Chapter 6-THEORY OF LUBRICATION 



contact occurs. As long as the lubricant film 

 remains unbroken, sliding friction and rolling 

 friction are replaced by fluid friction. 



In any process involving friction, some power 

 is consumed and some heat is produced. Over- 

 coming sliding friction consumes the greatest 

 amount of power and produces the greatest 

 amount of heat. Overcoming rolling friction con- 

 sumes less power and produces less heat. Over- 

 coming fluid friction consumes the least power 

 and produces the least amount of heat. 



LANGMUIR THEORY 



A presently accepted theory of lubrication is 

 based on the Langmuir theory of the action of 

 fluid films of oil between two surfaces, one or 

 both of which are in motion. Theoretically, there 

 are three or more layers or films of oil existing 

 between two lubricated bearing surfaces. Two of 

 the films are boundary films (indicated as I and 

 V in part Aof fig.6-1), one of which clings to the 



STATIONARY 

 VIEW 



STARTING 

 VIEW 



RUNNING 

 VIEW 



47.78 

 Figure 6-1.— Oil film lubrication. (A) Stationary 

 position, showing several oil films; (B) surface 

 set in motion, showing principle of oil wedge; 

 (C) principle of (A) and (B) shown in a journal 

 bearing. 



surface of the rotating journal and one of which 

 clings to the stationary lining of the bearing. 

 Between these two boundary films are one or 

 more fluid films (indicated as II, III, and IV in 

 part A of fig. 6-1). The number of fluid films 

 shown in the illustration is arbitrarily selected 

 for purposes of explanation. 



When the rotating journal is set in motion 

 (part B of fig. 6-1), the relationship of the journal 

 to the bearing lining is such that a wedge of oil 

 is formed. The oil films II, III, and IV begin to 

 slide between the two boundary films, thus con- 

 tinuously preventing contact between the two 

 metal surfaces. The principle is again illustrated 

 in part C of figure 6-1, where the position of the 

 oil wedge W is shown with respect to the position 

 of the journal as it starts and continues in motion. 



The views shown in part C of figure 6-1 rep- 

 resent a journal or shaft rotating in a solid 

 bearing. The clearances are exaggerated in the 

 drawing in order to illustrate the formation of 

 the oil film. The shaded portion represents the 

 clearance filled with oil. The film is in the 

 process of being squeezed out while the journal 

 is at rest, as shown in the stationary view. As 

 the journal slowly starts to turn and the speed 

 increases, oil adhering to the surfaces of the 

 journal is carried into the film, increasing the' 

 film thickness and tending to lift the journal as 

 shown in the starting view. As the speed 

 increases, the journal takes the position shown in 

 the running view. Changes in temperature, with 

 consequent changes in oil viscosity, cause 

 changes in the film thickness and in the position 

 of the journal. 



If conditions are correct, the two surfaces are 

 effectively separated, except for a possible mo- 

 mentary contact at the time the motion is started. 



FACTORS AFFECTING LUBRICATION 



A number of factors determine the efficacy 

 of oil film lubrication, including such things as 

 pressure, temperature, viscosity, speed, align- 

 ment, condition of the bearing surfaces, running 

 clearances between the bearing surfaces, start- 

 ing torque, and the nature and purity of the lubri- 

 cant. Many of these factors are interrelated and 

 interdependent. For example, the viscosity of 

 any given oil is affected by temperature and the 

 temperature is affected by running speed; hence 

 the viscosity is partially dependent upon the run- 

 ning speed. 



A lubricant must be able tosticktothe bear- 

 ing surfaces and support the load at operating 



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