Principles of naval engineering - Historic Naval Ships Association

Chapter 6-THEORY OF LUBRICATIONcontact occurs. As long as the lubricant filmremains unbroken, sliding friction and rollingfriction are replaced by fluid friction.In any process involving friction, some poweris consumed and some heat is produced. Overcomingsliding friction consumes the greatestamount of power and produces the greatestamount of heat. Overcoming rolling friction consumesless power and produces less heat. Overcomingfluid friction consumes the least powerand produces the least amount of heat.LANGMUIR THEORYA presently accepted theory of lubrication isbased on the Langmuir theory of the action offluid films of oil between two surfaces, one orboth of which are in motion. Theoretically, thereare three or more layers or films of oil existingbetween two lubricated bearing surfaces. Two ofthe films are boundary films (indicated as I andV in part Aof fig.6-1), one of which clings to thesurface of the rotating journal and one of whichclings to the stationary lining of the bearing.Between these two boundary films are one ormore fluid films (indicated as II, III, and IV inpart A of fig. 6-1). The number of fluid filmsshown in the illustration is arbitrarily selectedfor purposes of explanation.When the rotating journal is set in motion(part B of fig. 6-1), the relationship of the journalto the bearing lining is such that a wedge of oilis formed. The oil films II, III, and IV begin toslide between the two boundary films, thus continuouslypreventing contact between the twometal surfaces. The principle is again illustratedin part C of figure 6-1, where the position of theoil wedge W is shown with respect to the positionof the journal as it starts and continues in motion.The views shown in part C of figure 6-1 representa journal or shaft rotating in a solidbearing. The clearances are exaggerated in thedrawing in order to illustrate the formation ofthe oil film. The shaded portion represents theclearance filled with oil. The film is in theprocess of being squeezed out while the journalis at rest, as shown in the stationary view. Asthe journal slowly starts to turn and the speedincreases, oil adhering to the surfaces of thejournal is carried into the film, increasing the'film thickness and tending to lift the journal asshown in the starting view. As the speedincreases, the journal takes the position shown inthe running view. Changes in temperature, withconsequent changes in oil viscosity, causechanges in the film thickness and in the positionof the journal.If conditions are correct, the two surfaces areeffectively separated, except for a possible momentarycontact at the time the motion is started.FACTORS AFFECTING LUBRICATIONSTATIONARYVIEWSTARTINGVIEWRUNNINGVIEW47.78Figure 6-1.— Oil film lubrication. (A) Stationaryposition, showing several oil films; (B) surfaceset in motion, showing principle of oil wedge;(C) principle of (A) and (B) shown in a journalbearing.A number of factors determine the efficacyof oil film lubrication, including such things aspressure, temperature, viscosity, speed, alignment,condition of the bearing surfaces, runningclearances between the bearing surfaces, startingtorque, and the nature and purity of the lubricant.Many of these factors are interrelated andinterdependent. For example, the viscosity ofany given oil is affected by temperature and thetemperature is affected by running speed; hencethe viscosity is partially dependent upon the runningspeed.A lubricant must be able tosticktothe bearingsurfaces and support the load at operating113