Machinery Lubrication magazine published by Noria Corporation
Issue link: http://www.e-digitaleditions.com/i/388231
www.machinerylubrication.com | September - October 2014 | 23 The weight of the sulfate ash is affected by metallic abrasion from bearing wear, hard impurities such as silicon (dust), levels of solid lubri- cants like molybdenum disulfide, metal organic extreme-pressure additives, and other metal soaps and inorganic thickeners from other greases with which they have mixed. Shear Stress, visible viscosity A rheometer is used to measure a grease's visible viscosity at different temperatures. For this purpose, a small amount of grease is placed on a temperature-controlled plate. The plate-based testing cone, which develops a gap between the upper and the lower plates, moves onto the film of grease. The force between the plates and the cone is measured as dynamic viscosity, which for grease is also called "shear viscosity." The stability after the shearing, which assesses characteristics such as a grease's deformability, can be described in terms of apparent viscosity. The rheometer provides the shear viscosity at the beginning and end of the testing procedure and displays the shear viscosity decrease in terms of percentage. By using the index and comparing with different fresh greases or through trend analyses, you can determine the lower temperature limit at which the grease can be used, whether the grease is suitable for high rotation speeds and whether it is acceptable for specific types of bearings. Dropping Point As temperatures rise, greases behave differently than edible fats. They do not melt like butter or coconut oil when warmed but hardly change at all as the temperature rises because the thickener holds the base oil firm. Only when the thickener's critical temperature has been reached does the soap structure dissolve. To establish the dropping point, a grease sample is warmed in the testing device until liquid drops fall to the bottom of the test tube through an opening in a nipple. Gel- or powder-based greases, which are measured at temperatures of more than 300 degrees C, are considered as not having a dropping point. There will not always be a correlation between a grease's dropping point and its maximum operating temperature. Of course, the permissible tempera- ture is always less than the dropping point value. Along with the thickener, the oil type will dictate a grease's maximum operating temperature. A lower dropping point of a used grease in comparison to a fresh grease may be caused by greases ML Measuring grease consistency with a cone penetrometer