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. 11.1. 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.
