing cages. Contact Oil Manufacturer to ask if it's part of the additive package and/or compare with virgin oil analysis. Lead W Considered a hazardous material. Not commonly found on gearboxes or lubricants. Tin W Unlikely to be found gearboxes. Cadmium W Unlikely to be found gearboxes. Silver W Unlikely to be found gearboxes. Vanadium W Unlikely to be found gearboxes. Contaminant Metals Silicon C,A Considered mostly as a contaminant. Even if found in very small quantities in reduction components, it is not considered as wear metal. Contact Oil Manufacturer to ask if it's part of the additive package and/or compare with virgin oil analysis. Sodium C,A Found in greases as thickener. Contact Oil Manufacturer to ask if it's part of additive package and/or compare with virgin oil analysis. Potassium C - Multi-source Metals Lithium C Found in greases as a thickener. Boron C, A Contact Oil Manufacturer to ask if it's part of the additive package and/or compare with virgin oil analysis. Titanium W Unlikely to be found in gearboxes. Molybde- num A Not considered as wear metal even if it can be found in reduction components in very small quantities. Contact Oil Manufacturer to ask if it's part of the additive package and/or compare with virgin oil analysis. Antimony W Unlikely to be found in gearboxes. Manganese W Found in reduction components in very small quantities. Additive Metals Magnesium A - Calcium C,A Found in greases as a thickener. Contact Oil Manufacturer to ask if it's part of the additive package and/or compare with virgin oil analysis. Barium A - Phospho- rus A Considered mostly as additive. Found in reduction components in very small quantities. Zinc A - cases, Acid Number (AN) may be a more appropriate test. Water Content Water Content Description: "Determines the amount of water contamination — reported in percentage (%) or parts per million (ppm)." One percent of water equals 10,000 ppm. Interpretation: Minimizing water contam- ination is critical to the life of the gearbox. AGMA recommends having a maximum limit of 300 ppm (0.03%) of water contamination.