Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/388231
22 September - October 2014 | www.machinerylubrication.com Bleeding Test The sponge-like structure of the thickener or soap used in grease holds the base oil firm and allows it to transfer slowly to the lubri- cation point. However, if the oil flows too quickly and uncontrollably from the thickener, the grease "bleeds" away. If the remaining thick- ener contains too little oil for the lubrication tasks, the remaining grease dries out. If the residual oil content of a grease falls too quickly, either the grease is unsuitable or must be reapplied more frequently or in larger quantities. Determining the residual oil content provides the necessary information to decide which is the case. This test shows the percentage of base oil that the soap struc- ture has lost over the course of six hours at a temperature of 60 degrees C. The residual oil content of used grease should then be compared with the fresh grease sample. If the values are between 5 and 25 percent, and the difference between the used and fresh grease is plus or minus 15 percent or less, the grease can still be used without changing the relubrication inter- vals. If the used grease loses considerably more oil than the fresh oil, the thickener is no longer able to maintain the base oil in its sponge- like structure. If significantly less oil is released from the used grease, it has already begun to dry out. The bearing surface is "hungry" for lubrication and must be lubricated again. Residual oil content levels that are too low may be the result of too much time in use and insuffi- cient relubrication; strong vibrations, loads or rotation speeds; mixing of greases that have been saponified differently; impurities caused by water, acids or lyes; inadequate temperature resistance; or oxidation and aging (acetifying) of the base oil. Penetration Test While viscosity describes the ability of a lubri- cating oil or hydraulic fluid to flow, consistency refers to the extent to which a lubricating oil is stiff. However, the consistency of a grease is not directly linked to the viscosity of its base oil or the kind of thickener used. The National Lubricating Grease Institute (NLGI) divides lubricating greases into classification groups based on their consistency. Along with the base oil's type and viscosity, the consistency cate- gory to which it belongs provides another important value to classify the firmness of the grease. The consistency shows whether the grease is feedable or has become too firm as a result of bleeding. Consistency is measured using a standardized cone. Grease is spread into a small pot. The tip of the cone touches the grease. The penetration depth is reached in 5 seconds and measured in 0.1 millimeters. This produces the grease penetration number used to determine the NLGI class. The softer the grease, the deeper the cone penetrates. This indicates a high penetration number and a low NLGI class. Comparing fresh and used grease penetration allows certain conclusions to be drawn. For example, if the used grease has become softer and therefore has a higher penetration number than the fresh grease, it may have mixed with another grease or been contaminated by water or another liquid. The grease may also have been mechanically sheared and loaded. A significant drop in the penetration number indicates that the grease may have been bled by excessively strong vibrations or that high temperatures may have resulted in oil separation. High levels of pressure in the central lubrication system may have also disturbed the balance between the base oil and the thickener. Sulfate Ash Investigating sulfate ash is a process used to identify inorganic levels in organic samples. This is achieved by heating the sample to temperatures of 775 degrees C. At this temperature, the organic elements in the samples "combust." All that remains are ashes consisting of metallic oxides (soap, additives, etc.) and impurities. By smoking this with concentrated sulfuric acid, the oxides in the ashes are transformed into sulfates. The weight of the remains is then recorded. If the proportion of sulfate ashes in the used grease has increased in comparison to the fresh grease, this is a clear indication of impu- rities and/or wear. If the metal values established by AES are consulted at this stage, the cause of the weight increase can be clarified. High levels of iron and chromium point to wear, while increased proportions of silicon and calcium hint at impurities. GREASES nLGI CLASS PEnETRATIOn nuMBER COnSISTEnCY AT ROOM TEMPERATuRE 000 445-475 Very liquid 00 400-430 Liquid 0 355-385 Semi-liquid 1 310-340 Very soft 2 265-295 Soft 3 220-250 Semi-solid 4 175-205 Solid 5 130-160 Very solid 6 85-115 Extremely solid Establishing a grease's dropping point The National Lubricating Grease Institute (NLGI) divides greases into classification groups based on their consistency.