ABSTRACT
Grease is a semi-solid, high viscosity type of lubricant that is well-suited for use in bearings, couplings, and open gears where shock loads, high temperatures, and/or good adhesion to bearing surfaces are important performance features. Consider the problem of journal bearing lubrication in Fig. 20.1. The outer cylinder (journal) is fixed and is separated from the center rotating shaft by a layer of grease. When the shaft is at rest or turning at low speeds and/or high loads, metal-to-metal surface contact can occur. Wear protection under these conditions can be provided by lubricant decomposition products or surface-active additives which form thin, soft tribo-films which retard metalto-metal adhesion and reduce friction. As the shaft begins to rotate at higher speeds, it climbs the journal surface in a direction opposite to the direction of rotation. Layers of grease cling to the journal and rotating shaft surfaces, the former remaining stationary and the latter moving in concert with the shaft. Additional grease is carried into the contact zone, and the system enters the hydrodynamic lubrication regime. The choice between oil and grease lubrication depends upon the ratio of
journal speed to viscosity. In general, contacts moving at relatively low relative velocity have higher viscosity requirements than surfaces moving at high velocity. Bearings designated for low speed operation usually are designed with relatively large clearance between the shaft and journal housing to facilitate introduction of high viscosity grease lubricants. High-speed bearings with small clearance are lubricated with lower viscosity oils. Other bearings commonly lubricated by grease include rolling-element bear-
ings such as ball bearings, cylindrical roller bearings, tapered roller bearings, spherical barrel-shaped roller bearings, and needle bearings. In these types of bearings, extremely high local pressures are formed between the relatively small rotating rolling elements and their raceways (support housings). We learned in Chapter 1 that lubricant viscosity increases rapidly at high pressures that enable the lubricating film to withstand high contact stresses while preventing contact between the rolling surfaces. Greases are complex formulations consisting of base fluids, thickeners, struc-
tural components, and additives designed to meet specific application
for Line and Point
FIGURE 20.1
Development of a hydrodynamic film with increased shaft speed in a plain journal bearing: (a) at rest, (b) low rotational velocity, and (c) high rotational velocity. (after Kudish and Covitch [1]). Reprinted with permission from CRC Press.
