Machinery's Handbook, 31st Edition
Bearing Lubricants
2405
Table 2. NLGI Consistency Numbers
NLGI a Consistency No.
Consistency of Grease
Typical Method of Application
0 1 2 3 4 5 6
Semifluid Very soft
Brush or gun Pin-type cup or gun
Soft
Pressure gun or centralized pressure system Pressure gun or centralized pressure system Pressure gun or centralized pressure system
Light cup grease Medium cup grease Heavy cup grease
Pressure gun or hand Hand, cut to fit
Block grease
a NLGI is National Lubricating Grease Institute strength, they shear more readily than the bearing material and thereby allow relative mo- tion. So long as solid lubricant remains between the moving surfaces, effective lubrication is provided and friction and wear are reduced to acceptable levels. Solid lubricants provide the most effective boundary films in terms of reduced friction, wear, and transfer of metal from one sliding component to the other. However, there is a significant deterioration in these desirable properties as the operating temperature of the boundary film approaches the melting point of the solid film. At this temperature the friction may increase by a factor of 5 to 10 and the rate of metal transfer may increase by as much as 1000. What occurs is that the molecules of the lubricant lose their orientation to the surface that exists when the lubricant is solid. As the temperature further increases, additional deterioration sets in with the friction increasing by some additional small amount but the transfer of metal accelerates by an additional factor of 20 or more. The final effect of too high temperature is the same as metal-to-metal contact without benefit of lubricant. These changes, which are due to the physical state of the lubricant, are reversed when cooling takes place. The effects just described also partially explain why fatty acid lubricants are superior to paraffin base lubricants. The fatty acid lubricants react chemically with the metallic surfaces to form a metallic soap that has a higher melting point than the lubricant itself, the result being that the breakdown temperature of the film, now in the form of a metallic soap is raised so that it acts more like a solid film lubricant than a fluid film lubricant. Journal or Sleeve Bearings Although this type of bearing may take many shapes and forms, there are always three basic components: journal or shaft, bushing or bearing, and lubricant. Fig. 1 shows these components with the nomenclature generally used to describe a journal bearing: W = ap- plied load, N = revolution, e = eccentricity of journal center to bearing center, θ = attitude angle, which is the angle between the applied load and the point of minimum film thick - ness, d = diameter of the shaft, c d = bearing clearance, d + c d = diameter of the bearing and h o = minimum film thickness. Grooving and Oil Feeding.— Grooving in a journal bearing has two purposes: 1) to establish and maintain an efficient film of lubricant between the bearing moving surfaces and 2) to provide adequate bearing cooling The obvious and only practical location for introducing lubricant to the bearing is in a region of low pressure. A typical pressure profile of a bearing is shown by Fig. 2. The arrow W shows the applied load. Typical grooving configurations used for journal bearings are shown in Fig. 3a through Fig. 3e. Heat Radiating Capacity.— In a self-contained lubrication system for a journal bearing, the heat generated by bearing friction must be removed to prevent continued temperature
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