Regimes of Fluid Film Lubrication Introduction Sliding between clean solid surfaces generally results in high friction and severe wear. Clean surfaces readily adsorb traces of foreign substances, such as organic compounds, from the environment. The newly formed surfaces generally have a much lower coefficient of friction and wear than the clean surface. Presence of a fluid film between two surfaces in relative motion prevents solid-solid contact and can provide very low friction (~ 0.001001-0.003) and negligible wear. The presence of a layer of foreign material can not be guaranteed during a sliding process; therefore, lubricants are deliberately applied to produce low friction and low wear. A lubricant is any substance (solid or fluid film) that reduces friction and wear and provides smooth running and a satisfactory life for machine elements. Most lubricants are liquids (such as mineral oils, synthetic esters, silicone fluids, and water), but they may be solids (such as polytetrafluoroethylene, or PTFE) for use in dry bearings, greases for use in rolling-element bearings, or gases (such as air) for use in gas bearings. The physical and chemical interactions between the lubricant and the lubricating surfaces must be understood in order to provide the machine elements with satisfactory life. As an aid in understanding the features that distinguish the lubrication regimes from one another, a short description of each regime is given. 1
Regimes of Fluid Film Lubrication Hydrostatic lubrication requires an external pumping agency. Other lubrication regimes without an external pumping agency (self-acting) are found in the Stribeck Curve. Hydrostatic Lubrication In Hydrostatic lubrication, a thick fluid film is maintained between two surfaces with little or no relative motion by an external pumping agency, a pump, which feeds pressurized fluid to the film. Since hydrostatic bearings do not require relative motion of the bearing surfaces to build up the load-supporting pressures as necessary in hydrodynamic bearings, hydrostatic bearings can be used in applications with little or no relative motion between the surfaces. Hydrostatic bearings provide high stiffness. However, hydrostatic bearings have the disadvantage of requiring highpressure pumps and equipment for fluid cleaning which adds to space and cost. Fig. 8.4.1 Schematics of (a) a hydrostatic thrust bearing with circular step pad, and (b) fluid supply system (Williams, 1994). 2
Other Lubrication Regimes The Stribeck curve shows all other lubrication regimes. It presents coefficient of friction as a function of Hersey number (ηn/p) absolute viscosity x rotational speed in rev./s divided by load per unit projected area. The regimes of lubrication are sometimes also identified by a lubricant film parameter (h/σ) mean film thickness/composite standard deviation of surface heights of two surfaces. A high Hersey number usually means a relatively thick lubricant film, whereas a small number results in a very thin film. At an extremely low Hersey number, no real lubricant film can develop and there is significant asperity contact, resulting in high friction. Fig. 8.2.1 Lubricant film parameter (h/σ) and coefficient of friction as a function of ηn/p (Stribeck curve) showing different lubrication regimes observed in fluid lubrication without an external pumping agency. 3
This high h friction value is persistent t with increasing Hersey number until a first threshold value is reached. This represents the dominance of boundary lubrication in determining i load transfer and friction between surfaces. As the Hersey number increases, a noticeable and rapid decrease in friction values is observed. This is explained by an increasing lubricant film thickness and shared load support between the surface asperities and the pressurized liquid lubricant present in the conjunction. In this regime, widely varying friction values can be measured and are strongly dependent on operating conditions. With a further increase in Hersey number, friction reaches a lower plateau value, corresponding to the onset of hydrodynamic lubrication. At this point, the surfaces are effectively separated by the liquid lubricant, and asperity contact has negligible effect on load support and friction. Fig. 8.2.1 Lubricant film parameter (h/σ) and coefficient of friction as a function of ηn/p (Stribeck curve) showing different lubrication regimes observed in fluid lubrication without an external pumping agency. 4
The Stribeck curve shows a slight increase in friction with respect to Hersey number in the hydrodynamic regime. This is a common occurrence for certain bearing geometries, such as journal bearings, where film thickness is limited by geometric influences and not operating conditions. Increased friction can be attributed to increased redundant d work in the lubricant or to increases in shear strength, but these are seen to be relatively minor effects. Similarly, the plateau value of friction in the boundary regime may not be present if the lubricant does not contain the proper chemistry, and the friction may continue increasing with decreasing film thickness. A further distinction is often made between elastohydrodynamic and full film lubrication in a Stribeck curve. This is useful for some bearing types, gears or cams, but it should be recognized that conformal contacts do not encounter elastohydrodynamic lubrication. Therefore, this regime may or may not be identified in the Stribeck curve. Fig. 8.2.1 Lubricant film parameter (h/σ) and coefficient of friction as a function of ηn/p (Stribeck curve) showing different lubrication regimes observed in fluid lubrication without an external pumping agency. 5
Hydrodynamic d (HD) Lubrication It is sometimes called fluid-film or thick-film lubrication. As a bearing with convergent shape in the direction of motion starts to move in the longitudinal direction from rest, a thin layer of fluid is pulled through because of viscous entrainment and is then compressed between the bearing surfaces, creating a sufficient (hydrodynamic) pressure to support the load without an external pumping agency. A high load capacity can be achieved in the bearings that operate at high velocities in the presence of fluids of high viscosity. These bearings are also called self-acting bearings. Fluid Film can also be generated only by a reciprocating or oscillating motion in the normal direction towards each other (squeeze). This load-carrying phenomenon arises from the fact that a viscous fluid cannot be instantaneously squeezed out from the interface with approaching surfaces. It takes a finite time for the surfaces to meet. During that period, because of the fluid s resistance to extrusion, a pressure is built up and the load is supported by the fluid film. When the load is relieved or the two surfaces move apart, the fluid is sucked in and the fluid film can often recover its thickness in time for next application. The squeeze phenomena controls the build up of a water film under the car tires on wet roadways (hydroplaning). HD lubrication is often referred to as the ideal lubricated contact condition because h/σ > 5. The coefficient of friction can be as small as 0.001. The friction increases slightly with the sliding speed because of viscous drag. Physical contact occurs during start-stop operations at low surface speeds. 6
Elastohydrodynamic (EHD) Lubrication (EHL) EHL is a subset of HD lubrication in which the elastic deformation of the contacting solids plays a significant ifi role in the HD lubrication process. The film thickness in EHL is thinner (~0.5 5 μm) than that in conventional HD lubrication. In isolated areas, asperities may actually touch. Therefore, in liquid lubricated systems, boundary lubricants that provide boundary films on the surfaces for protection against any solid- solid contact t are used. EHL is important in heavily loaded contacts (such as machine elements of low geometrical conformity) and low elastic modulus contacts of high geometrical conformity (such as lip seals, journal and thrust bearings with soft liners). In EHL, in addition to adhesive wear and corrosive wear as in HD lubrication, in well designed heavily loaded contacts (e.g. rolling element bearings and gears) fatigue wear is most common. 7
Mixed Lubrication The transition between the HD/EHD lubrication and boundary lubrication regimes is known as mixed lubrication. There may be more frequent solid contacts, but at least a portion of the bearing surface remains supported by a partial HD film. Boundary Lubrication As the load increases, speed decreases or the fluid viscosity decreases in the Stribeck curve, the coefficient of friction can increase to high levels (~ 0.1 or higher) in the boundary lubrication regime. Boundary lubrication is that condition in which the solid surfaces are so close together that surface interaction between molecularly thick films of lubricants (liquids or gases) and the solid asperities dominate the contact. The failure in boundary lubrication occurs by adhesive and chemical (corrosive wear). 8
Hydrodynamic d Bearings Hydrodynamic action occurs in the bearings with convergent clearance space through the length of the bearing. The loads carried by a rotating machinery may be radial load and/or an axial or thrust loads. The thrust load is carried by a thrust bearing. The surfaces of a thrust bearing are perpendicular to the axis of rotation. Thrust bearings consist of multiple pads. The pad geometry is selected such that it results in a convergent clearance. The radial load is carried by a journal bearing. The surfaces of a journal bearing are parallel to the axis of rotation. Eccentricity of the shaft with respect to the journal bearing during rotation results in formation of convergent clearance. Fig. 8.5.2 852 Schematics of typical thrust and journal bearing configurations. 9
Thrust Bearings Fixed-inclined-pad thrust bearing Multiple-pivoted-pad thrust bearing Fig. 8.5.3 Schematics of various shapes for pads in thrust bearings (Raimondi and Boyd, 1955). Fig. 8.5.4 Schematic of a multiple-pivoted-pad thrust bearing. 10
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