Micro-pitting Fatigue Lives of Lubricated Point Contacts Experiments and Model Validation

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  • o. 19

    Life predictionSurface roughness

    o-dcro-ratial Fll racon

    quantify micro-pitting failure, the micro-pitting severity index (MSI) which is dened as the cumulative

    of botine gehe relily altertionali

    Extensive experimental studies have been conducted in litera-ture to investigate the inuences of various potential factors on mi-cro-pitting. Using a twin-disk set-up, Tokuda et al. [4] showed thatsurface roughness was a key parameter inuencing micro-pittingeven under full lm lubrication condition. Ariura et al. [5] studiedthe roughness effect on micro-pitting for gear contacts, conrmingthe critical role of surface roughness. Webster and Norbart [6] per-

    concerns in mind, various biodegradable lubricants were testedby Cardoso et al. [10] for their micro-pitting performance.

    In a recent paper, these authors proposed a physics-based mi-cro-pitting prediction methodology [1]. This methodology em-ployed a mixed elastohydrodynamic lubrication (EHL) model of apoint contact [11] to determine the transient surface traction dis-tributions. These surface traction distributions were applied to aboundary element based rough surface stress prediction model tond the histories of the multi-axial stress components for all thematerial points passing through the contact. The boundary Corresponding author. Tel.: +1 614 247 8688; fax: +1 614 292 3163.

    International Journal of Fatigue 48 (2013) 918

    Contents lists available at


    lsE-mail address: li.600@osu.edu (S. Li).certain operating conditions, the amount of micro-pits might stabi-lize after a certain number of loading cycles as the surface devia-tions due to micro-pitting redistribute and relieve the contactpressure. However, the continued cyclic contact can result in thefatigue failure in the form of macro-pitting, which often initiatesform the boundaries of the micro-pitted zones [2,3]. On the otherhand, the prole changes of gear teeth due to excessive micro-pit-ting activity increase the motion transmission error amplitudes tocause elevated vibration levels and dynamic tooth contact forces,further accelerating the rate of micro-pitting.

    cro-pitting reduction. Ahlroos et al. [7] performed the fatigue testsusing a twin-disk machine to study the inuences of different steelmaterials, surface roughness amplitudes, surface treatments (sur-face hardness and coatings) as well as lubricants on micro-pitting.Several other experimental works focused on the effects of lubri-cant additives on micro-pitting. Brechot et al. [8] reported thatanti-wear (AW) and extreme-pressure (EP) additives typicallyaggravated micro-pitting. A study by Laine et al. [9] suggested thatfriction modier agents alleviated the occurrence of micro-pitsthrough the reduction of boundary friction. With environmental1. Introduction

    Micro-pitting of contact surfacesautomotive, aerospace and wind turbjor problem that adversely impacts tprocess of micro-pitting progressivecontact surfaces, affecting the func0142-1123/$ - see front matter 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.ijfatigue.2012.12.003probability of fatigue failure is proposed. The micro-pitting test outcomes are compared to the predic-tions of a recently developed physics-based micro-pitting model [1] to describe the failure mechanismand assess the model accuracy.

    2012 Elsevier Ltd. All rights reserved.

    h gears and bearings ofarboxes has been a ma-ability of products. Thes the geometries of thety of gearboxes. Under

    formed twin-disk fatigue tests and found micro-pits tended to ap-pear on the surface with negative sliding. It was also shown thatthe reduction of micro-pitting could be achieved through thereduction of slide-to-roll ratio and/or roughness amplitude. How-ever, increasing the k ratio (the ratio of lm thickness to roughnessamplitude) by simply increasing the lm thickness while keepingthe roughness amplitude unchanged had limited benets in mi-Rolling contact fatigueMicro-pitting

    For the normal operating stage that follows, lower roughness amplitude, lower slide-to-roll ratio, higherrolling velocity and lower contact pressures are observed to lead to reduced micro-pitting activity. ToMicro-pitting fatigue lives of lubricated pand model validation

    Sheng Li , Ahmet KahramanDepartment of Mechanical and Aerospace Engineering, The Ohio State University, 201 W

    a r t i c l e i n f o

    Article history:Received 19 September 2012Received in revised form 12 November 2012Accepted 1 December 2012Available online 12 December 2012


    a b s t r a c t

    This study employs the twvarious parameters on mimatrix that spans the opestructed using the Fractionrolling velocity, slide-to-roa run-in stage with higher

    International Jo

    journal homepage: www.ell rights reserved.int contacts: Experiments

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    isk rolling contact fatigue test methodology to investigate the impacts ofpitting performance of lubricated point contacts of rough surfaces. A testng ranges of the sun-planet gear pair of a wind turbine gearbox is con-actorial technique to rank the order of the inuences of contact pressure,tio, roughness amplitude and run-in process. The test results indicate thattact pressure and lower rolling velocity reduce the amount of micro-pits.

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  • the contact in the rolling (tangential) and axial directions, respec-tively. The specimens are made out of AISI 4620 low carbon gearsteel and are case hardened to a surface hardness of 6062 HRCto represent a typical gear tooth surface hardness. A special surfacenishing process was developed to simulate the roughness laydirection of the actual ground gear tooth surface which is perpen-dicular to the direction of sliding, as the previous rolling contact fa-tigue tests that simulated gear contacts found such treatment to benecessary [2]. The axially directed roughness pattern on the rollerand disk surfaces as shown in Fig. 1b are the direct results of thisspecial nishing process that ensures not only the amplitudesbut also the directionality of ground gear surface roughness canbe simulated. Two batches of specimens with the average root-mean-square (RMS) surface roughness amplitudes of 0.3 lm and0.5 lm (composite roughness amplitudes of about 0.4 lm and0.7 lm) are procured. These roughness values are representativeof roughness ranges of typical ground gear tooth surfaces.

    2.2. Design of experiments test matrix and test procedure

    Table 1 lists the test matrix constructed using the FractionalFactorials technique [12]. This Design of Experiment (DOE) ap-proach uses a fraction of all the combinations of levels for all thefactors considers, allowing statistically meaningful measurementswith substantially reduced number of test runs. In order to studythe inuence of run-in on micro-pitting occurrence, a run-in stageis implemented before each normal test stage. This is especiallyrelevant to wind turbine gearboxes that are put through a run-inprocess. The run-in process that consists of 0.2 million roller con-

    Pneumatic cylinder

    l Jouelement model included the full description of the microroughnessgeometries in the stress computation such that surface asperity in-duced local stress concentrations can be captured fully. The fatiguedamage was then evaluated using a multi-axial fatigue criterion asdescribed in Refs. [13].

    This study focuses on the experimental investigation of micro-pitting with two main objectives. The rst objective is to quantifythe inuences of the contact pressure, rolling velocity, slide-to-rollratio, surface roughness amplitude and a run-in process on micro-pitting failure with the ranges of these parameters to be represen-tative of gears, establishing a statistically meaningful data set. Thesecond objective is to simulate the experiments using the micro-pitting model of Li and Kahraman [1] in order to assess the accu-racy of the model through comparing its predictions to the resultsof micro-pitting experiments.

    A test matrix is dened using the Fractional Factorial techniqueto bring certain statistical meaning to the measurements with rel-atively small number of test runs [12]. The operating conditionsare dened based on the sun-planet gear mesh of a wind turbinegearbox [13]. The tests are performed on a two-disk rolling contactfatigue machine with the contact pairs whose surfaces are axiallyground in order to simulate the surface roughness textures of gearsin relation to the direction of rolling. To quantify the degree of mi-cro-pitting after a certain number of contact cycles, Nf0, the micro-pitting severity index (MSI), W, is dened as the cumulative prob-ability of micro-pit crack initiation at Nf0. The probability distribu-tions are constructed using the predicted fatigue lives [1]. NotingthatW is equivalent to the micro-pitting area percentage, the pre-dictions are allowed to compare with the measurements to showreasonably good agreement.

    2. Micro-pitting experiments

    2.1. Test set-up and specimens

    The two-disk set-up as shown in Fig. 1a is employed in thisstudy to evaluate the inuences of various potential factors,including contact pressure, rolling velocity, slide-to-roll ratio, sur-face roughness amplitude and run-in process, on the fatigue failureof micro-pitting. The larger component of the contact pair inFig. 1a, referred as the disk, is fastened axially against its shaftshoulder using a retaining lock nut. The smaller one of the twodisks, referred as the roller, is shrink-tted onto its shaft and in-stalled into the pivoted loading arm, which is pushed against thedisk by a pneumatic cylinder. The lubricant is provided throughan overhead lubrication jet in an into-the-mesh manner. The rollerand the disk are driven independently by two AC motors at therotatio