Machinery Lubrication

Machinery Lubrication May-June 2017

Machinery Lubrication magazine published by Noria Corporation

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44 | May - June 2017 | ilm strength is one of the most critical lubricant prop- erties for protecting a machine's internal compo- nents from wear and degradation. It is heavily influenced by the lubricant's base stock and additives. This article will discuss the importance of a lubricant's film strength and what impacts its effectiveness. Film Thickness When you think of lubrication, what comes to mind? It might be the base oil creating a film thickness to separate two metal surfaces. After all, the primary intent is to avoid metal-on-metal surface contact. For the base oil to provide separation in this situation, there must be a balance of three contributing factors: the relative velocities, the base oil viscosity and the amount of load. These three factors are also influenced by other elements such as temperature and contamination. When the film thickness is the result of a balance between these factors, it is called hydrody- namic lubrication. In applications with rolling contact (and thus negligible relative sliding motion), film thickness between the metal surfaces can still occur, even with greater localized pres- sure points. In fact, these pressure points play an important role. The base oil's pres- sure- viscosity relationship allows the oil's viscosity to increase temporarily due to the higher pressures. This is called elastohy- drodynamic lubrication. A full film separation remains, albeit a very thin one. In practice, it is best to keep machine surfaces separated, with the film thickness providing the best opportunity for reduced friction and wear. But what happens if these film thickness conditions are not met, such as when there is insufficient relative velocity, inadequate viscosity or too much load? Most machine designs and operating parameters will require instances when insufficient velocity exists, such as starts and stops or changes in directional motion. There may also be concerns when the temperature increases too much, causing the viscosity to decrease, or excessive contamination contributes to abrasive contact in the film gap. When the hydrodynamic or elastohydro- dynamic lubrication prerequisites are not met, the base oil will require support during F Benne t t Fi t ch | Nori a Corpor at ioN LESSONS IN LUBRICATION IMPORTANCE of Lubricant FILM STRENGTH S y n t h e t i c L u b r i c a n t s The Boundary lubrication 90% of load rests on surface peaks Sliding Sliding

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