Why Use Grease?
Grease lubricates more bearings than any other lubricant, and is used when oil is either not convenient or not practical. Grease can lubricate accessibility-challenged equipment for extended lengths of time. It can form a thicker film than oil in high temperature and shock loading conditions. If a machine only runs intermittently, grease can instantly form a lubricating film. Another advantage is that grease can effectively create a seal preventing contaminant ingression.
Viscosity
Viscosity is the most important physical property of any lubricating medium, however, with grease it seems to be ignored. Maintenance personnel tend to think that one NLGI #2 EP grease is the same as any other #2 EP grease which does not indicate viscosity. Great care is taken in the viscosity selection of lubricating oils and the same care has to be taken with grease. Grease is oil that is thickened which reverts back to the base oil viscosity when it is under shear force. Lubricant viscosity is a design factor and is published in OEM manuals. To get the life out of the equipment it is paramount to get the viscosity right.
A common grease application would be centrifugal pump bearings in which the general purpose grease would be used. ITT Industries recommends a NLGI #2 grease at about 46 centistokes at 40°C (ISO VG) for their centrifugal pumps. Most general purpose grease is around an ISO VG 150 to 220. Using the general purpose grease in this application would cause churning and overheating leading to reduced bearing life.
NLGI Grades
The National Lubricating Grease Institute created a numeric scale based on consistency of worked greases. The consistency ranges from 000 which is semi-liquid (soft) up to 6 which is semi-solid (hard). The NLGI grades indicate how the grease flows and how well they tend to ‘stay put'. To specify grease in any application the NLGI grade alone is not enough information.
Thickeners
The purpose of a thickener is to hold the oil in a ‘sponge' like matrix until it is drawn in and the oil is squeezed out due to pressure in such areas as gear tooth contact and rolling element bearings. Thickeners can be broken down into three groups: Soap Thickeners, Complex Soap Thickeners and Non-Soap Thickeners. Soap thickeners are created through a chemical reaction between a fatty acid (derived from plant or animal fat) and a metal. Complex soap thickeners are created through a chemical reaction between a fatty acid, a metal and another acid. Each thickener has its own properties and benefits which is summarized in the chart below.
Mixing grease of different thickeners should never be performed unless adequate testing has been done on the mixture. Even if the thickeners are compatible the properties of the mixed grease is unknown and the properties may be less desirable than either grease on its own.
The two most notable thickeners are Lithium Complex and Polyurea Complex. These two thickeners have great overall properties and are closest to being an all purpose grease. Polyurea greases are very popular motor bearing greases due to the ‘built-in' extreme pressure and anti-wear properties (EP additives can damage motor windings).
Base Oils and Additives
As mentioned grease is an oil combined with a thickener. Any base oil can be combined with a thickener so there are both synthetic and mineral greases. The same additives in oils can also be found in grease, the most common being EP additives. If the type of grease is being changed due to different suppliers or otherwise the additives and base oil compatibility is just as important as thickener compatibility.
As with all oils, grease does not have an unlimited shelf life because the base oil gradually oxidizes. Grease should be stored in a clean dry environment and stock should be rotated using the First-In-First-Out principle. Each grease in use should have dedicated grease guns in order to avoid cross contamination.
Conclusion
Grease plays an important role in industry yet it is misunderstood. One all purpose grease cannot accomplish the full range of lubrication tasks assigned to it. Each application has specific requirements from the grease and the selection has to reflect this. There is room for consolidation of greases a plant uses, but consolidation should not be a higher priority than equipment reliability. Viscosity of a lubricant is a part of the machine design an advantage can be taken from the use of complex greases with synthetic base oils to perform within viscosity specifications over a broad range of temperature. As equipment maintainers we all have to work in order to give grease the respect it deserves and to stop treating grease like oil's ugly cousin.
