A lubricant is any substance that is used to reduce friction and wear and to provide smooth running and a satisfactory life for machine components. Lubrication fundamentals are discussed and the various lubrication mechanisms are defined. These include: hydrodynamic, elastohydrodynamic, mixed, boundary, and extreme pressure. Before the various lubrication mechanisms are presented, it is desirable to define conformal and nonconformal surfaces

Hamrock, B. J.

English

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Lubrication Fundamentals
(NASA-TM-81762} LUBBIC/_TIOI_ FUNDAMENTALS N81-23_90
(_S_.) 3 p HE _02/MF _.01 CSCL 11ti
Unclas
G3/39 _234_
Bernard J. Hamrock
Lewis Research Center
Cleveland, Ohio
i I • t
%' . ,.
April 1981
https://ntrs.nasa.gov/search.jsp?R=19810014957 2020-03-21T13:53:01+00:00Z
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LUBRICATION FUNDAMENTALS
i'ili Bernard J. N_m_rock
i_i:i'ii'i_Ii,i Natlonal AeronautiCSLewisRes archandSpaCecenterAdministrationClev land, Ohio
_ _ SUMMARY
A lubricant is any substance that is used to reduce friction a_d wear
and to provide smooth running and a satisfactory life for machine
components. This section deals with lubrication fundamentals and, in
particular, defines the various lubrication mechanisms:, hydrodynamic,
elastohydrodynamlc, mixed, boundary, and extreme pressure. Before the
various lubrication mechanisms are presented, it is desirable to define
conformal and nonconformal surfaces.
CONFORMAL AND NONCONFORMAL SURFACES j
l
Hydrodynamic lubrication is generally characterized by surfaces that
are conformal. That is, the surfaces fit snugly into each other with a high
degree of geometrical conformity so that the load is carried over a
relatively large area, Furthermore the load-carrying surface area remainscO
, essentially constant while the load is increased, Fluid-film journal andL4J
" _ slider bearings exhibit conformal surfaces. In journal bearings the radial
clearance between the shafl and bearing is typically one-thousandth of the
shaft diameter; in slider bearings the inclination of the bearing surface to
'r'"°'" the runner is typically one part in a thousand.
_;c._!_"L,_,_,_I Many m_chine elements have contacting surfaces that do not conform to
_-i!l_ii"_:°i_i_il each other very well. The full burden of the load must t_n"e'n"b'ecarried by avery small contact area. In general the contact area betwe n nonconformal
_,;_i_il surfaces enlarge considerably with increasing load but are still small
,, _,_ compared with the contact areas between conformal surfaces. Some examples
_i:i!ii!_,:_ of these nonconfon_al surfaces are mating gear teeth, cams and followers,
,_:_,_i_ and roiling-element bearings.
_,:_' The ]dad per unit area in conformal bearings is relatively low,
_i:_ typically only 1MN/m 2 and seldom over 7 MN/m2. By contrast, the load
,,,,,,_, per unit area in nonconformal contacts, such as those that exist in ball
- ,_J_:i_, bearings, will generally exceed 700 MNim2, even at modest applied loads.
_"_"",. These high pressures result in elastic deformation of the bearing materials
i_:i,,,,:,c such that the elliptical contact areas are formed for oil film generation
_-_,,_...... and load support. The significance of the high contact pressures is that
--':" they result in a considerable increase in fluid viscosity within the contact
_rea. Inasmuch as viscosity is a measure of a fluid's resistance to flow,
_ , _, this increase greatly enhances the lubricant's ability to support load
c,_,_, without bel,g squeezed out of the contact zone
....° , Figure I illustrates the two distinctly different geometries. On the
' l_ft is a typical journal bearing; on the right is a rolling-element
bearing. A universal characteristic of the journal bearing - and other
sllding-surface bearings such as pad thrust bearings - is a high degree of
conformity t,etweenthe surfaces, relativel/ large effective contact areas,
--_I-. _nd low unit loading. In contrast, as mentioned earlier, the
r '_ rolling-element bearing has poor conformity between surfaces, very small
'_- Ll contact are,_s,and very high unit loads.
1
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' ' ' '" " _', . ' " i;
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,i
'!ii HYDRODYNAMIC LUBRICATION
Fluid-film lubrication occurs when the lubricant fihn is sufficiently
)ii thick to prevent the opposing solids from coming into contact. This
_:_ condition is often referred to as the ideal form of lubrication since it
• provides low friction and a high resistance to wear. The lubrication of the
!:_'IZ,_ contact is goverened by the bulk physical properties of the lubricant,k
notably viscosity; and the frictional characteristics arise purely from the
_i'I./_" shearing of the viscous lubricant. The characteristics of hydrodynamic
lubrication are illustrated in figure 2. The pressure developed in the oil
film of hydrodynamically lubricated bearings is due to two factors:
) (I) The geometry of the moving surfaces produces a convergent wedge of
the liquid.
(2) The liquid is viscous which results in a resistance to flow.
The lubricant films are normally many times thicker than the surface
roughness. The physical properties of the lubricant dictate contact
behavior. The film thickness normally exceeds 10-6 m. The effect of the
surface finish can be related to the film thickness by the following
equation:
h
A=
where
......i h film thickness
fa,fh ms surface finish of solids a and b
!!!!i__ Thert,forethe film parameter A is a ratio of the film thickness to the
_i composite rms. For hydrodynamic lubrication the film parameter is in excessof 10 and may even rise to 1OO. Films of this thickness are clearly
,,_,_, insensitive to chemical action in surface layers of molecular proportions.
ELASTOIYDRODYNAMIC LUBRICATION
E1astohydrodynamic lubrication is a form of fluid-film lubrication
where elastic deformation of the bearing surfaces becomes significant. It
is usually associated with highly stressed machine components of low
conformity, such as gears and rolling-element bearings. Besides the elastic
effect there is a viscous effect, that is. for example, at a pressure of
700 MNIm2 (which is moderate since the pressure can be as high as
2100 MNIm2) the viscosity may be increased IO O00-fold. The liquid
entering the gap between the gear teeth is trapped between the surfaces, and
....' at the high pressures existing in the contact region, behaves virtually like
,,_,_, a solid separating layer.
Figure 3 demonstrates the contact effects found in elastohydrodynamic
lubrication. As the contacts approach, lubricant is forced Irom between
them because of the hydrodynamic effect. This flow is resisted by viscous
' forces, and there is an accompanying pressure rise that, in turn, raises the
viscosity of the trapped lubricant. As the pressure increases, the surfaces
l:, deform elastically. The increased pressure resulting from the contact
interaction represents load support by the contacts through the fluid film.
Elastohydrodynamic lubrication normally occurs in contacts where the film
thickness is in the range 10-7 m • h < 10-6 m and the film parameter
!i _1.
!
• ,)
, .-4:
is in the range 3 < A < 10. The characteristics of elastohydrodynamtc
!, lubrication are t'_ustrated in ftgu:'e 4.
":i_ilj! The elastohydrodynamic lubrication mechanism is also encountered withsoft bearin_t mate ials, such as elastomeric seals and rubber tires, lhe
_;:,:_,l: common fact(_rs in these applications are that the local elastic deformation
_:,:/_: of the solids provides coherent fluid film and that contact of asperities
"_ (high points on the surface) is largely prevented, Another feature of the
_..:';i,.';_, _ elastohyarodynamtc lubrication of low-elastic-modulus materials is the
_,:"_ negligible effect of the relatively low pressures on viscosity of the
':;i;,:i_I lubricating fluid
•_._:...i BOUNDARY LUBRICATION
If in (_lubricated contact the pressures become too high, the running
speeds too low, or the surface roughness to great, penetration of the
lubricant film will occur. Contact will take place between the asperities.
The friction will rise and, more importantly, wear will take place. Addin_
a small quantity of a certain active organic compound to the lubricating oII
can extend the llfe of machine elements. These additives are present in
small quantities ((i percent). They function because they form low shear i
strength surface films that are strongly attached to the metal surfaces. !
Although they are generally only one or two molecules thick, they are able
to prevent n_tal-to-metal contact.
c_ _ Some boundary lubricants are long-chain molecules with an active end
,_;_ii group, typically an alcohol, an amine, or a fatty acid. When such a
material, dissolved in a mineral oil, meets a metal or other solid surface,
_,i_, the active end group attaches itself to the solid and graduallX builds up a i
-,C&:i:(,,,,o surface layer. The surface films vary in thickness from 5xi0-_ to I0-8 '
m, and the film parameter A is less than unity (A < I).
MIXED-FILM LUBRICATION
_!:__,::,il.;l The behavior of the conjunction in a mixed-film lubrication regime is
....i_:_i":_ governed by a combination of boundary and hydrodynamic or-elastohydrodynamic
-: ..... effects. Some asperity contact may occur. Interaction takes place between
"_'_- one or more molecular layers of boundary lubricating films. A partial
i_. _1 fluid-film lubrication action develops in the bulk ot the space between the
_'.'_._-_,I solids. The,film thickness in a mixed-film lubrication contact is less than
=:i::_i_i!!i!I between10-Bm andlandgreater4,than 10-6 m. The film parameter A is normally
Figure 5 illustrate> the film conditions existing in fluid film,
mixed-film, and bound,,'ylubrication. The surfaces shown are greatly
distorted for purpofes of illustration. To scale, real surfaces would
appear s gt,ntly rolli g hills rat r than sh rp peaks.
The variation of the coefficient of friction _ with the film
parameter ._ is shown in figure b. In this figure the approximate
locations of the various lubrication regimes already discussed are shown.
This figure shows that, as the film parameter A increases, there is
initially a decrease in the coefficient of friction in the
elastohydrodynamic regime and then an increase in the coefficient of
friction in the hydrodynamic regime.
c
:I,
EXTREME-PRESSURELUBRICATION
The best boundarylubricantscease to be effectiveabove 200" to 250° C
becausethe softeningor meltingpoint of the surfacefilm is exceeded. If
surfacesare to operateunder more severeconditions,other types of
lubricantsmust be used. Extreme-pressure(EP) lubricantsusuallyconsist
of a small quantityof an EP additivein a lubricatingoil. The most widely
used additivesfor this purposecontainphosphorus,chlorine,and sulfur.
In generalthese materialsfunctionby reactingwith the surfaceto form a
surfacefilm that preventsmetal-to-metalcontactat high temperatures. If,
in addition,the surfacefilm formedhas a low shear strength,it will not
only protectthe surface,but it will also give a low coefficientof
friction. A word of caution- althoughEP additivesfunctionby reacting i
with the surface,they must not be too reactiveor chemicalcorrosionmay be
more troublesomethan frictionand wear.




Lubrication fundamentals

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