Hydrodynamic lubrication regime

Hydrodynamic injection, capillary electrophoresis, 4 633-634 Hydrodynamic lubrication regime, 15 210-211... 

In the hydrodynamic lubrication regime the properties of the continuous lubricant film that separates the two surfaces, in particular its viscosity, determine the friction coefficient. The average thickness of the liquid film h is much larger than the average roughness of the contact surfaces ... 

Table I compiles the relationships between measured forces, impact on syringe application and physics involved. Sustaining forces at velocities encountered during manual use of syringes are related to the hydrodynamic lubrication regime. Lubricant is...

Fig. 28—Different stages in transition of lubrication regimes, (a) Full-film lubrication with film thickness much larger than roughness h/cr> ). (b) Surfaces are separated but roughness effect becomes significant (5>/i/cr>3). (c) Asperities interfere with each other but hydrodynamic films carry the most load (h/cr 3). (d) Typical mixed lubrication with load shared by lubrication and asperity (h/cr<3). (e) Boundary lubrication when asperities carry the most part of load (h/a-<0.S).

Depending on the thickness of the lubricating layer, we distinguish between two different lubrication regimes. In hydrodynamic lubrication the lubrication layer is thicker than the maximum height of the surface asperities resulting in a complete separation of the friction partners. In boundary lubrication the lubrication layer is typically only a few molecular layers thick and therefore thinner than the surface roughness. In many practical applications we are between the two extremes, which is referred to as mixed lubrication. 

Figure 11.12 Stribeck diagram Plot of friction coefficient /u versus ijkv/P,where rjk is the kinematic viscosity, v the velocity, and P the contact pressure. From left to right there are three different friction regimes Boundary lubrication with high friction and wear, mixed lubrication with medium friction and wear, and hydrodynamic lubrication with low friction and wear.

Nascent surface Explain the difference in the concept of liquid lubrication mechanism in (a) hydrodynamic, (b) elastohydrodynamic and (c) boundary lubrication. Which of the following characterize (a), (b), and (c) lubrication regime continuous fluid film, negligible deformation, complete separation of the surfaces, elastic and plastic deformation, no wear takes place, no contact between the sliding surfaces, involving surface topography, physical and chemical adsorption, catalysis and reaction kinetics, and tribochemical film formation ... 

A qualitative model for lubrication of the condensed alcohol layer is not fully developed yet. We present here the best qualitative model based on the literature. As the applied pressure is very large and the scanning speed is very low in AFM, one can rule out the hydrodynamic lubrication even in the presence of condensed alcohol layer on the substrate. The AFM tip and substrate interface must be in the boundary lubrication regime. Figs. 7-10 show that the adhesion force reduction upon alcohol adsorption is accompanied by reduction of the friction force. Part of the adhesion force reduction is because of the surface tension decrease of the water layer on the substrate. When alcohol is dissolved in water, the surface tension decreases significantly because of segregation of alcohol molecules to the liquid-air interface. ... 

Hydrodynamic lubrication is characterized by relatively large film thickness, typically greater than 0.25 pm, which is substantially greater than surface asperity heights. In a hydrodynamic regime, surfaces are generally conformal with a positive fluid pressure. 

In hydrodynamic lubrication, the load is supported by the pressure developed due to relative motion and the geometry of the system. In the regime of hydrodynamic or fluid film lubrication, there is no contact between the solids. The film thickness is governed by the bulk physical properties of the lubricants, the most important being viscosity friction arises purely from shearing of viscous lubricant. 

Several examples of experimental approaches to thin-film lubrication have been reported [3]. It is important in examining these techniques to make the distinction between methods that are used to determine lubricant film thickness under hydrodynamic or elastohydrodynamic conditions (e.g., optical interference, electrical capacitance, or x-ray transmission), and methods that are used to determine the occurrence or frequency of contact. As we will see later, most experimental studies of synovial joint lubrication have focused on friction measurements, using the information to determine the lubrication regime involved this approach can be misleading. 

Elastohydrodynamic Lubrication. With liquid lubricants, as the thickness of the lubricant film decreases, one may reach the point at which the properties of the lubricant are no longer those of the bulk material, but rather those of a special film only a few molecules thick, which is penetrated by large surface asperities leading to excessive wear. Such a situation does not correspond to normal hydrodynamic lubrication, but neither does it indicate the onset of classic boundary lubrication (see discussion below). This intermediate regime is referred to as elastohydrodynamic lubrication. ... 

F ure 10.8 Lubrication regimes (a) hydrodynamic lubrication (b) mixed lubrication and (c) boundary lubrication. 

Hydrodynamic lubrication is favored by a high sliding velocity, a weak normal force and a high viscosity of the lubricant. In machines, lubricated contacts normally operate in the hydrodynamic regime, minimizing thus friction and wear. 

Here, tj designates the viscosity of the lubricant, Vgi is the sliding velocity, and Fn is the normal force. The positions of the different lubrication regimes on the Stribeck curve are indicated in Figure 10.9. For hydrodynamic lubrication, the firiction coefficient can be described by the relation (10.16), where kf is a proportionality factor. 

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