Machinery's Handbook, 31st Edition
2500 Lubricating Films Full Film Hydrodynamic Lubrication (HDL): HDL is the desired lubrication condition for plain style bearings in which both surfaces are fully separated by a working or specific film thickness lambda, λ of more than 2 at the point of pressure distribution loading shown in Fig. 2 . A fluid wedge is created in which the asperities will not collide. Both surfaces are said to be “metal-contact” free at all times.
Bearing
Journal (Shaft)
– –
Full Film Thickness Magnified Surfaces
Fig. 2. HDL Hydrodynamic Lubrication of a Journal Bearing As the shaft speed accelerates, rotation of the journal acts as a pump, forcing lubricant into the pressure distribution area. Providing the lubricant is of a high enough viscos- ity, the wedge-shaped lubricant channel will create a load-carrying pressure sufficient to completely separate the two surfaces and support the moving journal. Full film thickness will vary between 5 and 200 microns depending on speed, load, and viscosity. As the speed increases so does the lubricating action and ability to carry heavier loads. Inversely, slow speeds do not allow the lubricant wedge to form, causing breakdown of the hydrody- namic action and an undesirable Boundary Layer Lubrication state to prevail. Boundary Layer Lubrication (BL): When a journal shaft is at rest in the bearing, any full film lubricant wedge has collapsed leaving a residual film of lubricant in its place, insufficient to prevent metal-to-metal contact from occurring. Upon subsequent start up, the bearing surfaces partially collide and ride on the thin lubricant film (start up condi - tions promote heavy wear). When lubricant supply is inadequate, or heavy loads coupled with low shaft speeds is the only design possible, the boundary layer lubrication must rely heavily on the composition of the lubricant to provide specific anti-wear and extreme pressure sacrificial additives, designed to retard premature wear. These surface-active additives act to form a thin surface laminate that prevents metal adhesion. Boundary layer lubrication also occurs when a lubricant of too low a viscosity is chosen. Mixed Film Lubrication (MF): Mixed film lubrication state is generally encountered under shock load conditions when a minimum thickness hydrodynamic film momentarily breaks down or “thins out” into a boundary layer condition under severe shock load. Mixed film condition is also encountered as a shaft accelerates toward full speed and the film thickness transforms from boundary to full hydrodynamic condition. Choosing too light a viscosity lubricant can lead to momentary or full time mixed film condition. Mixed film condition is encountered when the specific film thickness lambda λ is between 1 and 2. Lubricating Film Transition: Boundary layer condition is encountered when the specific film thickness lambda λ ratio is less than 1, mixed-film when the lambda λ ratio is between 1 and 2, and hydrodynamic when the lambda λ ratio is more than 2. Once the lambda λ ratio surpasses 4, relative bearing life is increased four-fold as depicted in Fig. 3. Lambda λ < 1 1 < Lambda λ < 2 2 < Lambda λ < 4 Boundary Layer Film Lubrication Mixed Film Lubrication Hydrodynamic Lubrication
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