Most users are aware of the importance of selecting the right lubricant for a given application, and it’s made easier when following original equipment manufacturer (OEM) recommendations. OEM specifications for a lubricating oil normally include viscosity at operating or ambient temperature, additive requirements, base oil type and sometimes special considerations for different environmental conditions. Grease specifications on the other hand, often lack the necessary detail to make a proper selection. Due to the lack of specificity in most grease recommendations, it is important to learn how to properly select greases for each application in your plant.
Proper grease specification requires all of the components of oil selection, and more. Other special considerations for grease selection include thickener type and concentration, consistency, dropping point and operating temperature range, worked stability, oxidation stability, wear resistance etc. Understanding the methods for correct grease selection will go a long way toward improving your lubrication program.
The most important property of any lubricant is viscosity. A common mistake when selecting a grease is to confuse the grease consistency with the base oil viscosity. As most grease-lubricated applications are element bearings, viscosity selection is important. There are several common methods for determining minimum and optimum viscosity requirements for element bearings, most of which use speed factors.
Once the appropriate viscosity has been determined, it’s time to consider additives. The additive and base oil types are components of grease which should be selected in a similar way to lubricating oil applications. Most performance enhancing additives found in lubricating oils are also used in grease formulation, and should be chosen according to the demands of the application.
The consistency of grease is controlled by the thickener concentration, thickener type and the viscosity of the base oil. Even though base oil viscosity affects consistency, it is important to note that a grease can have a high consistency and a low base oil viscosity or vice versa. The NLGI has established a scale to indicate grease consistency which ranges from grades 000 (semi-fluid) to 6 (block grease). The most common NLGI grade is 2.
The final criteria is performance properties. Grease performance properties include many of the same properties used for lubricating oils, as well as others exclusive to grease. Those exclusive to grease include dropping point, mechanical stability, water washout, bleed characteristics and pumpability. The most important performance properties are determined by the application.
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