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5866 Life Predictions

Aug 07, 2018



  • 8/21/2019 5866 Life Predictions






    By: Michael Kotzalas and Gerald F

  • 8/21/2019 5866 Life Predictions



    Recent advances in rolling bearing technology have spawned a flurry of activity aimed at reassessing bearing

    life prediction algorithms. This has become particularly important for the wind turbine gearbox industry, where

    20 years of calculated bearing L10 life is a standard requirement. However, it is a common observation that

    numerous, proprietary approaches have evolved to predict bearing life, and these varying methods can provide

    significantly different predictions. In an effort to create an advanced, publicly accessible method of predicting

    bearing L10 life that might provide some uniform basis, the Deutsches Institut für Normung e.V. (DIN) has created a

    standard utilizing assumptions for what constitutes a typical bearing design, manufacturing process, as well as the

    expected damage mechanisms. Validation of the standard was only accomplished to the extent that the member

    companies shared test results or comparisons with their prediction algorithms.

    To further consider the effectiveness of the DIN algorithms to accurately predict the fatigue life of rolling bear-

    ings, this paper compares test data of standard production tapered roller bearings (TRB) from six top manufac-

    turers, including Timken, with the DIN and Timken proprietary algorithms. The test data was selected to include

    varying operating conditions in recent test programs; thick and thin lubricant films, misalignment, variable loading

    and debris denting of the raceway surfaces. The results of this investigation show a bearing manufacturer’s propri-

    etary algorithms, in this case Timken, more accurately predict the actual performance of their products. In fact, the

    DIN algorithms tended to over-predict bearing fatigue life for low load and under-predict for debris contaminated

    operating conditions.

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    Recently, there have been significant advances in

    rolling bearing technology. These advances can be seen

    in design and manufacturing through the use of cleaner

    steels, new surface finishing and texturing techniques as

    well as the use of tribological coatings. Also, advances

    have been made in the fundamental understanding and

    modeling of rolling bearing performance. The use of com-

    puters has increased the level of sophistication available

    for bearing analyses to the point that what used to be im-

    practical is now standard practice. As such, there has

    been a large amount of recent activity in the area of pre-

    dicting bearing performance [1-7]. This has become par-

    ticularly important for many demanding and highly sophis-

    ticated applications, such as the wind turbine gearbox

    industry where 20 years of calculated bearing L 10

     life is a

    standard requirement.

    One significant problem with all of the recent activ-

    ity in bearing analyses is the numerous, proprietary ap-

    proaches that have evolved to predict fatigue life. These

    methods can vary significantly causing difficulty for engi-

    neers selecting bearings, as seemingly identical bearings can have vastly different predicted lives depending on the

    approach used.

    In an effort to create an advanced, publicly accessi-

    ble method of predicting bearing L 10

     life that might pro-

    vide some uniform basis, the Deutsches Institut für Nor-

    mung e.V. (DIN) [5 and 6] has created a standard utilizing

    assumptions for what constitutes a typical bearing design,

    manufacturing process, as well as the expected damage

    mechanisms. As with any standardization activity, vali-

    dation was only accomplished to the extent that member

    companies shared test results or comparisons with their

    prediction algorithms. As the DIN 281 Addendum 4 stan-

    dard is being utilized in a slightly modified fashion for bear-

    ing selection within the American Gear Manufacturer’s As-

    sociation (AGMA) Wind Turbine Gearbox standard 6006

    [8], an investigation of its accuracy compared to bearing

    fatigue test data is desired, and is the aim of this paper.

    Bearing Fatigue Tests

    Bearing fatigue life test results from the author’s labo-

    ratory were collected. Only tests of standard production,

    or “off the shelf” tapered roller bearings (TRB) were used,

    as more scientifically controlled tests would not be rele-

    vant to the community of bearing users. The tests were

    selected to include bearings from six top manufacturers,

    including Timken, and varied operating conditions. The

    different general operating conditions included thick and

    thin lubricant films, bearing misalignment, variable loading

    and debris denting of the raceway surfaces.

     All of the tests were conducted in the author’s labo-

    ratory using a first-in-four test scheme; See Figure 1. In this scheme, the center bearings are radially loaded with

    a hydraulic cylinder, while the end bearings are loaded

    through the reaction with the shaft and housing. The test

    is shut down when one bearing has a spall subtending 6

    mm2 (0.01 in2 ) in area. At this time, the remaining 3 bear-

    ings are suspended, yielding the L 15.91

     life for this sample

    of four bearings.



    The standard test setups use an ISO viscosity grade

    mineral oil supplied via a circulating system. The lubri-

    cants contain only rust and oxidation (R&O) additives to

    prevent any alteration of the bearing fatigue performance

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    due to the additives in fully formulated oils. The circulat-

    ing oil is filtered with a 40 mm, absolute filter. Even with

    such course filtration, due to the general laboratory envi-

    ronment, the ISO 4406 oil cleanliness level has been con-

    sistently measured at 15/12 for all test housings.

    Utilizing the previously described standard test setup,

    variable parameters within the test rigs were controlled to

    obtain the desired operating environment. Such as, the

    lubricant supply temperature was set at different values to

    produce the desired lubricant film conditions. Thick film

    tests were typically run at 37.8ºC (100ºF) oil inlet temper-

    ature while thin film tests at 82.2ºC (180ºF). To produce

    misalignment conditions, the cup-housing adapters were

    manufactured with a predisposed misalignment. Control-

    ling the hydraulic pressure applied to the loading cylinder

    created the variable load conditions.

    Finally, the debris-dented conditions were created in

    a more elaborate setup, by pre-denting the bearing as-

    semblies. This was conducted by rotating a single bear-

    ing under a 4448 kN (1000 lb) pure thrust load for 2000

    revolutions in a debris-laden lubricant. The debris parti-

    cles were hardened T15 tool steel from 25 µm to 53 µm

    in size. The debris was mixed into highly filtered ISO VG

    032 mineral oil containing only R&O additives in a ratio

    of 0.5 mg/ml. After pre-denting, the bearings were ul-

    trasonically cleaned to remove any residual dirt or debris

    from the components and then assembled into the fa-

    tigue test rig.

    The final matrix of selected tests for this study is list-

    ed in Table 1. Table 1 represents 48 different sets of bear-

    ing tests, consisting of 1228 tapered roller bearings from 6

    top manufacturers. The different sets have been grouped

    into 5 general categories of operating conditions: thick lu-

    bricant films, thin lubricant films, misaligned; varied load-

    ing and debris.

    Fatigue Life Prediction

     After the bearing fatigue test data was collected, the

    lives were predicted using the Timken catalog and Timken

    advanced proprietary methods as well as the DIN 281 Bei-

    blatt 1 and 4 algorithms. In doing such, the average mea-

    sured cup OD temperature was used for determining the

    lubricant properties, and the average measured load zone

    to determine an applied bearing thrust load for each of the

    48 test sets. The bearing misalignment was estimated

    through shaft bending analysis coupled with the pre-man-

    ufactured misalignment in the cup-housing adapters. The

    above inputs were necessary for all prediction algorithms,

    however the Timken advanced and DIN 281 Addendum 4

    methodologies also require the input of profiles and sur-

    face finishes. As the design profiles and finishes were not

    known for all of the test bearings, as is the case for most

    end users, the default profiles and finishes built into each

    tool were used. The default profiles in both algorithms use

    some form of a modified (e.g. logarithmic) profile.

     After the lives were predicted for each of the 48 test

    sets, Weibull analyses were performed to combine data

    into the 5 groups of operating conditions. To do this, the individual bearin

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