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Fundamentals of Lubrication
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Agenda
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02
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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
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Introduction to Tribology
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Tribology is about understanding, analysing, predicting and controlling interactions between
moving surfaces
Tribology Solving Real World Problems
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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
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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
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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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Surface Examination Tools
Optical microscope
White-light interferometer
ProfilometryScanning electron microscopy
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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
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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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