Lubricant Film Formation in Rough EHL Contacts

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Keyword: Engineering and Technology, Mechanical Engineering, Tribology (Interacting Surfaces including Friction, Lubrication and Wear), Teknik och teknologier, Maskinteknik, Tribologi (ytteknik omfattande friktion, nötning och smörjning), Machine Elements, Maskinelement
Publication year: 2020
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SDG 9 Industry, innovation and infrastructureSDG 3 Good health and wellbeing
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Downsizing of machine elements and thinner lubricating oils, with simultaneous improved power density, is an on-going pursuit in automotive industry and lubrication science. The outcome is shifting the performance of e.g. gear contacts to operate under more severe lubricating regimes. In extension, this sets the role of surface finish in the view of increased importance since roughness amplitude in combination with thin films generally is considered to activate surface degradation mechanisms such as wear, fatigue, and ultimately machine failure. Thus, in order to meet the present demands, it is of critical importance to better understand the interplay between surface roughness and elastohydrodynamic oil film (EHL) formation.

Recently, our research [1] has shown that the default model [2] for estimation of lubrication quality strongly deviates from the supposed proportional relationship between film thickness and composite surface roughness (). Thus, with the present approach, false assumptions about lubrication quality are possible. An adequately run-inned surface may operate under elasto-hydrodynamic performance even at such conditions when  suggests substantial contact interference. The latter suggest that the more detrimental opposite situation also would be possible under a certain surface roughness configuration. This research was therefore set out as part of a long term goal of improving the present engineering design tool so that better and more safe estimates of the lubrication quality can be made.

This work explores the mechanisms involved in the formation of an elasto-hydrodynamic (EHD) oil film under heavily loaded mixed rolling/sliding operation of circular contacts. A WAM ball-on-disc machine was operated under conditions representative for those found in heavy duty transmission assemblies. Specimens were prepared with isotropic engineering surface finishes to capture the effect of roughness amplitude on the contacts capability to form a protective EHD film. Electrical contact resistance (ECR) and the coefficient of friction was monitored during running-in tests to reveal how surface roughness affects the number of cycles to EHL lift-off, and the associated response to friction. Specific emphasis was set out to investigate what changes surface topographies must undergo in order to reach a steady state in EHL when starting in mixed lubrication. A surface-re-location technique was developed to enable for detailed examination of the most active sites that typically involve the most prominent asperity features. Surface analysis was conducted with the aim of clarifying the importance to lubrication quality of parameters from all families, i.e. spatial, feature and hybrid parameters in addition to the present approach that only accounts for the surfaces height by root-mean-square-average (RMS/). A strong correlation between friction change and the modification of surface topography was found as the contact went through the running-in process for EHL lift-off. Additionally, it was observed that surface roughness still significantly affect the coefficient of friction at EHL steady state operation.


[1]      Hansen J, Björling M, Larsson R. Mapping of the lubrication regimes in rough surface EHL contacts. Tribol Int 2018. doi:10.1016/j.triboint.2018.11.015.

[2]      ISO/TR 15144-1:2014. Calculation of micropitting load capacity of cylindrical spur and helical gears. Geneva: n.d.


Jonny Hansen

Luleå tekniska universitet; Maskinelement; Gear Technology, Transmission Development, Scania CV AB, Södertälje, Sweden
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Marcus Björling

Luleå tekniska universitet; Maskinelement
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Roland Larsson

Luleå tekniska universitet; Maskinelement
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