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Technical Topic
Grease — Its Components and Characteristics
Depending on the application, grease can present several benefits over fluid lubrication. Greases provide a physical seal preventing contamination ingress, resist the washing action of water, and can stay in place in an application even in vertical y mounted positions. Greases are uniquely suited for use in applications where relubrication is infrequent or economical y unjustifiable, due to the physical configuration of the mechanism, type of motion, type of sealing or the need for the lubricant to perform al or part of any sealing function in the prevention of lubricant loss or ingress of contaminants. Due to their semisolid nature, greases do not provide application cooling and cleaning functions associated with the use of a fluid lubricant. With these exceptions, greases perform al other functions of a fluid lubricant. While fluid lubricants are typical y preferred by design, the aforementioned mechanical circumstances will always exist and thus the need for grease remains. As a result, greases are used in approximately 80% of Picture 2: Penetrometer cone released in grease (soluble or finely dispersed particles such as molybdenum Grease Components
disulfide and graphite) and dyes or pigments. Dyes or pigments Greases are manufactured by combining three essential impart color ONLY having no ef ect on grease’s lubricating components: base oil, thickener, and additives.
Base Oils: Base oil comprises the largest component of a grease, Grease Consistency
representing 80 - 97% by weight. The choice of base fluid may Consistency is defined as the degree to which a plastic material be mineral oil, synthetic oil, or any fluid that provides lubricating resists deformation under the application of force. In the case properties. It must be noted that the base oil portion of a grease of lubricating greases, this is a measure of the relative hardness performs the actual lubrication except in very slow or oscillating or softness and has some relation to flow and dispensing applications. The same rules applied to determine proper viscosity properties. Consistency is measured by ASTM D 217, Cone grade in a fluid lubricant apply to the selection of the base oil Penetration of Lubricating Grease and is often reported in Thickeners: The thickener may be any material that, in Cone Penetration: Grease consistency is measured at 25°C combination with the base oil, will produce the solid to semi-fluid after the sample has been subjected to 60 double strokes structure. Simply put, a grease thickener in combination with the in the ASTM grease worker (picture 1). After the sample has base oil acts much the same way as a sponge holding water. been prepared, a penetrometer cone (picture 2) is released Principal thickeners used in greases include lithium, aluminum, and allowed to sink into the grease under its own weight for calcium soaps; clay; polyurea; either alone or in combination. 5 seconds. The depth the cone has penetrated is then read, Lithium soap is the most common thickener in use today.
in tenths of a mil imeter. The further the cone penetrates the grease, the higher the penetration result and the softer the Additives: As in lubricating oil additives, grease additives and modifiers impart special properties or modify existing ones. Additives and modifiers commonly used in lubricating greases NLGI Grade: The NLGI (National Lubricating Grease Institute) are oxidation or rust inhibitors, polymers, extreme pressure (EP) has standardized a numerical scale for grease consistency additives, anti-wear agents, lubricity or friction-reducing agents based upon ASTM D 217 worked penetration ranging from 000 for semifluid to 6 for block greases (table 1). The most In application and use, ingress of environmental contaminants common grease grade is NLGI 2 representing a smooth, buttery is unfortunately a common reality that often adversely consistency. It must be noted that grease consistency is related to affects the mechanical stability of the grease. It is important thickener content and has no relationship to base oil viscosity. that greases not only be developed to provide excellent structural stability in a pristine state, but also in the presence of environmental contaminants such as water, process fluids, or other contaminants. This can be assessed by means of laboratory bench tests operating in a variety of conditions with Dropping point: The dropping point of grease is the temperature at which the thickener loses its ability to maintain the base oil within the thickener matrix. This may be due to the thickener melting or the oil becoming so thin that the surface tension and capillary action become insufficient to hold the oil within the thickener matrix. ASTM D2265 (preferred over the older and less precise ASTM D566) is the standard method used to determine the dropping point of grease. A small Grease Structural Stability
grease sample is placed in a cup and heated in a control ed Mechanical stability: This is an essential performance manner in an oven-like device. When the first drop of oil characteristic of lubricating grease as it is a measure of how the fal s from the lower opening of the cup, the temperature is grease consistency wil change in service when it is subjected to recorded to determine the dropping point (picture 4). Dropping mechanical stress (shear) resulting from the churning action caused point is a function of the thickener type. High drop points, by moving elements or vibrations generated by, or external to, the typical y above 240°C, are commonly observed for lithium application. Grease softening in a bearing may eventually cause complex, calcium complex, aluminum complex, polyurea grease to leak out from the housing, requiring more maintenance and clay greases while much lower dropping points are and frequent grease replenishment to avoid premature failure typical of conventional lithium (180°C), calcium (180°C) and resulting from lack of lubricant on the rolling elements. In order to sodium (120°C) soaps. The dropping point is one of the have good mechanical stability, greases are developed through determinations that characterise the grease’s thermal stability. careful selection of the thickener composition and optimization of However it is NOT an accurate prediction of the grease’s the manufacturing process. Mechanical stability is often measured upper operating temperature limit which is a function of using the ASTM D217 prolonged worker test (e.g., 100,000 double many variables such as base oil oxidation stability, additive strokes), or the ASTM D1831 Rol Stability test. ASTM D1831 degradation, thickener shearing, oil separation and so forth. subjects the grease to shearing by rotating a cylinder containing a A high dropping point, while not a predictor of upper operating 5kg rol er at 165 rpm for 2 hours. The change in penetration at the temperature, is an indicator of the maximum peak temperature end of the tests is a measure of the mechanical stability. Picture 3 that the grease may be subjected to for a short duration while illustrates extreme mechanical softening of one grease on the left not releasing oil excessively and therefore drastical y reducing compared to little softening of another grease on the right. This test the life of the grease and potential y damaging the equipment produces low shearing forces approximately equal to those found in Picture 3: Grease at end of ASTM D1831 Roller Test Picture 4: Dropping Point Determination — Thermometer positioned in cup for determining the temperature at which www.mobilindustrial.com
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