Machinery's Handbook, 31st Edition
PLAIN BEARINGS
2397
Full Film
ZN/P
Fig. 3. Three Modes of Bearing Operation. 1) Full-film, or hydrodynamic, lubrication produces a complete physical separation of the sliding surfaces resulting in low friction and long wear-free service life. To promote full-film lubrication in hydrodynamic operation, the following parameters should be satisfied: a) Lubricant selected has the correct viscosity for the proposed operation; b) proper lubricant flow rates are maintained; c) proper design methods and considerations have been utilized; d) and surface velocity in excess of 25 fpm (7.62 m/min) is maintained. When full-film lubrication is achieved, a coefficient of friction between 0.001 and 0.005 can be expected. 2) Mixed-film lubrication is a mode of operation between the full-film and boundary modes. With this mode, there is a partial separation of the sliding surfaces by the lubricant film; however, as in boundary lubrication, limitations on surface speed and wear will result. With this type of lubrication, a surface velocity in excess of 10 fpm (3.05 m/min) is required with resulting coefficients of friction of 0.02 to 0.08. 3) Boundary lubrication takes place when the sliding surfaces are rubbing together with only an extremely thin film of lubricant present. This type of operation is acceptable only in applications with oscillating or slow rotary motion. In complete boundary lubrication, the oscillatory or rotary motion is usually less than 10 feet per minute with resulting coef ficients of friction of 0.08 to 0.14. These bearings are usually grease lubricated or periodi cally oil lubricated. In starting up and accelerating to its operating point, a journal bearing passes through all three modes of operation. At rest, the journal and bearing are in contact, and thus when starting, the operation is in the boundary lubrication region. As the shaft begins to rotate more rapidly and the hydrodynamic film starts to build up, bearing operation enters
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