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Fluid Film LubricationIntroduction

• Sliding between clean solid surfaces generally results in high friction & 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.

• The presence of a layer of foreign material can not be guaranteed during a sliding process; therefore, lubricants (solid or fluid film) are deliberately applied to produce low friction and low wear. Fluid can be liquid or gaseous (vapor phase lubrication).

• A thin fluid film, even on the order of surface roughness of moving surfaces, results in relatively low friction and wear, as compared to solid-solid contact.

• A thick fluid film between two surfaces in relative motion prevents solid-solid contact and can provide very low friction (~ 0.001 - 0.003) and negligible wear.

• Fluid film lubrication can be technological nuisance since filters, pumps, and cooling systems are required to maintain the performance of the lubricant. Limitations include loss of load carrying capacity at high temperatures and degradation in service.

• Performance depends on its composition and its physical and chemical characteristics .

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•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.

•Hydrostatic lubrication requires an external pumping agency.

•Other lubrication regimes without an external pumping agency (self-acting) are found in the Stribeck Curve:

•The Stribeck curve shows all other lubrication regimes.

•It presents coefficient of friction as a function of (N/P) –absolute viscosity x rotational speed in rev./s divided by load per unit projected area.

•The regimes of lubrication are sometimes identified by a lubricant film parameter (h/) – mean film thickness/composite standard duration of surface heights of two surfaces.

Regimes of Fluid Film LubricationStribeck Curve

Fig. Lubricant film parameter (h/) and coefficient of friction as a function of N/P

(Stribeck curve) showing different lubrication regimes observed in fluid lubrication without

an external pumping agency.

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•Hydrostatic (HS) pad bearings support load on a thick fluid film (up to 100 m) supplied from by an external pressure source, a pump, which feeds pressurized fluid to the film, i.e. externally pressurized. •Since HS bearings do not require relative motion of the bearing surfaces to build up the load-supporting pressures as necessary in hydrodynamic (HD) bearings by viscous shear/drag, HS bearings can be used in applications with little or no relative motion between the surfaces. •HS bearings may also be required in applications where for one reason or another touching or rubbing of the bearing surfaces cannot be permitted at startup and shutdown.•However, HS bearings have the disadvantage of requiring high-pressure pumps and equipment for fluid cleaning which adds to space and cost. •HS bearings provide high load-carrying capacity, such as in machine tool guideways, large telescopes and radar tracking units (antenna bearings)

•By supplying high pressure fluid at a constant P or V to a recess relief or pocket area at the bearing interface, the two surfaces can be separated and the frictional force reduced to a small viscous force.•The fluid under pressure, ps, dropped to some low value, pr, after passing through restrictor element, which depends on applied load, W.•The fluid then passes out of the bearing through the narrow gap of thickness hbetween the bearing and the opposing bearing surface.

(1) Hydrostatic (HS) Lubrication(a) Hydrostatic Liquid Bearing

Fig. Schematics of (a) a hydrostatic thrust bearing with circular step pad, and (b) fluid supply system .

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(1) Hydrostatic (HS) Lubrication(b) Hydrostatic Gas Bearing

•In some applications a gas is used to lubricate the bearings – aerostatic bearings.

•The mechanism of film generation is the same as in liquid bearings except gas compressibility and ambient operating pressure have to be incorporated into the analysis.

•Gas, typically He, N, O, air, or CO2, lubricated bearings offer some advantages over liquid such as:

•Gas viscosity increases with temperature, thus reducing heating effects during overload or abnormal operating conditions. •Some gases are chemically stable over a wider temperature range than e.g. hydrocarbon lubricants.•Gases can offer greater cleanliness and non-toxicity than fluid lubricants.

•Disadvantage is lower load capacity compared to fluid film lubrication of same size bearings.

•Applications in precision machine tool industry, computer spindle bearings, dental drills, etc.

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Regimes of Fluid Film Lubrication(2) Hydrodynamic (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 withoutan external pumping agency. Hydrodynamic journal/thrust bearings.•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.•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). 5

•HD lubrication is often referred to as the ideal lubricated contact condition because h/ > 5, i.e. the lubricated films are many times thicker (typically 5 to 500 m) than the height of the surface roughness on bearing surface.

•The coefficient of friction HD regime 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.

•The behavior of the contact is governed by bulk physical properties of the lubricant, notably viscosity, and the frictional characteristics arise purely from the shearing of viscous lubricant.

•In HD lubrication, adhesive wear occurs during start-stop operation and corrosive (chemical) wear of the bearing surfaces can also occur as a result of interaction with the lubricant.

•One way to reduce corrosive wear is by forming an inert film on the bearing surface, e.g.

for ferrous bearing systems phosphate containing additives are used.

(2) Hydrodynamic (HD) Lubrication(continued)

•As the bottom surface moves it drags the lubricant along it into the converging wedge.•A pressure field is generated as otherwise there would be more lubricant entering the wedge than leaving it. •Thus at the beginning of the wedge the increasing pressure restricts the entry flow and at the exit there is a decrease in pressure boosting the exit flow.

Fig. Principle of HD lubrication.

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•Examples of HDL: automobile and aircraft engines, pumps, gear boxes, computer disk drives,...•Hydrodynamic action occurs in the bearings with convergent clearance space through the length of the bearing. The loads carried by rotating machinery may be radial load and/or an axial/thrust loads in the direction of shaft axis of rotation.•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 pivoted pad bearing was first used in supporting the thrust of a ship propeller shaft and the load from a hydroelectric rotor (see next page). At the present level of technology, load of several thousand tons are carried at sliding speeds of 10 to 50 m/s in hydroelectric power stations. COF~0.005.•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. •The wedge on previous slide can also be wrapped or curved around this shaft to form journal bearing.

(2) Hydrodynamic (HD) Lubrication(a) Hydrodynamic Bearing

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Regimes of Fluid Film Lubrication(3) Elastohydrodynamic (EHD) Lubrication

•EHL is a subset of HD lubrication where there’s elastic deformation of contacting solids, and changes in with Pplay significant roles in the HD lubrication process. •The typical film thickness in EHD is thinner (~0.5 – 5 m) than that in conventional HD lubrication. •In isolated areas, asperities may actually touch. •The 2 following effects play major roles in EHD film formation:1. HD film formation – basic principles of HD lubrication apply, however unlike classical HD, both contact geometry and lubricant viscosity are a function of HD pressure.2. Lubricants viscosity and rheology change under pressure –Non-conformal geometry causes intense concentration of load over a very small Hertzian area. Extreme lubricant pressures (1-4 GPa), > HD, are exhibited with associated increases in elastic deformation of the contacting bodies and lubricant , known as piezoviscosity.

•EHD is important in heavily loaded contacts (such as machine elements of low geometrical conformity), where loads act over relatively small contact areas (point contacts of ball bearings, and point/line contacts of gear teeth).

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(3) Elastohydrodynamic Lubrication

•In EHD, the contact pressures in nonconforming contacts are very high and the of most liquid lubricants is also very high, ~108 times, so that a change to a solid phase occurs in the contact. •In EHD, the elastic deformation equations, and the equation relating viscosity and pressure [typically for liquid lubricants, 0 exp (p)] must be accounted for. •In presence of piezoviscous fluid such as oil, static Hertz pressure distribution profile changes:

•The greatest changes to the pressure profile occur at the entry and exit regions of contact. •The combined effect of rolling & a lubricating film results in a slightly enlarged contact area.•Consequently at the entry region, the HD pressure is < the value for a dry Hertzian contact. •The lubricant experiences a precipitous rise in as it enters the contact followed by an equally sharp decline to ambient levels at the exit of the contact.•To maintain continuity of flow & compensate for the loss of lubricant at the contact exit, a constriction is formed close to the exit with thickness ho, which is important since it controls the likelihood of asperity interaction. declines even more sharply at the exit than at the entry.•A large pressure peak (>max. Hertzian contact pressure) is generated next to constriction on upstream side, and decreases to less than dry Hertzian values on downstream side.•Size & steepness of the large pressure peak depend strongly on the lubricant’s P- characteristics.

Fig. HD pressure distribution in an EHD

contact, hc is the central film thickness

and ho is the minimum film thickness.

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