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LUBRICATION
AND LUBRICANTS
1.Introduction
Lubrication is a process in which afilm of lubricant is inrted between rubbing surfaces for the purpo of controlling friction and/or to reduce wear of the sur-faces.Thefilms are designed to minimize contact between the rubbing surfaces and to shear easily so that the frictional force opposing the rubbing motion is low. Lubricants may be liquids,solids,gas,or greas.Lubricating oils and greas contain refined or synthesized ba oils from animal,vegetable or mineral (petroleum)origin,and a variety of additives to improve their lubricating and other characteristics.
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Lubrication is a major component of tribology,defined as the science and technology concerned with interacting surfaces in relative motion,including fric-tion,lubrication,wear and erosion(1).王者训练营
Tribology and lubrication are ancient arts.In his splendid History of Tribo-logy(2),Professor Dowson traces the development of the arts and sciences,and describes the outstanding artists and scientists responsible,from the paleolithic age to the end of the twentieth century.He reports archeological eviden
ce that bitumen was ud to lubricate potters wheels5000years ago.Water-lubricated sliding bearings were ud in Egypt$2400BC to transport large objects.A char-iot wheel from$1400BC was found with traces of tallow as lubricant,and the Chine had lubricated metal wheel bearings with leather als to hold the lubri-cant in place in the fourth century BC(2).
The word‘‘tribology’’first appeared in Lubrication(Tribology)Education and Rearch—A Report on the Prent Position and Industries Needs,Depart-ment of Education and Science(UK),1966.This is often called The Jost Report, after H.Peter Jost,the chairman of the British Lubrication Engineering Work-ing Group,which prepared the report.The word is derived from the Greek tribein,meaning‘‘to rub’’,and logos,meaning‘‘reading’’or‘‘study’’.Tribology is literally the study of rubbing.The Working Group defined it more precily as‘‘the science and technology of interacting surfaces in relative motion and the practices related thereto’’(3).
The Jost Report was part of an effort to focus attention on the‘‘scientific, technological,economic and environmental issues’’(2)involved in the study and practice of tribology.Another purpo was to bring together the many, and often splintered engineering and scientific disciplines that deal with this technology.Dowson speculates that the dramatic progress in thisfield in the final third of the twentieth century may have been significantly influenced by the efforts in the1960s(2).
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The1966report by the British Lubrication Engineering Working Group demonstrated to industry and government the impact of friction,wear,and lubri-cation on the nation’s economy,and the value of further rearch in tribology. That report showed,eg,that the most significant value of better lubrication (91%)comes from incread productivity,lower maintenance and replacement costs,and lower investment cost.Direct energy savings(5%),and savings in the cost of lubrication,in manpower and material(4%),account for the remainder.
1
Kirk-Othmer Encyclopedia of Chemical Technology.Copyright John Wiley&Sons,Inc.All rights rerved.
2LUBRICATION AND LUBRICANTS Vol.15 Tribology is a multidisciplinary science that embraces lubrication,friction, wear,properties of lubricants,surface characterization,bearing materials,and the lection and design of lubricating systems.The lubrication engineer would add to this list lubricant and coolant lection,plant lubrication and maintenance programs,and machine condition monitoring.
2.Fundamentals of Lubrication
Tribology,by definition,is concerned with interacting surfaces in relative motion.It is appropriate,therefore,to begin the discussion of lubrication funda-mentals by describing the characteristics of tribological surfaces.
2.1.The Nature of Interacting Surfaces in Relative Motion.Tribo-logical surfaces are the load-bearing surfaces on the moving parts of machines. They include surfaces on crankshaft rod and main bearings,radial and thrust bearings on steam and gas turbines,cams and valve lifters,pistons and cylin-ders,natural and artificial hip joints,ball and roller bearings,machine tool slideways,cutting tools,magnetic information storage devices,and microelectro-mechanical systems(MEMS).Despite their appearance andfinishing efforts, the surfaces are not perfectly smooth.There are microscopic irregularities; gently sloping hills and valleys called asperities on them.
If an imaginary surface is drawn through a real surface,such that the volume of all of the material above the imaginary surface is equal to the volume of voids below that surface,the roughness of the real surface,R a,can be defined as
R a¼ðj y1jþj y2jþÁÁÁþj y n jÞ=nð1ÞWhere R a is the center line or arithmetic average of the absolute distances,y i, from the imaginary surface(mean line)for a given sampling length(usually 0.80mm).
Roughness(R a)values of machined surfaces range from0.025m m for ball bearing surfaces to25m m clearance surfaces on rough machine parts(4–6). The roughness of computer hard disk surfaces is measured in angstroms(A˚)or nanometers(nm)(7).
The total profile of a surface consists of a‘‘waviness’’and a roughness com-ponent.The parameter R a,although it is the most common measure of surface roughness,is innsitive to the shape or waviness of the profile.A more uful parameter is the root-mean-square(rms)roughness,R q.
R q¼½ðy21þy22þÁÁÁþy2nÞ=n 1=2ð2ÞWhere R q is the rms deviation of y i from the mean line for a given sampling length(4).The rms roughness of computer hard disk surfaces is<2nm(7).
The most repeatable of the roughness parameters is the10points height, R z.
Vol.15LUBRICATION AND LUBRICANTS3 R z¼½ðP1þP2þÁÁÁþP5ÞÀðV1þV2þÁÁÁþV5Þ =5ð3ÞWhere R z is the average distance between thefive highest peaks(P i)and thefive deepest valleys(V i)within the sampling length.It is also linked with the machin-ing parameter S2/8r,where S is the feed rate and r is the tool radius(4).
Another index of roughness is R t,the maximum peak-to-valley height.
R t¼R pþR vð4ÞWhere R p is the maximum peak height and R v is the maximum valley depth within the sampling length(4).
The description of the test disk ud in the ASTM D6425-02test method for measuring friction and wear illustrates the impact of surface texture on the quantities(8)and is shown below and ud with ASTM’s permission.
Test Disk,AISI52100Steel,62Æ1HRC Hardness.The surfaces of the disk are lapped and free of lapping raw materials.The topography of the disk will be determined by four values:0.005m m<R z>0.65m m;0.035m m<R a>0.050m m;
0.020m m<R p>0.035m m;0.050m m<R v>0.075m m
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Four different measures of surface topography are specified for the disk in order to get acceptable reproducibility of the test method(9).
Approximate roughness indexes obtained with various metal-working process are
Production process R t,m m R a,m m
turning 4.00–25.00.500–3.0
grinding 2.00–6.00.400–0.8
milling 1.50–20.00.200–2.0
boring0.50–20.00.050–1.6
honing0.03–1.00.015–0.2
lapping0.03–0.60.015–0.1
Engineering surfaces also differ in composition from the underlying bulk material.A metal bearing,eg,will have a work hardened layer at the surface, over which an oxide layer forms,on top of which is an adsorbed layer of moisture and gas.When two such surfaces come in moving contact,their surface struc-tures,compositions,and the interaction between opposing asperities accounts for a major portion of the friction between them and much of the wear that inevita-bly occurs(10–12).政治提纲
For example,without the oxide and adsorbed layers,coefficients of friction>4have been measured in a vacuum of0.133mPa on surfaces cleaned by abrasive cloth and heated.The coefficient decread considerably when oxy-gen was admitted to a pressure of0.133Pa(13).The oxide and adsorbed layer东晋时期
s on metal surfaces can,therefore,be considered as lubricatingfilms.
2.2.Friction.When two of the surfaces are brought together,they initially touch at the highest asperities.The load,N,normal to the surfaces at
4LUBRICATION AND LUBRICANTS Vol.15
the contact points caus the asperities to deform until the pressure in the result-ing contact areas just equals the yield pressure,p,of the asperities.The sum of the contact areas is the real contact area,A r.The yield pressure is equivalent to the Brinell Hardness Number(BHN),in consistent units,measured at the surface of the material(4).
A r¼N=p¼N=BHNð5Þ
The real area of contact is a minute fraction of the total surface area.For example,with a typical bearing contact stress of3MPa and a bronze bearing asperity yield pressure of500MPa,<1.0%of the nominal area would involve asperity contact(14).
As the load on the surfaces increas,the asperities continue to deform,the softer surface more than the other.More of the asperities come in contact and the real area of contact grows.The opposing sur
faces also tend to adhere or bond to each other in the contact area.The shear strength of the bonds depends on the time of contact and the difference in composition of the two surfaces.
Sliding of one of the nonlubricated surfaces across the other requires a friction force,F,to displace the contacting asperities.This force includes veral components,among them:a shear or adhesion component arising from bonding of the contacting asperities;a plowing or deformation component,arising from the interlocking of asperities;a lifting component to rai asperities over the roughness of the mating surfaces.
The shearing component,F s,may account for90%or more of the total fric-tion force(14).This component is proportional to the shear strength,s,of the asperity junctions:
F s¼A r sð6Þ
More detailed descriptions of surface texture,surface structure,and compo-sition and the real area of contact will be found in Refs.(4–6,12,13,15,16).
2.3.Coefficient of Friction.The coefficient of friction,f,of a pair of con-tacting surfaces is defined as the ratio of the total frictional force to the normal force or load.It can also be expresd as the ratio of shear strength,s,to the yield pressure,p,at the asperity junctions.
f¼F=N¼s=pð7Þ
If there is a lubricatingfilm on the surfaces,the coefficient of friction is the ratio of the shear strength of the surfacefilm,s f,to the yield pressure,p m of the substrate or backing material.
f¼s f=p mð8Þ
If a shear force is gradually applied to one of two dry,unlubricated surfaces in contact,the surface will not move until the force is great enough to overcome
Vol.15LUBRICATION AND LUBRICANTS5 the shear strength of the asperity contacts.The ratio of the shear force required to start motion to the normal force on the surfaces is the static coefficient of friction.Once motion starts,less force is needed to keep the surface moving at a constant velocity.The coefficient of friction during sliding is the kinetic or dynamic coefficient.
The static coefficient measured for a hard steel surface on another hard steel surface is0.78.The dynamic coefficient measured for hard steel on hard steel is0.42.When a thinfilm of light mineral oil is applied to the surfaces, the static coefficient drops to0.23.The dynamic coefficient with a light oilfilm drops to$0.1.Adding a friction modifier to the oil can reduce or rever the dif-ference between
the two coefficients.Adding stearic acid to the lubricant,eg,for hard steel on hard steel,reduces the static coefficient to0.0052,which is lower than the dynamic coefficient,0.029(17).
An extensive Friction and Wear Databank is found in Ref.18.Tables of the coefficient of friction values for a wide variety of material combinations are also available in Refs.19and20and many other sources.The data,however, should be ud with caution.The coefficient of friction varies with changes in humidity,gas pressure,time,temperature,sliding speed,surface quality, the shape of the contact region,the method of testing,and other variables. Where high reliability is needed,the friction should be measured using a proto-type device under design conditions(20).
2.4.Wear.The principal types of wear in sliding contacts are adhesive, abrasive,and corrosive wear.Fatigue wear occurs in concentrated contacts(ball and roller bearings,gears,cams,and automotive valve lifters)under the combi-nation of sliding and rolling(21).
The Archard equation reported by Rabinowicz(21)gives a simple,quanti-tative relationship for predicting the adhesive wear rate:
V¼kNx=pð9Þwhere V¼wear volume,k¼wear coefficient,N¼normal load,x¼sliding dis-tance,p¼yield stress or indentation hardness.
Values of k for veral unlubricated material combinations are shown in Table1(14).Others will be found in Ref.18and in Refs.(21–24).
Adhesive wear is material parated or transferred during the shearing of asperity contacts.The wear particles,and other particulate surface contami-nants that are hard enough to damage the surface,cau abrasive wear.Abra-sive wear is the removal of material by ploughing,cutting,or scratching.Its rate generally obeys equation9and the wear coefficients tend to be higher than the adhesive coefficients(21).Corrosive wear is the wearing away of the products of galvanic or chemical corrosion of the surface.There is no simple equation that characterizes this type of wear.
Current broader discussions of the friction and wear phenomena are found in Refs.13,25,and26.
2.5.Viscosity.Viscosity,or resistance toflow is the most important property of a lubricating oil.It was defined by Newton(27)as the ratio of the绍兴儿童公园
