5866 Life Predictions

8/21/2019 5866 Life Predictions

1/12

TECHNICAL PAPER

COMPARISON OF DIN 28 BEARING FATIGUE LIFE PREDICTIONS

WITH TEST DATA

THE TIMKEN COMPANY

By: Michael Kotzalas and Gerald F

8/21/2019 5866 Life Predictions

2/12

 Abstract

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.

8/21/2019 5866 Life Predictions

3/12

Introduction

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.

FIGURE 1. BEARING TEST SETUP

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

8/21/2019 5866 Life Predictions

4/12

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