Fundamentals of Lubrication - Infineum Insight€¦ · almost all lubricants behave as non-Newtonian fluids under high shear rates Shear rate y Newtonian Shear stress e Pseudo-plastic

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Fundamentals of Lubrication

1



Agenda

01

02

03

State what the word tribology means

List the main regimes of lubrication and state the

conditions under which each tends to occur

Understand Infineum approach to lubricant

performance testing

2



Introduction to Tribology

3



Tribology is about understanding, analysing, predicting and controlling interactions between

moving surfaces

Tribology Solving Real World Problems

4



Tribology is the study of friction, wear and lubrication between surfaces sliding against each other

Tribology History and Definition

Leonardo Da Vinci was the first to enunciate the laws of friction

The word ‘Tribology’ was first coined in 1964

Source: WSJ

5



Why Does Tribology Matter?

Environment

Energy

Reliability

Safety

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Multi-disciplinary Aspect of Tribology

Tribology

Fluids mechanics

Material science

Physical-chemistry

Solid mechanics

7



Stribeck Curve and Lubrication Regimes

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Stribeck Curve

Stribeck curve is used to represent friction response of a tribological contact across different lubrication regimes

LogViscosity x speed

Load

Boundary lubrication

Mixed lubrication

Elasto-hydrodynamic

Hydrodynamic

Min

imu

m film

thickn

ess (h)C

oef

fici

ent

of

fric

tio

n (

µ)

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The contact is designed such that their relative motion drags lubricant between them, forcing the lubricant to high pressures of up to 200 MPa (30,000 psi), large enough to support external loads

Hydrodynamic Lubrication

Oil Flow

Moving Surface

Pressurised film creating a 'lift' of the top surface

Bearing Bearing load

Journal

Lubrication gap

Minimum oilfilm thickness

Hydrodynamicoil film pressure

profile

Lube oil supply

Oil film thickness typically 1 to 100 μm

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Plain journal bearings Aquaplaning

Hydrodynamic Lubrication Examples

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• High localised stresses cause elastic deformation of the surfaces

• There is an exponential rise in viscosity of the lubricant as it is squeezed through the contact

Elasto-hydrodynamic Lubrication (EHL)

Thin fluid film is formed due to surface deformation and viscosity increase

When all the loading is concentrated on a small contact area:

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EHL Examples

Toyota 1HZ timing gear Valve train – cam and lifter experience EHL at cam nose

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Oil film thickness typically 1 to 100 nm

Boundary Lubrication

Insufficient film to separate the surfaces

Tribo-reacted film on contacting asperities

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Mixed Lubrication

Enough film to separate most of the rubbing surfaces

Metal-to-metal contact between asperities

Oil film thickness typically 1 to 100 nm

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Top Dead Centre (TDC)

Bottom Dead Centre (BDC)

Piston rings experience all lubrication regimes as piston slides from TDC to BDC

Metallic contact (%)0 100

Cran

k angle

(de

g)

Hydrodynamic

Mixed/boundary

Mid-stroke

Mixed/boundary

Mixed and Boundary Lubrication Examples

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Applied Tribology and Key Properties

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Parameters to Consider in Tribology

Surface finish

Metallurgy

Hardness

Coating

Temperature

Pressure

Relative velocity

Chemical species

Viscosity and Viscosity index

Behaviour under extreme pressure

Newtonian vs. non- Newtonian

Surface active additives

Material properties

Operating conditions

Lubricant properties

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Material PropertiesTop ring and cam

Real example of top ring roughness discrepancy

Chilled cast iron

Grey cast iron

Surface Finish

Metallurgy Hardness

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Operating Conditions Main bearings

Critical operating conditions• High load

• Low engine speeds

Main challenges• Increasing contact pressures and

temperatures

• Stop and start and hybridisation

• High soot content in certain applications

Main types of wear• Abrasion

• Corrosion

• Cavitation

Typical temperature and contact pressure:

• 120 °C

• 60 MPa

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Lubricant Properties Viscosity

Dynamic viscosity:

resistance to shearing flow

Kinematic viscosity:

flow response to gravity

Viscosity index:

𝜂 =𝜏

ሶ𝛾

𝑆ℎ𝑒𝑎𝑟 𝑠𝑡𝑟𝑒𝑠𝑠 (𝑁

𝑚2)

𝑆ℎ𝑒𝑎𝑟 𝑟𝑎𝑡𝑒 (𝑠−1)[Pa.s]

𝜗 =𝜂

𝜌

𝐷𝑦𝑛𝑎𝑚𝑖𝑐 𝑣𝑖𝑠𝑐𝑜𝑠𝑖𝑡𝑦

𝑑𝑒𝑛𝑠𝑖𝑡𝑦[𝑚𝑚2/𝑠]

300

250

200

150

100

50

0Kin

em

atic

Vis

cosi

ty (

mm

2/s

)

Temperature (°C)

30 40 50 60 70 80 90 100 110

VI > 250

VI ~ 65

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Viscosity pressure relationship:

lubricant viscosity increases with pressure and this effect is generally greater than the effect of temperature.

Barus equation is most commonly used to show the relationship:

𝜂𝑝 = 𝜂0𝑒𝛼𝑝

𝜂𝑝 viscosity at pressure ‘p’ [Pa.s]

𝜂0 viscosity at atmospheric pressure [Pa.s]

𝛼 is the pressure viscosity coefficient [m2/N]

Lubricant Properties High pressure

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Viscosity-shear rate relationship:

almost all lubricants behave as non-Newtonian fluids

under high shear rates

Shear rate

Vis

cosi

ty

Newtonian

Shear stress

She

ar r

ate

Pseudo-plastic behaviouralso referred as shear thinning

Thixotropic behaviouralso referred as

shear duration thinning

Lubricant Properties Non-Newtonian behaviour

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How to Test and Measure

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Why Study Friction and Wear in an Engine?

Several tests are used to qualify formulated oils against well-defined performance criteria

Durability, performance retention and efficiency

Internal combustion engines contain many moving metal parts

Movement between parts generates friction and wear

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Representativeness, complexity, time, cost

How to Select the Right Level of Testing

Field Testing

Representativeness, complexity, time, cost

Control of variables, fundamental understanding

Chassis Dyno

Fired Engine

Motored Engine

Motored Sub-

assembly

Bench Rigs

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http://resources/sites/CorpImages/Test/Exhaust emission pipe ISP.jpg



Surface Examination Tools

Optical microscope

White-light interferometer

ProfilometryScanning electron microscopy

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http://www.google.co.uk/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=ERhEPVF8bWtoMM&tbnid=MpdgD69dyhs23M:&ved=0CAUQjRw&url=http://www.dia-m.ru/lab/mikroskopy-pryamye-issledovatelskie/carl-zeiss-4749-axio-scope-a1/&ei=bwC_Ue_WLInM0QX56IGQDA&bvm=bv.47883778,d.d2k&psig=AFQjCNHB1rkRXyhJTnVcsqxP2Nb9lAnN8w&ust=1371558325015126

http://www.google.co.uk/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=FCca2ne6wLm4_M&tbnid=hUX9_SN51WiPqM:&ved=0CAUQjRw&url=http://arboretum.harvard.edu/research/policies-and-instrumentation/microscopy-lab/?wpmp_switcher=mobile&ei=kXYoUv2TII-Z0AXA4YHQCg&bvm=bv.51773540,d.ZG4&psig=AFQjCNF8m6v1mhVgX0Tf_Bc2jzZaGo28FA&ust=1378469850751472



ExampleRing/liner wear tested in the SRV

Liner couponTop ring section

Oil supplyLiner temperatureReciprocating frequencyStrokeNormal load appliedTest duration

Material properties

Lubricant properties

Operating conditions

Severity Discrimination Repeatability

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Example - Ring/liner wear tested in the SRV

On-test monitoring Liner surfaceElements on ring surface

Liner surface

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Is That Everything for the Lubricant?

• Keep surfaces separate

• Protect surfaces

• Act as a cooling fluid

• Fulfil detersive and dispersive functions

• Act as a sealant

• Remain adequately stableFuel economy

Durability

Performance

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Summary

01

02

03

You should now have a better understanding of:

04The different levels of testing and

surface analysis techniques

The key parameters to consider for

a system approach

The main lubrication regimes and the conditions

under which they occur

Tribology: definition and implications

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Fundamentals of Lubrication - Infineum Insight€¦ · almost all lubricants behave as non-Newtonian fluids under high shear rates Shear rate y Newtonian Shear stress e Pseudo-plastic