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Initiation of rolling contact fatigue from asperities in elastohydrodynamic lubricated contacts
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).ORCID iD: 0000-0002-5101-9532
2018 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Rolling contacts are utilized in many technical applications, both in bearings and in the contact between gear teeth. These components are often highly loaded, which makes them susceptible to suffer from rolling contact fatigue. This work focuses on the rolling contact fatigue mechanism of pitting. In order to attain a better understanding of why pitting initiates and grows, detailed simulations of rolling contacts have been performed. In particular the contact between two gears in a truck retarder was here used as a case study. The investigated contact experienced elastohydrodynamic lubrication conditions since the load was high enough to causes the surfaces in contact to deform and the viscosity of the lubricant to increase significantly.

In Paper A it was investigated if surface irregularities in the size of the surface roughness are large enough to cause surface initiated fatigue. The investigation focused on the pitch line since small surface initiated pits were found here even though there was no slip present. Since there were pits present at the pitch line, it is important that the theories of pitting can explain the development of pits also in the absence of slip. The conclusion of the work was that surface irregularities of the size of normal surface roughness are enough to cause surface initiated fatigue at the pitch line.

In Paper B it was investigated why pits are more likely to initiate in the dedendum of pinion gears than in the addendum. In both areas slip is present but in different directions. In the dedendum the friction from slip is against the rolling direction which enhances the risk for pitting. The investigation was performed by studying the effect of the temperature rise in the contact caused by the slip. The conclusion drawn was that the temperature rise in the contact explained why pitting was more common in the dedendum than in the addendum.

Abstract [sv]

Rullande kontakter förekommer i många applikationer, till exempel i lager och mellan kugghjulständer. Både lager och kugghjul utsätts ofta för höga laster vilket gör att dess ytor löper stor risk att utmattas, vilket kallas rullande kontaktutmattning. Denna studie fokuserar på pitting, även kallat spalling, vilket är en typ av rullande kontaktutmattning där en utmattninsspricka växer fram som får delar av ytan att ramla av. För att få en bättre förståelse varför pittingskador uppkommer har noggranna simuleringar utförts av rullande kontakter. Kontakten mellan två tänder på kugghjul i en lastbilsretarder har används som underlag då många pittingskador påträffats på dem. 

För att minska friktionen och nötningen i kontakten mellan kuggtänderna användes smörjmedel. De höga lasterna lastbilsretardern utsattes för deformerade kuggarnas ytor elastiskt samtidigt de kraftigt ökade viskositeten hos smörjmedlet. Dessa förhållanden gör att kontakten kallas för elastohydrodynamiskt smord, vilket på engelska förkortas till EHL.

I Artikel A undersöktes om små ytojämnheter kan orsaka ytinitierade pittingskador. Eftersom skadan påträffats i friktionslösa kontakter, så som vid rullcirkeln på de undersökta kugghjulen, är det viktigt att teorierna som förklarar uppkomsten inte är beroende av friktion. Undersökningen fokuserade därför på förhållandena vid rullcirkeln.  Slutsatsen från arbetet var att små ytojämnheter, av samma storleksordning som ytojämnheterna på de undersökta kugghjulen, är tillräckligt stora för att orsaka utmattningsskador.

I Artikel B undersöktes varför det är vanligare att pitts initieras i dedendum än addendum på drivande kugghjul. Kontakten på båda sidorna om rullcirkeln slirar svagt åt olika håll. Att kontakten slirar skapar friktion som är motriktad rullriktningen i dedendum vilket ökar risken för pittingskador. För att undersöka varför dessa förhållanden ökar risken för skador fördjupades analysen av kontakten genom att inkludera temperaturfältet. Simuleringarna visade att temperaturen ökar genom kontakten vilket orsakar en asymmetrisk spänningsfördelning. Denna asymmetriska spänningsfördelning gör att ytojämnheter i dedendum är troligare att orsaka skador än ytojämnheter i addendum. 

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2018. , p. 19
Series
TRITA-SCI-FOU ; 2018:05
Keyword [en]
Spalling; Pitting; Slip; Elastohydrodynamic; Thermal elastohydrodynamic; Fatigue; Contact mechanics.
Keyword [sv]
Pitting; Glidning; Elastohydrodynamik; Termisk elastohydrodynamik; Utmattning; Kontaktmekanik.
National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-222371ISBN: 978-91-7729-688-1 (print)OAI: oai:DiVA.org:kth-222371DiVA: diva2:1182344
Presentation
2018-03-14, Seminarierummet, KTH Hållfasthetslära, Teknikringen 8D, 100 44 Stockholm, 10:15 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 2012-5922
Note

QC 20180213

Available from: 2018-02-13 Created: 2018-02-13 Last updated: 2018-02-13Bibliographically approved
List of papers
1. Contact fatigue initiation and tensile surface stresses at a point asperity which passes an elastohydrodynamic contact
Open this publication in new window or tab >>Contact fatigue initiation and tensile surface stresses at a point asperity which passes an elastohydrodynamic contact
2017 (English)In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464Article in journal (Other academic) Published
Abstract [en]

Contact mechanics and tribology was combined with fundamental fatigue and fracture mechanics to form a new mechanism for surface initiated rolling contact fatigue. Following, fatigue was investigated numerically for single asperities and craters in lubricated rolling contact surfaces. The hypothesis suggests that asperity point contacts can create sufficiently large tensile stresses for fatigue. The investigated case corresponded to a heavily loaded truck gear with ground surfaces. Reynolds equation resolved the elastohydrodynamic effect of the asperity in the transient three dimensional contacts. The Findley critical plane criterion was used for multiaxial and non-proportional fatigue evaluation. The simulations confirmed the new mechanism for rolling contact fatigue and showed how asperities can create contact fatigue in the lubricated contacts even without slip.

Place, publisher, year, edition, pages
Elsevier, 2017
Keyword
Spalling; Elastohydrodynamic; Fatigue; Contact mechanics.
National Category
Applied Mechanics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-222360 (URN)10.1016/j.triboint.2017.10.013 (DOI)
Funder
Swedish Research Council, 2012-5922
Note

QC 20180212

Available from: 2018-02-08 Created: 2018-02-08 Last updated: 2018-02-13Bibliographically approved
2. The asperity point load mechanism for rolling contact fatigue considering slip and thermal elastohydrodynamic lubrication
Open this publication in new window or tab >>The asperity point load mechanism for rolling contact fatigue considering slip and thermal elastohydrodynamic lubrication
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Rolling contact fatigue was investigated numerically for a single asperity and crater passing through lubricated rolling contacts with slip. The purposes was to explain why rolling contact fatigue pits develop in the forward rolling direction when slip is negative and to further verify the asperity point load mechanism for pitting. The simulations included thermal effects from friction and compression of the lubricant. It was concluded that heating of the lubricant decreased the viscosity and the contact shear stress profile in the second half of the contact, which provided an asymmetric stress profile and explained the pitting damage behaviour in relation to slip direction. 

Keyword
Spalling; Pitting; Thermal elastohydrodynamic; Slip.
National Category
Applied Mechanics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-222370 (URN)
Funder
Swedish Research Council, 2012-5922
Note

QC 20180212

Available from: 2018-02-08 Created: 2018-02-08 Last updated: 2018-02-13Bibliographically approved

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