Commentary

Introduction of Rolling Bearings for Electric Drives
Corresponding of Automobiles

Takashi KAWAI *Tomohisa UOZUMI **Continued electrification of automobiles has introduced the need for electric drive bearings with new anddiverse performance capabilities. This article introduces the efforts made at NTN to design rolling bearings
specifically to meet these requirements.

1. Introduction
The automotive industry is undergoing an
unprecedented transition from gas powered to fully
electric vehicles. The shift to electric vehicles (EVs) isaccelerating worldwide as one solution on the path toachieving carbon neutrality1). Rolling bearings, which
support power transmission in electric drives, are a
key component. They must respond to the demandfor higher speeds in addition to smaller size, lighter
weight, and lower torque. Here NTN introduces rolling
bearings for electric drives that have been designed
specifically to meet these requirements.

Fig. 2 shows an example of an “e-Axle”. An e-Axle
is a drive unit which integrates a motor, inverter, andreduction gear for electric vehicles (EVs). An e-Axle witha parallel three-axis structure uses two bearings forthe motor and six bearings for the gear reducer. Fig. 3
shows a typical example of a parallel triaxial e-Axle witha rolling bearing arrangement supporting the motor andreduction gearbox. Since the combination of the motor
torque and reduction ratio affects the bearing load oneach axis of the reducer, deep groove ball bearings andtapered roller bearings are used on the second andthird axes, depending on the load.

2. Performance Requirements of
Rolling Bearings for Electric Drives

Motor

There are two main types of rolling bearings forautomobiles: deep groove ball bearings and tapered
roller bearings. These rolling bearings consist of thefour parts shown in Fig. 1 (inner ring, outer ring,rolling elements [balls and tapered rollers], andcage) and sometimes seals. With the recent shift toelectric vehicles, bearings are required to have new
performance characteristics to meet the operating
conditions.
Seal

ReducerFig. 2 Example of e-Axle structure and arrangementof rolling bearings3)

Outer ring

Cage
Deep
groove ballbearings
appliedRolling
element(a) Deep groove ballbearings

Inner ring(b) Tapered roller
bearings

Fig. 1 Rolling bearing cross-section and part names

2)(a) Using deep groove ballbearing for all

Taperedrollerbearings
applied(b) Using tapered roller bearingsin shaft 2 and shaft 3

Fig. 3 Example of rolling bearing arrangement
in a simplified cross-sectional view of an e-Axle(motor and reduction gear)4)* Automotive Bearing Engineering Dept., Automotive Bearing Products Unit, Automotive Business Headquarters** NTN BEARINGS (UK) LTD.

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Introduction of Rolling Bearings for Electric Drives Corresponding of Automobiles
The rolling bearings used in each of the e-Axleʼsaxes are required to have the following performance
characteristics.
(1) To meet the demands for higher power output
and downsizing of the e-Axle, driving motors areoperating at higher speeds. The deep groove ballbearings that support the motor shaft and the
directly connected motor shaft of the first reducer
shaft are required to offer high-speed performance.
(2) As bearing raceways become thinner due tothe e-Axle becoming smaller, creep (a sliding
phenomenon in which the outer ring rotates relative
to the housing) is more likely to occur on the outer
diameter of the outer ring. Each reduction gear
shaft must therefore have anti-creep measures.
(3) The high voltage of the batteries used in e-Axle
motors requires countermeasures against electricalpitting of the bearings (a phenomenon in whichelectrical discharge occurs in the bearing, causing
microscopic melting of the raceway surface) in the
motor shaft and the first shaft of the reduction gear.
(4) To improve the efficiency of the e-Axle under low
viscosity lubricating oil, the tapered roller bearing
supporting the second shaft of the reducer requires
heat generation countermeasures at the area of
sliding contact on the large end rib.
(5) The increased bearing load associated with thehigher output of the e-Axle requires the tapered
roller bearings supporting the second and third
shafts of the reducer to have a higher load carrying
capacity than conventional bearings.
Table 1 summarizes the performance requirements for
e-Axle rolling bearings.
Table 1 Performance Requirements for e-Axle rolling
bearings
Markettrend

Technology
trendsPerformance requirements forrolling bearings

Low torque3.1 Deep groove ball bearing
with ultra-low friction seal
Higher efficiency 4.1 Low temperature rise andlow torque tapered roller
bearing
Support for high-speed

Higher speed

3.2 Deep groove ball bearing

Environment
for high-speed rotation
ally friendly
technology Downsizing and Improved creep resistancelightweighting 3.3 Creepless bearings
Motor-driven
electrification Higher voltage Preventing electrical pitting
3.4 Ceramic rolling element
HigherShareddeep groove ball bearings
frequency

3. Introduction of high-performance
deep groove ball bearings
This section introduces the technology NTN has
developed for deep groove ball bearings to meet theperformance requirements for e-Axles as shown in
Table 1.
3.1 Deep groove ball bearing with ultra-low friction seal
Due to increased demand for higher efficiency
and more compact e-Axles, the deep groove ballbearings used in these applications are required tohave both high operating life and low operating torque.
Optimizing e-Axle bearing operating life requires
preventing hard particulate material present in the
lubricating oil of the application from contaminating
the bearing rolling surfaces. Although contact seals canprevent the intrusion of particulate contamination, it
is difficult to optimize both bearing life and operating
torque with this seal type because sliding torque (which
accounts for most of the running torque for deep
groove ball bearings with contact seals) is generated
at the contact area between the seal lip and the innerring.
NTNʼs newly developed ultra-low friction seal5)
employs a contact-type seal with arc-shaped(half-cylindrical shaped) microscopic convexes at equalintervals on the sliding contact area of the seal lip, asshown in Fig. 4. During rotation, the wedge film effectof the microscopic convexes forms an oil film (Fig. 5)
between the sliding surfaces of the seal and inner ring,
significantly reducing seal drag torque compared toa more conventional contact-type seal. Furthermore,because the clearance between the seal and the innerring sliding surface is extremely small, the intrusion ofparticulate contamination of a size that would affectbearing operating life can be prevented.
Under the conditions shown in Table 2, NTN
compared running torque between the ultra-low
friction seal and conventional contact/non-contact
seals. As shown in Fig. 6, The running torque of the ballbearing with ultra-low friction seals was 80 % lower
than that of the ball bearing with contact seals dueto the wedge film effect of the micro convexes on theseal lip. Running torque of the ball bearing with ultralow friction seal was also equivalent to that of the ballbearing with non-contact seals.

Lower viscosity and Lower temperature risesmaller volume of 4.1 Low temperature rise and low
torque tapered roller bearings
lubricating oil
Long operating life

High reliability 4.2 Tapered roller bearings withlong operating life

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NTN TECHNICAL REVIEW No.89

Arc-shaped micro convexes (5 points in the upper cutout)

Fig. 4 Cross section of deep groove ball bearing withultra-low friction seal5)

Arc-shaped micro
convexes
Pressure distribution

Seal
Oil film

3.2 Deep groove ball bearing for high speeds
By adopting a new shape of resin cage that takes
into account the deformation of the cage duringhigh-speed rotation, NTN found operation was
possible under oil lubrication at d mn*1 = 180 × 104.
Here NTN introduces a high-speed deep groove ballbearing for EV/HEV that uses high-speed rotation
technology.6) 7)*1 Bearing rolling elements pitch diameter and rotational speed
mm min-1

Inner ring
Direction of rotation

Fig. 5 Diagram of wedge film effect5)
Table 2 Test condition5)
Test bearing

6010 equivalent

Radial load

Basic dynamic load rating of 5 %(JIS B 1518:2013)

Rotational speed

1 500 min-1

Bearing temperature

35 to 120 C

Lubricating oil

CVTF(1) Adoption of high-strength materials⇒ Improved rigidity and strength of cage(2) Thickening of the pocket bottom to suppress
deformation⇒ Improved rigidity of cage ring(3) Thinned wall between cage pockets (weight reduction)⇒ Reduction of centrifugal force deformation(4) Installation of oil grooves on the inner surface ofthe cage pockets⇒ Improved lubricity of cage and rolling elements

Deep groove ball bearing with ultra-low friction seal
Ball bearing with non-contact seal
Ball bearing with contact seal
0.40

Running torque N･m

0.35

A temperature rise qualification test (oil lubrication)
was conducted on high-speed deep groove ballbearings for EVs and HEVs under the conditions shownin Table 3. Fig. 8 shows the test results. The outerring temperature of the high-speed deep groove ballbearings could be operated stably below the upper
temperature limit for d mn values up to 180 × 104.

0.30
0.25

80 %
reduction

0.20
0.15
0.10
0.05
0.00

Deep groove ball bearings, which support the motor
shaft of e-Axles and the directly connected motorof first axis reduction gears, are required to support
high rotational speeds. As the rotational speed ofdeep groove ball bearings increases, centrifugal force
causes contact between the cage pockets of the resin
cage and the rolling elements (balls), which may result
in seizure.
In response to this, NTN developed high-speed
deep groove ball bearings for EVs and HEVs by adding
improvements (1-4) to the resin cage.
Fig. 7 shows a comparison between theconventional and improved resin cage.

Table 3 Test condition2)
0

20

40

60

80

100

Bearing temperature °
C

120

140

Fig. 6 Relationship between running torque andbearing temperature5)(a) Conventional resin cage

Tested bearing
characteristics

Basic dynamic load rating= 16 800 N(JIS B 1518:2013)

Radial load

264.5 N

Axial load

147 N

Rotational speed

d mn 160 to 180×104(b) Modified resin cage

Fig. 7 Exterior of resin cage for high-speed deep groove ball bearings for EV/HEV 4)

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Relative percentage of outerring temperature

Introduction of Rolling Bearings for Electric Drives Corresponding of Automobiles

Undercut

Judgment
upper limit
temperature

100
95
90
85
80

0

160

170

180

dmn ×104

Fig. 8 Relationship between d mn and outer ringtemperature2)
Recently, by optimizing the lubrication conditions
and the internal elements of the bearing, NTN has
confirmed that the deep groove ball bearing can
support high-speed rotation with a d mn value of 220
× 10 4 8).
3.3 Creepless bearing
As bearing raceways become thinner due to thedemand for smaller and lighter e-Axles, the rigidity ofthe outer ring decreases, increasing the likelihood ofrippling deformation on the outer ring outer diametersurface as the rolling element passes through. This
repetition results in a progressive wave that causes
the outer ring to rotate relative to the housing(progressive wave type creep).10)
The structure of the creepless bearing that NTN
has developed is shown in Fig. 9. As shown in Fig. 10,
by providing a full-width arc-shaped relief area on a
portion of the outer ring outer diameter surface, thehousing inner diameter surface and outer ring outer
diameter surface lose contact when the undercut of
the outer ring is positioned in the load-bearing zone,
thereby blocking the transmission of traveling waves.
This suppresses creep. Although creep occurs when
the undercut of the outer ring is positioned in the
non-load-bearing zone, creep stops at that position
when the undercut of the outer ring moves to theload-bearing zone. Therefore, when the bearing isassembled into the housing, there is no need to alignthe undercut with the load-bearing zone. The only
design change is in the undercut of the outer ringouter diameter, making it easy to replace an existing
bearing.
Creep rate evaluation tests were conducted on
creepless bearings and standard deep groove ballbearings11) under the conditions shown in Table 4 ina testing machine for inducing creep. The test results
are shown in Fig. 11. The creep rate of the standard
deep groove ball bearing tended to increase withincreasing load. By contrast, it was confirmed thatcreep did not occur in the creepless bearing under anyof the loading conditions due to the effect of the outerring undercut.

Fig. 9 Cross section of creepless bearings9)

Red line part:
Outer
diametershape of outerring underload

Housing

9)68

NTN TECHNICAL REVIEW No.89

Undercut

No contact with
deformation of outer
diameter surface

Fig. 10 Relationship between creepless bearing
outer ring shape and housing9)
Table 4 Test condition9)6208

Radial load

Basic dynamic load rating(JIS B 1518:2013)
10 % - 40 %Inner ring rotational speed

6 000 min-1

Lubricating oil

CVTF

Bearing outer ringtemperature

50 ℃

Creep speed Degree/min

Bearing number

100Standard type deep
groove ball bearing
Creepless bearing

80
60
40

Creep speed zero

20
0

0

10

20

30

40

50

Radial load (ratio to basic dynamic load rating) %Fig. 11 Creep speed evaluation test results9)

Table 5 Test condition14)

3.4 Ceramic rolling element deep groove ball bearings

Bearing being tested
Rolling element

Fig. 12 Ceramic rolling element deep groove ball bearings
Inverter-driven motors are shifting toward higher
power output and smaller size, and the drive power
supplies are becoming higher voltage. In the case of
inverter drives, a potential difference is generated
between the shaft and housing. The oil film that forms
between the bearing raceways and rolling elements is
known to break down, causing damage due to electricaldischarge. 12) 13) The higher the voltage, the greater the
potential difference generated, which increases the riskof bearing damage due to dielectric breakdown. This
leads to acoustic deterioration and a shorter operating
life. As a countermeasure, a ceramic rolling element
deep groove ball bearing (Fig. 12) using silicon nitride
ceramic balls as rolling elements can be applied toinsulate the inner and outer ring raceways and preventthe occurrence of electrical pitting.
Rolling tests were conducted under the conditions
shown in Table 5 on the test machine shown in Fig. 13
to investigate bearing spalling life in an energizedenvironment. The results are shown in Table 6.
Compared to standard ball bearings using steel balls,
the ceramic rolling element deep groove ball bearing
can suppress the occurrence of spalling on the rolling
element surface due to electrical pitting.
Testbearing

Pulley

Pin A

Pin B
Supportbearing

Insulating
material

Fig. 13 Rolling life test machine

Deep Groove Ball Bearing(6203)
Steel ball

Ceramic ballGreaseNon-conductive grease

Amount of grease injected (g)0.86

Rotational speed (min-1)

0-20 000(rapid acceleration/
deceleration)

Atmosphere

Room temperature

Pulley load (N)1 617

Bearing load (N)Current (A)

2 332
0.5

Stopping conditions−

Vibration 10 times initial level

Table 6 Rolling life test results for energized state14)
Life (h)Spalling area

Deep groove ball bearings(steel balls)

19.6

ball

Deep groove ball bearings(ceramic balls)

>200

None

4. Introduction of high-performance
tapered roller bearings
Fig. 14 shows the structure of a tapered roller
bearing. Compared to ball bearings, tapered roller
bearings can support higher radial and axial loads,
but also generate more torque. NTN has developed atapered roller bearing that achieves both low torque(low temperature rise) and long operating life by
optimizing the geometry of the rolling surfaces andthe inner ring large end rib face. 16)
Inner ring large
end rib
Outer
ring

Cage

Inner
ring
Rolling element (tapered
roller bearing)

Fig. 14 Structure of tapered roller bearing15)
4.1 Low Temperature Rise and Low Torque
Tapered Roller Bearing
The lubricating oil used in e-Axles is becoming lower
in viscosity and volume in order to achieve higher
efficiency in automobiles. When standard tapered
roller bearings are used under these conditions, it
becomes difficult for an oil film to form in the sliding
contact area of the bearing rib, increasing the risk of
rapid temperature rise. Our newly developed Low

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Introduction of Rolling Bearings for Electric Drives Corresponding of Automobiles
Temperature Rise and Low Torque Tapered Roller
Bearing improves anti-galling and reduces torque bymaking the design changes described in (1) to (4)below (Figs. 15 and 16).

Tapered roller pitch
circle diameter(1) Compact design oftapered roller pitch
circle diameter

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NTN TECHNICAL REVIEW No.89

Table 7 Test bearing16)
Standard tapered roller
bearing 32007X
18

13.5

φ58.5

Scale
drawing of
bearing
crosssection

Low Temperature
Rise and Low Torque
Tapered Roller Bearing

φ34

A rotary torque test was carried out under theconditions shown in Table 8 to compare performance
between a conventional tapered roller bearing11) andthe Low Temperature Rise and Low Torque Tapered
Roller Bearing shown in Table 7. The test results
are shown in Fig. 17. The Low Temperature Rise
and Low Torque Tapered Roller Bearing achieved asignificant torque reduction of 66 % compared to theconventional design.
Temperature rise characteristics were evaluatedfor the two types of bearings shown in Table 7 under
the conditions shown in Table 9. In this test, tosimulate an oil starved environment in the e-Axle, thebearings were operated after only an extremely small
amount of lubricating oil was applied to the bearing
surface. The bearings were tested until the outer ring
C. The test results aretemperature reached 100 °C for theshown in Fig. 18. The time to reach 100 °Low Temperature Rise and Low Torque Tapered Roller
Bearing was approximately 10 times longer whencompared to the standard tapered roller bearing.

Fig. 16 Structure of “Low Temperature Rise and Low
Torque Tapered Roller Bearing”16)

φ62(1) Compact size and low torque enabled bytapered roller and reduced pitch diameter(2) Reduced clearance between cage inner ring andinner ring cone front face rib outer diameter
reduces torque caused by agitation of lubricatingoil(3) Recessed shape added to large diameter end ofcage pocket improves lubricity(4) Optimal shape of tapered roller end face andinner ring large end rib face improves lubricity(4) Optimal shape between
the tapered roller end
face and inner ring large
end rib(2) Smaller clearance
between cage innerdiameter and innerring small side rib
outer diameter

φ35

Fig. 15 Exterior of Low Temperature Rise and Low
Torque Tapered Roller Bearing16)(3) Concave shape
applied to pocket
large side edgeSize

φ35 φ62 18

φ34 φ58.5 13.5

Dynamic
load rating

46 000 N

28 500 N

Cage

Standard steel plate cage

New shape resin cage

Table 8 Test condition16)
Axial load

3 000 N

Rotational speed

5 000 min-1

Lubrication conditions

Oil bath
ATF (50 °C)

Table 9 Test condition16)
Lubricating oil conditions

ATF (25 °C)

Inner ring large end rib plane
contact stress

Approx. 200 MPa

Inner ring large end rib planesliding velocity

Approx. 2.5 m/s

Roller Bearings to achieve a dynamic load rating whichis 1.3x higher than conventional product and a rated
life which is 2.5x higher.*2 The geometry of the contact
area on the inner ring large end rib surface was also
optimized to improve the allowable rotational speed
by approximately 10 %*3.

50
40

66 %
reduction30*2: Comparison with tapered roller bearings to which thebasic dynamic load rating is applied according to JapaneseIndustrial Standards B 1518:2013.

20
10
0

Standard
tapered roller
bearing*3: Comparison with tapered roller bearings in NTN Corporation
Ball and Roller Bearings Catalog (CAT.No.2203/J 20.08.199
NI/NI)

Low Temperature Rise
and Low Torque Tapered
Roller Bearing

Fig. 17 Tapered roller bearing running torque test results
Time for outer ring temperature toreach 100 °
Cs

16)

800
700

Fig. 20 compares the contact stress distribution of
ULTAGE Tapered Roller Bearings and a conventional
tapered roller bearing design.11) The ULTAGE Tapered
Roller Bearings do not have excessive stress (edgestress) at the edge of the contact area due to the
optimized crowning geometry.

600
500
400

Large

Approx. 10
times

Standard tapered roller bearings
ULTAGE Tapered Roller Bearing

300
200
100
0

Standard
tapered roller
bearing

Low Temperature Rise
and Low Torque Tapered
Roller Bearing

Fig. 18 Tapered roller bearing temperature rise
evaluation test results16)
4.2 Long operating life tapered roller bearings
Here we introduce ULTAGE Tapered Roller
Bearings,15) which feature an optimized internalbearing design to achieve high load capacity andhigh-speed rotational performance (Fig. 19).

Contact stress

Running torque N･cm

60

Small

Rolling contact surface of tapered
roller Shaft direction widthFig. 20 Contact stress distribution on the raceway
surface of a tapered roller bearing15)
Service life tests were conducted under the highmisalignment (0.002 rad) conditions (Table 10)
assumed for e-Axle. Fig. 21 shows the test results.
The ULTAGE Tapered Roller Bearings achieves a long
operating life approximately 16 times longer thanthat of standard tapered roller bearings due to theuniformity of the contact stress distribution.
Table 10 Test condition15)

Fig. 19 ULTAGE Tapered Roller Bearing

15)

ULTAGE Tapered Roller Bearings use optimized
tapered roller surface geometry to maximize bearing
operating life. (This technique, which involves
reducing the diameter of tapered rollers toward
the end in micrometer increments, is known as“crowning.”) Roller crowning allows for uniform
contact stress between the rolling elements and the
raceways when an unbalanced load is applied. This
design technology has enabled ULTAGE Tapered

Test bearing size

φ23×φ55×20

Bearing material heat
treatment

Standard bearing steel heat
treatment

Test load

Basic dynamic load rating of 26 %(JIS B 1518:2013)

Misalignment

0.002 radRotational speed

4 000 min-1

Lubricating oil

ISO VG100 gear oil
equivalent

JIS basic rating life

73 hours

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Introduction of Rolling Bearings for Electric Drives Corresponding of Automobiles
8)

500

400

300

L10h
h

Approx. 16
times

200

100

0

Standard
tapered roller
bearing

ULTAGE Tapered Roller
Bearing for Automotive
Application

Fig. 21 Clean oil life test results for tapered roller bearings
under high misalignment conditions15)

5. Conclusion
As the shift to electric vehicles (EVs) continues aspart of the pursuit of carbon neutrality, the functions
required of bearings are becoming more diverse. In
this article, NTN introduced rolling bearings for electric
drives that can meet the technical requirementsof e-Axles. Looking towards the future, NTN willcontinue to develop technologies that anticipate theevolving needs of the automotive market.
References
1) Ministry of the Environment website,
Decarbonization Portal,
https://ondankataisaku.env.go.jp/carbon_
neutral/2) Takashi Kawai, Introduction of Recent Rolling
Bearings for Automotive Applications,
Automotive Technology, Vol. 9, No. 10, (2022)
43-50.
3) NTN Corporation to expand mass production anddelivery of high-speed deep groove ball bearings
for EVs and HEVs, 2020
https://www.ntn.co.jp/japan/news/new_
products/news202000044.html
4) Takashi Kawai, Introduction of Rolling Bearings
for EVs (Electric Vehicles), Journal of Economic
Maintenance Tribology, No. 692, (2022) 37-44.
5) Katsuaki Sasaki, Takahiro Wakuda, Tomohiro
Sugai, Ultra-low Friction Sealed Ball Bearing forTransmission, NTN TECHNICAL REVIEW, No. 85,(2017) 62-66.
6) Katsuaki Sasaki, Technical Trend of High-Speed
Deep Groove Ball Bearings for Motors, Monthly
Tribology, No.404, (2021) 12-15.
7) NTN Corporation wins Nippon Brand Award of2021 “CHO” MONODZUKURI Innovative Parts
and Components Award, 2021
https://www.ntn.co.jp/japan/news/press/
news202100067.html

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NTN TECHNICAL REVIEW No.89

NTN Corporation achieves high-speed rotation of
2.2M with deep groove ball bearings for EVs andHEVs, 2022
https://www.ntn.co.jp/japan/news/new_
products/news202200027.html
9) Hayato Kawaguchi, Toshiki Masuda, Marina
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12) Shoji Noguchi, Electric Pitting Prevention
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13) Yoshinori Isomura, Tatsuo Maetani, Akihiko
Watanabe, Keisaku Nakano, et al., A Study ofElectrical Pitting for Small Ball Bearing of PWM
Inverter Drive (Part 2), The Tribologist, Vol. 58,
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Ohira, New Technology of Rolling Bearings
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Photo of authors

Takashi KAWAI

Tomohisa UOZUMIAutomotive Bearing NTN BEARINGS (UK)LTD.
Engineering Dept.,
Automotive Bearing
Products Unit,
Automotive Business
Headquarters