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TABLE 20.11 Some Components Used in Grease Manufacture
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tance in tenths of a millimeter to which a standard metal cone penetrates the grease under a standard load; the result is known as the penetration. A widely used classification of greases is that of the American National Lubricating Grease Institute (NLGI), and Table 20.12 shows the relationship between NLGI number and penetration.
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TABLE 20.12 NLGI Grease Classification
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The consistency of a grease varies with temperature, and there is generally an irregular increase in penetration (softening) as the temperature increases. Eventually a temperature is reached at which the grease is soft enough for a drop to fall away or flow from the bulk of the grease; this is called the drop point. The drop point is usually taken to be the maximum temperature at which the grease can be used in service, but several factors confuse this situation: 1. The drop point is measured in a standard apparatus which bears no resemblance to any service equipment, so that the correlation with service use may be poor. 2. Some greases will never give a drop point because chemical decomposition begins before the thickener structure breaks down. 3. A grease may be a satisfactory lubricant above its drop point, although then it will behave like an oil rather than a grease.
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Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
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4. Some greases can be heated above their drop points and will again form a grease when cooled, although normally the re-formed grease will be markedly inferior in properties. At high temperature greases will decompose thermally or oxidatively in the same way as lubricating oils. In addition, the grease structure may break down, as explained previously, or the thickener itself may decompose. Table 20.13 depicts the general effects of temperature on lubricating greases. A grease behaves as an extreme form of non-Newtonian fluid, and its viscous properties change when it is sheared in a feed line or a bearing. Occasionally the viscosity increases with small shear rates, but more commonly the viscosity decreases as the shear rate increases, until eventually the viscosity reaches that of the base oil. For this reason, the viscosity of the base oil may be important if the grease is to be used in high-speed equipment. The mechanism by which a grease lubricates is more complicated than that for an oil, and it depends partly on the geometry of the system. Some part of the total grease fill distributes itself over the contacting surfaces and is continually sheared in the same way as an oil. This part of the grease performs the lubricating function, giving either hydrodynamic lubrication or boundary lubrication according to the load, speed, and effective viscosity. The remainder of the grease is swept out of the path of the moving parts and remains almost completely static in the covers of a bearing or the upswept parts of a gearbox. Because of the solid nature of the grease, there is virtually no circulation or exchange between the static, nonlubricating portion and the moving, lubricating portion. In a plain bearing or a closely fitting gearbox, a high proportion of the grease fill is being continuously sheared at the contacting surfaces. In a roller bearing or a spa-
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TABLE 20.13 Temperature Limits in Degrees Celsius for Greases as a Function of Required Life
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LUBRICATION 20.21
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cious gearbox, a small proportion of the grease is continuously sheared and provides all the lubrication, while the larger proportion is inactive. If a rolling bearing or gearbox is overfilled with grease, it may be impossible for the surplus to escape from the moving parts. Then a large quantity of grease will be continuously sheared, or churned, and this causes a buildup of temperature which can severely damage the grease and the components. It is, therefore, important with grease lubrication to leave a void space which is sufficient to accommodate all the surplus grease; in a ball bearing, this could be more than 60 percent of the total space available. The static grease which is not involved in lubrication may fulfill two useful functions: It may provide a very effective seal against the ingress of dust or other contaminants, and it can prevent loss of base oil from the grease fill. In addition, the static grease may form a reservoir from which to resupply the lubricated surfaces if the lubricating portion of the grease becomes depleted. If the void space in the system is large, i.e., in a large bearing or gearbox, then usually it is desirable to use a stiffer grease to avoid the surplus grease slumping into the moving parts and being continuously churned. The advantages and disadvantages of grease lubrication are summarized in Table 20.14. The selection of a grease for a specific application depends on five factors: speed, load, size, temperature range, and any grease feed system. For average conditions of speed, load, and size with no feed system, an NLGI no. 2 grease would be the normal choice, and such a grease with a mineral-oil base is sometimes known as a multipurpose grease. The effect of the various factors on selection can then be summarized in a few paragraphs. 1. Speed For high speeds, a stiffer grease, NLGI no. 3, should be used except in plain bearings, where no. 2 would usually be hard enough. For lower speeds, a softer grease such as no. 1 or no. 0 should be used.
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