Download - Lubrication 2012
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FLUID FILM LUBRICATION
Reproduced mainly from
Introduction to Tribology byBharat Bhushan
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Gas-Lubricated Bearings
Kingsbury (1897).
The gas is generally air.
This avoids the need for a liquid lubricationsystem, simplifies the bearing design, and
reduces maintenance.
Gas bearings are also called aerodynamic orself-acting gas bearings.
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Applications
Gyroscopes where precision and constanttorque are required,
Machine tool spindles,
Turbomachinery Dental drills
Food
Textile machinery and Tape and disk drives as part of magnetic
storage devices
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The gas is compressible and the change in
density as a function of pressure cannot be
neglected in the solution of Reynolds
equation.
Due to low viscosity of gas, the film thickness,
pressures, and load capacities of gas bearings,
which are proportional to the fluid viscosity,
are much lower than with a liquid.
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The frictional force is reduced in roughly the
sameproportion; therefore, the value of frictionforce is very low.
However, the coefficient of friction iscomparable to that of liquid bearings.
Since the energy dissipated by friction losses islow, the temperature rise is low in the gasbearings
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Elastohydrodynamic Lubrication (EHL)
EHL is a subset of HD lubrication
Elastic deformation of the contacting solids plays a significant role
The film thickness in EHD lubrication is thinner (typically 0.5 -5 um)
In isolated areas, asperities may actually touch.
Therefore, in liquid lubricated systems, boundary lubricants thatprovide boundary films on the surfaces for protection against any solid
solid contact are used.
Bearings with heavily loaded contacts fail primarily by a fatiguemode that may be significantly affected by the lubricant
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EHL is most readily induced in heavily loadedcontacts(such as machine elements of low
geometrical conformity), where loads act overrelatively small contact areas
EHL phenomena also occur in some low elastic-
modulus contacts of high geometricalconformity, such as
lip seals,
conventional journal and thrust
bearings with soft liners,
and head-tape interface in magnetic
recording tape drives
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In EHL-the simultaneous solutions of the
Reynolds equation, the elastic deformation
equation, and the equation relating viscosity
and pressure.
Thermal and shear rate effects also become
important and need to be taken into account
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In EHL, adhesive wear occurs during start-
stop operations and corrosive wear of the
bearing surfaces can also occur as a result of
interaction with the lubricant
In well-designed heavily loaded bearings,
fatigue wear is most common.
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In the absence of boundary lubricants andgases (no oxide films), friction may becomevery high ( > I).
All self-acting bearing interfaces duringcontact start-stops (CSS), before a fluid film as
a result of HD or EHL is developed, operate inthe boundary lubrication regime.
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The failure in boundary lubrication occurs by
adhesive and chemical (corrosive) wear
Boundary lubricants form an easily sheared
film on the bearing surfaces, thereby
minimizing adhesive wear and chemical wear
The important physical properties of the films
are the melting point, shear strength and
hardness.
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Under these conditions, the lubricant viscosity isrelatively unimportant and the physical and
chemical interactions of the lubricant with thesolid bodies control friction and wear
Even a monolayer of adsorbed molecules may
provide some protection against wear.
Lubrication in some situations can be achieved bythe use of multimolecular lubricant films.
Monolayer lubrication is referred to asBOUNDARY LUBRICATION and multimolecularlubrication is referred to as MIXED LUBRICATION.
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Occurs under high-load and low-speedconditions in machine components such as
bearings, gears, cam and tappet interfaces,
and piston ring and liner interfaces.
Boundary lubrication forms a last line of
defense
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The boundary films are formed by physical adsorption,chemical adsorption, and chemical reaction; for typical
examples, see Fig. NEXT.
The physisorbed film can be of either monomolecular(typically < 3 nm) or polymolecular thickness.
The chemisorbed films are monomolecular
In general, the stability and durability of surface filmsdecrease in the following order:
chemical reaction films, chemisorbed films, andphysisorbed films.
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Properties of the solid surface that are
desirable for good lubrication
A solid should have a strong tendency formolecules to adsorb on the surface.
Consequently, metals tend to be the easiest surfaces to
be lubricated.
The solid surfaces should have a high wetting sothat the liquid lubricant wets the solid easily.
For better lubrication, the surface should bereactive to the lubricant under test conditions sothat durable, chemically reacted films can form.