Presented for the NLGI 2019 conference by Prof. Piet Lugt / Frank Berens

SKF – a world of reliable rotationGrease Lubrication in Rolling Bearings

© SKF Group

• Established 1907

• Sales 2018 SEK 85,713 million

SKF – some figures

SEK 87 billion assets

SEK 2.6 billion new investments

SEK 2.6 billion R&D investments

SKF – a truly global company

Financial resources Social resources Physical resources

94 manufacturing units

15 technology centers

1.814 GWh energy

535.000 tonnes metal

Customers in 40 industries

17.000 distributors worldwide

44.428 employees

800 application engineers

2.200 service engineers

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Product examples

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Beyond products…

Customer need:“I want on-time delivery, quality

and field performance, flawless

launches of new products,

technology and price”

Product

RIGHT

PRODUCT

RIGHT

COST

RIGHT

TIME

Customer need:“I want your products and my

assets to reach technical end of

life with trouble-free operation”

Rotating

equipment

performance RIGHT

SOLUTION

RIGHT

PERFORM-

ANCE

RIGHT

TOTAL COST

OF OWNER-

SHIP

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Interacting …

Local presence with

Global Industry & Application Knowledge for

Technologies around the shaft

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No. 3 on Google for

“Global Industry Drivers”

Global trends and Industry Drivers

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From Global Trends…

DigitalizationPopulation growth &

increased wealth

Environmental

impactGlobalization Urbanization

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• Connected

• Feedback

• Improvement

Looking at Industrial change drivers…

Digitalization &

connectivity

New manufacturing

& technology

Electric & connected

vehicles

• Flexibility

• Reliability

• Productivity

• Safety

• Entire Value Chain

• Transformation

• Electrification

• Autonomous drive

• Connectivity

• Shared mobility

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• Application specific

• Rotational speed

• Energy efficient

• Environmental

SKF Lubrication related…

New application

demandsFocus Reliability Development speed Knowledge based

• Consistent performance

• Model = Reality

• Proactive + Reactive

• Anticipation

• Knowing not testing

• Ready for changes

• Underlying base line

• Global

• Available

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• Performance need will grow

Challenge for Lubrication / Lubricants:

• Reliability is a must

• Development has to be agile

• Knowledge is the key

… with “Grease life” as a core part!

Know-how to rotate the world

Understanding Grease Lubrication in Rolling Bearings

© SKF Group

• Speed/temperature/load window

• Long Grease Life

• Oscillating Movements

• Fretting and False Brinelling

• Fast Outer Ring Rotation

• Vibrations and Shock Loads

• Radiation Resistance

• Low Friction

• Rust Protection

• Water Resistance

• Biodegradability

• Food Compatibility

• Pumpability

Most Used Grease Performance Criteria

© SKF Group

Structural / automated grease selection

LubeSelect

Expert system on grease lubrication

www.skf.com

Includes Grease Life/Relubrication interval

Calculation

© SKF Group

Predictability of Grease Life: Traffic Light System

Similar concept for

speed and load range

Grease Life can be calculated

Theoretical Developments Grease Life

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Flow/Reservoir Formation

Replenishment and

mixing

Rheology

Film thickness

Degradation; oil release

and oil/grease loss

Grease Life / Grease

Performance

Internal

bearing

geometry

Grease physics

and chemistry

GreaseNo Grease

Only grease

on one side

Internal Bearing Design Chaotic Behaviour Starved Lubrication Grease Micro-Structure

and RheologyUpscaling

Bearing Life

Grease, located on various parts of the bearing, forms containers of lubricant,

each with its own properties/importance, providing the contacts with lubricant.

© SKF Group

Outline Grease Life

Mechanical Shear

Heat

Centrifugal Forces

Heat (Oxidation)

Destruction

Microstructure

Evaporation

Oil separation

Contact Pressure

Decreasing Oil

Content

Hardening of

Grease

Softening of

GreaseLeakage

Reduced Lubricity

Oxidation of base oilIncrease Viscosity

Destruction

Microstructure

Lacquer / Sludge

FormationReduced Lubricity

Hardening of

Grease

Me

ch

an

ica

lC

he

mic

al

Co

nta

min

ati

on Wear Particles

WaterSoftening/

Hardening of

Grease

Starvation

Change

Microstructure

Thin Layer Flow

Leakage

Film Thickness

Extension of the map given in H. Osawa and T. Okaniwa. Lubrication of Japanese bullet train traction motor. NLGI Spokesman, 62(10):22–27, 1999.

Grease Life

© SKF Group 18 June 2019Slide 21

Grease lubrication mechanism

Churning phase Bleeding phase

Reservoir formation:

rheology

Film thickness: fully flooded

Oil bleeding from grease reservoir

Film thickness: starved EHL

Replenishment by centrifugal forces

Grease dynamics causing

replenishment

Possible oxidation

Severe film

break down

Flow/Reservoir Formation

Replenishment and

mixing

Rheology

Film thickness

Degradation; oil release

and oil/grease loss

Grease Life / Grease

Performance

Internal

bearing

geometry

Grease physics

and chemistry

GreaseNo Grease

Only grease

on one side

Internal Bearing Design Chaotic Behaviour Starved Lubrication Grease Micro-Structure

and RheologyUpscaling

Bearing Life

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Reservoir formation: Grease flow and rheology

J.M. Madiedo, J.M. Franco, C. Valencia, and C. Gallegos. Modelling of the non-linear rheological behavior of a lubricating grease at low-shear rates. ASME Journal of Tribology, 122:590–596, 2000.

© SKF Group

Macroscopic flow during the churning

phase (Channeling)

Microscopic flow during the bleeding

phase.

Channeling

A.C. Horth, J.H. Norton, and Paltenden W.C. Temperature rise characteristics of greases in rolling

element bearings. Lubrication Engineering, 27(11):380–385, 1971.

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Flow Properties of Grease: Rheology

J.M. Madiedo, J.M. Franco, C. Valencia, and C. Gallegos. Modelling of the non-linear rheological behaviourof a lubricating grease at low-shear rates. ASME Journal of Tribology, 122:590–596, 2000.

P.M. Lugt, Grease Lubrication in Rolling Bearings, Wiley.

© SKF Group

The Yield Stress

Oil behaviour

Yield

F. Cyriac, P.M. Lugt, and R. Bosman. On a new method to measure yield stressin lubricating greases. Tribology Transactions, 58(6):1021–1030, 2015.

P. Baart, P.M. Lugt, and B. Prakash. Non-Newtonian effects on film formation in grease-lubricated radial lip seals. STLE Tribology Transactions, 53(3):308–318, 2010.

© SKF Group

Grease Flow: Rheology

F. Cyriac, P.M. Lugt, and R. Bosman. On a new method to measure yield stress in lubricating greases. Tribology Transactions, 58(6):1021–1030, 2015.

© SKF Group

Grease Flow in Bearings: CT Scan and CFD

T. Noda, S. Miyata, M. Taniguchi, and H. Aramaki. Liquid-gas two-phase flow analysis of lubricating grease in ball bearing and validation of visual experiments using X-ray CT. Proceedings 2011 STLE Annual Meeting & Exhibition, Atlanta, Georgia, USA, 2011.

© SKF Group

Initial filling: Chaotic Nature of Grease Lubrication

Chaotic behaviour → initial conditions important

(filling)

Lyaponov exponent >1:

chaotic behaviour

P.M. Lugt, S. Velickov, and J.H. Tripp. On the chaotic behaviour of grease lubrication in rolling bearings. Tribology Transactions, 52:581–590,

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Film thickness: EHL

Fully

flooded

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Starved EHL

Cann, P. M. (1996), “Starvation and Reflow in a Grease-Lubricated Elastohydrodynamic Contact,” Tribology Transactions, 39(3), pp 698-704.

M.T. van Zoelen, C.H. Venner, and P.M. Lugt. Prediction of film thicknessdecay in starved elasto-hydrodynamically lubricated contacts using a thin film layer model. Proceedings of the Institution of Mechanical Engineers.Part J: Journal of Engineering Tribology, 223(3):541–552, 2009.

© SKF Group 31

Fil

m t

hic

kn

es

s [

nm

]

y

q

ResultsFilm thickness distributionSpherical Roller Bearing 22317

Fr = 10kN, Ω = 3000 rpm

M.T. van Zoelen, C.H. Venner, and P.M. Lugt. The prediction of contact pressure induced film thickness decay in starved lubricated rolling bearings. Tribology Transactions, 53(6):831 – 841, 2010.

© SKF Group 32

ResultsFilm thickness distribution

Fil

m t

hic

kn

es

s [

nm

]

y

q

Spherical Roller Bearing 22317

Fr = 10kN, Ω = 3000 rpm

© SKF Group

Progress in Single Contact Film Thickness

S.Y. Poon. Experimental study of grease in elastohydrodynamic lubrication. J Lubr Technol Trans ASME, 94(1):27–34, 1972

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Low Speeds

Thickener material travelling through the

contact.

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Very Low Speeds

H. Cen, P.M. Lugt, and G.E. Morales-Espejel. Film thickness of mechanically worked lubricating grease at ultra-low speeds. Tribology Transactions, 57(6):1066–1071, 2014.

© SKF Group

Thicker films:

• Thickener concentration high

• Particle size small

Medium Speeds: Effect of Thickener Particle Geometry on Film Thickness

F. Cyriac, P. M. Lugt, R. Bosman, C. J. Padberg, and C. H. Venner. Effect of thickener particle geometry and concentration on the grease EHL film thickness at medium speeds. Tribology Letters, 61(2): 1–13, 2016.

© SKF Group 18 June 2019Slide 37

Grease film thickness: Mass Balance Between Feed and Loss Mechanisms

V. Wikström and B. Jacobson. Loss of lubricant from oil lubricated near-starved spherical roller bearings. Proceedings of the Institution of Mechanical Engineers. Part J: Journal of Engineering Tribology, 21(1):51–55, 1997.

© SKF Group

Measured Film Thickness in a Ball Bearing

Film thickness is a

function of the base oil

viscosity and bleed

Film thickness is

independent of speed!

H. Cen and P.M. Lugt. Film thickness in a grease lubricated ball bearing. Tribology International, 2019.

M. Barz, Die Schmierfilmbildung in fettgeschmiertenschnellaufenden Spindellagern, PhD Thesis, University of Hannover, Germany (1996).

© SKF Group October 30, 2007 © SKF Group

Slide 39

The significance of bleed

S. Hibino, T. Hosoya, K. Nakamura, K. Matsuoka, T. Nagayama, M. Kitamura, and T. Sunohara. A new grease-pocket shape to extend the service life of grease. Tribology Online, 3(2):54–58, 2008.

© SKF Group

The grease in the “housing” actively

contributes to extending grease life.

More grease is better….

But not in the swept area of the bearing

The Significance of Grease Bleed

S. Komatsuzaki and T. Uematsu. Estimation of service life of large size roller bearings. LubricationEngineering, 50(1):25–29, 1994.

© SKF Group

Grease Bleed: University of Akron / Timken model

A. Saatchi, P.J. Shiller, S.A. Eghtesadi, T. Liu, and G.L. Doll. A fundamental study of oil release mechanism in soap and non-soap thickened greases. Tribology International, 110:333 340, 2017.

© SKF Group 18 June 2019Slide 42

Grease bleed: SKF Model

Simulate grease as a porous

medium with collapsing structure

where flow is driven by centrifugal

forces

P. Baart, B. Van der Vorst, P.M. Lugt, and R.A.J. Ostayen. Oil bleeding model for lubricating grease based on viscous flow through a porous microstructure. Tribology Transactions, 53(3):340–348, 2010.

© SKF Group

Model based on the (wetting) affinity between grease

matrix and base oil.

Grease Bleed: University of Twente Model

Courtesy University of Twente

Surface area of fibers

© SKF Group

Too much bleed:

Reduced oil content

Short shelf life

Too little bleed:

Severe starvation

The Bleed DilemmaBleed rate + Long Time Bleed Matters!

D.F. Wilcock and M. Anderson. Grease-an oil store-house for bearings. Symposium on Functional Tests for Ball Bearing Greases, ASTM No 84, 1949.

© SKF Group 18 June 2019Slide 45

Chemical aging: oxidation

Induction time due to active anti-oxidants.

P.M. Cann. Starvation and reflow in a grease-lubricated elastohydrodynamiccontact. STLE Tribology Transactions, 39(3):698–704, July 1996.

H. Ito, M. Tomaru, and T. Suzuki. Physical and chemical aspects of grease deterioration in sealed ball bearings. Lubrication Engineering, 44(10):872 879, 1988.

C.C.J. Wang, J.L. Duda, and E.E. Klaus. A kinetic model of lubricant deposit formation under thin film conditions. STLE Tribology Transactions, 37:168–174, 1994.

© SKF Group

Oxidation takes

primarily place in the

base oil and not in

the thickener

Grease Oxidation

carboxyl peak at 1750 cm−1

J.P. Kaperick, W.B. Anderson, M.T. Devlin, K.J. Garelick, J.M. Guevremont, and K.W. Kvasnicka. The effect of grease stability on torque increases in bearing tests. Proceedings of the 19th International Colloquium Tribology, 21-23 January, TA Esslingen, 2014.

© SKF Group

Mechanical Aging: Louisiana State University Model

A. Rezasoltani and M.M. Khonsari. On the correlation between mechanical degradation of lubricating grease and entropy. Tribology Letters, 56(2):197–204, 2014.A. Rezasoltani and M.M. Khonsari. An engineering model to estimate consistency reduction of lubricating grease subjected to mechanical degradation under shear. Tribology International, 103:465 – 474, 2016.

© SKF Group 18 June 2019Slide 48

Mechanical Aging (University of Twente/SKF)

Y. Zhou, R. Bosman, and P.M. Lugt. A master curve for the shear degradation of lubricating greases for rolling bearings. Tribology Transactions, pages 1–10, 2018.

© SKF Group

Application of the Aging MasterCurve to a Bearing

Y. Zhou, R. Bosman, and P.M. Lugt. A master curve for the shear degradation of lubricating greases for rolling bearings. Tribology Transactions, pages 1–10, 2018.

© SKF Group

• Grease life is one of the most important parameters for grease selection

• Great steps have been made in understanding the physics and chemistry of grease

lubrication but there is still much that is unexplored

• For the bearing industry predictability is crucial

• For the grease industry new developments in grease formulation for rolling bearing

greases should be directed towards extending grease life

• For the prediction of grease life under practical conditions, an improved understanding of

the physics and chemistry in grease lubrication is required:

Conclusions

Thank You!