TECHNICAL REPORT

Development of Needle Roller Bearing for High-efficiency
Planetary Gear
M. TANIYAMA K. KOTANI Y. NAKASHIMA T. SATOU

In order to improve the fuel efficiency of vehicles to counter the current environmental crisis, the development of
automatic transmission (AT) has continued to progress for increasing gear steps to multispeed, requiring planetary
bearings for high-speed revolution. Moreover, planetary bearings for ATs are used under severe conditions such as
high moment and centrifugal force. This paper describes the characteristics and design technique of planetary gear
bearing developed by our company which facilitates high efficiency and high-speed revolution.
Key Words: needle roller bearing, planetary gear, automatic transmission, cage and roller

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Small

Automatic transmissions are becoming the mainstream
transmission for passenger vehicles as they facilitate acomfortable driving experience. To help solving the global
environmental problems highlighted in recent years, a
lot of alternatives to vehicles which use reciprocating
engines have emerged on the market, including hybrid
vehicles combining an engine and motor (HEV) andelectric vehicles powered only by motors. There are also
efforts to improve the fuel economy of conventional
reciprocating engines, such as the advancement of
automatic transmission (AT), etc.
Recent years have witnessed an increase in car models
adopting continuously variable transmission (CVT), andthe market is diversifying with the emergence of dualclutch transmissions, etc., however in regards to hightorque, large-engine capacity vehicles, a step-type AT
which changes speed by a planetary gear set, is becoming
mainstream.
Measures to improve fuel economy relating to AT
are weight reduction, higher efficiency and a greater
reduction ratio by adopting multi-speed. For this reason,
the needle roller bearings (planetary bearings) used in AT
planetary gear inners are required to rotate at even higher
speeds, as shown in Fig. 1. In line with this, there is anincreasing number of cases in which radial type needle
roller bearings with cages (cage and roller) are used as analternative to the full-complement needle roller bearing,which used to be mainstream.
Planetary gear mechanisms are also being used in the
power switching unit of HEVs. HEVs are characterized
by the fact lubricant supply is stopped when the oil pumpstops during motor-based driving, therefore in order toenhance the lubrication of the planetary bearing, the cageand roller type is increasing in this position.
This paper introduces the development activities
underway for the needle roller bearing used in planetary
gears which facilitates high efficiency and high speed.

Centrifugal acceleration, G

1. Introduction

Multi-speed AT

4AT

Low
Carrier rotation speed

Fig. 1 Effects of multi-speed AT

2. Improvement for Planetary Bearings
Figure 2 shows the mechanisms of a single piniontype planetary gear. Recently, many multi-speed ATs use
various types of structures, such as double pinion andRavigneaux, however the issues relating to bearings are
the same in any type of planetary structure as in the single
pinion-type.
In response to the global environmental problems
highlighted in recent years, the centrifugal acceleration
and rotational speed of bearings are increasing.
Accordingly, there is an increased potential requirement

JTEKT ENGINEERING JOURNAL English Edition No. 1014E (2017)

−Development of Needle Roller Bearing for High-efficiency Planetary Gear−

Ring gear
Fa

Ft

Fa

Fr

Ft
WasherRing gear

Moment loaded area

Fa

Radial loaded area
Fa

Ｍ

Sun gear

Pinion gear

Ring gear

Bearing

Sun gear

Bearing
Sun gearFig. 2 Mechanism of single pinion type planetary gear
Unit trends

Bearing technical issues

Weightreduction

DownsizingHighefficiency

Low friction

Bearing functional requirements
Long life
Shift to cageand roller

Reduction of supply
lubricant,low-viscosity lubricant
High
speed

Fig. 4 Forces acting on pinion gear

Pinion gear

Carrier

Multispeed

Ｍ

ｒ

Fr

Ｒ

for planetary bearings to have longer life, stronger cages
and anti-seizure performance (Fig. 3). This section
introduces the long-life planetary bearing, high-speed
planetary bearing and ultra-thin thrust needle roller
bearing developed to satisfy such demands.

Higher centrifugal
acceleration
Higher rotation speed

Reductionof rotating
torqueImproved
anti-seizure
performance
Higher cagestrength

Fig. 3 Required functions of planetary bearing

2. 1 Long-Life Planetary Bearing
Helical gears are used as AT planetary gears therefore
the planetary gear, as the gear inner, is subjected totangential force (Ft), radial force (Fr) and axial force(Fa) due to the helix angle (Fig. 4). The axial force (Fa)acts on the bearing as moment load. Moment load acts
together with radial load and causes the gear to tilt. For
that reason, if the contact surface pressure on rollingsurface of roller end portion increases and exceeds thelimit, edge load will occur. Edge load drastically reduces
life and therefore must be controlled by appropriate
crowning shape on roller.

JTEKT ENGINEERING JOURNAL English Edition No. 1014E (2017)

One feature of the conventional full complement
roller bearing for planetary is the large load capacity
compared to the cage and roller bearing. However, theadjacent rollers repeatedly slide and collide, therefore if
the bearing rotates at high speed, problems such as large
rotational torque and seizure occur.
Meanwhile, in the case of the cage and roller, there isno direct contact between the rollers therefore high-speed
rotation is capable. However, cage and roller bearings
had the issue of a short life due to having less rollers than
the full complement roller bearing, hence a small load
capacity. In light of these circumstances, the life of thecage and roller has been improved.
Of the components comprising a planetary gear, pinion
gear (outer ring), rollers, and shaft (inner ring), the shaft
is the most susceptible to damage. Typical radial needle
roller bearings are used with the outer ring fixed andthe inner ring rotating. In this case, the contact surfacepressure of the inner ring is greater than that of the outerring due to curvature, however the number of cycles tofailure is greater on the fixed outer ring than the innerring. As such, from the perspective of impact on life, there
is no significant difference between the load on the innerring and the load on the outer ring. In contrast, planetary
bearings are normally used with the pinion gear (outerring) rotating and the shaft (inner ring) fixed. Not only is
the contact surface pressure of the shaft high, it also hasa large number of cycles to failure, therefore has the most
severe load conditions.
Moreover, due to moment load, the contact surfacepressure of the roller rolling surface end portion is high,causing the lubricant film to become thinner, therefore
surface spalling may occur due to the oil film shortage.
JTEKT has focused on these issues and developed abearing with significantly longer life than conventional
bearings. Bearing life was improved by combining thetwo measures explained below.
The first measure was to improve the lubricant film
parameter, Λ, by improving roller surface roughness andachieving a mirror finish. But this alone is not enough to
improve life satisfactorily. As such, a second measure of
improving fatigue strength of the shaft was implemented.

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−Development of Needle Roller Bearing for High-efficiency Planetary Gear−

Conventionally, life was improved by applying special
heat treatment to the shaft, however by adding surface
modification with shot peening, it was possible to apply
a high residual compressive stress (Fig. 5) thereby
extending life by 8 times that of standard bearings(Fig. 6).

−400

Conventional bearing

−600
−800

Developed bearing−1 000−1 200−1 400−1 600

0

50

100

150

200

250

300

Depth from surface, µm

Fig. 5 Example of residual compressive stress measurement
10
Developed bearing

Life ratio

8
6
4
2

Conventional Conventional
bearingbearing0
RollerStandard

Standard

Shaft

Standard

Special heat Special heat Special heat
treatment
treatment treatment ＋
shot peening

Mirror-like finish Mirror-like finish

Centrifugal force of cage

Residual stress, MPa0−200

cages available, depending on the operating condition.
In development projects to date, there is a tendency forthe stress at the cage rib and cage pillar base to be high,and it was recognized that performing design verifications
on these sections was difficult. In order to reduce thisstress, JTEKT applied its proprietary forming technology
to form the optimal cage cross-section and developed ahigh-strength cage.
Figure 8 is a table comparing the performances ofthe standard, high-speed (conventional bearing) andhigh-speed (developed bearing) cages. The cage of thedeveloped bearing has high strength, therefore stress is
reduced by 12% compared to that of the conventional
bearing. This makes it possible for the developed bearing
to respond to 12% higher centrifugal accelerationcompared with the conventional bearing.
Moreover, the cage has higher strength under the same
operating conditions as those of the conventional bearing,making it possible to increase the number of rollers andraise the dynamic load ratings of the bearing, therefore it
is also possible to improve bearing life.
Based on the increase in dynamic load ratings, C, life
was calculated as being around 1.25 times longer, andby combining this with the long-life planetary bearing
technologies introduced in the preceding section, the lifeof the developed bearing is 10 times greater than that of
the standard bearing.

Fig. 6 Life test results

2. 2 High-Speed Planetary Bearing
Due to centrifugal acceleration, centrifugal force actson the cage depending on the mass of the cage itself.
As such, the cage is pressed to the pinion gear bore
and rotates together with the pinion gear. However, therollers revolve at around half the speed of pinion gears,
hence push the cage back with a force equivalent to thefrictional force of the cage’s outer face and pinion gear’sbore.
Moreover, the roller is subjected to centrifugal force
due to the revolution around the sun gear, therefore eachroller applies a load which comes from this centrifugal
force on the cage.
These forces act repeatedly due to cage rotation,
therefore the bearing design must be such that the stresswhich occurs on the cage is less than the fatigue limit.
JTEKT uses the types of cages shown in Fig. 7 dependingon load. A resin cage will be used if centrifugal force issmall and a steel cage will be used if centrifugal force ishigh. There are standard types and high-speed types of

56

Large
High-speed typeHigh-speed type (developed bearing)(conventional bearing)Standard type Standard type
Small resin cage resin cage
Current status Development
aim

Fig. 7 Types of planetary bearing cages
High-speed type
Standard type

Conventional
bearing

Developed
bearing

Mass, g

4.9

4.4

4.2

No. of rollers

12

12

13

Stressoccurring at
2 000 G, MPa

270

170

149

Rated load
C/C0, kN17.5/22.9

18.0/24.0

19.3/26.0

Shape

Fig. 8 Comparison of steel cage performance

JTEKT ENGINEERING JOURNAL English Edition No. 1014E (2017)

−Development of Needle Roller Bearing for High-efficiency Planetary Gear−

3. Ultra-Thin Thrust Needle Roller Bearing
JTEKT develops the peripheral components ofplanetary bearings also.
Planetary bearings generate axial load internally asa result of skew, which is unique to roller bearings (theroller rotating axis tilts relative to the bearing centralaxis). This is referred to as skew force. It is possible thatthe skew force may increase due to the impact of pinion
gear accuracy and carrier rigidity. Skew force acts on
the side washer from the cage end face. The side washer
has high frictional torque due to being used as a sliding
portion, therefore the compact, ultra-thin thrust needle
roller bearing (Fig. 9) was developed to reduce torqueby replacing to a rolling bearing. The thrust needle roller
bearing developed by JTEKT is one of the smallest in
the world with two races totaling a thickness of 2mm. It
adopts small diameter rollers with an O.D. of 1mm andlength of 2.5mm.
Typically, thrust needle roller bearings use rollers
with an O.D. of 2mm as the standard dimension. As
such, JTEKT aimed to reduce the roller O.D. by half to1mm, however the machining of thrust bearing rollers
was difficult initially due to their short length. However,
JTEKT succeeded in developing an ultra-thin thrustneedle roller bearing through the establishment of amachining technology enabling the roller O.D. to 1mm.
Figure 10 provides the test results comparing thefrictional torque of the current copper washer and thrustneedle roller bearing, respectively. Unit test of bearingshows torque has been reduced by 80% or more. If
there is a large skew force in the planetary bearing, thefrictional torque on the pinion gear end face can also be
reduced by adopting the developed bearing.

JTEKT ENGINEERING JOURNAL English Edition No. 1014E (2017)

u1mm (Roller)
0.5mm (Race)
2.5mm(Roller)

0.5mm (Race)

Replace toroller bearing

Fig. 9 Ultra-thin section thrust needle bearing
0.5
Frictional torque, N㨯m

In line with the shift to multi-speed ATs of recent years,
cage centrifugal force is increasing due to a larger carrier
revolution radius and bearing size. It is considered that,
if there is a large cage centrifugal force and the cage isrotating at high speed, then there will be a large frictional
force between the pinion gear bore and cage outer face,hence seizure may occur on the outer face of the cage.
To prevent such seizure, it is necessary to increase theamount of lubricant supplied to the planetary bearing inan AT unit however from the perspective of increasing AT
efficiency, there are cases where significant increases inlubricant supply are not possible. In such cases, the cage
may be coated in order to reduce the friction between thepinion gear bore and cage outer face.
It has been confirmed through evaluation that, by
coating the cage, the friction index, PV, which is themultiplication of contact surface pressure between thepinion gear bore and cage outer face, P, and the sliding
speed, V, improved by 1.3 times.

Load: 0.5 kN, Lubricant: ATF

0.4
0.3

Copper washer (slide bearing)

0.2
ً92％

0.1
0

ً83％
Thrust needle roller bearing

0

1 000

2 000

3 000

4 000

−1

RPM, minFig. 10 Bearing bench test results

4. Conclusion
Figure 11 summarizes the effects of the needle roller
bearings for planetary gears developed to satisfy highspeed and high efficiency demands. The need to enhancefuel economy further will continue to intensify, therefore
JTEKT wishes to develop bearing products in response to
future trends based on the developments introduced here.
BearingLong-life
planetary
bearingEﬀect of developed bearing
Life improvement:
8 times of standard
bearing
No. of rollers: Life
1 additional improvement:roller
1.25 times

High-speed
planetary
bearing

Life improvement(total):10 times of standard
bearing

Cage stress: 12% less
→ Centrifugal acceleration response: 12% higher
Coating of the cage outer face
→ Friction index PV: 1.3 times improved

Ultra-thin thrustneedle roller
bearing

Replace to side washer rolling bearing→ Over 80% reduction in frictional torque (bearing
bench test result)Fig. 11 Summary of effects of developed bearing57

−Development of Needle Roller Bearing for High-efficiency Planetary Gear−

References
1)
2)

K. Chihara, T. Satou : Koyo Engineering Journal,
No.168E (2005) 39.
M. Kitamura : Koyo Engineering Journal, No.164E(2004) 19.

M. TANIYAMA*

K. KOTANI**

Y. NAKASHIMA*

T. SATOU****

NRB Business Development Dept., Bearing Operations
Headquarters** Central JAPAN Technical Center, Bearing Operations
Headquarters*** East JAPAN Technical Center, Bearing Operations
Headquarters

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JTEKT ENGINEERING JOURNAL English Edition No. 1014E (2017)