TECHNICAL REPORT

Improvement in Reliability of Spherical Roller Bearings
H. YASUDA Y. TAKEDA

K. OTSUKA

Ever increasing productivity through reliability is a requirement for mechanical elements used in industrial
applications, where spherical roller bearings are subjected to severe operating conditions. In order to meet thesevere application requirements, JTEKT has optimized spherical roller bearings with symmetrical rollers to achieve
larger load ratings. Physical testing was completed to evaluate the performance and confirm the theoretical design.
As a result, it was confirmed that the rollers move steadily, which led to less temperature rise and contributed toboth high speed and better axial loading performance. After the bench testing was complete, both the physicalevaluation and the theoretical analysis (CAE) show very good correlation.
Key Words: spherical roller bearing, roller, temperature rise, axial load, high speed, CAE, reliability
improvement

Table 1 Types of spherical roller bearings

1. Introduction
Due to their superior alignment, large load rating
and good ability to integrate, spherical roller bearings
are widely used in general industrial machinery suchas reducers and construction machines as well as
1, 2)
steel production equipment . These bearings arealso commonly used as main shaft bearings in wind3)
turbine generators . Recently, with the improvementin productivity of industrial machinery, bearings must
perform in increasingly severer conditions. This reportintroduces spherical roller bearings developed by JTEKTthat have upgraded high speed performance and axial
loading performance to satisfy these requirements.

2. Types of spherical roller bearings
Table 1 shows the different types of spherical roller
4)bearings , which are the asymmetrical roller type andthe symmetrical roller type. In the asymmetrical type,which contains guide ribs in the center of the inner ring,
the roller load is balanced at three points on each outerring raceway, inner ring raceway, and inner ring centerrib. This concept enables rollers to stabilize at high speed.
In contrast, with the symmetrical type, there are no innerring center rib because the roller load is balanced at two
points on each outer ring raceway and inner ring raceway.
Therefore the longer rollers, which lead to higher load
rating, are implemented to the bearings, which are applied
to high loading applications.

38

Types

Convexasymmetrical roller

Convexsymmetrical roller

Structure
Roller load:
Roller load:
Balanced at 3-points
Balanced at 2-points
Inner
ring

With center rib

Without center rib(Guide ring)

3. Purpose of developed product
The asymmetrical roller type spherical roller bearing
is believed to have a large temperature rise due to sliding
contact between center rib of the inner ring and roller
end face, and therefore inner center rib was not includedin the developed bearing. In addition, suppression oftemperature rise was achieved for the symmetricalroller type bearing, which can withstand a high load
rating, by stabilizing roller movement. High speed
rotation performance and axial load performance werealso upgraded within the bearings to improve bearing
reliability.

JTEKT ENGINEERING JOURNAL English Edition No. 1012E (2015)

−Improvement in Reliability of Spherical Roller Bearings−

The balance of sliding friction force is described asfollows.

4. Forces influencing roller movement
In the symmetrical roller type spherical roller bearing,
the roller is supported by two points, which include
the outer ring raceway and the inner ring raceway. The
moment of the force influencing roller movement isderived in the order shown below.
Figure 1 shows the procedure for calculating the
moment of the force influencing roller movement. The
details of this procedure are explained in order.

Calculate circumferential speed distribution
Calculate the exact rolling points

Calculate friction force distribution (contact stress × friction coefficient)
Remove cancelled out areasVo（x ）−Vr（x ）Po（x ）dx ＝ 0
Vo（x ）−Vr（x ）⑸Outer ring side∫

Pi (x ): Contact stress distribution of roller and inner ring
Po (x ): Contact stress distribution of roller and outer ring

Subsequently, the sliding velocity distribution of the
bearing ring is determined at each position of the contact
area.
Inner ring side Vi (x )−Vr (x )Calculate moment of force

Outer ring side Vo (x )−Vr (x)Fig. 1 Calculation flow of moment of force

Figure 2 shows the x-axis and dimensions used in theformula. In both the inner ring contact and outer ring
contact, the origin is set as the center of the contact area,
and the x-axis is set along the central axis of the roller inthe direction of the diameter widening.

Considering the direction of the sliding velocity, thefriction force distribution S (x ) is determined.
Inner ring side Si（x ）＝ li

Vi（x ）−Vr（x ）Pi（x ）＝ 0 ⑹Vi（x ）−Vr（x ）Outer ring side So
（x ）
＝lo

Vo（x ）−Vr（x ）
Po（x ）＝0 ⑺Vo（x ）−Vr（x ）

ｘli : Inner ring friction coefficient, lo : Outer ring
friction coefficient
The moment M (x ) of the sliding friction force isobtained by integrating the product of friction force
distribution and distance from the origin within thecontact area. Therefore,

Ro
F

Ri

OriginDw

Outer ring
Rr

Inner ring⑷

Vi (x ) =Vr (x )，Vo (x ) =Vr (x)Calculate sliding velocity distributionx

Vi（x ）−Vr（x ）Pi（x ）dx ＝ 0
Vi（x ）−Vr（x ）The values of x are determined for the inner ring side
and outer ring side. These values of x represents the exact
rolling points.
The exact rolling points are described by

Calculate contact stress distribution

OriginInner ring side∫

RollerFig. 2 x axis and dimensions

The circumferential speed nearby the contact points
can be described using the below formula.

｛ F− Ri 2−x 2 cos h＋x sin h ｝
xi ⑴
（x ）
Inner ring Vi＝

Inner ring side Mi（x ）＝∫x ・Si（x ）dx⑻

Outer ring side Mo
dx（x ）＝∫x ・So（x ）⑼It is thought that roller movement can be controlled bybalancing the moment of force from the outer ring sideand inner ring side. Therefore, we confirmed the behaviorof the roller due to the changes in bearing internal
geometry through an evaluation test.

Roller Vr（x ）＝｛ Dw / 2 −Rr ＋ Rr 2 −x 2 ｝
xr⑵

5. Evaluation Test

｛ Ro − x cos h ＋x sin h ｝
xo（x ）＝
Outer ring Vo⑶

5. 1 Confirmation of bearing inner element behavior

h: Contact angle, xi : Angular velocity of inner ring,
xr : Angular velocity of roller, xo : Angular velocity ofouter ringF , Ri , Dw , Rr , Ro : Refer to Fig. 2

A parameter study was implemented to confirm thebehavior of the roller due to the difference in internal
geometry of the bearing. First, the factors stabilizing the
movement of the spherical roller bearing were selected

2

2

JTEKT ENGINEERING JOURNAL English Edition No. 1012E (2015)39

−Improvement in Reliability of Spherical Roller Bearings−

from the FTA related to roller movement shown in Fig. 3.
Five factors were selected as a result, and a parameter
study was conducted using a total of 27 combinations
taken from these five factors and three levels. The
contribution ratios of each factor influencing the roller
movement were calculated in a design of experiments.
Roller movement
instability

Large moment ofroller force

Roller curvature radius/
Outer ring curvature radius
Roller curvature radius/
Inner ring curvature radius
Outer ring roughness
Inner ring roughness
Contact angle
Radial clearance

movement stabilization was confirmed when both thecurvature ratio between inner ring raceway and rollers(roller curvature radius/inner ring curvature radius) issmaller than the ratio between outer ring raceway androllers (roller curvature radius/outer ring curvatureradius), and inner ring raceway roughness is smootherthan the outer ring raceway. A tendency of roller
movement instability was observed only when the ratio of
axial to radial load (axial load/radial load) was in between
0.2 and 0.5.
The calculation of the contribution ratios from thepractical test of the 27 bearings using the design ofexperiments revealed large contribution ratios forroughness and the ratio of curvature radius.
Table 2 Summary of evaluated bearings

Cage pocket clearance
Axial load/Radial load

Ratio of curvature radiusParameter study of a total of 27 combinations from 5 factors and 3 levels

Fig. 3 Fault tree analysis of roller movement

The test measured roller movement and was conductedwith 27 evaluated bearings created from five factors andthree levels. Figure 4 shows the method for measuring
roller movement. Gap sensors were placed vertically tothe roller end-face to conduct measurement, and roller
movement was analyzed from the output waveform. The
measured position was at the load applying row side,and gap sensors were installed at six locations along theperimeter.
Sensor installation position:load applying row side

Fr

1

Small

Large

Large Small

2

Small

Large

Large Small Medium Medium

3

Large

Small

Large Small

Small
Large

Small
Large

Test conditions
1) Main dimensions of evaluated bearing: u110 ×u180 × 69
2) Rotational ring: Inner ring3) Rotational speed: 72 min−1
4) Radial load: 37 kN Constant5) Lubrication: Grease lubrication

Roller inclination, °• Ratio of curvature radius (roller curvature radius/outer ring curvature radius,
roller curvature radius/inner ring curvature radius)• Contact angle• Radial clearance• Outer ring roughness• Inner ring roughness

Roughness

Evaluated
Contact Radial
Roller curvature Roller curvature
bearingradius/Outerradius/Inner Outer Inner angle clearanceNo.
ring curvature ring curvature ring ringradiusradius

1.2
1
0.8
0.6

Evaluated bearing 1
Evaluated bearing 2
Evaluated bearing 3

0.4
0.2
0
0

0.2

0.4

0.6

0.8

1

Axial load/Radial load

Fa

Gap sensor installation position: Evaluatedbearing
6 locations along perimeter
Overview of testing machine

Fig. 4 Measurement method of roller movement

The results of three roller movements are shown toexemplify the measurements of the 27 evaluated bearings.
Table 2 shows a summary of the evaluated bearings,
and Fig. 5 shows the results of the roller measurements.
A difference in roller movement was observed due todiffering internal geometry. A good tendency of roller

40

Comparison of measured results

Evaluated bearing 1(Unstable)

Evaluated bearing 2(Slight disturbance)

Evaluated bearing 3(Stable)Fig. 5 Measurement result of roller movement

The combination ratios of the factors influencing roller
movement were confirmed through the results of theparameter study of 27 combinations, as shown above.
Furthermore, a comparison of the practical test results
was made through CAE. Figure 6 shows the comparisonof the practical test results and theoretical analysis.
The practical test results and theoretical analysis are

JTEKT ENGINEERING JOURNAL English Edition No. 1012E (2015)

−Improvement in Reliability of Spherical Roller Bearings−

Inner ring rotation

In addition, the kinematic viscosity of the lubricant
during operation is greater for the developed bearing due
to better suppression of temperature rise than the massproduced bearing, and therefore oil film thickness canbe maintained. As a result, the state of the lubrication issignificantly improved compared with the mass-produced
bearing, leading to longer life. Figure 8 shows a life
comparison of the mass-produced bearing and developed
bearing with consideration to the state of the lubrication.

Fa

Fr

Theoretical analysis results

1.5
1

2

0.5
0

Mass-produced asymmetrical roller type bearing

Practical test results
090Developed symmetrical roller type bearing

180

Roller revolving phase, °Fig. 6 Results of correlation between practical test andtheoretical analysis

practically consistent, allowing for future simulation ofroller behavior through an analysis technique utilizing
CAE.

Life comparison

Roller inclination, °

2

1.5

1

1.2 times
greater0

5. 2 Bearing performance evaluation
Roller movement stability was achieved within thedeveloped symmetrical roller type spherical roller
bearing. To confirm this result, the performance of thedeveloped bearing was compared with the performanceof the asymmetrical roller type bearing, which is mass
produced bearings.
Figure 7 shows the results of the high speed test.
The results of the practical test show that the developed
spherical roller bearing has superior high speed
performance and better suppression of temperature risethan the bearing undergoing mass production.
Test conditions
1) Main dimensions of evaluated bearing: u110 ×u180 × 69
2) Rotational ring: Inner ring3) Radial load: 46.9 kN4) Axial load: 17.4 kN5) Axial load/Radial load: 0.37
6) Lubrication: Oil lubrication

110

440

Bearing bore × Rotational speed, × 1000 mm･min−1

Fig. 8 Life comparison between mass-produced and upgradedbearings considering influence of lubrication

Moreover, the results of the developed sphericalroller bearing were also confirmed on super large
bearings. Figure 9 shows the results of the axial loading
performance test. The test results confirm that thedeveloped bearing has better suppression of temperaturerise than the mass-produced bearing and superior axial
load performance.
Test conditions
1) Main dimensions of evaluated bearing: u240 ×u400 × 160
2) Rotational ring: Inner ring3) Rotational speed: 33 min−1
4) Radial load: 175 kN Constant5) Lubrication: Grease lubrication60

50

Mass-produced asymmetrical roller type bearing
Developed symmetrical roller type bearing
Temperature rise, °C

Mass-produced asymmetrical roller type bearing
Developed symmetrical roller type bearing
40
Temperature rise, °C

1.5 times
greater

30

50

40

30

20

0

0
0.5
0

100

200

300

400

500

0.6

0.7

0.8

0.9

1

1.1

Axial load/Radial load

Bearing bore × Rotational speed, × 1 000 mm･min−1

Fig. 7 Test results of high speed rotation

JTEKT ENGINEERING JOURNAL English Edition No. 1012E (2015)

Fig. 9 Test results of axial loading performance41

−Improvement in Reliability of Spherical Roller Bearings−

6. Conclusion
In response to the increasingly severe application
conditions of spherical roller bearings, JTEKT has
improved bearing performance of the symmetrical roller
type spherical roller bearing, which can withstand large
load rating. The evaluation test results of the developed
bearing demonstrated that the bearing has stable roller
movement with suppression of temperature rise, as wellas superior high speed rotation performance and axial
load performance. Furthermore, an evaluation techniquefor measuring roller behavior was developed, and, as themeasurement results were closely consistent with CAE
analysis results, an analysis technique using CAE wasalso established.
JTEKT will strive towards further improving theperformance of the entire spherical roller bearing throughthe utilization of the developed analysis technique as well
as evaluations using testing machines of equal size to theactual machines of use, and by expanding the scope of
application to ultra-large spherical roller bearings.

References
1)

2)
3)
4)

JTEKT CORPORATION: Mechanism and structure ofrolling bearings, SHUWA SYSTEM CO., LTD. (2011)
17 (in Japanese).
R. Hosaka, N. Yasuda: JTEKT ENGINEERINGJOURNAL, No.1004E (2008) 42.
Y. Kobayashi: JTEKT ENGINEERING JOURNAL,
No.1010E (2013) 48.
JTEKT: Ball & Roller Bearings, CAT No. B2001E.

H. YASUDA*
*
**

42

Y. TAKEDA*

K. OTSUKA**

Industrial Machinery Application Engineering Dept.,
Bearing Operations Headquarters
Experiment & Analysis Dept., Bearing Operations
Headquarters

JTEKT ENGINEERING JOURNAL English Edition No. 1012E (2015)