资料内容教案讲稿skf lubrication management theory fundamentals

1、SKF Lubrication ManagementLubrication theory & fundamentalsPresented to: SKF Lubrication Management -Technical TrainingPresented by: SKF Maintenance products16 20 April , 2012SKF BTP Nieuwegein, The NetherlandsOctober 30, 2007 SKF Group Slide 1 Agenda1. Introduction 2. Friction3. Wear4. Lubrication

2、& lubricant function5. Lubrication regimes6. Application: The stribeck curve7. ExercisesOctober 30, 2007 SKF Group Slide 2 Introduction1October 30, 2007 SKF Group Slide 3 Why a theoretical part? Knowledge of theory will help in the other training parts Help to minimize trial & error in the field Pro

3、vides a framework to apply lubrication concepts effectively & efficientlyOctober 30, 2007 SKF Group Slide 4 Tribology Tribologyis the science and engineering of interacting surfaces in relativemotion. It includes the study and application of the principles of:- Friction- Lubrication- Wear Origins: P

4、eter Jost, 1966, British department of education & sciences From the Greek: Tribos = To “rub”October 30, 2007 SKF Group Slide 5 2FrictionOctober 30, 2007 SKF Group Slide 6 What is friction? Definition:“ Friction is the force that resists one surface from sliding, or rolling over another” Must be con

5、trolled:- Minimize energy losses ( ex: Combustion engines)- Keep friction level constant ( Ex: brakes)October 30, 2007 SKF Group Slide 7 Application Rubbing hands Sliding friction:- Creates heat- Creates noise- Creates vibrationsOctober 30, 2007 SKF Group Slide 8 Different types of friction ( Dry &

6、solid surfaces) Pure sliding friction - Static friction- Kinetic friction Pure Rolling friction- Lower friction coefficient- Deformation of surfacesSource: Da Vinci friction rig drawingFaFnFrVu2u1FNMD MRSource: Herguth laboratoryOctober 30, 2007 SKF Group Slide 9 What causes friction?Source: for-lub

7、rication/ SEM 150X Surfaces are rough in reality- Asperities Source: WikipediaOctober 30, 2007 SKF Group Slide 10 What causes friction? When stress is applied to these asperities (under load), the asperities will shear, weld, scarp surfaces, The energy required to displace or break these asperities

8、is called surface resistanceSource: Pall corporationOctober 30, 2007 SKF Group Slide 11 The asperities (or roughness) - Implications Surface contact is limited to the surface area of the asperities in contact. Under loads, pressures are thus far higher locally. As these asperities are removed, surfa

9、ce area increases (running-in) As the load increases, surface area increases and friction force increasesOctober 30, 2007 SKF Group Slide 12 How to measure friction? The ratio of the force needed to overcome the weight Coefficient of friction:=Frictional force / Applied forceSource: Dept of physics,

10、 Univ. of Illinois, 1996Source: Da Vinci friction rig drawingOctober 30, 2007 SKF Group Slide 13 Friction measurementsSource: 2./211_fall2002.web.dir/ben_townsend/staticandkineticfriction.htm Surfaces (static) (kinetic)Steel on steel0.740.57Glass on glass0.940.40Ice on ice0.100.03Teflon

11、on Teflon0.040.04Tire on concrete1.000.80Tire on wet road0.600.40Tire on snow0.300.20 Static friction is usually higher than kinetic friction. For dry surfaces, ranges in general from 0.2 to 0.8. Depends on :- Type of surfaces- Environment ( Vacuum)- CoatingsOctober 30, 2007 SKF Group Slide 14 Commo

12、n ways to reduce friction Replace sliding friction by rolling friction ( plain bearings rolling element bearings) Combination of surfaces that are themselves sacrificial Improve lubrication (see later in this chapter)October 30, 2007 SKF Group Slide 15 3WearOctober 30, 2007 SKF Group Slide 16 What i

13、s wear? Definition:“ Erosion of material from a solid surface by the action of another solid” We understand (and accept) it from our life experience:- Tires- Lawn mover blades- October 30, 2007 SKF Group Slide 17 Wear relation with friction Coupled, but difficult to understand:- Controlling friction

14、 can reduce wear- Controlling wear can reduce friction Created by different phenomenon:- Possibility to create high friction Low wear- Possibility to create low friction Low wearOctober 30, 2007 SKF Group Slide 18 Economic impact of wear Estimated to be 5% GNP ! Costs split between:- Direct costs: E

15、quipment replacement, equipment rental, standby maintenance.- Indirect costs: Loss of production, product liability, incomplete work, damage or loss of additional equipmentOctober 30, 2007 SKF Group Slide 19 Economic impact of wear - Example Study 1976 “ Wear control to achieve product durability”Oc

16、tober 30, 2007 SKF Group Slide 20 Influencing factors of wear. Surface chemistry Fracture mechanisms Elastic & plastic deformations Heat generation Transfer Adhesion .October 30, 2007 SKF Group Slide 21 but difficult to observe & quantify in live Wear scar Debris Noise Thermal effect Hardness change

17、 Microstructure alterationsSource: Elsevier publicationSource: Elsevier publicationOctober 30, 2007 SKF Group Slide 22 Predicting wear Archards equation V = KWL / H Applies only to sliding systems Has serious limitations- Takes only into account hardness- Only for dry contacts- Does not predict wear

18、-in- Predicts only two types of wear ( see next slide)V: Volume of wear, cubic inchesK: Wear coefficientW: Normal load, poundsL: Sliding distance, feetH: Penetration hardness of the softer material, psiOctober 30, 2007 SKF Group Slide 23 Different types of wear Abrasive wear Adhesive wear Erosion-Ca

19、vitation Fretting Contact fatigue Corrosive wearCan be predicted by Archards equationOctober 30, 2007 SKF Group Slide 24 Different types of wear Abrasive wear Sliding, rolling Adhesive wear Sliding, rolling Erosion-Cavitation NA Fretting Vibrations (standing still) Contact fatigue Rolling Corrosive

20、wear NADuringOctober 30, 2007 SKF Group Slide 25 Different types of wear Abrasive wear Hard particle contamination, surface finish Adhesive wear Asperity contact, cold welding Erosion-Cavitation High velocity particle, inadequate flow, air and water Fretting Loose fits, moisture, Contact fatigue Str

21、uctural deformation Corrosive wear Chemical and moisture contaminationCaused byOctober 30, 2007 SKF Group Slide 26 Why bother about wear mechanisms? An understanding of the wear mechanisms will help to determine which actions should be taken, essentially:- Lubrication technology- Materials substitut

22、ion- Load reduction- Removal of impact conditions- Cleanliness of lubricants- October 30, 2007 SKF Group Slide 27 Abrasive wear (Cutting wear, gouging, scratching, broaching) -1Two body abrasion: Sliding movement Poor surface finish - One or both surfaces - Soft metal against hard metal Too thin oil

23、 film - Asperities are allowed to collide Source: Surface engineering forumSource: STLE12October 30, 2007 SKF Group Slide 28 Abrasive wear (Cutting wear, gouging, scratching, broaching) -2Three body abrasion: Sliding and rolling Contamination scratches surface Oil film = particle size - Film is too

24、small to allow contaminant to pass without touching surfaces Source: Surface engineering forumSource: SKF bearing failures and their causes123October 30, 2007 SKF Group Slide 29 Abrasive wear (Cutting wear, gouging, scratching, broaching) -3Abrasive wear occurs in virtually all machines Control it b

25、y:- Clean lubricant (particles larger than film thickness-to an extent)- Large oil film thickness (will be described later)- Decrease machine loads (remember Archards eq.)- Removal of impact conditions- Material substitution- October 30, 2007 SKF Group Slide 30 Adhesive wear (galling, seizure, scuff

26、ing, smearing) -1 Sliding and rolling movement, primarily rolling Poor surface finish - One or both surfaces Too thin oil film - Asperities allowed to collide - Film breakdown Additives (EP/AW) not adequate Low surface speedsSource: SubstechSource: SKF bearing failures and their causesOctober 30, 20

27、07 SKF Group Slide 31 Adhesive wear (galling, seizure, scuffing, smearing) -2 Adhesive wear examples:- Pistons- Gears- Sliding bearings- Rolling element bearings- Control it by:- Larger oil film or effective additives- Decrease loads- Improve surface finishSource: InternetSource: SKF bearing failure

28、s and their causesOctober 30, 2007 SKF Group Slide 32 Erosion & cavitation -1 Erosion ( Impact erosion)- Results from particle impingementor cavitation shocks against a surface(Not in between surfaces) Three main typesSolid particle erosion ( particles in oil/gas under high flow helicopter blade in

29、dusty environment)Liquid drop erosion (compressor blades withmoisture)Cavitation erosion ( air explosion ships propeller) Hardness of material does not helpSource: Source: October 30, 2007 SKF Group Slide 33 Erosion & cavitation -2 Erosion & cavitation also occurs on:- High flow outlets, valves, noz

30、zles (particles)- Engines, plain bearings and sliding surfaces, pump inlets (Cavitation) Erosion & cavitation can be controlled:- Removal of solid particles- Changing the angle at which particles are projected ontoa surface.- Control air entrainment, moisture levels, foam, pump shaft seals, flow con

31、strictions, decrease oil viscosity, maintain corrosion control additive health,Source: webwormcptOctober 30, 2007 SKF Group Slide 34 Fretting (fretting corrosion, false brinelling) Occurs on:- Rolling element bearings seats & elements- Couplings When low amplitude oscillatory motioncreates fine debr

32、is which oxidizeAnd influence fatigue life Cars transported by trains example. Can be avoided via:- Tighter fits ( occurs in loose fits under vibrations)- Use better lubricants ( ex: LGBB 2)- Block bearings during transportationSource: SKF bearing failures and their causesOctober 30, 2007 SKF Group

33、Slide 35 Fatigue (Stress wear, cyclic fatigue, compressive fatigue, pitting-surface intiated) Sliding and rolling, primarily rolling Causes:- Normal fatigue (SKF life theory)- Subsurface fatigue- Cyclic loading/unloading- Accelerated by overloading, indentations acting as stress risers (see later)-

34、Lubrication of little influence, except contamination control!Source: SKF bearing failures and their causesOctober 30, 2007 SKF Group Slide 36 Corrosive wear (Rust, chemical wear, oxidative wear, stress corrosion) Sliding and rolling, primarily rolling Causes:- Water or reactive chemical (acids)- Re

35、moval protective layer- Loss of additives (anti corrosion) Can be controlled via:- Corrosion resistant materials ( Ex: stainless steel)- Reduce temperature- Remove water, oxidation products, fuel, bases,- Lubricant anti corrosion propertiesSource: SKF bearing failures and their causesOctober 30, 200

36、7 SKF Group Slide 37 4Lubrication & lubricant functionOctober 30, 2007 SKF Group Slide 38 What is lubrication? Definitions “Lubrication is the science of reducing friction through the application of a suitable material between rubbing surfaces in relative motion”“ The suitable material can be a lubr

37、icant, defined as a substance capable of reducing friction and wear when introduced as a film between solid surfaces” (Webster)The lubricant can be: Liquid (oils), Semi-solid (Grease), Solid (Graphite) or gaseous (Air, helium,)October 30, 2007 SKF Group Slide 39 How does lubrication reduces friction

38、 and wear?Lubricant film 0,130 m By creating a lubricant film between surfaces in relative motion. This lubricating film separates the surfaces:- Prevents asperities to get in contact.- Shear stress is localized into the lubricant, not on the surfaces. LoadOctober 30, 2007 SKF Group Slide 40 Lubrica

39、nt Motion The shear stress is located in the lubricant.- The amount of friction created in the film depends on viscosity & lubricant type. Analogy: Playing cards- Can stand high loads- Easily slide against each otherOctober 30, 2007 SKF Group Slide 41 Viscosity (unit Cst, mm2/s,)Viscosity is a measu

40、re of a fluids resistance to flowDynamic viscosity is the measure of the force needed to shear one fluid layer over the othergap)/ velocity (fluid rateshear area)unit / force (applied stressshear viscosityDynamicOctober 30, 2007 SKF Group Slide 42 Influence of temperature on viscosity Viscosity decr

41、eases when temperature increases Quantified by the VI (Viscosity index)Source: AmsoilOctober 30, 2007 SKF Group Slide 43 Functions of a lubricant In addition of controlling friction and wear, the lubricant should:- Control heat ( removal by circulation oils) - Control corrosion ( Surface coating ant

42、i rust)- Control contamination (seal out, grease; Flush, oils)- Hydro-mechanical power ( transmit pressure)- Transmit electrical current ( electrical motor for instance)October 30, 2007 SKF Group Slide 44 5Lubrication regimesOctober 30, 2007 SKF Group Slide 45 Lubrication regimes: A bit of History.

43、Mr. Beauchamp Tower 1883, experiments on plain bearings (Light load)Source: History of tribology, D. DowsonOctober 30, 2007 SKF Group Slide 46 Lubrication regimes: A bit of History. Discovery of pressure in the oil filmSource: History of tribology, D. DowsonOctober 30, 2007 SKF Group Slide 47 Lubric

44、ation regimes: A bit of History. Discovery of pressure in the oil film Lifting forceSource: History of tribology, D. DowsonOctober 30, 2007 SKF Group Slide 48 Hydrodynamic lubrication Also referred as “Full film lubrication” The asperities do not touch low friction, no wear If these condition is mai

45、ntained, components could last very long. However:- Start-up- Shock loads- Source: History of tribology, D. DowsonSource: SKFOctober 30, 2007 SKF Group Slide 49 Hydrodynamic lubrication: How does it work? Example: Sliding bearingLubricant film thickness separate the two surfaces when a relative spee

46、d is imposed.HeHsUWViscous fluidW121,2 : Solid bodiesW : Applied load : Viscosity of the fluidU : SpeedViscous fluidOctober 30, 2007 SKF Group Slide 50 Hydrodynamic lubrication: How does it work? Example: Sliding bearing Qe = He*U/2Qs = Hs*U/2UHeHsFlow continuity: Qe = QsAssuming a Couette speed pro

47、fileFlow = Speed * surface (m3/s)Qe not equal to Qs !October 30, 2007 SKF Group Slide 51 Hydrodynamic lubrication: How does it work? Example: Sliding bearingUHeHsCreated pressure profileSpeed profileUQe = Qs !October 30, 2007 SKF Group Slide 52 Example: Hydrodynamics Journal bearing exampleSurfaces

48、fully separated by a lubricant film thickness.PUShaftHousingPressure generationLubricantOctober 30, 2007 SKF Group Slide 53 Example: Hydrodynamics Journal bearing example- At rest- Start-up ( Additives, materials,.)- In operation (Viscosity, speed, load)- Note: The most critical operation of journal

49、 bearing is start & stops!Source: Machinery lubricationOctober 30, 2007 SKF Group Slide 54 Parameters - Hydrodynamics The film thickness is mainly influenced by the following parameters :- The speed : U - The viscosity of the fluid : - The wedge created by the two surfaces - The inlet- The applied l

50、oad : WAnalogyOctober 30, 2007 SKF Group Slide 55 Hydrodynamics - behavior Film thickness increases when :- Speed increases ( film breaks down at low speeds)- Viscosity increases ( to such an extent- starvation)- Load decreases- Temperature decreases ( influencing viscosity, and thus film thickness)

51、Friction is governed by the film thickness height- No film thickness = Very High friction (solid/solid)- Too high film thickness = “Higher” friction (Viscous forces)Optimal film thickness has to be foundOctober 30, 2007 SKF Group Slide 56 Applications & components Internal combustion enginesTurbines

52、 and compressorsTransmissionsJournal (plain) bearingsThrust bearingsBushingsGas lubricated bearingsPiston ringsPorous bearingsLightly loaded roler bearingsCage/ roller contactSource: October 30, 2007 SKF Group Slide 57 Lubrication regimes: A bit of History (2). Hydrodynamics started to be well under

53、stood, however When applied to highly loaded components (especially in non conformal contacts), it did not work! This was found out when applied to gears (Martin, 1916)Source: Gear designNon conformal contactConformal contactOctober 30, 2007 SKF Group Slide 58 Lubrication regimes: A bit of History (

54、2). 2 additional thoughts:- Surfaces deform due to high pressure (Gpa, elasto)- The oil viscosity increases exponentially with pressure, creating a near- solid film.Source: Gear designSource: SKFOctober 30, 2007 SKF Group Slide 59 Elasto-hydrodynamicsF extremely highUP Deformation of the surfaces un

55、der load. Piezoviscous fluid become a solid under high loadsElastoHydrodynamics(EHD)BallFlat Inner ringhcOctober 30, 2007 SKF Group Slide 60 Elasto-Hydrodynamics: some numbersFilm thickness is around 0.1 - 1 micrometer - 100 times less than human hairContact pressure is around 20 000 bars (2 Gpa)- 1

56、000 times a car tire pressure ! Surfaces are still separated, but film is very sensitive to variations due to its thin thickness! October 30, 2007 SKF Group Slide 61 Boundary & mixed lubrication Sometimes, it is not possible to separate the surfaces.- Start-up, stops- Very low speeds, very high load

57、s- Low oil supply to the contact- Shock loads Friction & wear are controlled by properties of surfaces & by properties of the lubricant, other than viscosity.October 30, 2007 SKF Group Slide 62 This happens in: Gears, during the sliding part of tooth engagement. Rolling bearings, when sliding during

58、 low speeds, high loadswpSource: LubconOctober 30, 2007 SKF Group Slide 63 Mixed & boundary lubricationMixed lubricationBoundary lubrication Mixed lubrication:- Incomplete film separating surfaces. Metal to metal contact. Boundary lubrication:- Surfaces separated only by molecules attached to steel

59、surfaces.- High friction October 30, 2007 SKF Group Slide 64 Use of additives and/or solid lubricantsSolidE.P (Extreme pressure)AW (Anti wear)Chemical reaction: Surfaces of material changesPhysical adsorption: “Carpet” lubricationOctober 30, 2007 SKF Group Slide 65 6Application: the stribeck curveOctober 30, 2007 SKF Group Slide 66 The stribeck curveOctober 30, 2007 SKF Group Slide 67 Kappa v