Special Issue; Machine Tool Bearings andPrecision Apparatus Products
October 2004

ISSN 0915-0528 OSAKA,JAPANCONTENTS

Preface

Contribution

Bearings for
Machine Tools

Osamu KATO

Latest Trend of Main Spindle for NC Machine Tool
Yoshiaki KAKINO

Development of Eco - friendly Oil Jet Lubricated Angular Contact Ball Bearings for Machine Tool

1

2

6

Yoshinobu AKAMATSU and Masatsugu MORI

Minimizing Lubricant Supply in an Air-Oil Lubrication System

12

Yoshinobu AKAMATSU and Masatsugu MORI

Development of Long Life Grease for High Speed Application“ME-1” Grease for Motor Bearings

20

Hidenobu MIKAMI"ULTAGE" Series Precision Bearings for Machine Tools
Hiroshi TAKIUCHI and Futoshi KOSUGIULTAGE Standard Angular Contact Ball Bearings, 79U/70U type26

30

Keiichi UEDAHTA U Type ULTAGE Angular Contact Ball Bearings for Axial Loads

36

Yasuyuki HIROTAHigh Speed and Long Life Double-Row Cylindrical Roller Bearings

42

Naota YAMAMOTO and Mamoru MIZUTANINext Generation Deep Groove Ball Bearing for High-Speed Servomotor

46

Chikara KATAGIRI and Kenichiro NAITO

Precision
Apparatus
Products

Multi-Repair System for Color Filters52

Masahiro SARUTA

Repair System for Sixth and Seventh Generation LCD Color Filters

56

Akihiro YAMANAKA and Akira MATSUSHIMAPDP Rib Repair System

64

Shizuka YAMAZAKI and Yuji YADA

Improvement of the Accuracy of an Air Spindle of the Nano-meter Order- Improvement of Rotational Accuracy by Optimization of Feed Hole Positioning Yoshio FUJIKAWA, Takashi HARAGUCHI, Kazuyuki AONO and Teruyoshi HORIUCHI

Nanometer accuracy positioning system
Akio NAKAJIMA

Our Line of New Products

70

78

84

NTN TECHNICAL REVIEW No.72（2004）[ Preface ]Special Issue

Machine Tool Bearings andApparatus Products
Osamu KATO
Director

We are in the fourth year of the 21st century or the century of the environment. Safety and humanity must
be carefully planned and designed into industrial products, too. At NTN CORPORATION, ‘Coexisting inHarmony with the Environment’ is our underlying philosophy as we undertake programs to improve ourenvironmental management. In the automobile and industrial machinery fields, NTN is offering to ourcustomers environmentally friendly products that are designed for longer life, low torque, light weight, andenergy conservation.
The 22nd Japan International Machine Tools Fair (JIMTOF2004) will open on November 1, 2004 through the8th. The theme for this year is “MONO-ZUKURI” innovation. NTN will present our own solution to the needsof the machine tool market under a theme of ‘Proposals for “MONO-ZUKURI New Age”. Our ULTAGE series
new bearings were developed with a basic concept of ‘Realizing ultra high speed, high precision, and highreliability while maintaining harmony with the environment.’ We now have Grease-lubricated Sealed bearingsand Eco-conscious Air-Oil Lubricated bearings available on the market. Every one of these products has
been highly accepted as ‘being people-friendly and ecologically sound.’In the coming JIMTOF2004, NTN will present our unique user-friendly, highly functional, and ecologicallysound technologies that are designed for machine tools of today and tomorrow. We plan to exhibit productsthat will add more functionality to machine tools and, at the same time, provide improvement in the customer’s
work environment and reduce the environmental burden.
In this special issue, we will review the development of new technologies and introduce some of our brandnew technologies to you.
Since the founding of our company in 1918, NTN has been engaged in the manufacture and sales of rolling
bearings. In 1963, we began and have since been producing and supplying constant velocity joints toautomobile and various equipment manufacturers. We entered the field of precision apparatus products in1985 with our mechatronics technology, which is a combination of mechanical and electronic technologies.
NTN now manufactures and sells component products such as hydrostatic bearings, magnetic bearings, andpositioning tables, and system products based on these components such as FPD (Flat display) repair units
for liquid crystal, PDP, organic EL, etc. that are equipped with image processing function and laser. Growth in
this field has been remarkable. Some of the leading component products meet the accuracy requirements ofnanometers and sub nanometers, while the system products must accommodate even larger manufacturing
devices and provide finer fabrication technology. In addition to the traditional engineering issues of ‘faster’,‘more precise’, and ‘more functional,’ NTN will strive to find solutions to ‘challenge of “ultra” in the advanced
fields.’ This special issue also carries an introduction to the related new type repair units and the nanometer
control technology.
Under the concept of ‘For New Technology Network,’ NTN is committed to make all out effort in expanding
our business as a full-line manufacturer of bearings, constant velocity joints, and precision apparatus
products.

-1-

NTN TECHNICAL REVIEW No.72（2004）[ Contribution ]Latest Trend of Main Spindle for NC Machine Tool

Yoshiaki KAKINOKAKINO Technical Laboratory, Chief
Professor of kyoto University

This paper overviews today's technologies and future trends in high-speed spindles formachine tools. High-speed machine tools have been popularized in order to meet the
progress of machining technologies led by the technical progress in tools and in workpiecematerials. We consider that such progress will continue for a while, and that spindle
systems will get faster. The dmn value may reach as high as 4 million in the near future.

1. Introduction

2. Changes in machining technology

The machine tool manufacturers today are excitedabout good business for the first time in several years
and are loaded with an unprecedented amount oforders. The order amount for this year appears tosurpass that of the previous year by more than 160%.
The orders include not only compound machine tools,
5-axis control machining centers, or other multi-axis
control, complex, and high-end machine tools, butalso such popular equipment as common 2-axis NC
lathes and vertical machining centers. The primaryreason for this huge order volume is that the domestic
manufacturers have finally rebounded from the recent
spell of low market demands and begun their fullcapital investment drives.
When business is good, manufacturers tend to tone
down technology development and place priority on
the production of easy-to-manufacture equipment.
Nevertheless, the technological innovation in the NC
machine tools was staggering up until a year ago. It
all began with the pursuit for high-speed main
spindles, and then came the high-speed feeding
systems. Soon, the high-speed feeding systems
caught up with and passed the high-speed main
spindles, creating a situation where they are pushing
the main spindles to even higher speeds.
In the following sections, we will review the changes
that have taken place in the machining technology,
which are the true cause for this pursuit of highspeeds. We will then look at the today's status of thehigh-speed trend in the main spindle and feedsystems.

Progress in machining technology is generally
triggered by development of new cutting tool material(lately including new coating technologies) and,indirectly, by the changes in the workpiece materials
and profiles at the end users. The fabrication method
for hardened dies, for example, has suddenly changed
to the past 10 years. In the past, the raw material of
about HRC25 was machined and hardened by heat
treatment. Since the material was too hard tomachine, it was finished with electrical dischargemachining.
With the advent of (Al, Ti) N-coated ultra hard end
mill cutting tools, machining of the hardened materials
was made possible. High-speed machining
technology that cuts hardened SKD61 die steel ofabout HRC53 at an amazing 200m/min wasdeveloped, which brought a big change in theconfiguration of the die machining processes andmachine tools. Furthermore, there has been a rapidtransition in the complex, free-form finishing processfrom the use of electrical discharge machining to smallbore ball end mill cutting tools. This change
accelerated a shift towards high-speed main spindles
in the machining centers for die fabrication.
In the parts fabrication works, a major change took
place in the workpiece material for energy
conservation from steel to aluminum, which could bemachined at high speeds. These two developments
brought significantly fast main spindles and highspeed, high-acceleration feeding system to medium
and small size machining centers.
-2-

Latest Trend of Main Spindle for NC Machine Tool

These changes may be described as changes "fromlow-speed, heavy cutting to high-speed, light cutting."
Fig. 1 shows the historical transition in the machining：High-speed machining range for carbon steel

100

Feeding speed F m/min

speeds of various materials, which depicts thecompound effects of the above developments.

■ Facing (rough)□ Facing (finishing)◆ End mill (insert type)◇ End mill (solid type)● Drill◎ Tap (M10)○ Boring

10

Synchronizedtapping

JIMTOF'96◆◆ EMO'93
EMO'97
Noris◎'96 ● ■
'96
■
'93

'96
◎

1
'93

◎
'93

◆ '98

●

◆
'93

'93

'99
JIMTOF'98
EMO'97JIMTOF'96

'93

0.1
10

100

1000

10000

Cutting speed Vc m/min(a) Carbon steel：High-speed machining range for aluminum alloy

Feeding speed F m/min

100■ Facing (rough)□ Facing (finishing)◆ End mill (insert type)◇ End mill (solid type)● Drill◎ Tap (M10)○ Boring

10

'95
'94
'93

1

EMO'95
EMO'97
●
'94
●

'95

'94

'97

Synchronizedtapping'93
EMO'97

'93

'95
'94

'94

■

'96
'93 ●

EMO'97 PCD

IMTS'94●

EMO'97■■

EMO'95Compound tapping
'96
○
○
'93

0.1
10

100

1000

Cutting speed Vc m/min(b) Aluminum alloy
Fig.1 Improvement of cutting speed

-3-

10000

NTN TECHNICAL REVIEW No.72（2004）

balls. Now, with increased production, the quality of
the ceramic balls has stabilized and the price hascome down. As a result, more bearings with theceramic balls are being used. It is not an
overstatement when they say that almost all the rolling
bearings rated for 1.5 million in the dmn value use theceramic balls.
Another recent trend is the customer's preference
for environmentally friendly lubrication methods. The
environmentally friendly lubrication methods are often
low-cost lubrication methods, too. For this reason,
this trend appears to proliferate faster. NTN reevaluated grease lubrication in preference to therelatively popular air-oil lubrication method andsuccessfully made the grease-lubricated sealed
angular contact ball bearings available for service at
up to 1.4 million in the dmn value. This speed is often
sufficient for use in steel parts fabrication, andbearings of this type are increasing their applications.
Leading the race for faster bearing speed, the air-oil
lubrication method has been improved by NTN for low
noise and less consumption of air and oil. Ecoconscious type bearings are contributing to reduction
in environmental burdens.

3. Evolution of main spindle speed
The dmn values are often used as an index toindicate high-speed performance of the main spindle.
"dm" represents the ball pitch circle diameter of the
bearing rollers in millimeters, and "n" is a product of arealizable maximum revolution of the main spindle
represented by /min. The limit of the dmn value
naturally varies with the methods of lubrication and thetypes of bearings.
Fig. 2 shows the history of the bearing speeds for
NTN's main spindle bearings in the past 20 years.
Around 1990, the maximum dmn value of the air-oil
lubricated bearings was about 2 million. Today, withadvancement in optimization of the bearings internal
specifications, the dmn value has reached 3 million.
Since, as mentioned above, heavy cutting has
declined, demands for rolling bearings of high rigidity,
which can be used at low-speeds only, have notincreased much. Instead, more and more ballbearings, especially those using Si3N4 ceramic balls ofsmaller specific gravity, are used very frequently. In
the past, the ceramic balls were considered more
expensive and less accurate as compared to the steel

300

Angular contact ball bearing

Allowable dmn value ×104

Orange: ULTAGE series

Air-oil lubrication200

HSB CAEX1 type

300
100

HSA type

HSF type
HSFL type (eco-conscious type)

HSC type

HSB type

HSE type
HSL type (eco-conscious type)
70U type

70 type2000Grease lubrication
100

BNS type (sealed)
HSE type

HSB CAEX1 type
HSB type

HSA type

70LLB type (sealed)
70U type

70 type
0
1980

1990

Cylindrical roller bearing
Orange: ULTAGE series

1996

2000

2004

（Year）

300

200

Allowable dmn value ×104

N10HSR typeAir-oil lubrication

300

NN30HSR type

100

NN30HS type N10HS type
NN30 type NNU49 type N10 type

200
0

NN30HSR type N10HSR type

Grease lubrication

100

NN30HS type N10HS type
NN30 type NNU49 type N10 type
0
1980

1990

1996

2000

2004

Fig.2 Improvement of NTN bearing speed for main spindle-4-（Year）

Latest Trend of Main Spindle for NC Machine Tool

4. Faster feeding speed

5. Conclusion

As mentioned above, it is desirable for the main
spindles and the feeding systems to advance hand-inhand in the race for faster speeds in view offabrication technology. However, they do not always
progress in synchronization subject to the status of thetechnological development. Recently, more significant
advancement is visible in high-speed, highacceleration feeding systems. This progress was led
by the introduction of linear motors into machine tools,
and then followed by the use of high-lead ball screws.
Fig. 3 shows the status of development. There aremachining centers in the market that utilize the parallelmechanisms without the use of guides.
Thanks to these technologies, the maximum feeding
speed of 60m/min and the maximum acceleration of
1G in the machining centers no longer draw special
attention. (Nevertheless, in many actual applications,this performance is sufficient.) The maximum speedof over 100m/min and acceleration reaching 2G havealready been achieved on actual equipment.
Considering that 24m/min and 0.1G were thestandards some 10 years ago, there has been great
progress. On the other hand, the speed of thecorresponding main spindle for BT40 is 40,000/min,though is still in the testing stage and has not provento be service worthy.

As mentioned above, it is desirable for the main
spindles and the feeding systems to advance hand-inhand in the race for faster speeds. Lately,achievement in making faster main spindles issomewhat behind that of the feeding systems. I hopethat NTN will develop a high-speed technology formain spindles to break this cycle, and become theleader of the world's machine tool industry.

120
Cutting feed speed

Fast feeding speed m/min

100◆

Feeding speed

80

60

◆ ◆
◆

40
◆

20
◆
◆
◆

0
1955

◆

1960

1965

1970

1975

1980

1985

1990

1995

2000

Fig.3 Improvement of feeding speed for machining center-5-2005 Year

NTN TECHNICAL REVIEW No.72（2004）[ Technical Paper ]Development of Eco - friendly Oil Jet LubricatedAngular Contact Ball Bearings for Machine Tool

Yoshinobu AKAMATSU*
Masatsugu MORI*

The lubrication system is critical for the high-speed operation of a machine tool. In 1992, NTN
developed a complex lubrication system with oil jet lubrication and an under-race lubrication system
for angular contact ball bearings with dmn ≤ 390 million (dm = ball pitch circle diameter, n = speed). In
order to increase the speed limit of angular contact ball bearings, the “Eco-friendly Oil Jet Lubrication
System” was developed. In this paper, the eco-friendly jet lubricated bearing and the lubrication
system are introduced. This paper summarizes a test that measured the power loss and temperatureof bearings (70mm bore, dmn value = 500 million) using this new lubrication system.

1. Introduction

lubrication mechanism supplied a lubricant jet from theouter ring spacer to the cylindrical surface of the
scoop, which was formed by enlarging the ID of theinner ring end surface. This scoop is connected to the
lubricant supply hole leading to the inner ring raceway
through the axial through-hole in the inner ring, thus
accomplishing supply of the lubricant. Both lubrication
mechanisms lubricate the bearing. At the same time,
the oil jet lubrication contributes to the cooling effect of
the outer ring and the under-race lubrication to thecooling of the inner ring.
As mentioned above, adoption of oil jet lubrication
and under-race lubrication increased the speed of the
bearings. However, if even faster bearings aredesired, these lubrication mechanisms require adriving unit of larger capacity to compensate for the
power loss on the bearings caused by a large amountof lubricant that passes through the bearings. This
paper describes a new eco-conscious oil jet
lubrication mechanism that has reduced the power
loss of the bearing and achieved ultra high-speed
rotation.

Faster rotation of machine tool main spindles isneeded to improve the surface quality and machining
efficiency of machine tools. The key technologies toaccomplish this will increase the speed and accuracyof the rolling bearings that support main spindles.
NTN has already established a technology thatanalyzes the relationship between the accuracy of the
bearing components and the bearing vibration underpreload 1), 2), 3). This technology makes it possible tostudy how to reduce bearing vibration. The speed ofrolling bearings for machine tool main spindles
depends on lubrication methods which include, in theascending order of the limiting speed, greaselubrication, air-oil lubrication, oil jet lubrication, andunder-race lubrication. In 1992, NTN combined oil jet
lubrication and under-race lubrication mechanisms
into an angular contact ball bearing of 100mm bore
and achieved a dmn value of 3.9 million 4), 5), 6). The oil
jet lubrication system supplied a jet of lubricant
through a nozzle that was facing the inner ring
raceway from the outer ring spacer. The under-race*Research & Development Center

Technical Research Dept.

-6-Development of Eco - friendly Oil Jet Lubricated Angular Contact Ball Bearings for Machine Toolthen flows along the inner diameter, shown on theright side in the figure, of the ring that establishes theclearance. The outer ring spacer consists of a bearing
side spacer with a nozzle and a counter bearing side
spacer. As shown in Fig. 2, only the nozzle portion of
the bearing side spacer protrudes towards the innerdiameter and the remainder forms an annulus ring of alarger diameter than the inner diameter of the nozzle.
Consequently, the lubricant moving along the innerdiameter of the smaller annulus ring moves towards
the counter bearing side spacer as shown in Fig. 2.
A special lubrication device is typically installed forthe bearings of machine tool main spindles. Fig. 2
shows the lubrication system used for this study
where a jacket-cooling device for the machine tool
was used as a lubrication device. Here, the oil fromthe cooling oil delivery device is filtered and fed to theouter ring spacer for lubrication. Oil that has cooled
the inner ring is pumped out from the outer ring
spacer, and oil that has lubricated the bearing, cooled
the outer ring, and then come out of the bearing isreturned to the oil recovery tank by the discharge oilpump. The discharged oil is returned from the oilrecovery tank back to the cooling oil delivery device
for circulation. The lubrication system shown in Fig. 2
does not require any special lubrication devices otherthan the filter and the discharge oil pump. Thus, thislubrication system incurs less power loss as comparedto the traditional oil jet lubrication system, and it
requires no cost for the peripheral equipment. That is
why this system is called an eco-friendly oil jet
lubrication mechanism.

2. Eco-friendly oil jet lubrication
mechanism
The technical issue to be resolved in achieving
higher speeds with rolling bearings is reduction in
power loss and preload fluctuation. In the oil jet
lubrication mechanism, reduction in power loss would
require a reduction in the drag against the rotation of
the rolling elements aside from the friction torque ofthe rolling elements at their contact surfaces. To dothis, the technology that controls the amount of
lubricant inside the bearing is important. Additionally,
high-speed operation of a bearing causes the bearing
to produce heat, which leads to thermal expansion of
the outer ring and inner ring raceways. Furthermore,
centrifugal force causes the inner ring raceway to
expand and increases preload. Oil jet lubrication
cools the outer ring, but its cooling effect on the innerring is low. Therefore, the oil jet lubrication requires acooling mechanism for the inner ring.
To resolve these issues, the eco-conscious oil jet
lubrication mechanism shown in Figs 1 and 2 wasdeveloped. Lubricant supplied to the outer ring spacer
is injected towards the scoop formed at the inner ringend surface through the nozzle. Inside the scoop,
centrifugal force of the rotating inner ring causes the
lubricant to stick to the inner diameter of the scoop.
Then the lubricant, moves from the inner ring end
surface to the conical surface by centrifugal force andsurface tension. At the conical surface of the innerring, a portion of the outer ring spacer has a ring thatcreates clearance to allow oil passage. A portion of
the lubricant on the inner ring passes through thisclearance into the internal structure of the bearing,
while the remaining lubricant cools the inner ring. If

FilterJacket cooling

Oil flow

Cooling oil supply
deviceNozzleOuter ring
spacer

Scoop

Oil recovery tank

Discharge oil pumpFig.1 Bearing design

Jacket-cooling
returnFig.2 Eco-friendly oil jet lubrication system-7-

NTN TECHNICAL REVIEW No.72（2004）

preload was used. To compare the test lubrication
mechanism to the air-oil lubrication mechanism, thesame performance test was conducted on the air-oil
lubrication mechanism. The air-oil lubrication wasestablished by 0.03cc/3min of oil and 30NL/min of air.

3. Test results
3.1 Test conditions and test equipment
The outer ring temperature of a bearing and power
loss of a motor were measured at different bearing
speeds using a 70mm bore angular contact ballbearing equipped with the eco-friendly oil jet
lubrication mechanism. Fig. 3 shows the structure of
the test spindle and Photo 1 shows the appearance ofthe test equipment. A built-in motor powered thespindle. The spindle was supported by two test
bearings and two support bearings. The innerdiameter of the support bearings was 35mm.
Lubricant was supplied to the test bearings throughthe orange oil hole at the top indicated in Fig. 3, and it
was discharged through the orange discharge hole atthe bottom. The test equipment also supplied jacketcooling oil to the test bearings, motor, and the support
bearings. For lubrication of the support bearings, ecofriendly air-oil lubrication was used. A constant

Test bearings

The primary test conditions are shown in Table 1.

Table 1 Test conditions
Test bearing

φ70×φ110×20

Contact angle

30˚

LubricantLubricating oil for machine tools ISO VG2

Initial preload

1kN

Jacket-cooling rate

3L／minJacket-cooling oil temperature Room temperature±1˚C
Race material

Carburized steel, High-speed tool steel

Ball material

Si3N4

Cage material

PEEK

Support bearings

Oil hole

Discharge
oil hole

Fig.3 Schematic construction of test spindle

Photo 1 Test spindle-8-Development of Eco - friendly Oil Jet Lubricated Angular Contact Ball Bearings for Machine Tool

within the operating temperature range.
Fig. 5 shows a comparison of the temperature riseof the outer ring between the eco-friendly oil jet
lubrication and the air-oil lubrication methods. The
delivery rate of the eco-friendly oil jet lubrication
mechanism was 3L/min. In this figure, it is clear thatthe eco-friendly oil jet lubrication method is superior to
the air-oil lubrication method in terms of thetemperature rise in the outer ring. Also confirmed was
the fact that the temperature rise in the outer ring forthe eco-friendly oil jet lubrication mechanism at a dmn
value of 5 million was below 60˚C.
For the eco-friendly oil jet lubrication method, theamount of the lubricant delivered inside the bearing
seems to determine the friction torque of the bearing
as well as the cooling capability of the outer ring. Fig.
6 shows the measurements of power loss for the ecofriendly oil jet lubrication method with different oil
delivery. In this figure, the delivery rate of the ecofriendly oil jet lubrication mechanism was measured at
the nozzle, and it was not the amount of the lubricant
that passed through the bearing. The discussion of
the measurement results, however, assumed that theoil flow through the bearing is proportional to thedelivery rate at the nozzle. Fig. 6 shows that the more
oil was delivered inside the bearing, the larger the

3.2 Test results

40
30
20
Eco-friendly oil jet lubrication

10

Air-oil lubrication

0

200

100

300

dmn value

400

500

600

4

×10

Fig.5 Effect of lubrication type on outer race temperature

8

6Power loss kW

Power loss kW

50

0

8

4
Eco-friendly oil jet lubrication

2

0

60

Outer ring temperature rise ˚C

The test spindle was set at predetermined rotational
speeds starting with the lowest speed using an
inverter controlled built-in motor. When the bearing
speed stabilized, the motor's power loss and the outerring temperature were measured. After themeasurement, the rotational speed was raised to a
pre-determined speed and the test continued until a
dmn value of 5 million was reached. For the air-oil
lubrication mechanism, the test was terminated whena maximum speed of 2.5 million in the dmn value wasreached.
Fig. 4 shows a comparison of the motor's power
loss between eco-friendly oil jet lubrication and air-oil
lubrication. The delivery rate of the oil jet lubrication
was 3L/min. The measured power loss values
included friction torque of the support bearing.
However, when comparing the eco-friendly oil jet
lubrication to the air oil lubrication mechanisms, thedifferential between the two is simply the differential
between the two lubrication methods for the test
bearing. In the same figure, the power loss of ecofriendly oil jet lubrication is about the same as that of
air-oil lubrication. The test for the air-oil lubrication
mechanism finished at the dmn value of 2.5 million, buteven if the measured value is extended up to 5 million
in the dmn value, the power loss in the eco-conscious
oil jet lubrication is about the same as that of the air-oil
lubrication. The support bearings were smaller in sizeas compared to the test bearings. If the power loss inthe support bearings is zero, the power loss in the
eco-conscious oil jet lubrication at the dmn value of 5
million was below 8kW.
A quantitative discussion of the preload changes
associated with the rising rotational speed would
require measurements of the outer ring and the innerring temperatures as well as calculations of thecentrifugal expansion of the inner ring. In this test,
however, the outer ring temperature in the ecoconscious oil jet lubrication method was verified to be

Air-oil lubrication

0

100

200

300

400

500

dmn 0.9 million
dmn 1.8 million
dmn 2.7 million
dmn 3.6 million

6Air-oil
4

2

0

600

dmn value ×104

0

1

2

3

Oil volume L/min

Fig.4 Effect of lubrication type on power loss

Fig.6 Effect of amount of lubricant on power loss

-9-4

NTN TECHNICAL REVIEW No.72（2004）

Outer ring temperature rise ˚Cpower loss. It also shows that, at higher revolutions,the rate of power loss became greater as the oildelivery increased.
Fig. 7 shows the temperature rise of the outer ringat different lubricant delivery rates using the ecofriendly oil jet lubrication method. The outer ringtemperature dropped as the lubricant delivery
increased. At low revolution rates, increase in the
lubricant delivery rate did not significantly cool theouter ring, but as the revolution rate became higher,
the cooling effect on the outer ring increased. In the
figure, the delivery rates in the air-oil lubrication at dmn
0.9 million and dmn 1.8 million were plotted as being
close to zero. However, since these points are almoston the extension lines of the measurements of oil jet
lubrication, we can say that the amount of lubricant
delivered into the bearing determines the coolingefficiency of the outer ring.
Since the relationship of the lubricant delivery rate
for oil jet lubrication and friction torque on the bearing
is opposite that of the lubricant delivery rate for oil jet
lubrication and cooling efficiency, the optimal
lubrication conditions for machine tool main spindle
bearings can be determined by selecting a proper
lubricant delivery rate for the eco-friendly oil jet
lubrication mechanism.

4. Conclusion
To meet the needs for increased speeds in main
spindle bearings for machine tools, we havedeveloped an eco-friendly oil jet lubrication
mechanism with less power loss and fewer changes inbearing temperatures. This lubrication mechanism
was successfully applied to a 70mm bore angularcontact ball bearing at an operating speed of dmn 5
million. It is our hope that this lubrication mechanism
will contribute to the development of faster machine
tools.

References
1) T. Sakaguchi and Y. Akamatsu, Proc. Int. Trib. Conf.
Nagasaki (2000)1795.
2) T. Sakaguchi and Y. Akamatsu, NTN TECHNICALREVIEW No.69(2001)69, (in Japanese)
3) Y. Akamatsu and T. Sakaguchi, Proc. JAST Trib
Conf.(Tokyo 2004-5) 383, (in Japanese)4) M. Mori and M. Niina, NTN TECHNICAL REVIEW
No.60(1992)41, (in Japanese)5) K. Hibi, M. Mori and M. Niina, Proc. JAST Trib
Conf.(Tokyo 1992-5) 633, (in Japanese)
6) K. Fujii, Proc. JAST Trib Forum '98 (1998) 95, (inJapanese)60
dmn 0.9 million
dmn 1.8 million
dmn 2.7 million
dmn 3.6 million

Air-oil

50
40
30
20
10
0

0

1

2

3

4

Oil volume L/min

Fig.7 Effect of amount of lubricant on outer race temperature

Photos of authors

Yoshinobu AKAMATSU

Masatsugu MORITechnical Research Dept.
Research & Development Center

Technical Research Dept.
Research & Development Center-10-

NTN TECHNICAL REVIEW No.72（2004）[ Technical Paper ]Minimizing Lubricant Supply in an Air-Oil Lubrication System

Yoshinobu AKAMATSU*
Masatsugu MORI*
Rolling bearings for machine tool main spindles must havelow vibration, low friction, and ability to operate at high speed.
NTN’s environment-friendly ULTAGE series HSL-type air-oil
lubricated angular ball bearing meets all of these requirements.
The HSL-type bearing supplies lubricant via the inner ring
surface by delivering an air-oil mixture to the cone-shaped
counter bore of the inner ring. The traditional lubrication
method of supplying the air-oil mixture to the inner ring/roller
contact surface produces an air-piercing sound as the rollers cut through the flow of the air-oil mixture, and this new
structure successfully eliminates the air-piercing sound. It also successfully reduces the required amounts of oil andair.
This paper describes the technology for minimizing the lubricant supply used in the HSL-type angular ball bearing,and more specifically, evaluates the limit of the counter bore tilt angle, swash clearance, lubricant viscosity, effects of
air flow rate, uniformity of oil, and the factors for the minimum required oil amount. It then concludes that the swash
angle is determined by the dmn value and that smaller wettability of the tube results in more uniform lubricant supply.

diameter of the inner ring from the end surface to the
raceway to make it a conical shape and supplies airoil mixture to this counter bore surface. Thislubrication method eliminated the air-piercing noisecaused by passage of the rolling elements in the
traditional structures 2) and successfully reduced suchnoise by about 10dBA between the dmn 1.3 million
and 2.65 million. This high-performance air-oil
lubrication mechanism also reduced oil consumption
down to 1/10 and air consumption to 1/4 at dmn 2.65
million 1), 4). NTN is offering this new bearing as an
eco-conscious air-oil lubricated bearing 1).
Rolling bearings for machine tool main spindles
demand minimal heat generation. In this respect, it isnecessary to determine the minimum necessaryamount of oil to hold the bearing torque at a minimum
and to establish lubricant delivery technology. This
paper describes our studies on the technology forminimizing the lubricant delivery, and more
specifically, evaluates the inner ring swash angle thatdelivers the lubricant, swash clearance, viscosity of
lubricant, effects of air flow rate, and leveling of
pulsation of the lubricant.

1. Introduction
Faster rotation of the machine tool main spindles isneeded to improve the surface quality and themachining efficiency of works. The key technologies
to accomplish them are those that increase the speed
and accuracy of the rolling bearings that support themain spindles. The speed of the rolling bearings formachine tool main spindles depends on lubrication
methods which include grease lubrication, air-oil
lubrication, oil jet lubrication, and under-race
lubrication. Presently, the revolving spiral limit of agrease-lubricated angular contact ball bearing with theshaft diameter of 100mm is 1.4 million in the dmn
value, and this bearing has reached its service life of
20,000 hours or longer 1). The oil consumption in the
air-oil lubrication method is a low 1/100,000 that of oil
jet lubrication or under-race lubrication, and thislubrication method does not lose power due to theagitation resistance of the lubricant. Nevertheless, themarket demands reduction in the oil and air
consumption in the air-oil lubrication method. In 2000,
NTN developed high-performance air-oil lubrication
mechanism 2), 3), 4) that increases the counter bore**Research & Development Center

Technical Research Dept.

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Minimizing Lubricant Supply in an Air-Oil Lubrication System

2. Structure of air-oil lubricatedangular contact ball bearing

Fig. 1 (c) shows the structure of an eco-conscious
type bearing that was developed for further energy
conservation. The low noise bearing has acircumferential groove at the exit of the nozzle torelease pressure. It was confirmed that, when air
supply is reduced, lubricant in this circumferential
groove entered the inner section of the bearing,causing temperature rise. With the eco-conscious
type bearing, the pressure release space at the exit of
the nozzle was formed on the rotating inner ring to
avoid collection of lubricant in the circumferential
groove, thus improving the lubricant delivery to therolling elements 4).

A traditional air-oil lubricated angular contact ballbearing, shown in Fig. 1 (a), is so structured to inject
air-oil mixture from the nozzle to the rolling elements.
Fig. 1 (b) is a low-noise bearing developed to reduce
the air-piercing sound of air-oil mixture. The low-noise
bearing injects air-oil mixture onto the tilted surface of
the inner ring counter bore and delivers lubricant to
the rolling elements by centrifugal force 2). This
structure reduced noise, improved the delivery
capability of the lubricant, and reduced the necessaryamount of oil and the minimum delivery air flow 3).

(a) Standard HSB type bearing(b) Low noise SF type bearing(c) Eco-friendly HSL type bearing

Fig.1 Air-oil lubrication angular contact ball bearings

3. Designing inner ring swash angle
In the air-oil lubrication mechanism for the low-noise
and the eco-conscious bearings, lubricant moves
towards the rolling elements by the component forceof centrifugal force along the slope of the inner ringcounter bore. Therefore, the swash angle is a key
design factor. For this reason, a test was conducted
to determine factors that impact the lubricant flow
along this swash surface. The test used bearings withan inner diameterφ100mm and inner diameterφ70mm with different swash angles and clearances,lubricant viscosity, and rotating speeds.

Marks indicativeof oil splashing
off edge
Oilabsorbingpaper

3.1 Test method
To confirm the flow of lubricant on the swash
surface, a model test was carried out by installing an
inner ring on the rotating shaft and an oil absorbing
paper (filter) on the outside of the inner ring, Fig. 2.
The oil absorbing paper was held against the housing
by way of an acrylic plate. This allowed an
observation of the oil splash during the bearing
operation. Fig. 2 shows an oil absorbing paper after
the test, indicating oil collection at the inner ring.
When oil collects on the inner ring, it moves to theedge of the swash surface and splashes by centrifugal
force. Some oil was observed to have splashed off
the outer face of the inner ring by centrifugal force.

Fig.2 Test for oil flow on swash surface of inner ring

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NTN TECHNICAL REVIEW No.72（2004）

Fig. 3 shows the structure of the test equipment.
The components with which to observe oil adhesion
and flow were installed at the end of the spindle thatwas supported by an air-oil lubricated bearing. To
study impacts of the swash angle α, the shoulder
dimension of the swash surface at the inner ringgroove was kept consistent and the swash angle waschanged. Impacts of the swash surface clearanceδ(radial clearance) between the nozzle spacer and theswash surface were also studied. The swash surface
clearance was adjusted by changing the shimthickness, thereby moving the nozzle spacer in theaxial direction.

3.2 Test results
3.2.1 Swash surface angle and oil adhesion andflow of lubricant
Figs.4 and 5 show the test results of the lubricant
adhesion and flow using sample inner rings ofφ100mm
ID andφ70mm ID and different swash angles.
Lubricant readily adheres and flows if the swash angle
is large and the rotating speed is fast. These results
indicate that a larger swash angle produces a larger
component force of centrifugal force, making lubricant

24
Oil adhesion

Rotating speed 103min−1

21

End of swash surface (boundary with groove)
No oil adhesion
Air-oil delivery

Oil adhesion
Acrylic plate
Oil absorbing
paper

Shim
α
δ

Enlarged

Constant

Nozzle spacer

No oil adhesion or flow

18
15
12
9
6
3
0

5

0

10

15

20

Swash angle °Sample inner ringFig.4 Effect of swash angle and rotational speedon oil adhesion ( d=100mm)

Location where
adhesion andflow of oil was
verified(Test conditions)
Swash surface clearanceδ: 0.16 ~ 0.2mm
Nozzle diameter :φ1.2
Air supply rate : 20NL/min
Oil supply rate : 0.01mL/5min shot interval
Oil used : Mobil Velocity No. 6 (VG10)

Drive side

Fig.3 Test equipment for lubricant flow observation

Rotating speed 103min−1

35

Table 1 shows the different bearing sizes, inner ringswash angles, swash surface clearances, lubricant
names, and viscosity used in the test. The injection
angle of the nozzle with respect to the center of themain spindle was 30˚30

Oil adhesion
No oil adhesion or flow

25

20

15

Table 1 Test conditions for lubricant flow observation
BearingNozzle diameter (mm)Swash angle α(˚)
Swash surface clearance δ(mm)Air supply rate (NL/min)
Oil supply rate (mL/shot interval min)Inner Diameter Inner Diameter
φ100mm
φ70mm
φ1.2

φ0.8

2〜16

10，12，14

0.16〜1.0

0.1〜0.3

2020

0.01/50.01/5

Viscosity of oil (mm2/s)

10，32，68

32

Rotating speed (min-1)〜21000〜30000

10

0

4

8

12

16

Swash angle °Fig.5 Effect of swash angle and rotational speedon oil adhesion ( d=70mm)(Test conditions)
Swash surface clearanceδ: 0.15mm
Nozzle diameter :φ0.8
Air supply rate : 20NL/min
Oil supply rate : 0.01mL/5min shot interval
Oil used : Mobil DTE OilLight (VG32)

-14-

20

Minimizing Lubricant Supply in an Air-Oil Lubrication System

more readily adhere and flow and that faster rotating
speeds cause swirling of air around the rotating shaft,preventing air-oil from adhering. Reading thesefigures, the adhesion limit swash angle at dn 2.1
million (dmn 2.63 million) is about 12.3˚. This indicatesthat the limit angle at which adhesion and flow can beestablished may be affected by the circumferential
speed (dmn value). Fig. 6 is a dmn value conversion of
the adhesion limit speeds at different swash angles
shown in Figs. 4 and 5.

3.2.2 Swash surface clearance and lubricant
adhesion and flow
The study results of the impacts of the swash
surface clearance (δ) on the lubricant adhesion andflow are shown in Fig. 7. The test used a sample withan inner diameter φ100 and swash angles of 10, 13,
and 16˚. All other test conditions were identical. With
the sample set at 13˚ and 16˚ swash angles, adhesion
and flow of oil were verified when the swash surface
clearance was increased up to 1mm. On the other
hand, the sample that was set at 10˚ swash angle did
not show adhesion or flow at rotating speeds at or
above 19000min-1 irrespective of the swash surface
clearance (δ=0.19, 0.52mm). Adhesion and flow
were confirmed at 18000min-1 or lower. These results
concluded that the swash surface clearance did notaffect the adhesion or the flow of lubricant.

20
d= 70
d=100

24

12

Rotating speed 103min−1

Oil adhesion limit angle °

16

8

4

21
18
15
12

Swash angle 16°OK

9

Swash angle 16°NG

6

Swash angle 113°NG

0100

dmn value

200

Swash angle 10°OK

300Swash angle 113°OK

Swash angle 10°NG

0

0.2

0.4

0.6

0.8

1

Swash surface clearance (radius) mm

300

×104

Fig.7 Effect of swash angle, rotational speed andclearanceδon oil adhesion

Fig.6 Relation between dmn value and swash angle(Test conditions)
ID :φ100mm
Nozzle diameter:φ1.2
Air supply rate: 20NL/min
Oil supply rate: 0.01mL/5 min shot interval
Oil used: Mobil Velocity No. 6 (VG10)

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1.2

NTN TECHNICAL REVIEW No.72（2004）

4. Leveling air-oil pulsation
To further reduce the usage of air and oil in the airoil lubrication method, it is critical to have a technologythat evenly distributes the amount of oil delivered tobearings. The following sections describe the resultsof the model test that was conducted to determine an
optimal air-oil delivery system for air-oil reduction.
The test evaluated the materials and sizes of the airoil tubing and air-oil delivery methods (intermittent andcontinuous delivery) that affect the oil transfer
characteristics in the air-oil lubrication method.

4.2 Test results
4.2.1 Effects of tube diameter and material on
oil flow
Fig. 9 compares the time from the start of air-oil
supply to oil delivery by different sample tubing to the
air supply rate in intermittent supply of 0.03mL oil per
shot at 5min shot interval.
The larger the tube ID and the less air supply rate,
the longer it took for the air-oil to come out of thenozzle and the harder the flow became. With aφ4×
φ6 polyurethane tube, the air supply rate wasincreased to 20NL/min and the oil took more than 40
minutes to come out (the test was terminated at 40
minutes). A Teflon tube ofφ2×φ4 delivered oil
within the shortest period of time and offered easy
flow because of it small surface tension and lipophobic
characteristic.

4.1 Test method
The test used a 2m tube one end of which wasconnected to an air-oil unit and the other to a nozzle.
Time for lubricant to come out of the nozzle wasmeasured and the delivery of the lubricant observed.
Fig. 8 shows the set-up of the test equipment. The
delivery status of the lubricant was recorded by arecorder, where a recording paper was moved at aconstant speed for evaluation of the level of oil
absorption. The nozzle profile used for this test is
shown in Fig. 8. The recorder used was Rectigraph.
Table 2 shows the test conditions.

Oil supply rate: 0.03mL/5min

Polyurethaneφ4×φ6
Polyurethaneφ2.5×φ4
Polyurethaneφ1.5×φ3.5
Polyurethaneφ2×φ4

Oil travel time min

50

Air-oilTube

53

Fig.8 Equipment for oil flow observation

Table 2 Test conditions for oil flow observation
Polyurethane, Teflon

Tubing Inner Diameter× φ1.5×φ3.5，φ2.5×φ4，φ4×φ6
Outer Diameter (mm)Air supply rate (NL/min)
Oil shot interval (min)
Oil

10
5

10

15

20

Air flow rate NL/min

φ0.8
Recordingpaper

Length of tubing (m)20

Fig.9 Air flow rate and oil travel time

Recordingpaper

Material of Tubing

30

0
0

46

φ3

30

Nozzle402
7.5〜20Continuous ~ 20
Mobil DTE OilLight (VG32)

-16-

25

Minimizing Lubricant Supply in an Air-Oil Lubrication System

4.2.2 Fluctuation in oil delivery
A study was made to identify how the air-oil supply
conditions and different sample tubes affected theconsistency in oil delivery. The test used anintermittent oil supply unit and recorded chronologicalchanges in the oil amount (fluctuation in oil delivery)
delivered from the nozzle by way of oil absorption on
the recording paper. Fig. 10 shows the recorded oil
absorption. As shown in (a) and (b), with a Teflon
tube, when air supply rate was small, its capability totransport oil decreased and the oil moved in lumps,

causing fluctuation in oil delivery. With the air supply
rate kept constant, when the oil supply time was
extended, oil was delivered in correspondence withthe shot interval as shown in (b) and (c). If the interval
is long, it causes fluctuation in oil delivery. On theother hand, polyurethane tubes showed lessfluctuation in oil delivery even though the shot interval
was extended, (c) and (d). It was because
polyurethane tubes had large resistance on theinternal walls that stopped the oil flow inside the tubes.

2min(a) φ2×f4 Teflon tube, Air supply rate: 7.5NL/min, Oil supply rate: 0.03mL/5min

2min(b) φ2×f4 Teflon tube, Air supply rate: 12.5NL/min, Oil supply rate: 0.03mL/5min

2min(c)φ2×f4 Polyurethane tube, Air supply rate: 12.5NL/min, Oil supply rate: 0.03mL/15min

2min(d)φ2.5×f4 Teflon tube, Air supply rate: 12.5NL/min, Oil supply rate: 0.03mL/15min

Fig.10 Example of lubricant jet soak on recorder chart-17-

NTN TECHNICAL REVIEW No.72（2004）

4.2.3 Impacts of continuous and intermittent
oil supply
Ease of air-oil flow was studied using a system thatwas capable of continuously supplying a very small
amount of oil and aφ2×φ4 Teflon tube. Fig. 11
shows the results. Like intermittent oil supply,
continuous oil supply showed a tendency of losing airoil flow as the air supply rate became smaller. Easeof air-oil flow was also compared between thecontinuous and the intermittent oil supply methods
with the oil delivery rate per hour maintained thesame. As a result, the continuous oil supply method
took longer time than the intermittent oil supply
method in delivering oil with poorer oil flow. This may
have been caused by poor oil transport efficiency of
air because the continuous oil supply method makes
the oil grains that adhere to the tube wall smaller.

Figs. 12 and 13 show the fluctuation in oil delivery,
defined as Variance β (mm) of the maximum and theminimum absorption marks, in the continuous oil
supply method using aφ2×φ4 Teflon tube. In thecontinuous oil supply method, oil delivery rate wasless subject to air and oil supply rates. As comparedto the intermittent oil supply method, the continuous oil
supply method showed less fluctuation in oil delivery
at air supply rate of 7.5NL/min.
The continuous oil supply method has lessfavorable oil transport efficiency within tubes than theintermittent oil supply method. However even if the air
supply is small, it has less fluctuation in oil delivery
and offers stability in the supply of lubricant.

φ2×φ4 Teflon tube used

10
0.1

0.2

0.3

0

Intermittent
oil supply

0.09mL/hContinuous
oil supply

0.4

Fig.12 Variation β for intermittent and continuous
oil supply (air 7.5NL/min)

Oil flow rate mL/h

Fig.11 Oil flow rate and oil travel time40

0Intermittent
oil supply

0.09mL/h

50.12mL/h

100.18mL/h

150.36mL/h

200.03mL/20min

25

0.03mL/15min

30

0.01mL/1.7min

35

0.03mL/10min

05

Oil supply：0.03mL/5min
rate

0

10

0.12mL/h

20150.18mL/h30200.36mL/h

40

250.03mL/10min

530Fluctuation in oil delivery(maximum width – minimum width) mm

Oil travel time min

1035

0.01mL/1.7min

20 15

50

min

40

Oil supply：0.03mL/5minrate

Shot interval in intermittent oil supply method (0.03mL)

Fluctuation in oil delivery(maximum width – minimum width) mm

Air supply rate: 7.5NL/min, Continuous oil supply method
Air supply rate: 12.5NL/min, Intermittent oil supply method
Air supply rate: 12.5NL/min, Intermittent oil supply method

Continuous
oil supply

Fig.13 Variation β for intermittent and continuous
oil supply (air 12.5NL/min)-18-

Minimizing Lubricant Supply in an Air-Oil Lubrication System

6. Conclusion
Faster rotation of the rolling bearings for machine
tools can be accomplished by optimization of the
bearings internal components and the development of
the lubrication technology. In order to minimize the oil
supply rate to air-oil lubricated rolling bearings that
support the machine tool main spindles, this reportconcludes that the oil supply swash angle of the innerring is determined by the dmn value and thatminimizing oil wettability of tubes is effective to reduce
the fluctuation of oil delivery into bearings at the oil
nozzle.

References
1) F. Kosugi, NTN TECHNICAL REVIEW, No.71 (2003)
18-27 (in Japanese).
2) K. Fujii, M. Mori and Y. Ohta, Proc. JSPE Autumn
Conf, (2000) 449 (in Japanese).
3) K. Fujii, M. Mori and Y. Ohta, Proc. JSPE Spring Conf,(2001) 392 (in Japanese).
4) K. Fujii and M. Mori, Proc. JSPE Autumn Conf. (2001)
561 (in Japanese)

Photos of authors

Yoshinobu AKAMATSU

Masatsugu MORITechnical Research Dept.
Research & Development Center

Technical Research Dept.
Research & Development Center-19-

NTN TECHNICAL REVIEW No.72（2004）[ Technical Paper ]Development of Long Life Grease for High Speed Application---- “ME-1” Grease for Motor Bearings ----

Hidenobu MIKAMI*

Electric motors are used in many household and industrial applications. They are essential to ourdaily lives. Motors are expected to be quiet and highly efficient. The motor bearings, too, shouldhave low noise and low torque. For these reasons, Li soap/ester greases are widely used in motor
bearings. They exhibit low torque and excellent acoustic characteristics. Several years ago, greases
having urea thickener and poly-α-olefin (PAO) or ester oils were developed to extend the life of
motor bearings, and today they are seeing increasingly wide use. The market, on the other hand, still
demands even longer bearing life. A new grease needs to be developed to meet this demand.
To extend the life of a grease, it is necessary to prevent oxidation of the base oil. Thermaloxidation stability of the base oil and oxidation inhibiting additives play a large role in extending thelife of the grease, so it is necessary to use the proper inhibitor. In this paper, the effectiveness ofoxidation inhibitors suitable for PAO and ester base greases is discussed. The paper also introduces
NTN’s “ME-1” long life electric motor grease that was recently developed based on this knowledge.

1. Introduction
Electric motors are also used by machine tools fordriving main spindles and feeding devices. In theseapplications, demands for longer life, higher speed,
and higher precision are growing around greases.
To extend the life of grease, it is necessary to
prevent oxidation of the base oil. Thermal oxidation
stability2) and oxidation inhibiting additives in the baseoil play a large role in extending the life of the grease,so it is necessary to use the proper inhibitor suitable
for the base oil used. In this paper, the effectivenessof oxidation inhibitors suitable for PAO and ester basegreases is discussed. The paper also introduces
NTN's "ME-1" long life grease for electric motors thatwas recently developed based on this knowledge.

Electric motors are used in many households andindustrial applications. They are essential to our daily
lives. Motors are expected to be quiet and highly
efficient. The motor bearings, too, should have low
noise and low torque. For these reasons, Li
soap/ester greases are widely used in motor bearings.
They exhibit low torque and excellent acoustic
characteristics. Recently, greases having urea
thickener and poly-α-olefin (PAO) or ester oils weredeveloped to extend the life of motor bearings, andtoday they are seeing increasingly wide use1). The
market, on the other hand, still demands even longer
bearing life. A new grease needs to be developed tomeet this demand.

*Research & Development Center

Technical Research Dept.

-20-

Development of Long Life Grease for High Speed Application ---- “ME-1” Grease for Motor Bearings ----

2. Extending grease life
Greases are made of thickener, base oil (lubricant)and additives. When grease is used at hightemperatures for an extended period of time, its baseoil suffers form oxidation. To extend the life of grease,
it is necessary to control deterioration of the base oil.
Consequently, it is critical to select a suitable base oiland oxidation inhibitor.
Fig. 1 shows weight changes of lubricant exposed
to high temperature. When lubricant is exposed tohigh temperature, it loses weight as time elapses. As
soon as the time period needed for the lubricant tobegin oxidation (induction time), the weight rapidly
decreases. Oxidation of lubricant makes it volatileand quickly reduces its weight. Therefore, lubricantswithout antioxidants have a shorter induction period
and those with antioxidant have a longer induction
period. Even if antioxidant is added, less effective
antioxidants can result in a shorter induction period.

Antioxidants are divided into two types based on
their functionality; radical scavengers and peroxide
decomposers3). Effectiveness of antioxidants added
to mineral oils4), 5) has been reported, but many
unknown factors remain with respect to how effective
they may be when added to synthetic oils such as
PAO and ester oils. To verify their effectiveness, PAO
and ester oils of different dynamic viscosity wereselected and an antioxidant of 1 wt% was added.
Then they were exposed to high temperature formeasurement of any weight changes. For ester oils,
traditional polyolester and a special ester oil that has
excellent heat resistance were selected. Tables 1
and 2 show the selected lubricants and antioxidants.

Table 1 Test oils
PAO-A

PAO-B

Ester A

Ester B

Type of oil

PAO

PAO

Polyolester

Specialester

Dynamic 40˚Cviscosity
mm2/s
100˚C

31

46

33

72

6

8

6

10

Viscosity index

135

138

128

125

Percent weight loss wt%

10

Lubricant: Ester oil
Temperature: 120˚C

8
6

No antioxidant
added

Inductiontime

4

Antioxidant added

2

Table 2 Test antioxidants
0

0

200

400

Type

600

A0-1

Elapsed time h

A0-2

Fig.1 Oil loss at high temperature

A0-3
A0-4

-21-

Radical scavengers

Peroxide decomposers

Name of chemical compound
Alkyl diphenylamine
Hindered phenol
Zinc dithiophoshate
Thioether

NTN TECHNICAL REVIEW No.72（2004）

Figs. 2 ~ 5 show the measurements. Lubricantswithout antioxidants began oxidation within a shortperiod of time and the induction time for all the
specimens was several tens of hours. In contrast,
those with antioxidants reduced weight loss andprolonged the induction time. PAO oils (Figs. 2 and3) extended the induction period with zincdithiophoshate (AO-3) or thioether (AO-4) added tothem. However, alkyl diphenylamine (AO-1) andhindered phenol (AO-2) showed little effect. Unlike
the PAO oils, the ester oils (Figs. 4 and 5) exhibited

30

40

Lubricant: PAO-A
Temperature: 150˚C

No additive
AO-1 added
AO-2 added
AO-3 added
AO-4 added

Percent weight loss wt%

Percent weight loss wt%

40

benefit when alkyl diphenylamine (AO-1) or hinderedphenol (AO-2) was added.
In order to inhibit oxidation, an appropriate
antioxidant must be added to suit the type of lubricant
such as PAO and ester oils. Peroxide decomposers
were found to have a greater effect in inhibiting
oxidation of the PAO oils and radical scavengers forthe ester oils. When radical scavenger was added toester-B (special ester oil), the greatest improvement inoxidation deterioration resistance was observed.

20

10

0

No additive
AO-1 added
AO-2 added
AO-3 added
AO-4 added

30

20

10

0
0

200

400

600

800

1000

0

200

Elapsed time h

40

800

1000

40

Percent weight loss wt%

30

600

Fig.3 Percent weight loss of PAO-B at high temperature

Lubricant: Ester-A
Temperature: 150˚C

No additive
AO-1 added
AO-2 added
AO-3 added
AO-4 added

400

Elapsed time hFig.2 Percent weight loss of PAO-A at high temperature

Percent weight loss wt%

Lubricant: PAO-B
Temperature: 150˚C

20

10

0

Lubricant: Ester-B
Temperature: 150˚C

No additive
AO-1 added
AO-2 added
AO-3 added
AO-4 added

30

20

10

0
0

200

400

600

800

1000

0

Elapsed time h

200

400

600

800

1000

Elapsed time hFig.4 Percent weight loss of Ester-A at high temperature

Fig.5 Percent weight loss of Ester-B at high temperature-22-

Development of Long Life Grease for High Speed Application ---- “ME-1” Grease for Motor Bearings ----

3. Grease for motor bearings
3.1 Traditional greases

3.3 High temperature durability

Greases for motor bearings must be low-noise, lowtorque and long life performance. Consequently, Li
soap grease (commercially available Grease A) madeof low-viscosity ester oil as the base oil is widely used,
as shown in Table 3. For applications that require a
broader temperature range, high-speed and long life
urea grease (commercially available Grease B, MP-1)
made of poly-α-olefin (PAO oil) or ester oil as thebase oil is used.

A high temperature endurance test (per ASTM
D3336) was conducted on ME-1 grease. Fig. 6
shows the test results of commercially available
greases and ME-1 grease. Endurance of ME-1
grease is more than five times that of commerciallyavailable grease A and 1.5~2 times that ofcommercially available grease B, urea grease, andMP-1. ME-1 grease uses a special ester as the baseoil and an appropriate antioxidant to inhibit oxidation
for longer life.

3.2 Composition and physical properties of
ME-1 grease"ME-1" grease uses urea compound as a thickener
and a mixture of ester and PAO oils as the base oil. A
suitable antioxidant is added to inhibit oxidation
deterioration of the base oil, thus extending its life.
Table 4 shows the representative physical properties
of ME-1 grease.

Test conditions
Bearing: 6204 bearing
Temperature: 150˚C

Rotating speed: 10000rpm
Load: Fa=Fr=67N

8000

Table 3 Greases for motor bearings
Life (average) h

6000
Commercially
Commerciallyavailable grease A available grease B

MP-1

Thickener

Li soap

Urea

Urea

Base oil

Ester

PAO

PAO+Ester

Viscosity of base oil
mm2/s 40˚C

25〜30

46

40

Worked penetrateion
60W 25˚C

250

4000

2000

220

250
0

Table 4 Typical properties of ME-1 grease
ME-1
Thickener

Ester + PAO
mm2/s

40˚C

60

JIS K2220.23

Worked penetrateion 60W 25˚C

250

JIS K2220. 7

250 or higher

JIS K2220. 8

Evaporation loss % 99˚C, 22h

0.2

JIS K2220.10

Oil separation mass % 100˚C, 24h

0.0

JIS K2220.11

Oxidation stability kPa 99˚C, 100h30

JIS K2220.12

Low temperature torquemN・m −30˚C

300

Dropping point ˚CStarting
Rotating

MP-1

Fig.6 Endurance test results

Urea

Base oil
Viscosity of base oil

Test method

Commercially Commerciallyavailable
available
grease A
grease B

JIS K2220.18

57

-23-

ME-1

NTN TECHNICAL REVIEW No.72（2004）

3. 4 Acoustic characteristics

3. 5 Rotational torqueSince low noise is a requirement for motor bearing
greases, the rotational acoustics of the grease sealed
bearings were measured. Fig. 7 shows themeasurements. The acoustic value of ME-1 greasemeasured the same as that of commercially available
grease A and was better than that of commerciallyavailable grease B. Generally, urea grease is difficult
to micro-disperse a thickener, and for this reason, it isconsidered poorer than Li soap grease in terms of theacoustic characteristics. However, both ME-1 andMP-1 greases have thickener micro-dispersed in the
products, their acoustic characteristics are better thanthose of ordinary urea greases.

Smaller rotational torque is also a requirement for
motor bearings for reduction in energy consumption.
Fig. 8 compares the rotational torque of ME-1
grease to that of commercially available greases.
Though the rotational torque of ME-1 grease waspoorer than that of commercially available grease A,
the performance was equal to those of commerciallyavailable grease B and MP-1. The smaller theviscosity of the base oil, the smaller becomes therotational torque. ME-1 grease uses special ester ofhigher viscosity in its base oil, but it maintains low
rotational torque to be equal in its performance tocommercially available grease B and MP-1, both ofwhich use the base oil of lower viscosity. For thisreason, ME-1 possesses torque performance
sufficient for field applications.

Test conditions
Bearing: 608 bearing
Load: Fa=19.6N

Test conditions
Bearing: 6204 bearing
Load: Fa=39.6N

Rotating speed: 2200rpm

50

Rotational torque mN・m

Acoustic value dBA

40

30

20

10

0

Rotating speed: 8000rpm

Commercially Commerciallyavailable
available
grease A
grease B

MP-1

40

30

20

10

0

ME-1

Fig.7 Noise levels of various greases

Commercially Commerciallyavailable
available
grease A
grease B

MP-1

ME-1

Fig.8 Rotational torque of various greases-24-Development of Long Life Grease for High Speed Application ---- “ME-1” Grease for Motor Bearings ----

4. Conclusion
ME-1 grease has excellent high-temperature
endurance and acoustic characteristics and exhibits
sufficient rotational torque for field applications.
Motors are expected to become more compact, lightweight and efficient. Today's market will demandgreases for motor bearings to last longer and perform
at a higher level. We sincerely hope that "ME-1"
grease will help machine tool and other various
motors achieve an even higher level of performance.

References
1) Seiji Okamura, THE TRIBOLOGY 9 (2001) 47.
2) Yuji Ohnuki, Hiroshi Kimura, TRIBOLOGY Kaigi
Yokoshuu (Tokyo, 1998.5)
3) Yasukazu Ohkatsu, TRIBOLOGISTS 40.4 (1995) 60.
4) Jinichi Igarashi, The Nisseki review 32,3 (1990) 5.
5) Jinichi Igarashi, The Nisseki review 32,3 (1990) 17.

Photos of author

Hidenobu MIKAMITechnical Research Dept.
Research & Development Center-25-

NTN TECHNICAL REVIEW No.72（2004）[ New Product ]"ULTAGE" Series Precision Bearings for Machine Tools

Hiroshi TAKIUCHI*
Futoshi KOSUGI*To meet the market needs for faster, more efficient, and more precise machine tool main spindles,
NTN has created new bearing designs and recently reorganized our product line into “ULTAGE
Series” bearings.
The paper explains various bearing types and their applications in plain terms. The paper also
describes the configurations of “ULTAGE Series” bearings.

ULTAGE is a term created by NTN combining"ULTIMATE" and "STAGE" to represent NTN's pursuit
for the highest level of precision in machine tool
bearings.

1. Introduction
To meet the market needs for faster, more efficient,and more precise machine tool main spindles, NTN
has been involved with the development andimprovement of various bearings. Recently NTN
adopted an approach of "What are the bearing
specifications that are truly desired of precision
bearings" and reviewed the bearing designs of
traditional product series. As a result, the traditional
product line has been reorganized into "ULTAGE
Series" bearings.
In the last Technical Review, Issue No. 71, NTN
introduced the "ULTAGE Series" bearings. Since
then, NTN added four new types, changed the namesof some of those products, and reorganized the
product line for the customers to readily understandthe characteristics of different types. In this issue, wehave re-defined the configuration of the "ULTAGE
Series" bearings.
*Industrial Engineering Department

2. Development history of precision
bearings for machine tools
The applications for the machine tool precision
bearings are divided largely into those for main
spindles and those for feeding systems. For the main
spindle application, there are angular contact ballbearings (for radial loads and axial loads) andcylindrical roller bearings. For the feeding systemapplication, there are ball screw support bearings.
Table 1 shows the chronological development ofprecision bearings for machine tools. (Y-axis is thescale of high-speed/high functionality.) Also added isa brief history of the development of the abovementioned three types of bearings.

Industrial Sales Headquarters-26-

Cylindricalroller bearings"ULTAGE" Series Precision Bearings for Machine Tools

Double-row cylindrical roller bearing

Single-row cylindrical roller bearing
N10

Angular contact ball bearings

NN30xxHSRT6NN30xxHS
NN30・NN49・NNU49

N10xxHSRT6・N10xxHSLT6

N10xxHSFor ball screw support

2A-BSTxx-1BLXL・2A-BSTxx-1B

BSTxx-1E

BSTxx-1B

For axial load
562xx・742xx

HTAxxxU

HTA・HTB

BNSxxxLLB
7xxxLLB

For radial load, sealed

For radial load

HSC

HSF・HSFL

HSBxxxCAEX1
HSA
7xxx

HSE・HSLHSB

79xxU・70xxU ※Red font indicates the

ULTAGE series bearings.
1980

1990

1996

2000

2004

（Year）

Table 1 Development history of precision bearings for machine tools

2.1 Angular contact ball bearing
2.1.1 Angular contact ball bearing for radial load

2.1.2 Angular contact ball bearing for axial load
For a long time, duplex thrust angular contact ballbearings (562xxx, 742xxx types) have been the mainstream of angular contact ball bearings for axial load.
In the late 1980's, when applications began to demand
faster operation, NTN announced the HTA and the
HTB types of bearings that combined two single-row
angular contact ball bearings. Since then, the HTA
type has replaced almost all the traditional bearings
for this use in Japan. However, recent requirements
for even faster operation prompted NTN to review thespecifications of the HTA type bearing. A new
HTAxxxU type bearing was created to meet this need
and added to the "ULTAGE" Series.

The standard angular contact ball bearing (7xxx
type) is the base for the angular contact ball bearing
for radial load. In the middle of the 1980's, the rolling
elements were made smaller in diameter to produce
the HSA type, and in the late 1980's, the HSA typewas improved to reduce heat generation and the HSB
type was announced. In the early to mid 1990's as
demands increased for faster and more rigid machine
tool main spindles, NTN added to this product line, theHSBxxxCAEX1 type and the HSC type, which uses
even smaller rolling element diameter than the HSB
type.
In 2000 JIMTOF, NTN announced the HSE typewith improved wear and seizure resistance and a lownoise type (predecessor to the HSL type).
Again, in 2002 JIMTOF, NTN announced, in
addition to the HSE type bearings, sealed angularcontact ball bearings (BNSxxxLLB type, 7xxxLLBtype) for longer life and eco-conscious angular contact
ball bearings (HSL type) for low-noise, low air-oil
consumption purposes, both of which are called the"ULTAGE Series" bearings.
NTN has added yet another type bearing based on
the standard angular contact ball bearing. The
standard 7xxx type was improved in its speed andfunctionality to build the new 70xxU/79xxU types,
which were added to the "ULTAGE" Series.

2.2 Cylindrical roller bearing
Cylindrical roller bearings come in double-row
cylindrical roller bearings (NN30, NN49, NNU49 types)
and single-row cylindrical roller bearings (N10xxxHS type).

2.2.1 Double-row cylindrical roller bearing
The NN30 type has been the most popular doublerow cylindrical roller bearing. NTN has been engaged
in the development of the cage material and profile.
Some bearings employ resin cages (T2), but thestandard bearings use high-strength brass cages.
Adoption of resin cages is effective to extend thegrease life, and for this reason NTN developed a newresin cage (T6). The new resin cage has been
adopted in the new high-speed NN30xxHST6 typeand super high-speed NN30xxHSRT6 type, and they
are added to "ULTAGE" Series product line.
-27-

NTN TECHNICAL REVIEW No.72（2004）

Table 2 ULTAGE series
By optimizing the internal design and utilizing a new resin cage,
NTN has increased axial capacity and decreased heat generation.
Available bore sizes: 10~130 mm.
(Contact Angles: 15˚, 25˚ & 30˚) Also available with ceramic balls ("5S" prefix).

Standard
70U-type & 79U-type

The use of special materials and surface improvements have greatly
increased wear resistance and seizure resistance.
The optimization of internal specifications has resulted in a high-speed,(Contact Angles: 15˚, 20˚ & 25˚) high-rigidity and highly reliable bearing.
Also available with steel balls (no “5S” prefix).

High-Speed
5S-2LA-HSE-type

For Main Spindles

Angular Contact Bearings(Contact Angle: 25˚)
Eco-Conscious Model
High-Speed
5S-2LA-HSL-type(Contact Angles: 15˚, 20˚ & 25˚)
Ultra High-Speed
5S-2LA-HSFL-type(Contact Angle: 25˚)

In addition to the features of the HSE-type bearing, this bearing alsouses small ceramic balls to achieve greater speed due to less heatgeneration.

These bearings use special materials and surface improvements for greatly increased wear
resistance and seizure resistance. Based on the high-speed HSE-type and ultra high-speed
HSF-type bearings, this design has an eco-conscious nozzle that should be used exclusively
with air-oil lubrication. These bearings provide the same high-speed operation as HSE and
HSF bearings, but operate more quietly and reduce air and oil consumption.
Therefore, they contribute to a cleaner working environment and save energy.
HSL-type is also available with steel balls (no “5S” prefix).

Standard
For this design, non-contact seals are utilized on both sides, special
Non-Contact Seal Model grease is used and the internal design has been optimized to create a
70 LLB-type & 79 LLB-type pre-greased bearing that is resistant to heat generation.
(Contact Angles: 15˚ & 25˚)

Also available with ceramic balls ("5S" prefix).

This bearing utilizes special materials and surface improvements for
greatly improved wear resistance and seizure resistance. Various
improvements have also been made to the internal design and thenon-contact seals at both ends, and the utilization of special grease.
The result is a pre-greased bearing that provides longer bearing life.
(Contact Angles: 15˚, 20˚ & 25˚) Also available with steel balls (no “5S” prefix).

High-Speed
Non-Contact Seal Model
5S-2LA-BNS LLB-type

For Axial Loads
HTA U-type

For Main Spindles

For Ballscrew Supports(Contact Angles: 30˚ & 40˚)

Cylindrical Roller Bearings

The ULTAGE Series

Ultra High-Speed
5S-2LA-HSF-type

Open Model
2A-BST-type

Internal specifications have been optimized to create an angular contact
bearing for axial loads that offers the same level of rigidity as theconventional HTA-type, but with increased axial capacity and a higher
operating speed.
Also available with steel balls (no “5S” prefix).

This open-model bearing provides longer service life and improved
fretting wear resistance due to bearing ring surface improvements.

(Contact Angle: 60˚)Light Contact Model
2A-BST LXL-type(Contact Angle: 60˚)

Bearing raceway surface improvements and special grease greatly
increase service life and fretting wear resistance. This pre-greased
bearing also has a low-torque, light-contact seal that provides greater
dust resistance, longer grease life and improves bearing handling andinstallation characteristics.

Ultra High-Speed
N10HSRT6(K)-type

Optimization of the internal design and utilization of a special resin cagethat allows for high-speed operation have resulted in a high-speed
bearing that is resistant to heat generation.
This bearing provides better speed performance than the standard
N10HS single-row cylindrical roller bearing.

Eco-Conscious Model
Ultra High-Speed
N10HSLT6(K)-type

This bearing is based on the N10HSRT6-type and has an eco-conscious
nozzle that should be used only with air-oil lubrication. This bearing
provides the same high-speed performance as the N10HSRT6-type,but operates more quietly and reduces air and oil consumption.
Therefore, it contributes to a cleaner working environment and saves
energy.

High-Speed
NN30 HST6(K)-type
Ultra High-Speed
NN30 HSRT6(K)-type

Optimization of internal specifications and the use of a new resin cage
result in bearings that achieve high-speed operation with low levels ofheat generation.
Both grease lubrication and air-oil lubrication are acceptable.

-28-

"ULTAGE" Series Precision Bearings for Machine Tools

into two types, open (BSTxx-1B type) and sealed(BSTxx-1BLXL type).

2.2.2 Single-row cylindrical roller bearing
The standard single-row cylindrical roller bearing
used to be the N10 type bearing. When high speed
performance was demanded in the late 1980's to early
1990's, NTN developed a high-speed N10xxHS typewhich used smaller rolling element diameter than thatof the N10 type. The single-row cylindrical roller
bearing was once again challenged by the needs foreven faster speed, NTN announced in the 2002
JIMTOF tow new type bearings for the main spindle
rear application. They are the super high-speed
N10xxHSRT6 type that adopted resin cages and theeco-conscious N10xxHSLT6 type that reduced noise
and air-oil consumption.

4. About publication and revision of thePrecision Rolling Bearings catalog
To become a comprehensive catalog for the
precision rolling bearings, this catalog was renewed in
the fall of 2003 (catalog No. 2260/J) with addition ofthe "ULTAGE" series product line and review of someof the technical contents.
The new catalog not only changed the bearing
product configuration by adding the "ULTAGE" series,
but also it made the following two major changes.
1) The bearing specifications were reviewed,
changes made to the allowable rotating speeds,
and the speed coefficients were adopted.
2) The allowable axial loads for the angularcontact ball bearings were changed.
The catalog has been revised incorporating the
new four types of the "ULTAGE" series
bearings. Please see the catalog for the details.

2.3 Ball screw support bearing
Demands for high-speed with oil lubrication
increased for ball screw support bearings in the late1980's. NTN developed the BSTxx-1B type bearing
then, and later in the early 1990's, announced the
BSTxx-1E type which used a larger rolling element
diameter to extend life. Then, during the 2000 JIMTOF,
NTN announced the open type 2A-BSTxx-1B and thelight contact seal type 2A-BSTxx-1BLXL for higher
speed, longer life, and better resistance against fretting.

5. Conclusion
For the past 4~5 years, NTN has made an overall
improvement and introduced new models to theprecision bearings line-up for machine tools. The final
product is the "ULTAGE" series product line. We
believe that the "ULTAGE" series is the mostadvanced precision bearing series that provides
improved reliability and high functionality to themachine tool main spindles.
We know that users will demand more. NTN iscommitted to perform our duties as a supplier ofprecision bearings and serve our customers as their
good partner. We will continue to seek improvement
and undertake development projects to become "NTN,
the maker of precision bearings."

3. Configuration of ULTAGE Series
product line
As shown in the preceding page, the new ULTAGE
series bearing product line has added four new types(indicated in bold characters below) and includes
name changes of the HSE type products. In the end,
the new series now has 14 types of products, which
are shown in Table 2 on the preceding page.
The configuration of the "ULTAGE" series has beenreorganized for easy understanding, comprising three
types of angular contact ball bearings for radial loads,
standard (7xxxU type), high-speed (HSE type), andsuper high-speed (HSF type), two types of sealed
angular contact ball bearings, standard (7xxxLLB type)and high-speed (BNSxxxLLB type, and two types of
eco-conscious angular contact ball bearings, highspeed (HSL type) and super high-speed (HSFL type).
Angular contact ball bearings for axial loads areavailable in only 1 type, standard (HTAxxxU type).
There are four types of cylindrical roller bearings.
The double-row resin cage model comes in highspeed (NN30xxHST6 type) and super high-speed(nn30xxHSRT6 type) types, and the single-row resin
cage model comes in super high-speed(N10xxHSRT6 type) and the eco-conscious(N10xxHSLT6 type) types.
The ball screw support bearings are largely divided

Photos of authors-29-

Hiroshi TAKIUCHI

Futoshi KOSUGIIndustrial Engineering Dept.
Industrial Sales headquarters

Industrial Engineering Dept.
Industrial Sales headquarters

NTN TECHNICAL REVIEW No.72（2004）[ New Product ]ULTAGE Standard Angular Contact Ball Bearings,
79U/70U type

Keiichi UEDA*

Machine tool bearings require high speed capability, long life, and consideration of the operating
environment. To meet such needs, NTN has developed a new standard angular contact ball bearing,the 79U/70U type, which has superb performance and adds rich variation to our ULTAGE Series
product line. This paper describes the features of NTN’s standard angular contact ball bearings thathave created a new epoch in the areas of high-speed, load resistance, and lubrication performance.
These improvements make the 79U/70U ULTAGE Series worthy of the title "The New World
Standard."

contact ball bearings and cylindrical roller bearings
which have superb high-speed performance to besuitable for the machine tool main spindles. Morerecently, NTN has improved the functionality of thestandard angular contact ball bearing and added it tothe "ULTAGE" series product line as a new series inthe bearing family. (Photo 1)

1. Introduction
Machining centers and machine tools are becoming
faster, more efficient, and more precise. Today, themachining centers that are used primarily for moldfabrication have reached a dmn value of 3.5 million(dm: ball pitch circle diameter of bearing rolling
elements mm, n: rotating speed min-1) 1). In addition,
increased productivity (higher efficiency achieved
through reducing the non-processing time) is actively
sought by consolidating multiple machining processes,
including the cutting process, into a single machine.
Recently, environmentally friendly technologies havebecome commercially available, which include "drymachining" where no cutting oil is used and "semi-dry
machining" where an extremely small amount of
cutting oil is used in the processes. The key words inthis field are "High-speed", "Compound", and "Ecoconscious technology."
To meet such market trends, NTN created the"ULTAGE" series machine tool bearings andintroduced them at JIMTOF (Japan International
Machine Tools Fair) in 2002. Since then, NTN has
been expanding this series mainly by adding angular*Industrial Engineering Department

Photo 1 ULTAGE standard angular contact ball bearing
79U/70U type

Industrial Sales Headquarters-30-

ULTAGE Standard Angular Contact Ball Bearings, 79U/70U typeballs).
Figs. 1 and 2 compare rigidity in the current andnew designs. The new design has a slight decreasein rigidity, but maintains the rigidity at nearly the samelevel.

2. Optimization of internal design
2.1 Adoption of optimal design for high-speed,
high-rigidity performance

2.2 Improvement in load resistanceThe bearings for machine tool main spindles can be
subject to large axial loads during tool change whilemachine is stopped. If the load exceeds the allowable
limit of the bearing, it can cause dents or otherdamage to the raceways. Such characteristics thatindicate limitations to axial loads are called "allowable
axial loads." NTN defines these as follows:¡The end of contact ellipse on the raceway surface
reaches the shoulder of either an inner or outer ring(contact ellipse formed between the rolling element
and the raceway protrudes out of the raceway: Fig.
3).
¡The contact surface pressure on the raceway
surface reaches 3650MPa in either the inner orouter ring raceway.

Fa

H
2a

H ：Shoulder heightof bearinga ：Semi major ofcontact ellipse
Fa ：Axial load

Fig.3 Rolling elements protrude over the raceway shoulder

The differences of displacement at Fa=3kN : 4μm

The differences of displacement at Fa=5kN : 3μm

60

60

New design
50

Axial displacement μm

Axial displacement μm

Standard angular contact ball bearings are superior
to high-speed angular contact ball bearings utilizing
small diameter balls in terms of rigidity and load
capacity (a parameter that indicates resistance torolling fatigue life). On the other hand, applications ofthese bearings are limited by speed. The recent
design change was aimed at improving high-speed
performance, thereby expanding its application. NTN
reviewed the internal design and solved the conflicting
high-speed performance and high rigidity at the sametime (establishing the specifications that achieve highspeed performance while retaining rigidity of thecurrent bearing).
More specifically, impact of the designs of the rolling
elements and raceways on rigidity were assessed.
Effort was made to change the design of the areasthat had less impact on rigidity to improve high-speed
performance. However, changes to those areas thathave definite impact on rigidity were kept at aminimum. "ULTAGE" 79U/70U type standard angularcontact ball bearings utilized the above approach andaccomplished both high-speed and high rigidity.
Furthermore, this bearing uses a new resin cage,
described later, and attained the dmn value of 0.95
million with grease lubrication, about 1.5 times that of
the current bearing, and 1.5 million with air-oil
lubrication, about 1.8 times that of the current bearing(both bearings used a contact angle of 15˚ and steel

Current design

40
30
20
10
0

0

1

2

New design
50
40
30
20
10
0

3

Current design01【New design】7005UCDB【Traditional design】7005CDB
φ25mm×φ47mm×12mm×Two rows
Contact angle 15˚Bearing［

Preload after
mounting

2

3

4

Axial load kN

Axial load kN【New design】7020UCDB【Traditional design】7020CDB
φ100mm×φ150mm×24mm×Two rows
Contact angle 15˚Bearing］
Preload after
mounting

200 N (Fixed position preloading)Fig.1 Axial rigidity (7005CDB)

880 N (Fixed position preloading)Fig.2 Axial rigidity (7020CDB)

-31-

5

NTN TECHNICAL REVIEW No.72（2004）

height and attained allowable axial load far greaterthan (about 2.5 times) that of the current design.
(Figs. 4 and 5)

12

120

10

100

Allowable axial load kN

Allowable axial load kN

With ULTAGE 79U/70U type standard angularcontact ball bearings, optimization of the internal
design (described earlier) took these characteristics,
into consideration. The review included the shoulder

8
6
4
2
0

New design

80
60
40
20
0

Current design

New design

Current design【New design】7005UC
Bearing 【Current design】7005C［φ25mm×φ47mm×12mm，Contact angle 15°］

【New design】7020UC
Bearing 【Current design】7020C［φ100mm×φ150mm×24mm，Contact angle 15˚］

Fig.4 Allowable axial load (7005CDB)

Fig.5 Allowable axial load (7020CDB)

2.3 Optimal design for lubrication

In addition, slits were created at four corners of theball pockets (where rolling elements are located) tosecure oil passage from the inner to the outer ring(flow of lubricant created by centrifugal force).
These measures will prevent stirring resistancecaused by stagnant lubricant.

In air-oil lubrication, oil delivery into the bearing isimportant, as is the discharge of used oil (exhaust of
air-oil). Unless there is a smooth discharge of usedoil, lubricant can collect inside the bearing, which
increases stirring resistance and results intemperature rise or seizure 1). Consequently, attention
must be paid not only to optimization of the rolling
elements, raceway profile, or other internal design but
also to the above issue in order to establish stabletemperatures under high-speed operation.
In grease lubrication, deterioration of grease by heatand lubrication life are critical. For this reason,internal structures having excellent grease retention
have an advantage.
ULTAGE 79U/70U type standard angular contact
ball bearings, have adopted designs that increase
reliability of lubrication both in air-oil and greaselubrication. The following sections describe the key
designs.

<Tapered profile of bore>[Air-oil lubrication] Easy to deliver and discharge lubricant.
[Grease lubrication] Easy to enclose grease inside bearing.

Fig.6(a) New resin cage (section of the ball pocket)

2.3.1 Molded cage with new profile(Polyamide resin: Figs. 6 (a) and (b))(a) Characteristics in air-oil lubrication<Slits at four corners ofball pocket>[Air-oil lubrication]
Easy to discharge usedlubricant.

To ease delivery of lubricant to the rolling element
and the raceway contact area (aiming) and to promote
the discharge of used oil outside the bearing(purging), the bore of the cage was tapered and thespaces for lubricant delivery and discharge inside thebearing were expanded.

[Grease lubrication]
Improves grease retention.

Fig.6(b) New resin cage (appearance)-32-

ULTAGE Standard Angular Contact Ball Bearings, 79U/70U typeFig. 8 compares the current cage to the new profile
cage in a high-speed operation test (greaselubrication). The graph shows equal or less bearing
temperature rise with the new profile cage than withthe current cage.

Fig. 7 compares the current cage to the new profile
cage in a high-speed operation test (air-oil lubrication).
The graph shows less bearing temperature rise with
the new profile cage than with the current cage.

(b) Characteristics in grease lubrication
ULTAGE 79U/70U type standard angular contact
ball bearings, with applications reaching the dmn valueof 1.05 million use this new cage profile as standard,
whereas those for high-speed application (air-oil
lubrication) beyond this speed use phenol machined
cages that have been field proven in many super highspeed applications.

In grease lubrication, the slits at the pockets
function to retain grease. Adoption of the tapered
profile increases the bearing space (increases
opening at bearing width), which makes it easy to
enclose grease inside the bearing. Thus, this profile
improved lubrication reliability and simplified grease
handling.

20

25

Outer ring temperature rise ˚C

Outer ring temperature rise ˚C

30

New profile cage
Current cage

20
15
10
5
0

New profile cage
15

Current cage
10

5

0
0

5000

10000

15000

20000

0

5000

0

0.25

0.50

0.75

1.00

0

1.25

0.25

15000

20000

0.50

0.75

1.00

1.25

dmn value ×106

dmn value ×106
【Test conditions】【Test conditions】
Bearing

10000

Speed min−1

Speed min−1

7010UCDB
φ50mm×φ80mm×16mm×Two rows
Contact angle 15˚［Bearing］

Air-oil lubrication
40 NL/min
0.03 mL/5 min/shot
ISO VG32

Jacket cooling

None

［

Preload after mounting 200 N (Fixed position preloading)

Preload after mounting 200 N (Fixed position preloading)
Lubrication method
Air supply rate
Oil supply rate
Oil type

7010UCDB
φ50mm×φ80mm×16mm×Two rows
Contact angle 15˚

Fig.7 High-speed test (air-oil lubrication)

Lubrication method
Brand name

Grease lubrication
ISOFLEX NBU15

Jacket cooling

None

Fig.8 High-speed test (grease lubrication)

-33-

］

NTN TECHNICAL REVIEW No.72（2004）

2.3.2 Changes in inner ring profile (Fig. 9)
The lubricant delivery space was expanded bydesigning the outer diameter of the inner ring of theback side, or non-load side, lower than that of thecurrent bearing and combining it with the above
mentioned new cage profile. This structure allows foreasier oil delivery to the cage-inner ring gap suitablefor high-speed operation (with excellent oil delivery in
air-oil lubrication) and allows oil delivery (aim of
nozzle) at an angle. This reduces the dimensional
restriction on the spacer with respect to the nozzle
and improves freedom of design for the bearing
peripheral structure.

The lubricant delivery space was
expanded by designing theouter diameter of the inner ring of thebackside or the non-load side lower thanthat of the current bearing andcombining it with the above-mentioned
new cage profile.

Inner ring outer diameterof the backside

Fig.9 Section of 79U/70U type

Like grease lubrication, air-oil lubrication evaluation(Fig. 11) also found lower temperature rise in the newseries with or without jacket cooling than that of thecurrent bearing. The current bearing showed unstabletemperature rise at around a dmn value of 1.05 million(indicated by ○＊, △＊), while the new series showedstable temperature rise at a dmn value of 1.5 million.

4. High-speed operation test results
The test results (grease lubrication) of ULTAGE
79U/70U type standard angular contact ball bearings,
are shown in Fig. 10. The temperature rise with the
new series is lower than that of the current bearings
with or without jacket cooling, and the speed reacheda dmn value of 0.95 million.

Outer ring temperature rise ˚C

Outer ring temperature rise ˚CULTAGE (new polyamide resin cage)
Current bearing (polyamide resin cage)20
15
No jacket cooling10
Jacket cooling

5
0

0

2000

4000

6000

8000

10000 12000 14000 16000

ULTAGE (phenol resin cage)ULTAGE (new polyamide resin cage)
Current bearing (polyamide resin cage)
50
40
30

＊

20

0

0.25

0.50

dmn value

0

2000

4000

6000

8000

10000

12000 14000

Speed min−1
0.75

0.50

0

1.00

1.00

1.50

dmn value ×106

×106
【Test conditions】【Test conditions】

Bearing

Jacket cooling＊10

Speed min−10

No jacket cooling【ULTAGE】7010UCDB【Current bearing】 7010CDB
φ50mm×φ80mm×16mm×Two rows
Contact angle 15˚［Bearing］【ULTAGE】7020UCDB【Current bearing】 7020CDB
φ100mm×φ150mm×24mm×Two rows
Contact angle 15˚［Preload after mounting 200 N (Fixed position preloading)

Preload after mounting 150 N (Fixed position preloading)

Lubrication method
Brand name

Grease lubrication
INSOFLEX NBU15

Jacket cooling

Provided, Not provided

Lubrication method
Air supply rate
Oil supply rate
Oil type

Air-oil lubrication
40 NL/min
0.03 mL/5 min/shot
ISO VG32

Jacket cooling

Provided, Not provided

Fig.10 High-speed test (grease lubrication)

Fig.11 High-speed test (air-oil lubrication)

-34-

］

ULTAGE Standard Angular Contact Ball Bearings, 79U/70U type

5. Series

6. Conclusion

ULTAGE 79U/70U type standard angular contact
ball bearings, series configuration is shown in Table 1.
The bearing models are either 0 series or 9 series,and each series has three different contact angles(15˚, 25˚, 30˚). The bearing sizes (bore diameter) for
these series are φ10mm ~ φ130mm (common to allseries). In addition to the current steel balls, thematerials for rolling elements include ceramic balls toprovide ample variation.

ULTAGE 79U/70U type standard angular contact
ball bearings, are products suitable for machine toolsthat are used under more and more severe
environments. NTN will continue to improve anddevelop machine tool bearings under the fundamental
philosophy of "people and environment-friendly
bearing technology" and "more advanced functionality
and ease of use."

Table 1 79U/70U Series

Reference
1) Keiichi Ueda, THE TRIBOLOGY No.188 (2003), 19 (inJapanese)

Contact angle 15˚ 79xxUC/70xxUC
Steel ball series Contact angle 25˚ 79xxUAD/70xxUAD
Contact angle 30˚ 79xxU/70xxU
Contact angle 15˚ 5S-79xxUC/5S-70xxUC
Ceramic ballseriesContact angle 25˚ 5S-79xxUAD/5S-70xxUAD
Contact angle 30˚ 5S-79xxU/5S-70xxU

Available bearing size (shaft diameter):
φ10mm ~ φ130mm (common to all series)

Photos of author

Keiichi UEDA
Industrial Engineering Dept.
Industrial Sales headquarters-35-

NTN TECHNICAL REVIEW No.72（2004）[ New Product ]HTA U Type ULTAGE Angular Contact Ball Bearings
for Axial Loads

Yasuyuki HIROTA*

A new angular contact ball bearing has been developed for machine tool applications, particularly
lathes, that require a high-speed main spindle with high rigidity and load resistance. This paper
introduces the HTA U-type angular contact ball bearing for axial loads, which offers high-speed
performance while maintaining the rigidity and load resistance of current bearings.

1. Introduction

2. Structure of lathe main spindle andbearing

Angular contact ball bearings for axial loads areused for machine tool main spindles, particularly forlathes, that require high rigidity and load resistance.
These bearings are primarily used at low and medium
rotating speeds. In recent years, however, there has
been a growing need for more efficient and precise
machining centers and other machine tools. As
machining processes are improved to achieve greater
efficiency. High-rigidity main spindles for lathes, etc.
Will likely be required to operate at higher speeds. To
answer the needs for high-speed operation, NTN
improved the high-speed performance of the current
HTA type angular contact ball bearing for axial loads
and developed the HTA U type bearing. This paper
describes the features of this product along with theevaluation results.

*Industrial Engineering Department

Fig. 1 shows an example of a lathe main spindle.
An angular contact ball bearing for axial loads is used
together with a double-row cylindrical roller bearing.
The former is to bear axial loads and the latter radialloads. To accomplish this division of load bearing, thebearing OD of the HTA type angular contact ballbearing for axial loads was made smaller than that ofthe double-row cylindrical roller bearing. This willcause the HTA type angular contact ball bearing foraxial loads to receive axial loads only. Table 1 shows
the tolerances for the bearing OD. The HTA type
bearings offer two contact angles, 30˚ and 40˚.
Bearings of 40˚ contact angle are typically used whenaxial rigidity of main spindles is critical. Bearings of
30˚ contact angle are used when low temperature riseduring operation is important.

Industrial Sales Headquarters-36-

HTA U Type ULTAGE Angular Contact Ball Bearings for Axial Loads

Front

Rear

Angular contact ballbearing for axial loads

Double-row cylindricalroller bearing

Cylindricalroller bearing

Fig.1 Structure of main spindle for turning machine

Table 1 Outside diameter deviation
Nominal bearing
outer diameter
mm
Over

Incl.

30
50
80
120
150
180
250
315
400

50
80
120
150
180
250
315
400
500

Outer diameter tolerances
μm Cylindrical
HTA typeroller bearing
P4 L accuracy
P4 accuracy
Upper limit Lower limit Upper limit Lower limit
-25
-30
-36
-43
-43
-50
-56
-62
-68

-36
-43
-51
-61
-61
-70
-79
-87
-95

0
0
0
0
0
0
0
0
-

-6
-7
-8
-9
-10
-11
-13
-15
-

Photo 1 HTA U type

3. Features
3.1 Allowable rotating speed
The HTA U type (Photo 1) is different from thecurrent bearings in the following areas.

Flow oflubricating oil¡Internal design (Fig. 2) that limits temperature riseat high-speed range¡Inner/outer ring profiles that improve oil discharge inoil lubrication¡Polyamide resin molded cage with its rolling
element contact profile that improves lubrication
efficiency in grease and air-oil lubrication

Space increased

Fig.2 Flow of lubricating oil

Table 2 Allowable dmn value

The design specifications described above madethe bearing run at high-speed and limit temperaturerise, thus reaching the allowable dmn value shown in
Table 2. The operation test results are explained in
Section 4.
However, the dmn value of 1.25 million (30˚ contact
angle, air-oil lubrication) in the table is of the phenol
machined cage specifications. The allowable dmn
value for the HTA U type standard polyamide molded
cage specifications is up to 1.05 million.

Grease lubrication

Air-oil lubrication

Contact angle 30˚ 100×104 over the current type 60％UP 125×104 over the current type 25％UP
Contact angle 40゜ 75×104 over the current type 30％UP 95×104 over the current type 10％UP-37-

NTN TECHNICAL REVIEW No.72（2004）

3.2 Axial rigidity

3.3 Load resistance (allowable axial load)

The internal design of the HTA U type was made to
improve high-speed performance at the expense of
axial rigidity. However, as shown in Fig. 3, thedifference in axial deflection between the HTA U andthe current HTA types was 1.5μm or smaller (at 30˚contact angle) when axial load of 5kN was applied,
hence the axial rigidity of the HTA U type can becalled almost equal to that of the current HTA type.

The allowable axial load for bearings is as importanta characteristic as axial rigidity. Fig. 4 compares thecurrent HTA type to the HTA U type in axial rigidity.
As you can see, by reviewing the internal design, theallowable axial load for the HTA U type was about 1.3
times that of the current HTA type at the contact angleof 30˚ and about 1.2 times at 40˚.

【Test conditions】
HTA020UADB，HTA020ADB
HTA020UDB，HTA020DB(φ100×φ150×22.5×double-row)[α:30˚] 880N
[α:40˚] 1470N

Bearing
Preloadafter mounting【Test conditions】
Bearing

HTA020UADB，HTA020ADB
HTA020UDB，HTA020DB(φ100×φ150×22.5×double-row)

HTA U type
HTA type

Allowable axial load kN

Axial deflection μm

15

10

α：30˚
5
α：40˚

100
80

HTA U type
71.3

60

HTA type
53.5

40

31.9

25.7

20
0

0
0

1

2

3

4

α＝30°

5

α＝40°

Axial load kN

Fig.3 Displacement in axial direction

Fig.4 Allowable axial load

4.1 Structure of test machine

4. Operation test results

Fig. 5 shows the structure of the test machine. To
simulate actual service conditions, the test bearing is
structured not to receive radial loads.

The structure of the test machine used in theoperation test and the test results are shown below.

Test bearing
No axial load is applied.
Preload only is applied.

Support bearing

Fig.5 Test rig for measuring temperature rise-38-

HTA U Type ULTAGE Angular Contact Ball Bearings for Axial Loads

4.2 Test results
All the test bearings showed steady temperaturerise up to the allowable dmn values as shown in Table
2.

Figs. 6, 7, 8, and 9 show the operation test resultsof the bearings with grease lubrication and air-oil
lubrication.

【Test conditions】【Test conditions】

HTA020UADBHTA020ADB(φ100×φ150×22.5×double-row)
Rotating speed〜10000min-1
Preload after mounting 880N
Method of lubrication Air-oil lubrication
0.03mL/shot
Oil supply rate
Oil shot interval 5min.
Air supply rate
40NL/minJacket cooling
Provided, Not provided
Bearing [α:30˚]

40
35

Outer ring temperature rise ˚C

Outer ring temperature rise ˚C

HTA020UADB
Bearing [α:30˚] HTA020ADB(φ100×φ150×22.5×double-row)
Rotating speed〜8000min-1
Preload after mounting 880N
Method of lubrication Grease lubrication
Grease used
MP-1
Jacket cooling
Provided, Not provided

HTA020UA without cooling
HTA020UA with cooling30

HTA020A without cooling
25

HTA020A with cooling

20
15
10
5
0
0

2000

4000

6000

Rotating speed min
0

25

50

40
35

HTA020UA without cooling30

HTA020UA with cooling
HTA020A without cooling

25

HTA020A with cooling

20
15
10
5
0
0

8000

2500

-1

75

100

0

25

50

dmn value ×104

Outer ring temperature rise ˚C

Outer ring temperature rise ˚C125

HTA020UDBHTA020DB(φ100×φ150×22.5×double-row)
Rotating speed〜7500min-1
Preload after mounting 1470N
Method of lubrication Air-oil lubrication
0.03mL/shot
Oil supply rate
Oil shot interval 5min.
Air supply rate
40NL/minJacket cooling
Provided, Not provided

HTA020U without cooling
HTA020U with cooling
HTA020 without cooling
HTA020 with cooling

15
10
5

2000

4000

30
HTA020U without cooling
25

HTA020 without cooling
20

HTA020 with cooling

15
10
5
0

6000

HTA020U with cooling

0

25

50

2500

5000

7500

Rotating speed min-1

Rotating speed min-1
0100

Bearing [α:40˚]

30

0

75

【Test conditions】

HTA020UDB
Bearing [α:40˚] HTA020DB(φ100×φ150×22.5×double-row)
Rotating speed〜6000min-1
Preload after mounting 1470N
Method of lubrication Grease lubrication
Grease used
MP-1
Jacket cooling
Provided, Not provided

0

10000

Fig.7 Results of high-speed test (air-oil lubrication)(α:30˚)【Test conditions】

20

7500

dmn value ×104

Fig.6 Results of high-speed test (grease lubrication)(α:30˚)

25

5000

Rotating speed min-1

0

75

25

50

75

dmn value ×104

dmn value ×104

Fig.8 Results of high-speed test (grease lubrication)(α:40˚)Fig.9 Results of high-speed test (air-oil lubrication)(α:40˚)-39-

NTN TECHNICAL REVIEW No.72（2004）

5. Standard series

6. Conclusion

The standard specifications of the HTA U type axial
load angular contact ball bearing for machine tool
main spindles include, like those of the current HTA
type, 30˚ and 40˚ contact angles and the 0-series and9-series diameters. The compatible bearing size for
the 0-series is φ50 ~ φ320, and that for the 9-seriesis φ100 ~ φ320. The rolling elements can be either
steel or ceramic balls.

The HTA U type bearings have achieved highallowable dmn values, 60% over the current type
bearings with grease lubrication and 25% over thesame with air-oil lubrication, while maintaining rigidity
and load resistance equal to those of the current
series. This bearing will meet the needs of highspeed and high-rigidity main spindles that are
expected to increase in the near future. NTN willcontinue to develop bearings of longer life andimproved handling.

Photos of author

Yasuyuki HIROTA
Industrial Engineering Dept.
Industrial Sales Headquarters-40-

NTN TECHNICAL REVIEW No.72（2004）[ New Product ]High Speed and Long Life
Double-Row Cylindrical Roller Bearings

Naota YAMAMOTO*
Mamoru MIZUTANI*NTN has developed a new high-speed and long-life double-row cylindrical roller bearing(NN30xxHSRT6 type) for the ULTAGE Series product line. High-speed is accomplished by
optimization of the internal design and adoption of a light-weight and high-strength PEEK® resin
retainer. Furthermore, the addition of grease reservoirs inside the retainer pockets extends the
lubrication life. This paper outlines the development process.

1. Introduction

2. Features

Double-row cylindrical roller bearings are widelyused in lathes, machining centers, and other machine
tool main spindles that require high rigidity and highaccuracy. Nowadays, these machines are increasing
their functionality and efficiency. To support theadvanced features of modern machinery, the main
spindle bearings must run faster and last longer.
Lubrication of the main spindle bearings is
primarily air-oil lubrication and grease lubrication.
Conventional double-row roller bearings can run up toa dmn value of 1.2 million under air-oil lubrication and
0.8 million under grease lubrication. Therefore, ifhigher speed was needed, either angular contact ballbearings or single-row cylindrical roller bearings had to
be used.
The ULTAGE Series double-row cylindrical roller
bearing offers dramatically faster and longer life
performance in both types of lubrication thanconventional bearings.

The development model focused on optimization of
the internal design. The retainer especially, went
through a comprehensive review. Fig. 1 shows theconfiguration of the development model and theconventional model.

*Industrial Engineering Department

Conventional model

Development modelFig.1 Conventional design and new design(HSRT6 Type)

Industrial Sales Headquarters-42-

High Speed and Long Life Double-Row Cylindrical Roller Bearings

deformation was noticed. To accommodate superhigh-speed operation, this ratio should be 30% or
greater. Furthermore, off-sets were installed at thecolumn ends that could make abnormal contact withthe rolling elements, especially during deformation.
These off-sets also serve as grease pockets.

1) Light weight
The retainer material was changed from theconventional high-strength cast brass to PEEK resin(polyether ether ketone). This material change
reduced the retainer weight to below 1/4 theconventional retainer weight and also
successfully limited heat generation andwear that results from contact between theretainer and the rolling elements during highspeed operation. PEEK is a highly rigid resin
and is excellent in heat, wear, and hydrolysis
resistance.

VALUE OPTION: ACTUAL
2.23D-01
1.98D-01
1.73D-01
1.48D-01

2) Super high-speed design for air-oil
lubrication

1.23D-01

To optimize the retainer profile, a FEM
analysis was conducted to determine
retainer deformation by centrifugal force insuper high-speed operation. (Fig. 2 shows
the analysis example.)
Fig. 3 is the analysis result of the ring
thickness. This dimension particularly affects
deformation. In the area where the ratio of
the ring thickness to the length of the rolling
element was below 30%, significant

9.77D-02
7.26D-02
4.75D-02
2.24D-02-2.67D-03-2.78D-02

Fig.2 FEM analysis of deformation（φ100×150×37）
Bearing size : NN3020K
15％

Ratio of retainer deformation to rolling
element diameter in direction of bearing radius (%)

20,000 min-1（dmn value of 2.5 million）
17,000 min-1（dmn value of 2 million）

13％

14,000 min-1（dmn value of 1.75 million）
12,000 min-1（dmn value of 1.5 million）

Deformation

Rotating speed of inner ringδ

Ringthickness

10,000 min-1（dmn value of 1.25 million）

10％

Broken line: Before deformation
Solid line: After deformation
8％

5％

3％

0％
15％

Conventional design
20％

25％

Development design HSRT6 types
30％

35％

40％

45％

Ratio of ring thickness to rolling element length (%)

Fig.3 Relation between ring thickness and deformation at super high-speed (FEM analysis of deformation)-43-

NTN TECHNICAL REVIEW No.72（2004）

3) Long-life design for grease lubrication

3. Test results in air-oil lubrication

Fig. 4 shows the outline of the new retainer
structure. As shown, grease pockets are created on
the retainer columns to improve grease retention for
longer grease life.

Fig. 5 shows the test results of this retainer in air-oil
lubrication. The conventional retainer (high-strength
cast brass retainer) showed a sudden temperaturerise at 13000min-1, but the development model (PEEK
retainer) was able to withstand high-speed operation
up to 15000min-1 (dmn value of 1.9 million).

Grease pockets

4. Test results in grease lubrication
Fig. 6 shows the test results of this retainer ingrease lubrication. A comparison of the conventional
retainer (PA66 retainer) to the development model(PEEK retainer) showed temperature rise of the outerring at 7000min-1 and a distinctive difference between
the two was observed at 8000min-1 (dmn value of 1
million). In the endurance test, the conventional
retainer developed high vibration after 1500 hours andthe test had to be terminated. Though the bearing
didn't sustain any major damage, the rolling elements
and the retainer columns had little lubrication.

Outer ring temperature rise ˚C

Fig.4 Drawing of the retainer

30

Development model
HSRT6 type (PEEK retainer)

25

Conventional model
High-strength brass retainer

20<Test conditions>[Development model HSRT6 type (PEEK retainer)][Coventional model High-strength brass retainer]¡Bearing size
: NN3020K(φ100×φ150×37)¡Rotating speed : 〜15,000min-1

15
10
5
0
4,000

6,000

8,000

10,00012,00014,00016,000175

200

Rotating speed min−1
50

75

100

125

150

(dmn = 1.9 million)¡Clearance after mounting : 0μm¡Oil supply rate : 0.02mL/20min（VG32）¡Air supply rate : 30NL/min¡Jacket cooling : Provided

dmn value ×104

Outer ring temperature rise ˚C

Fig.5 Temperature rise test results (air-oil lubrication)

12

Development model
HSRT6 type (PEEK retainer)
Conventional modelResin retainer (PA66)

10
8
6
4
2
0
2,000

3,000

4,000

5,000

6,000

7,000

8,000

88

100

Rotating speed min−1
25

38

50

63

dmn value ×10

75<Test conditions>[Development model HSRT6 type (PEEK retainer)][Conventional model Resin retainer (PA66)]¡Bearing size
: NN3020K(φ100×φ150×37）¡ Rotating speed : 〜8,000min-1(dmn = 1 million)¡Clearance after mounting : −5μm
¡Grease: MP-1¡Jacket cooling : Provided

4

Fig.6 Temperature rise test results (grease lubrication)-44-

High Speed and Long Life Double-Row Cylindrical Roller Bearings

Consequently, it was determined that the bearing hadreached its limit of operation.
On the other hand, the development model retained
grease in the retainer's grease pockets after 2000
hours of operation, and was able to continue operation(Photo 1).
To confirm the endurance performance of thedevelopment bearing, an endurance test is currently
on-going at 8000min-1 (dmn value of 1 million) (Fig. 7).

5. Bearing composition
The bearing specifications developed in this project
should apply to NN3013 ~ NN3026 (Inner Diameter φ
65 ~ φ130).

6. Conclusion
The ULTAGE series double-row cylindrical roller
bearing, bearing type NN30xx HSRT6, runs at highspeeds and lasts longer under air-oil and greaselubrication than conventional double-row cylindricalbearings. This bearing should contribute to increasing
functionality and efficiency needs of the lathe main
spindles. It will support the development of highspeed and high-rigidity main spindles that will see a
growing need in today's market.

Grease pockets
Photo 1 Grease remaining in the reservoirs
after 2000(h) running test

No.1

On-going test

On-going test

No.2

Conventional
retainer

Development model HSRT6 type (PEEK retainer)
Conventional model Resin retainer (PA66)

0

2,000

4,000

6,000

8,000

Hours of continuous operation h

Fig.7 Endurance test results

Photos of authors

Naota YAMAMOTO

Mamoru MIZUTANIIndustrial Engineering Dept.
Industrial Sales Headquarters

Industrial Engineering Dept.
Industrial Sales Headquarters-45-<Test conditions>[Development model HSRT6 type (PEEK retainer)][Conventional model Resin retainer (PA66)]¡Bearing size
: NN3020K(φ100×φ150×37)¡Rotating speed : 8,000min-1(dmn = 1 million)¡Clearance after mounting : −5μm
¡Grease: MP-1¡Jacket cooling : Provided

NTN TECHNICAL REVIEW No.72（2004）[ New Product ]Next Generation Deep Groove Ball Bearingfor High-Speed Servomotor

Chikara KATAGIRI*
Kenichiro NAITO*

In the past, servomotors have been developed by focusing mainly on machine tool and robot use. Recently, servomotors
are being more widely used in general industrial apparatus and in manufacturing equipment for products such as
semiconductors or electronic parts.
Due of the growth of these applications in recent years, demand has increased for high-speed servomotors with highaccuracy. High-speed angular contact ball bearings are typically used in these servomotors. Since they are easier to handle
than angular contact ball bearings, the demand for deep groove ball bearings has also increased for high-speed applications.
Because of this increased demand, NTN has developed the “Next Generation Deep Groove Ball Bearing for High-speed
Servomotor” that can be used in applications with dmn ≤ 950,000 (dm = ball pitch circle diameter, n = speed). In the past, only
angular contact ball bearings could be used at this high dmn value. This new bearing also has a longer life than the previous
deep groove ball bearing used in a servomotor application.
This report introduces the characteristics, including several test results, of NTN’s new design named the “Next Generation
Deep Groove Ball Bearing for High-speed Servomotor.”

1. Introduction

2. Bearing structure

In recent years, machine tools are becoming faster,
more efficient, more accurate, and more compact.
Accordingly, there is a great demand for faster, morerigid, longer-lasting, higher output, and smaller mainspindle servomotors. To meet the engineering needsof such servomotors, bearings must run faster or
achieve high dmn values. Today, a low-cost andeasy-handling deep groove ball bearing is desired tosatisfy those needs in the operation range traditionallyand primarily served by angular contact ball bearings.
Having realized the situations, NTN developed a"next generation deep groove ball bearing for highspeed servomotors" that provides "high-speed" and"long life."
This paper outlines this new bearing.

Sealed and greased deep groove ball bearings are
typically used for motors. Among those bearings,
some are used for high-speed motors. Because thecage of such a bearing is subject to large centrifugal
force, the bearings for high-speed motors typically usepressed steel cage that is made of highly rigid metallic
sheet.
The pressed steel cage can produce relatively large
collision noise (cage noise) upon impact with the balls.
For this reason, more synthetic-resin cages areused now. They are easy to manufacture, lightweight, and self-lubricating. The synthetic-resincrown-type cage, however, is subject to deformation
due to centrifugal force at high-speed ranges. The
deformation causes interference between the cage

*Industrial Engineering Department

Industrial Sales Headquarters-46-

Next Generation Deep Groove Ball Bearing for High-Speed Servomotor

and other parts, resulting in temperature rise of the
bearing and increase in the rotational torque.
The deep groove ball bearing developed this time
has resolved the above problems with the syntheticresin crown-type cages and improved its "high-speed"
performance. This new bearing also improved
endurance of the grease, thereby "extending bearing
life."

The configuration of the new cage assumed thestructure of a traditional pressed steel cage andcombined two resin-molded wave-type rings ofidentical shapes (see Photo 2).
Compared to the traditional crown-type cage, the
new cage does not have openings, which improves
rigidity of the pockets and dramatically reduces
deformation by centrifugal force in high-speed
operation.

(1) New cage
[Features of new cage]
1 High-speed
2 Improved lubrication
3 Improved productivity designing
1 High-speed
The traditional synthetic-resin crown-type cage (seePhoto 1) is open to one side. This profile causes theprongs on the open side to widen due to centrifugal
force in high-speed operation (see Fig. 1). This
deformation leads to interference with the balls andother components, which in turn contributes to the risein the bearing temperatures and the rotational torque.

Photo.2 New synthetic-resin cage design[FEM analysis of new cage deformation by
centrifugal force]
Table 1 shows the analysis conditions.
Table 1 Condition of FEM analysis
Bearing6308-1

Inner ring rotating speed, min

15,000 (dn value=0.6 million, dmn value=0.95 million)

Ambient temperature, ˚C 60
Material

PA66＋GF25%

Density
1.32
Modulus of vertical elasticity
2200
of material, MPa
Modulus of linear expansion 7×10-5
of material, /k*d: Bearing ID mm, dm: Rolling element pitch circle diameter mm,
n: Rotating speed min-1

Photo.1 Synthetic-resin crown-type cage

Outer
ring
side

[FEM analysis results]
The analysis was made to the pocket and a single
joint. The calculations assumed that the cage wassubject to centrifugal force only in high-speed
operations.
Fig. 2 shows the analysis results of the cage
deformation. Expansion was observed generally inthe radial direction. The largest deformation was
found at the joints due to the joint clearance (set in theradial direction for easy assembly). Nevertheless,deformation of the pockets is restricted and thecalculations confirmed that there would be nointerference between the cage and other components.

More
deformation

Inner
ring
side
After deformation by centrifugal force
Before deformation by centrifugal force
Less
deformation
Portion indicated by: Theoretical cage/ball contact area

Fig.1 Example of FEM analysis of synthetic-resin crowntype cage under centrifugal force-47-

NTN TECHNICAL REVIEW No.72（2004）

More displacement

Less displacement

Fig.2 Example of FEM analysis of new synthetic-resin cage design under centrifugal force (displacement in radial direction)

2 Improved lubrication
The new cage has grease grooves inside thepockets (see Photo 3), thus improving lubrication
inside the pockets. They also improve the bearing lifeand reduce the bearing noise.

3 Improved productivity designing
With the metallic-sheet wave-type cages, coupling
is typically accomplished by crimping the rivets (seeFig. 3). However, the new synthetic-resin cage simply
requires pressing together the coupling prongs of thetwo wave-type rings, reducing the assembly time.
The coupling portions are designed such that thetwo wave-type rings are of an identical profile, which
requires only one mold for manufacturing (see Fig. 4).

rivet

Before coupling

Photo.3 Grease groove

After couplingFig.3 Cross section of pressed-steel cage

Before coupling

After coupling

Fig.4 Cross section of new synthetic-resin cage design-48-

Next Generation Deep Groove Ball Bearing for High-Speed Servomotor(2) Long-life grease "ME-1"

3. Results of functionality confirmation
test(For details, see Page 20, "Development of highspeed, long-life grease.")The new long life grease "ME-1" was developedwith attention paid to oxidation stability of the base oiland selection of the most effective oxidation inhibitor.
This grease almost doubled the life of NTN's standard
motor grease (MP-1). This grease is now adopted as
standard for the "next generation deep groove ballbearing for high-speed servomotors."
1 Composition and properties of ME-1 greaseME-1 grease uses urea compound as thickener
and, for the base oil, it uses a mixture of special ester
oil and PAO. To prevent oxidation of the base oil, an
appropriate anti-oxidant is added, increasing thegrease life.
Table 2 shows the composition and properties of
ME-1 grease.

(1) High-speed operation
To evaluate the high-speed performance, atemperature rise test was conducted.
1 Bearing specifications: See Table 3.
Table 3 Test bearing
Bearing

Cage

GreaseAmount ofgrease sealed

Seal

6308

Synthetic resin
80% ofNon-contact typecrown type New grease(ME-1)
static spacerubber seal
cage

Same
as
above

New synthetic
resin cage

Same asabove

Same asabove

Same asabove※ Synthetic resin: PA66 + GF25%

Table 2 Typical properties of ME-1 grease
ME-1
Thickener

Test method

Urea

−

Ester/PAO

−

mm2/s

60

JIS K2220.23

Mixture thickness, 60W 25˚C

250

JIS K2220. 7

250 or higher

JIS K2220. 8

Base oil
Viscosity of base oil,

Dropping point, ˚C

2 Test conditions[Maximum rotating speed]
n=15,000min-1(dn value=0.6 million, dmn value=0.95 million)[Temperature]
Room temperature[Test machine]
Rapid acceleration/deceleration test machine(see Fig. 6)

2 High temperature endurance
A high-temperature endurance test (ASTM D3336)
was carried out on the new grease (ME-1). Fig. 5
shows the test results of a commercially available
grease and ME-1 grease.
As compared to commercially available grease A,
endurance of ME-1 grease is more than five times,
and to commercially available urea grease B and MP1 it is 1.5 to 2 times.

Test bearing 1
Thermocouple

Thermocouple

Motor
Coupling

Life before seizure (average value) hTestbearing2【Test conditions】
Bearing：6204
Rotating speed：10,000min-1
Temperature：150˚C Load：Fr=Fa=67N
8000

High-temperature, high-speed endurance test

Fig.6 Rapid acceleration/deceleration test machine

6000
4000
2000
0

A

B

MP-1

ME-1

Fig.5 Endurance test result-49-

NTN TECHNICAL REVIEW No.72（2004）

3 Test results
Fig. 7 shows the relationship between the rotating
speed and the outer-ring temperature rise.
Temperatures were read when it stabilized. The
test was terminated as soon as the temperature
exceeded 100˚C.
No significant difference was observed between thesynthetic-resin crown-type cage and the new
synthetic-resin cage up to n=12,000min-1 (dnvalue=0.45 million, dmn value=0.75 million). At
n=15,000min-1 (dn value=0.6million, dmn value=0.95
million), the crown-type cage showed abnormal
temperature rise and the test was interrupted. No
abnormal temperature rise was noticed with the newcage except for that possibly caused by grease
resistance.
The bearing inspection after the test and the FEM
analysis results confirmed that the abnormal
temperature rise found with the crown-type cage wascaused by its contact with the other bearing
components due to cage deformation by centrifugal
force.

(2) Endurance
To evaluate high-temperature endurance of thecages, a high-temperature, high-speed endurance test
was conducted.
1 Bearing specifications: See Table 4.

Table 4 Test bearing
Cage

GreaseAmount ofgrease sealed

A

6209

Pressedsteel cage

Traditional
grease(MP-1)

90% ofNon-contact typestatic spacerubber seal

B

Same New synthetic New greaseasresin cage
(ME-1)
above

Same as
above

Seal

Same asabove※ Synthetic resin: PA46 + GF25%

2 Test conditions[Rotating speed]
A: n=11,111min-1(dn value=0.5 million, dmn value=0.7 million)
B: n=13,333 min-1(dn value=0.6 million, dmn value=0.85 million)[Bearing temperatures]
130˚C (at bearing's outer diameter)[Test machine]
High-temperature, high-speed endurance test
machine (see Fig. 8)100

Outer ring temperature rise ˚C

Bearing

New cage
Crown type cage

80
60
40
20
0
3000

6000

9000

12000

Rotating speed min-1

Test bearing

15000

［

dn =0.6 million

dmn =0.95 million

Heater

］Fig.7 Temperature rise test result

High-speed motor

Fig.8 High temperature and high speed endurance test
machine-50-

Next Generation Deep Groove Ball Bearing for High-Speed Servomotor

3 Test results
Fig. 9 shows the relationship between the cage type
and the test duration.
The new synthetic resin cage + new grease (ME-1)
demonstrated three time and greater endurance thanthe pressed steel cage + traditional grease (MP-1),
despite the fact that it was run at even higher rotating
speeds.
This superb endurance is attributable to excellent
high-temperature endurance of the new grease (ME1) and to improved lubrication characteristics of the
new cage.

Test duration h

6000

4. Conclusion
By adopting the new synthetic-resin cage, the "Nextgeneration deep groove ball bearing for high-speed
servomotor" can successfully prevent abnormal
temperature rise at the dn value of 0.6 million (the dmn
value of 0.85~0.95 million) which the traditional design
was unable to resolve. Additionally, adoption of the
new grease "ME-1" realized a longer life of thisbearing. Furthermore, the unique cage profile has
reduced the assembly time and minimized the costincrease associated with the traditional types (pressedsteel cage, synthetic-resin crown-type cages) bycontrolling the mold costs.
We believe that this bearing can contribute to thedevelopment of the servomotors for machine-tool
main spindles in the areas of high-speed operationand longer service life. We will continue to develop
new products that support advancing performance ofthe latest machine-tool motors.

High-temperature, high-speed endurance test

5000
4000
3000
2000
1000

Pressed steel cage

New cage

Fig.9 High-temperature and high-speed endurance
test result

Photos of authors

Chikara KATAGIRI

Kenichiro NAITO

Industrial Engineering Dept.
Industrial Sales Headquarters

Industrial Engineering Dept.
Industrial Sales Headquarters-51-

NTN TECHNICAL REVIEW No.72（2004）[ New Product ]Multi-Repair System for Color Filters

Masahiro SARUTA*NTN has developed a color filter repair system (product name: Multi-Repair System) that is capable of repairing all of
the various production defects in liquid crystal color filters. This system was developed jointly with Takano Co., Ltd.
and offers new functions such as review and tape grinding in addition to traditional laser cut and ink pasting functions.
Integrating all of these functions into a single unit provides a great benefit in terms of improved repair quality, reduced
repair time and equipment cost, and smaller machine footpoint. It has already received acceptance by many users.
This paper outlines the system and explains the available functions.

The design concept of this system as well as its
field-proven performance was recognized in the 2004
Flat Panel Display Fair, and it won the ADY2004(Advanced Display of the Year) Grand Prix award in
the inspection device division.
Various functionalities of the system and the benefit
of function consolidation are discussed below.

1. Introduction
NTN has developed equipment that pastes color ink
on defective portions of LCD color filters (see "Repair
system for the 6th and 7th generation LCD color
filters") and has delivered it to many customers. NTN
recently conducted a joint development with Takano
Co,. Ltd. and added a polish repair function to thisequipment. The resulting product is a multi-repair
system. NTN has begun sales of this product.
The defects in the LCD color filter manufacturing
processes are largely divided into foreign particles,
black defects, and white defects. Until today,
separate repair machines were used to repair eachtype of defect. For the first time in the industry, thismulti-repair system has consolidated all these repairfunctions into a single piece of equipment. Thissystem will improve the productivity and quality of
production of ever-larger LCD color filters and will free
up floor space.

*Precision Equipment Division

NTN Corporation

Takano Co., Ltd.

No. 1
in coating repair field

No. 1
in inspection devices field

Laser cutting
repairtechnology
Ink coating
repairtechnology

ted
Patenology
techn

ted
Patenology
techn

Tape polishingrepairtechnology
Defect heightmeasuring
technology

Multi-repair system that can repair various defects

Product Engineering Department-52-

Multi-Repair System for Color Filters

employs a large glass surface plate at the glass
substrate chucking surface. It allows both backlightand incident lighting. In an observation using thebacklight lighting, the machined surface of the glass
surface plate may cause uneven light intensity,
which can create dead angles. To avoid such deadangles, the system has a function to offset thesubstrates to display the whole images without suchdead angles.
For profile measurement, the system utilizes
contact type height sensors to measure projection
heights. There are other functions in this systemthat allow dimensional measurement of the defects
and many other inspection functions.

2. Outline of multi-repair system
Photo 1 is the appearance of the system. The
system can handle large glass color filter substrates
up to the seventh generation (1870×2200mm) display
panels and have the following repair functions.
(1) Review function to assess defects(2) Tape polishing function to repair foreign particle
defects(3) Laser cutting function to repair black defects andforeign matters(4) Ink pasting function to repair white defects and to
fill laser cut portions(2) Tape polishing function
Photo 2 shows the appearance of the tapepolishing unit.
The polishing unit consists of a polishing head, tapedrive mechanism, two height sensors, etc. andperforms polishing of foreign particles by moving thetapes with appropriate grain size.
The pressure on the polishing head can be set atany pressure, which allows the system to run at themost suitable conditions for different materials. The
polishing head is also equipped with displacement
sensors that detect height. Consequently, theamount of material removal can be controlled not
only by time but also by height.
One of the two height sensors can scan thecontact point, and by selecting two nearby points ofa projection defect, the system can measure theheight of the defect.
The other height sensor is combined with theabove-mentioned height sensor in determining theinclination of the glass substrate. With thesemeasurements as basis, the system applies
numerical control of the height of the polishing headlocated in between the two height sensors, thus
performing highly accurate repair works.

Photo 1 Multi-repair system for color filters

The defect repairing process is as follows. First,
defect data is sent from a server or an inspection
device to the host computer of the multi-repair system.
The defect data contains the types and the locationsof the defects, which will be the basis for thesubsequent repair works. The system automaticallyrecognizes the alignment mark on the substrate,
calculates its inclination on the XY plane, converts theabove defect location into coordinates, and thendisplays it accurately on the monitor screen. At thispoint, the operator manually determines the method
and the range of repair. This system is alsocharacterized by abundance of software that will allow
users to deal with various situations.

Tape reel(1) Review function
The defect data received by the host computer isdisplayed on the monitor. Operators can select anyof these defects, and the computer will automaticallydisplay the selected defects on the monitor for
review.
In the review process, users may engage
themselves primarily in observing or measuring the
profiles of the defects.
To make defect observation easy, the system

Polishing head
Height sensor

Photo 2 Tape grinding unit

-53-

NTN TECHNICAL REVIEW No.72（2004）

positioning actuator via the slide. The coatingneedle picks up the paste in the ink container
located on the index disc and transfers it to a
defective portion on the substrate by touching it.
The needle tip diameter can be selected at will
depending on the pattern width of the defect, but theneedle tip diameter of φ30~φ70mm is typicallyused. If the defect portion is long, a continuous
pattern should be used by repeating coating of apitch smaller than the coating diameter.
For repairing color filters, up to four different
resists (ink) may be used. In preparation for needsof washing the needle tips along with the pastechange, the system is equipped with mechanismsthat agitate detergent, wash the needles, and air
purge them after washing.

(3) Laser cutting function
The YAG (Yttrium Aluminum Garnet) laser with an
EOQ (Electro Optic Q) switch is the most common
laser used in the LCD color filter repair. The basic
frequency is the second high-order harmonic at
532nm, but a laser of even higher-order harmonic
may be installed if necessary.
The laser optical unit shown in Fig. 1 is an imageformation optical unit that focuses the slit profile ofcutting on the color filter, and consists of a slit
mechanism which determines cutting profiles, apower control mechanism to adjust laser output, anda revolver mechanism which selects an appropriate
objective lens.
Since the pixel of a color filter may not always berectangle in shape and some pixels may be
deformed or all may be angled, the θ slit
mechanism which rotates the whole slit mechanism
is often installed.
Selection of the slit profile, angle, laser power, andthe objective lens can be done on the GUI(Graphical User Interface) screen or on the control
panel.

Positioning actuator

Index discInk container

YAG laser head
Electric power controller

Ink coating needle

Z stage
Electric revolver

Super-long stroke
objective lensAir-purging
Needle washing station

Electric slit controller(Equipped with θ slit
mechanism: ±45˚)

Fig.2 Pasting needle unit

Incident light

Photo 3 shows an example repair operation wherea foreign matter on the color filter was removed bylaser cutting followed by color coating using the ink
coating mechanism.
In this example, the defect is small and resides in
the R (red) region only. Therefore, it needed laser
cutting and coating repair of one color only.
However, as the area of a defect becomes larger,
repair would require multiple processes including
tape polishing, coating of RGB colors, etc.

Glass surface
plate

Fig.1 Optical unit with YAG laser(4) Ink coating function
Another feature of this system is the ink coating
mechanism using a coating needle. The methods of
ink coating include coating needle method,
dispenser method, injection method, etc. The
reason why this system has adopted the coatingneedle method is because it offers coating at finediameters, the paste does not clog, and because it
accommodates various types of paste used bycustomers.
Fig. 2 shows the structure of the coating needle
method. The coating needle is mounted on theForeign matter

Removing foreign
matter by laser
cutting

Coating color ink

Photo.3 Example of defect repair process-54-

Multi-Repair System for Color Filters

2. Advantages of consolidating
multiple functions into single
equipment

3. Conclusion
Demands for LCD's for large-size TV's are growing
rapidly and so are the demands for high-quality repair
equipment for various defects of color filters. NTN's
new multi-repair system should satisfy thosedemands.
This paper describes this system with a focus on
the mechanical aspects. However, the system isloaded with CIM (Computer Integrated
Manufacturing), GUI (Graphic User Interface), andother software that makes this system qualify as inline equipment. There has been an increase in the inline use of this system. We believe that the market
will be demanding even larger systems, reduction inrepair time, reduction in labor, and more efficient
repair performance. We are committed to improve thissystem so that it can offer greater productivity to ourcustomers.

Needless to say one of the advantages of
consolidating the three processes, polishing, laser
cutting, and ink coating, is to be able to repair all
defects with one machine. Another big advantage is
improvement in the quality of repair. If, for example,
multiple exposures to laser are a concern of damaging
the substrate, you can first apply tape polishing andthen cut out the defective portion with a low power
laser. The subsequent coloring process should
achieve good chromaticity because the laser has left
little irradiation damage.
Reduction in the repair time is another big benefit.
If multiple devices are used to repair defects, they
incur transfer time between the devices, time to alignthe glass substrates within the devices, and time toposition in each device. Time is saved, resulting insignificant repair time reduction. Other advantages
include reduction in the total equipment costs throughsharing the glass surface plate and other components
or saving in the required floor space.

Reference
1) SARUTA, Semiconductor FPD World 2004. 72) SATOU, Semiconductor FPD World 2004. 5

Photos of author

Masahiro SARUTA
Precision Equipment Division
Product Engineering Dept.

-55-

NTN TECHNICAL REVIEW No.72（2004）[ New Product ]Repair System for Sixth and Seventh Generation
LCD Color Filters

Akihiro YAMANAKA*
Akira MATSUSHIMA*

NTN's color filter repair system fixes defects in color filters, which are the primary components of liquid crystal
displays. The most important feature of this system is its ability to apply ink to a white spot, commonly referred to as a"white defect" on the color filter. The repair process of the liquid crystal color filter involves a backlight observation
mechanism that verifies the conditions before and after the repair. LCD screens have gotten larger in recent years
and, consequently, the size of the glass used for production is also increasing. Naturally, the repair system for these
new screens must also be larger. The growth in screen size could pose a problem for traditional backlight observation
mechanisms in the near future. This paper details a reflective-type backlight mechanism that NTN recently developed
for large-size substrates (Sixth and Seventh generation).

1. Introduction
The size of mother glass substrates used in the
production of FPD's is rapidly becoming larger. For
example, the Sixth generation has a size of 1500×1850mm and the Seventh generation measures
1850×2200mm. Consequently, the size of thepattern repair systems has increased dramatically.
This paper details NTN's newly developed backlightmechanism that is compatible for large-size
substrates.

In the past, televisions have primarily used CRT's(Cathode Ray Tubes) for their display screens. CRT's
are now being replaced by LCD's (Liquid Crystal
Displays) and PDP's (Plasma Display Panels), also
known as Flat Panel Displays or FPD's. By 2006,
domestic sales of FPD's are expected to exceed thatof the CRT's.
NTN has had a pattern repair system on the market
for well over 10 years. It repairs color filter defects
that occur during FPD production. Recently, as themarket has trended towards larger and higher
resolution FPD's, the repair process has been widely
recognized as an integral part of production. For thisreason, NTN's LCD color filter repair system has been
installed in many production facilities.

*Precision Equipment Division

Product Engineering Department-56-

Repair System for Sixth and Seventh Generation LCD Color Filters

pixel. Black defects are caused when colors are
mixied on a pixel or dust adheres to a pixel. They
may be repaired by first removing them by laser
cutting followed by coating them with ink of the samecolor.

2. Backlight mechanism
for LCD CF repair system
As shown in Fig. 1, an LCD panel is made up of
TFT and CF substrates bonded together with spacerballs and filled with liquid crystal and a backlight at thebottom of this structure. A CF glass substrate has red(R), green (G), and blue (B) pixels arranged in amatrix format (see Fig. 2).

2.1 Evolution of CF repair system configuration
As mentioned before, the size of FPD's isincreasing at an amazing rate. In response, the size
and the configuration of the repair systems are alsochanging rapidly. The primary purpose for thechanges is to limit the necessary floor space for thesystem inside clean rooms.
Fig. 3 shows the evolution of the CF system
configurations.

Viewing direction
Polarizing
plate

Color filter(CF) layer

SpacerCommonelectrode (ITO)

CF substrate(glass)
Seal

Seal

Repair head

TFT substrate (glass)
Polarizing plate Liquid crystal TFT
layer
Light

Glass substrate

Pixel electrodeY axisBacklight

X axisFig.1 LCD structurea. Third and earlier generations

A CF substrate adds color information to light and isturned "on" or "off" by the liquid crystal. Light going
through the CF substrate will display the color
information for that pixel: enabling an LCD to display
color images.
As described above, a CF substrate functions by
allowing light to pass through it. Consequently, a CF
substrate repair system must have a backlightmechanism to verify the quality of the repair.
A CF repair system is equipment that is designed to
repair defects on CF panels as shown in Fig. 2.
White defects are spots on pixels where color is
missing. These defects are repaired by coating thewhite spot with ink of the same color as the rest of the

X axisRepair head
Glass substrate
Y axisb. Fourth and Fifth generations

Y axisRepair head

X axisGlass substrateBlack
defectWhite
defectc. Sixth and future generations
Fig.3 Transition in the construction of repair systems
related to generation change of LCD substrate
Fig.2 Defects in a CF panel-57-

NTN TECHNICAL REVIEW No.72（2004）

In the repair system configurations for Third andearlier generations, substrates were placed on the XY
tables. The tables moved horizontally while the repair
heads were fixed. The systems for the Fourth and
Fifth generations generally had separate tables for the
X-axis and the Y-axis and the substrates were moved
in one axis while the repair head was moved in theother.
Most of the Sixth and Seventh generation repair
systems secures substrates in a fixed position andemploys the gantry method which moves the repair
head in both the X and Y directions.

Y axisX axisRepair head
Y axisLinear guideX axisLinear guide

Backlight
Linear guidea. XY moving backlight system

2.2 Traditional backlight mechanism and its
IssuesThe backlight mechanism, as shown in Fig. 4,
illuminates the back of the surface plate so the lighttransmitted through the CF substrate can beobserved. For this reason, the support for the CF
substrate needs to be transparent. Therefore, mostsurface plates are made out of glass.

Y axisX axisRepair head
Linear guide
Y axisBacklight
Linear guide

Repair headb. Y moving backlight system

CF substrate

Fig.5 Conventional backlight mechanism
Glass surface platesystems with this design.
In the configurations illustrated in Figs. 5-a and 5-b,
the light source of the backlight moves underneath theglass surface plate on which CF substrates areplaced. This means that support members must be
located at the perimeter of the glass surface plate.
To accommodate larger Sixth and Seventh
generation FPD substrates, the size of glass surface
plate must increase. However, as mentioned above,
the glass surface plate must be supported at its
perimeter. Therefore, the weight of the glass surface
plate can cause deflection in the glass. Also, impact
during transportation of the system can damage theglass surface plate.
The glass surface plate needs to have a mechanism
for securing the CF substrate in position. Today,
many systems use vacuum suction mechanisms forthis purpose. The vacuum suction mechanism
requires suction grooves to be cut and holes to be
drilled on the glass surface plate. It also requires
piping to be placed on the back of the glass surface
plate. The suction holes and piping can obstruct thebacklight.

Backlight

Fig.4 Glass surface plate

The gantry method used in the Sixth and theSeventh-generation systems moves the repair head inboth X and Y directions. It requires a synchronized
movement of the backlight mechanism. This results ina very complex system construction. Fig. 5 shows the
backlight mechanism that has been adopted for the
traditional gantry method. Fig. 5-a shows the complex
structure of the backlight mechanism that moves
synchronously with the repair head in X and Y
directions.
Fig. 5-b is a simpler design in terms of both its
structure and control where the linear backlight moves
synchronously with the repair head in a single axis.
This design does not require any additional control
mechanism and is used in many systems. However,this method presents many system configuration
issues when it comes to meeting the needs of Sixth
and Seventh generation FPD's. Also, it is difficult tobuild backlight mechanisms for increasingly larger-58-Repair System for Sixth and Seventh Generation LCD Color Filters

When placing a CF substrate onto a repair system,a transfer device normally is used. Today, robots areused as transfer devices most of the time.
Lift mechanisms need to be installed on the repair
systems so that they can receive the CF substrates
transferred by the robots. A typical lift mechanism
used for such an application is shown in Fig. 6. The
lift mechanism raises the lifter pins to receive thesubstrate from the robot, and then lowers the lifter
pins to place the substrate on the glass surface plate.
Holes must be drilled into the glass surface plate, as
shown in Fig. 6, so the lifter pins can pass through.
These holes affect the function of the backlight.
These are the major issues currently affecting thepresent backlight mechanism. At this time, thestrength of the glass surface plate is the biggest issue.

Robot hand

CF substrate

3. New backlight mechanism
In order to improve the function of the repair
system, NTN has developed a new backlightmechanism. Instead of lighting from underneath therepair head, the new mechanism lights from the top of
the repair head side to for improved inspection. With
this configuration, since the source of the backlight is
no longer located underneath the surface plate, theplate can be supported along its entire surface.

3.1 Outline of reflective type backlightmechanism
Fig. 7 shows a diagram of the new backlightmechanism (reflective type).
The reflective-type backlight mechanism has a ringlamp around the objective lens of the repair head.
Light is irradiated from around the observation point,
utilizing the translucency of a CF substrate. The lightis then reflected off the mirror surface on the backsideof the glass surface plate, re-entering the CF
substrate from the backside, thereby providing
backlight for the lens.
Let us now explain the potential issues andcorresponding solutions for backlight observation
using this method.

Lifter pin

Glass surface plate

Lift

Drive motor

Fig.6 Lift mechanism

Objective lensRing lamp
Converging lensCF substrate
Incident light
Glass surface
plate
Reflective light
Mirror finish

Fig.7 Reflective-type backlight mechanism

1 Securing reflective light intensity¡Irradiation angle of light
Since there is a constraint on the arrangement of
the ring lamp with respect to interference with theobjective lens, the angle of the incident light to theCF substrate was set at 30˚. Light emitted by thering lamp spreads out to about 30˚. A ring lens
was installed at the light emitter to improve the light
convergence, thus increasing the utilization of thelight from the ring lamp and increasing thereflective light intensity. Without these measures,the spreading light from the ring lamp reaches the
-59-

φD

Laser beamφD

Laser beam
（a）

（b）

Fig.9 Influence of glass plate thickness - 2(Damage on mirror by cutting laser)

3 Separation from incident light observation
As the ring lens prevents light from entering theobservation point, the incident light does not affect theobserved images. However, it is possible for theincident light emitted by the objective lens to bereflected by the mirror on the bottom of the surface
plate and impact the observation images.
As shown in Fig. 10 (a), if the thickness (t) of theglass surface plate is small, then the diameter (φD) of
light reflecting off the mirror surface and entering theback of the observation point is small and the lightdensity is high. If this occurs, the light that is reflectedon the image affects the transmitted light.
The test results indicate that, if the thickness (t) of
the glass surface plate is 5 mm or greater, as shownin Fig. 10-b, then the diameter (φD) of light reflecting
off the mirror surface and entering the back of theobservation point is large and the light density is low.
When this occurs, reflection on the mirror surface
does not affect the observed image.

（b）

Fig.8 Influence of glass plate thickness - 1(Backlight intensity)

2 Solution to impacts of laser cutting
As mentioned above, black defects on CF’s can be
repaired by cutting them with a laser and thencoating them with ink. The laser cutting could cut
the mirror on the back of the glass surface plate.
As shown in Fig. 9-a, if the thickness (t) of theglass surface plate is small, then the diameter oflaser convergence (φD) on the mirror surface issmall and the power density of laser is high. In thissituation, the mirror surface is vulnerable todamage by the laser.
The test results indicate that, if the thickness of theglass surface plate is 12 mm or greater, as shownin Fig. 9-b, then the diameter of laser convergence(φD) on the mirror surface is large and the laser
power density is low. Consequently, the mirror
surface should not sustain any damage duringlaser cutting.

Reflected
light

Incident light
φD

（a）

t

Incident light

t

t

t
（a）

t

observation point and prevents clear observation
images from forming.
¡Thickness of glass surface plate
In order to reflect the incident light from the ringlamp onto the mirror surface and then back onto
observation point, the glass surface has to be a
specified thickness. As shown in Fig. 8-a, if theglass surface plate is too thin, the reflected light
does not reach the back surface of the observation
point and cannot backlight observation images
cannot be formed.
The test results indicate that if the glass surface
plate has a thickness of 15mm or greater (Fig. 8b), it can direct the reflected light from the mirror
surface onto the backside of the observation point.
The tests also found that this backlight observation
was as good as that with a traditional backlightmechanism.

t

NTN TECHNICAL REVIEW No.72（2004）

Reflected
light

φD

（b）

Fig.10 Influence of glass plate thickness - 3(Reflection of incident light)-60-

Repair System for Sixth and Seventh Generation LCD Color Filters

6 Solution to electrostatic charge
Electrostatic charge is produced by the contact andseparation of the glass surface plate and the CF
substrate and is a common issue for all backlightmechanisms. Solutions to this issue were studied asa part of this development.
As mentioned above, electrostatic charge occurs
due to the contact and separation of the glass surface
plate and the CF substrate. Minimizing the contact
area between the plate and the substrate should
reduce the amount of electrostatic charge. As aresult, a special surface texture was used for the topof the glass surface plate (see Fig. 13).

4 Solution to lifter pinhole issue
The glass surface plate must have holes for thelifter pins to pass through for loading and unloading
CF substrates. Since these holes must pass throughthe entire plate, a mirror cannot be installed on theback of the glass surface plate at these locations. As
a result, light cannot be reflected at these holes andan image cannot be observed.
To resolve this issue, optical fibers were installedinside the lifter pins as shown in Fig. 11. Thismeasure successfully resolved the issue and madebacklight observation at the lifter pinholes possible.

Light

Lifter pin
Lighting optical fiber

Lifter pin holeFig.11 Lighting of lifter pin holes

CF substrate
Special processing

5 Solution to vacuum hole issue
Vacuum holes must be made on the glass surface
plate to secure CF substrates. Like the lifter pinholes,
these holes must pass through the entire plate so no
mirror process can be applied at the bottom.
However, if the hole diameter is small enough, shownin Fig. 12, backlight observation is possible by thelight reflected by the mirror surface around theseholes.
The reflected light at the center of the vacuum holeis refracted by the wall of the hole, distorting thebacklight observation image compared to the ordinary
surface. This distortion in the images was reduced by
optimizing the distance between the ring lamp and theCF substrate.

Glass surface plate

Fig.13 Glass plate surface special processing

Special processing makes micro dents and bumps
on the top of the glass surface plate. This special
process reduces the contact area between the glass
surface plate and the CF substrate, thereby reducing
the occurrence of electrostatic charge compared witha regular glass surface. Furthermore, light enteringfrom the back surface of the CF substrate is dispersedby the micro dents and bumps created by the special
process. As a side benefit, this dispersion increases
the brightness of the backlight observation image.

Vacuum holeFig.12 Lighting of vacuum holes-61-

NTN TECHNICAL REVIEW No.72（2004）

7 Solution to judgment time
The repair judgment is an important issue withrepair systems. In the preceding discussion aboutreflective-type backlight mechanisms, the ring lighting
was examined. The ring lighting has the following
problems.
Most of the current systems offer multiple objective
lenses that are exchanged by the rotational movement
controlled by the revolver mechanism. Since therevolver mechanism rotates the objective lenses, thering lamp must retreat during the rotation. This retreat
time must be included in the total judgment time. It
may only take a few seconds, but any additional
judgment time should not be accepted.
The objective of this study was to determine alighting layout that does not require the ring lamp toretreat during objective lens exchanges and thatprovides sufficient light intensity for backlightobservation.
The study concluded that multiple arc-type lamps
could achieve this objective. These lamps were madeby dividing a ring lamp in sections to avoid
interference with the objective lenses during theirrotation.

Photo 1 shows the backlight observation images
obtained using the reflective-type backlightmechanism. The images are as good as those of the
traditional backlight mechanism.

Photo 1 Image of CF pattern with reflective typebacklight

8 Meeting the needs of the Sixth and Seventh

generation FPD’s
Unlike the traditional backlight method, the reflective
type backlight method accommodates support
mechanisms under the glass surface plate.
Therefore, there would be no structural problem as thesystem becomes larger.
Considering all the above issues, the specificationsof a reflective type backlight mechanism were
determined as shown in Table 1.
Table 1 Reflective-type backlight mechanism
specifications
1540×1890 mm
Size of glass Sixth generation
surface plate Seventh generation 1910×2240 mm
Thickness of glass surface
plate

t=19 mm

Distance between reflective
light and CF substrate

W.D.=18 mm

Surface processing ofglass surface plate

Special processing

Back surface processingof glass surface plate

Mirror processing

Reflective light

Divided arc lighting-62-

Repair System for Sixth and Seventh Generation LCD Color Filters

4. Conclusion
This paper introduced a new backlight mechanism
for the CF repair system. The objective of the new
backlight mechanism was to meet the requirements of
Sixth and Senventh generation FPD's. However, FPD
manufacturers have concepts of even larger motherglass substrates. As described earlier, a backlightmechanism is a necessity for repairing color filters andthe traditional backlight mechanism would facestructural problems when dealing with increasingly
large glass substrates. The reflective-type backlightmechanism has resolved any structural issues
associated with the repair of large CF's substrate.
This mechanism is a new feature of NTN's CF repair
systems. This paper primary focused on backlightmechanism, but as glass substrates become larger,
numerous new issues will continue to challenge
equipment manufacturers. NTN will continue
developing our repair system components to solvethese new issues to improve the innovation andquality of our products.

Photos of authors

Akihiro YAMANAKA

Akira MATSUSHIMA

Precision Equipment Division
Product Engineering Dept.

Precision Equipment Division
Product Engineering Dept.

-63-

NTN TECHNICAL REVIEW No.72（2004）[ New Product ]PDP Rib Repair System

Shizuka YAMAZAKI*
Yuji YADA*In recent years, PDP is expected to rapidly penetrate into the market. However, rib defects occasionally occur during
the manufacture of PDP substrates. If the substrate has a rib defect, the image quality of the PDP is poor. Therefore,
equipment that can repair this rib defect is in demand. To meet this demand, NTN has added various functions to
NTN’s conventional repair equipment and developed a rib repair device.
This device makes repairs by repeatedly applying paste on the defective portion, baking it, and measuring the shapeof the repaired portion with a non-contact displacement meter. After filling the defective portion with paste, the uppersurface of the rib is finished by machining. Surplus paste is removed from the side surfaces with a cutting laser andscratch needle.
This paper provides an outline of this equipment.

1. Introduction
Recent increase in the PDP production has drawn
people's attention to the loss associated withscrapping substrates with these rib defects, the needs
for improved productivity and quality, and to theenvironmental issues relative to the scrapped
substrates. Thus, demands are growing for a rib
repair system.
To meet these demands, NTN expanded theexisting paste application and laser cutting
technologies and combined them with additional laser
curing, machining, contour measuring functions, etc.
to create a rib repair system. The following is an
outline of this system.

PDP's (Plasma Display Panel) have recently
become larger, improved the quality of images, andreduced the manufacturing costs. The civil market for
PDP's was first launched in Japan in 2002 and theEuropean as well as the American markets is
expected to follow. Plasma TV's are considered thewinner of flat and big screen TV's, and demands for
PDP's in the market of 30 inch or larger FPD are
expected to grow.
The PDP back plate glass substrate has ribs which
are subject to notch and projection defects due toadhesion of dust and foreign matters or inclusion of air
bubbles during the production processes of thesubstrates. These defects on the PDP ribs cause
color mixing and loss of image quality. Substrates
with these defects have been scrapped or repaired.

*Precision Equipment Division

Product Engineering Department-64-

PDP Rib Repair System

2. Structure of PDP

3. Outline of system

Fig. 1 shows the structure of a typical PDP
including ribs. A notch defect on a rib is shown in Fig.
2. A PDP consists of a rear glass substrate with ribs
(barrier walls) and a front glass substrate that isplaced against the top surfaces of these ribs. The
types of the rib profile include the stripe, waffle,
meander types, etc. The figure below shows typical
stripe type ribs.

3.1 System configuration

Front glass substrate

Photo 1 is the appearance of the system and Fig. 3
the system configuration.
This system applies repair paste to notch defects on
the ribs, bakes them, machine finishes the rib top, andremoves excess paste off the sides of the ribs by laser
cutting or use of a scrape needle.
The defect data is sent from the server or theinspection unit to the host computer of the system.
Repair operations are conducted based on these data.
As shown in Fig. 3, the head is equipped with an air
spindle for machining, laser displacement meter,
baking laser, observation optical unit, cutting laser,
scrape unit, and a pasting unit. Not shown in Fig. 3
and behind the scrape unit is a squeegee unit.

Discharge electrode

Dielectric layer

Protective layer
Rib
Rear glass
substrate
Fluorescent
substance

Rib

Address electrodeFig.1 Structure of PDP

Rib (barrier walls)

Notch defect100〜150（μm）

40〜80（μm）

Photo 1 Rib repair system

Fig.2 Rib defectBaking laser

Z-axis motor Observation optical unit

Cutting laser

Z-axisX-axisSpindle formachining
Pasting unit

Y-axisSubstrate

Laser
displacement
meter

Fig.3 Schematic of rib repair system-65-

Scrape unit

NTN TECHNICAL REVIEW No.72（2004）

A defective substrate placed on the stage moves in
the Y-axis direction while the head moves in the Xand Z-axis directions. In other words, the head has
relative displacement in the XYZ directions withrespect to the defective substrate. Table 1 lists thestage specifications.

localized heating, and for cutting. Short wave
continuous pulse YVO4 laser (wave length: 532nm)
which has less thermal effects is used.
Table 2 shows the major specifications of the above
functions.
Table 2 Specifications of functional elements

Table 1 Specifications of stages
Item

Items

Specifications

1. Laser displacement meter

1. X stage
Drive

Ball screw or linear motor

Control method

Full closed loop control

Resolution

1μm/pulse

Positioning repeatability±3μm

Specifications

Resolution

0.1〜0.01μm

Measuring point

About φ2μm

Measuring range± 0.3mm

2. Cutting laser

2. Y stageType

LD excited YVO4 laser

Wave length

532nm

Output250mW or higher (at 5KHz)

Drive

Ball screw or linear motor

Control method

Full closed loop control

Resolution

1μm/pulseType

CO2 laser

Positioning repeatability±3μm

Wave length

10.6μmMaximum output

10W

3. Baking laser

3. Z stage
Drive

Ball screw

4. Spindle

Control method

Semi closed loop control

・Type

Externally pressurized air bearing

Resolution

0.05μm/pulse

・Drive (rotation)

Air turbine

Positioning repeatability±1μm・Maximum rotating speed

40,000 rpm

・Tool diameter

φ0.4mm〜φ0.3mm

3.2 Feature of system(1) Numerous repair functions

5. Observation optical unit
・Revolver

Electric revolver

As mentioned above, this system offers various
functions necessary for repair works. Users may
select functions that are most suitable for specificdefects.
Different functions may be manually operated by
way of the buttons on the monitor screen or theswitches on the operation panel.
The observation optical unit has a CCD camera that
sends expanded images of the target areas to themonitor.
The spindle is an air bearing spindle driven by an air
turbine. This spindle generates no heat and highrotational accuracy. A small grinding wheel of φ0.4 ~0.3 mm in diameter is placed at the tip of the rotating
shaft for grinding.
When the coating unit has applied the paste at thedefective portion, the squeegee unit runs its end tip ofa cylindrical resin squeegee head of about φ3mm indiameter over the paste with reference to the adjoining
normal rib top, thus smoothing the repaired surface.
The scrape unit, then uses the scrape needle andmechanically removes the excess paste on the sidesof the rib.
For baking, continuous wave CO2 laser with longwave length (wave length: 10.4μm) is used for・Objective lensMagnification 2x〜20x(2) Height measurement using noncontact
displacement meter
This unit is equipped with a laser displacement
meter and off-sets between the Z-axis reference pointof the displacement meter and the focal point of theobservation optical unit, and between the measuring
point on the XY plane and the center of the optical unit
are preregistered. Therefore, operators need to viewthe rib image on the monitor screen and specify thestart and end points of measurement. Then the unit
automatically corrects each offset amount, moves thedisplacement meter to the specified location, andsimply measures the height.
Since the displacement meter uses a small spot
diameter (about φ2μm) and takes measurement
without contact, it can accurately measure the profileof the top of the filled portion even though the paste isstill soft.
These measurements are converted into graphsand displayed on the observation monitor screen by
the measurement locations.

-66-

PDP Rib Repair System

Fig. 4 shows one example of the rib defect repair
processes. Each repair process will be explained
following the sequence shown in Fig. 4.
This fugure does not show the profile measuring
process, but it is necessary to execute this process
whenever multiple repair process are necessary. This
is to remeasure the profile of the defect after the repair
processes.
1 Dip the needle in paste in container and apply
paste to defect by making contact. For longdefects, specify start and end points for coating
Then have the unit move needle up and down atconstant pitch to accomplish continuous coating.
2 Bake paste by CO2 laser. Repeat above coating
and baking several times to fill defects with paste.
3 Measure height of repaired portion by comparing
paste buildup to height of adjoining normal rib.
Determine depth of paste removal by amount ofbuildup, and then machine the rib surface by the tipof rotating tool. Typically, this operation requires
several round trip runs of tool to achieve sameheight as that of adjoining rib top.
4 If paste builds up beyond rib width in coating andbaking operations, remove excess paste withreference to width of the adjoining normal rib using
laser cutting and scrape needle. This completes
repair processes.

(3) Specifying positions in different functions
As described above, this unit automatically corrects
the reference points in all functions (tip position of therotating tool, focal point of the cutting laser, focal pointof the baking laser, etc.) and the offset amountbetween the displacement meter (Z-axis) and theobservation optical unit (X and Y axis's).
Consequently, this unit offers easy machining andbaking at the height and the positions specified on theobservation monitor screen.
The rib material, profile, and the material for the
dielectric layer vary by users and so does the mostsuitable paste. Before installation of this system, NTN
offers assistance with paste selection tests andoptimal conditions selection tests using the actual
substrates and repair paste.

4. Repair procedure
The actual repair works do not always use the sameprocesses. Some rib defects may require repair ofprojections only, and others may require only heightmeasurement. The system allows independent
operation of the functions so that users may combine
any of these functions to perform necessary defectrepairs.

Repeated operations
Coating needle
1 Rib (barrier wall)

Continuous
wave laser

Paste

Notch defect100〜150（μm）

40〜80（μm）

1Coating repair paste

Notch defect on rib

2Drying and baking

Grinding Wheel

3Polishing

Pulse laser

Scrape and vacuum

4Cutting excess paste

5Removing excess paste

Fig.4 Schematic of rib defect and repair process-67-

NTN TECHNICAL REVIEW No.72（2004）

5. Repair results
Photo 2 is the measurement of the defect and its
repair in the rib direction measured by thedisplacement meter. The defect on this sample
measured about 450μm in length and 105μm in
depth.
For this sample, the coating and the baking
processes were repeated about five times to build up
the paste above the normal rib height (about 27μm),
and then the top surface was polished. Table 3 lists
the baking and the polishing conditions.
As shown by the measurement results in Photo 2,
the repaired surface after the polishing process isalmost undistinguishable from the surrounding ribs.

Conditions

Item(CO2 laser)

1. BakingLaser output

0.2W

Spot diameter

φ200μm

2. Polishing
φ0.3, Electro deposition grinding wheel

Grinding wheel

Average particle diameter 22〜36μm
Feed speed

10μm/s (cutting: 2 paths)

Rotating speed

40,000 rpm

Defective rib section

200μm

Normal rib top

Table 3 Heat treatment and grinding conditions

720μm

a. Before repair

b. Pasting and heat treatment (once)c. Pasting and heat treatment (5 times)

d. After grinding

Photo 2 Shape of rib defect through repair process-68-

PDP Rib Repair System

6. Conclusion
This article introduced the "PDP rib repair system"that was designed to repair the rib defects on the PDP
back glass substrate.
As the market for PDP's is expected to grow rapidly,
demands for this system should also increase.
With the growing markets for Hi-definition TV
broadcasting and DVD or such high-quality graphic
media, PDP's must improve resolution, reduce power
consumption, and lower the production costs. The
repair systems will also have to improve the surface
and dimensional accuracy of the rib top,accommodate complex rib profiles and thinner ribs,
handle even longer defects, and reduce tact time.
NTN will continue to proactively deal with thesedemands so that we may improve this system to ahigher level of maturity.

Photos of authors

Shizuka YAMAZAKI

Yuji YADA

Precision Equipement Division
Product Engineering Dept.

Precision Equipement Division
Product Engineering Dept.

-69-

NTN TECHNICAL REVIEW No.72（2004）[ Technical Article ]Improvement of the Accuracy of an Air Spindleof the Nano-meter Order- Improvement of Rotational Accuracy by Optimization of Feed Hole Positioning -Yoshio FUJIKAWA*
Takashi HARAGUCHI*
Kazuyuki AONO*
Teruyoshi HORIUCHI*

NTN’s air spindles are used in production and inspection equipment for magnetic discs. The major issue of such air
spindles is how to improve the rotational accuracy. NTN studied the correlation of the rotational accuracy and the feedhole arrangement on an aerostatic journal bearing, and as a result NTN has developed a staggered triple-row feed
journal bearing. This paper presents the numerical analysis results of the performance of the staggered triple-row feed
journal bearing and describes the rotational accuracy of the prototype air spindle that is equipped with this bearing.

1. Introduction

2. Air spindles for magnetic disks

Aerostatic bearings offer dynamic accuracy of thenanometer order at realistic costs. They are widelyused in advanced production equipment for
semiconductors, optical disks, and for magnetic disks
as bearing/guide elements. NTN integrated thisaerostatic bearing and a motor to develop a highaccuracy air spindle. Today, these air spindles are in
production and fulfilling the needs for these products.
In this field of technology, however, progress has
been dramatic in terms of higher precision andrecording density. Air spindles are always expected toreduce the rotational errors.
To facilitate these demands, NTN has developed a
new staggered triple-row feed aerostatic journalbearing. This paper describes a case study where
this bearing was used on the air spindle for magnetic
disk inspection devices or servo track writers andsuccessfully reduced the major contributors to
repetitive run-out and non-repetitive run-out.

Magnetic head inspection devices for hard disk
drives or servo track writers may stack up magnetic
disks, as shown in Fig. 11), for rotation. Because thedisk stack overhangs significantly with respect to thespindle bearing part, the run-out at the top of the disk
stack is large. It is almost like the conical mode. Runout of disks causes errors in the positioning of thehead on the disk. Especially, non-repetitive run-outthat does not synchronize with the disk rotation has
been viewed as critical. Presently, needs for higher
storage density have caused the track pitch to
become smaller. Accordingly, allowable errors forhead positioning have become tighter. On someoccasions, it is necessary to assess repetitive run-out,
particularly on higher components.

*Precision Equipment Division

Fig. 2 shows the structure of an air spindle formagnetic disks. A shaft is supported by two journalbearings that support primarily radial load, and by a

Product Engineering Department-70-

Improvement of the Accuracy of an Air Spindle of the Nano-meter Order

2c
5

2a 2b
2

1

Shaft
Journal bearing
Non-contact seal section
for vacuum chuck4
3

Thrust bearing
Overall control
deviceThrust plate6

AC servomotor

1 ：Magnetic disk media
2 ：Head actuator mechanism
2a ：Head, 2b：Arm, 2c：Actuator
3 ：Spindle motor
4 ：Spindle hub
5 ：Clamp

Rotary encoder

Fig.1 Air spindle for magnetic disk 1)

Fig.2 Configuration of air spindle

pair of thrust bearings located on both sides of the
shaft-integrated thrust plate supporting primarily theaxial load. Restrictor has significant effects on thecharacteristics of aerostatic bearings and many types ofrestrictor are suggested. For applications that require
high accuracy, NTN primarily adopts complex restrictionthat combines an inherent restrictors and shallow
circumferential grooves. NTN also uses porous
restriction in some applications. The slit plate of the
optical rotary encoder and the AC servo motor rotor are
directly mounted on the shaft. Such structure makes
the rotating portion completely free from contact withthe fixed portions, thus minimizing fluctuation in friction
resistance with respect to the shaft rotation and making
it possible to accurately control the shaft rotation.
To accommodate a vacuum chuck mechanism for
mounting/dismounting of disks, etc., a non-contact
seal section is placed between the two journalbearings configuring a vacuum exhaust passage
between the rotating and the fixed portions. The noncontact seal section has a clearance about equal tothat of the journal bearing. Resistance at this small
clearance limits entry of outside air into this vacuum
exhaust passage. The seal surfaces of the rotating
and the fixed portions can be machined
simultaneously with the surface of the journal bearing,and therefore very high concentricity to the bearing
surface can be obtained, affecting little on the rotating
accuracy.
To ensure high rotating accuracy of an air spindle,
the motor control technology is important. If rotating
speed control of very high accuracy is required formagnetic disk or optical disk related applications, NTN
uses a driver that was developed internally by
combining PLL control (Phase Locked Loop) andsinusoidal linear drive.

3. Measurement of repetitive run-out
using 3-point method2)
3.1 Run-out measuring system
The run-out accuracy evaluation used the 3-point
method in order to eliminate influence of the profile
errors of the target object and to measure minute runout of the air spindle with good repeatability. Fig. 3
shows the measuring system. To evaluate the run-out
under the operating conditions closest to those on an
actual unit, dummy clamp and dummy disks were
fixed on the spindle by the vacuum chuck mechanism
to simulate a disk stack of multiple magnetic disks. In
addition, a steel ball of 25.4mm in diameter was
installed at the top to be the target for the
displacement sensor.

Steel ball

Displacement meter

Dummy clamp
Dummy disk

AD converter

P/C
processor
Encoder output

Fig.3 Measurement system for radial runout by 3 point method-71-

NTN TECHNICAL REVIEW No.72（2004）

The steel ball at the top of the dummy clamp allows
run-out measurement to be taken at a point where theamplitude is the largest. However, it adds mass to apoint away from the bearing, creating more severe
operating conditions in terms of run-out.
A capacitance displacement sensor was used tomeasure run-out. The output signal of thedisplacement sensor was stored into the memory via
an AD converter using the output pulses (1024pulses/rotation) from the optical rotary encoder thatwas built into the spindle as a trigger. Tshen, theaverage run-out wave pattern for 10 continuous
rotations was processed by the 3-point method. The
measurement results are the Fourier coefficients forthe profile error of the target and for the run-out at thecenter of rotation. This evaluation used the absolutevalues of the Fourier coefficients for run-out todetermine the actual run-out.
The 3-point method is known to include errors in themeasured value of the order if the "scaling factor"2),
which is defined by the arrangement of the three
displacement sensors (angle between the sensors)
and the order of run-out, is small. Our evaluation
calculated the "scaling factor" of each order at andbelow 32nd, and determined the sensor locations sothat the minimum value would be the largest.
Furthermore, after the measuring sytem was set up,
the angle between the three sensors was measured
by the rotary encoder that was built into the air
spindle, and this value was used in the calculations.

amplitude at the same orders except for the uniquedifferences in the sample populations.

4. Staggered layout of feed holes
Calculations indicate that a journal bearing with feedholes laid out at an equal pitch on its circumference
produces run-out of specific high orders depending on
the profile error of the rotating shaft and the number of
feed holes3). The profile errors are represented by δcos {m (θ−α)} having the m number of lobes on thecircumference, where d is the differential of theaverage radius and the maximum radius of the shaft
divided by the average radial clearance of the bearing.
The profile in the axial direction is assumed constant.
(See Fig. 7.)
Fig. 5 shows the position of the shaft center of ashaft having the m number of lobes rotating in a singlerow feed inherent restrictor journal bearing with the knumber of feed holes laid out on its circumference.
The position was obtained by equilibrium of staticforce. The position of the shaft center in Fig. 5
assumesδ= 0.1 and shows the k number of feed holeson the horizontal axis and the m number of the lobes ofroundness error on the vertical axis. It shows the
position of the shaft center corresponding to eachcombination. Run-out occurred at m=nk±1 (n is anatural number) and it was particularly large at
m=nk+1. During one shaft rotation, the shaft centerrotates the m number of times around the center of the
bearing and produces run-out of the mth order.
Unlike the configuration used in the reference3), thejournal bearing of the conventional spindle is adouble-row feed type having four feed holes laid out
on the circumference at an equal pitch at two axial
locations, and a circumferential groove is installed toconnect these feed holes. If this similar mechanism isto produce run-out of higher orders, it is possible that
it will produce run-out of the 5th, 9th, 13th, and the 17thorder, because k=4. This is in good approximation
with the run-out measurement shown in Fig. 4, andtherefore the repetitive run-out of higher order withconventional spindles may be produced by the profile

3.2 Measurement results

7

15

6

10

5
δ=0.1

m

Amplitude nm

Fig. 4 shows the measurement results of repetitive
run-out of a magnetic disk air spindle (hereinafterreferred to as "conventional spindle") that has been in
production at NTN. The bearing is of the complex
restriction type. The vertical axis represents theabsolute values of the Fourier coefficients for run-out
determined by the 3-point method, and the horizontal
axis represents the order of run-out with respect to theshaft rotation, or the amplitude of run-out
corresponding to each order. The peak of run-out are
seen at the 3rd, 5th, 9th, and at the 17th orders. The
maximum amplitude was recorded at the 5th order andwas about 13nm. The measurement of another air
spindle of the same design showed the peaks of

4

5

3

0
2

5

10

15

20

25

30

0.1

2

Order of rotation

k=3

Fig.4 Radial runout of conventional spindle
measured by the 3 point method

4

5

6

7

8

Fig.5 Effect of number of feed holes and lobes on radial runout-72-

Improvement of the Accuracy of an Air Spindle of the Nano-meter Order

totally symmetrical order or, as shown in Fig. 6 (c),arrange the feed holes at more than two locations on
the circumference and establish symmetry in eachbearings.

error of the shaft and the layout of the feed holes.
To reduce this run-out, it is effective to increase thenumber of feed holes on the circumference. However,if the feed holes are increased without carefulconsideration, it will reduce stiffness and the damping
factor, making the bearing vulnerable to external
disturbances.
The feed holes of a double-row feed journal bearing
are typically laid out in the same phase as shown in
Fig. 6 (a). On the other hand, the profile error of aspindle shaft is usually a copy of the run-out error of agrinder's work spindle during the finishing process,and it is considered consistent through the axial
direction. If the profile of a rotating shaft is consistentin the axial direction, feed holes on each row instaggered layout as shown in Fig. 6 (b) should
achieve the same effect as when the feed holes aredoubled over the circumference. However,asymmetrical layout of the feed holes in the axial
direction will produce the moment of force that works
to tilt the shaft. In order to reduce this the moment offorce, you may either layout the feed holes of the twojournal bearings that support the rotating shaft in a

5. Calculated bearing performance
To determine the specifications of a journal bearing
for prototyping an air spindle, bearing performancewas calculated. With a journal bearing for aconventional spindle as a basis (bearing a0: Shaft
diameter 30mm, Bearing width 25mm), the prototype
expanded the bearing width by 15mm to 40mm total,
primarily to improve the damping characteristic.
Performance was calculated for the three types offeed hole arrangements shown in Fig. 6. Table 1
summarizes the bearing specifications that were used
for this calculation.
The method of calculation used was finite difference
method using the DF method, which is generally used
to calculate performance of grooved aerostatic
bearings.
Table 1 Bearing specification(a) Double-row feedholes in same phase(b) Staggered
double-row feed holes(c) Staggered
triple-row feed holes

Fig.6 Feed hole arrangement of journal bearing

Bearing Rows of Number of feedwidth feed holes holes per row

Arrangement

a0

25mm

2

4

Same phase

a

40mm

2

4

Same phase

b

40mm

2

4

Staggered

c

40mm

3

4

Staggered

Feed hole

Cr=Average radial clearance
40.00

Cr・δ

θ
θ=0

Axial direction

35.00
30.00
25.00
20.00
15.00
10.00
5.000
0.000
0.000

45.00

90.00

135.0

180.0

225.0

270.0

315.0

360.0

Circumferential
direction（a）Bearing a
40.00
30.00
25.00
20.00
15.00
10.00
5.000
0.000
0.000

45.00

90.00

135.0

180.0

225.0

270.0

315.0

360.0

Circumferential
direction（b）Bearing b
40.00
35.00

Axial direction

Fig.7 Coordinates andshape error of shaft

Axial direction

35.00

shaft

30.00
25.00
20.00
15.00
10.00
5.000
0.000
0.000

45.00

（c）Bearing c

90.00

135.0

180.0

225.0

270.0

315.0

360.0

Circumferential
direction

Fig.8 Calculated pressure variation induced by the shape error-73-

NTN TECHNICAL REVIEW No.72（2004）

5.1 Calculated pressure distribution at bearing
clearance

5.2 Run-out by profile errors
Fig. 9 shows the amplitude of high order run-out
obtained by the simple method introduced in areference3). Force on the shaft is first obtained fromthe pressure distribution on the shaft when the centersof the shaft and the bearing match, and then it isdivided by radial stiffness obtained by perturbation
method assuming small displacement. The quotient is
the approximate value of the radial displacement of
the shaft center. As in the case of Fig. 7, the profile
error that is consistent in the axial direction but varies
in the sinusoidal wave pattern in the circumferential
direction was calculated within the profile error rangeof 3~13 in the lobe number m.
The amplitude of the profile error wasδ= 0.1. The
vertical axis is the nondimensional run-out amplitude
εnormalized by the bearing radial clearance. The
number of peaks m of the profile error is also indicatedin the figures.
Bearings a0, a, and c, with the feed holes of k=4 on
the same circumference, produce run-out when the
profile error lobe number is m=nk±1. The run-out
magnitude of the εis about 1/2 ofδat the largest andit is smaller than that of the single-row feed hole typewith no circumferential groove as shown in Fig. 5. Onbearing a which maintained the same phasearrangement of the feed holes but increased thebearing width,εis larger than that of the conventional
spindle bearing a0. On the other hand, run-out issmaller on bearings b and c where the feed holes are
staggered. Bearing b, in particular, shows no run-out
when m=3, 5, 11, and 13. The m value at which theshaft produces run-out is the same as when eight feedholes are laid out. However, as shown in Fig. 10,
bearing b produces the moment of force that tilts the
shaft when m=3, 5, 11, and 13. On the other hand,bearings a0, a, and c do not produce the moment offorce. Fig. 10 shows the nondimensional moment offorce M normalized by the air supply pressure ps andFig. 8 shows the effects of the shaft profile errors onpressure distribution in the bearing clearance aboutbearings a, b, and c. The contour lines represent thedifferences between the pressure distributions withand without profile errors.
The black dots indicate the feed holes. The redregion indicates pressure rise due to the profile error,
and the blue region indicates pressure drop. As
shown in Fig. 7, the shaft center is assumed to matchthat of the bearing, the shaft's profile errors are givenby the five peaks of the sinusoidal waves in thecircumferential direction, and the profile is assumed
consistent in the axial direction. The amplitude of the
profile error is δ = 0.1, a non-dimensional value
normalized by the bearing clearance, which is
supposed to be largest at 0˚ coordinate in thecircumferential direction. A bearing which has tworows of feed holes arranged in the same phase showshigh pressure at wide clearances or at 0˚ positions
and low pressure on the opposite side. As a result, it
produces a force that pushes the shaft in the 180˚
direction, causing run-out of the shaft with respect tothe bearing. When the shaft rotates 1/2 turn (36˚) onthe lobe cycle, the bearing clearance widens in the
180˚ direction, which is reverse what is shown in Fig.
7. This increases the pressure, causing the shaft torun-out in the 0˚ direction. If the shaft turns another72˚ from its original state, the clearance returns to theoriginal amount. Thus, the profile errors of five peaks
causes run-out of fifth orders.
With bearing b that has two rows of feed holes laidout in the staggered manner, regions of high and low
pressures exist symmetrically with respect to thecenter line of the axial direction and cancel eachother. As a result, this configuration does not produce
force that caused the shaft to run-out in the radial
direction. However, since there is force that pushes
the shaft in the 0˚ direction in the top half of the figureand force that pushes the same in the 180˚ direction in
the bottom half, a moment of force is present that tilts
the shaft.
Bearing c has three rows of feed holes and the feedholes on the central row of which are staggered withrespect to the rows on the both sides. With thisbearing, the pressure distribution is the same as thatof bearing a. However, the magnitude of pressurechanges are much smaller than that of bearing a dueto the staggered layout of the feed holes on thecentral row.

Non-dimensional amplitude ε

0.06
5
0.05
m=5
0.04

0.02

13

3
7

11

3

5

13
7

a0

7

7
11

3

0.01
0

9

9

9

0.03

a

b

13
11

c

bearingFig.9 Calculated nondimensional amplitude ofrotational error-74-

Improvement of the Accuracy of an Air Spindle of the Nano-meter Order

0.007

bearing a0 and bearing c is about the equal to that ofbearing a0.
Regarding the damping factor (Fig. 12), bearings a
and b show the largest values in the region where σ
is small (low frequencies), but when σ is at or above
50, the damping factor drops almost as low as that ofbearing a0. If σ is at or above 20, the damping factor
of bearing c is the largest or about three times that ofbearing a0. In conclusion, bearing c can achieve theleast run-out relative to external disturbance near thecharacteristic frequencies of the spindle.

m=5

0.006

M

0.005
13

0.004
3

0.003

11

0.002
0.001
0

a0

a

b

c

BearingFig.10 Calculated nondimensional moment
Bearing aO

Bearing a, b

Bearing c

1.8

the bearing diameter D.
(M＝m f ／ps D3, m f : Moment of force)

1.6
1.4
1.2

K

5.3 Radial stiffness and damping factor
Run-out accuracy is subject to a shaft's response toexternal disturbance. Damping characteristics near
the natural frequencies of air spindle are particularly
important. For this reason, stiffness and the damping
factors of each bearing type were calculated andcompared. In this calculation, it was assumed that ashaft would not rotate but produce fine translationalmotion near the center of a bearing, and then theperturhation method was used to obtain radial
stiffness k and radial damping factor c with respect toa small displacement. The calculation did notconsider the profile error. Fig. 11 and 12 show nondimensional radial stiffness K and non-dimensionalradial damping factor C, respectively, that werenormalized by the air supply pressure ps , bearing
diameter D, average radial clearance Cr, and by the
angular frequency ω of translation frequency.
（K ＝k・Cr/ps D2，C＝C・Cr・ω/ps D2）
The horizontal axis in both figures indicates thesqueeze numberσ.
12μωR2
σ＝―――――Pa Cr2

（

1.0
0.8
0.6
0.4
0.2
0.0

0

50

100

150

200

σ
Fig.11 Calculated nondimensional radial stiffness

Bearing aO

Bearing a, b

Bearing c

0.7
0.6

C

0.5
0.4
0.3
0.2
0.1
0.0

0

50

100

150

200

σ
Fig.12 Calculated nondemensional damping factor）

Bearings a and b have the same layout of the feedholes and almost the same stiffness and the damping
factors, and therefore they are represented by a single
line in the figures.
Radial stiffness (Fig. 11) of bearings a, b, and c isabout double that of bearing a0 of the conventional
spindle in the 50<σregion due to large dynamic
pressure caused by extended bearing width.
As to radial stiffness (static stiffness) at σ≒0,bearings a and b are slightly greater than that of-75-

NTN TECHNICAL REVIEW No.72（2004）

Fig. 14 and 15 show the measurement results of
non-repetitive run-out of the conventional spindle andthe prototype unit, respectively. The measurement of
non-repetitive run-out was carried out in the following
method.
In the identical measuring system shown in Fig. 3,
3-point method, one capacitance type displacement
sensor was used.
¡Using the origin signals (once/rotation) from therotary encoder as triggers, outputs of thecapacitance type displacement meter for 2000
rotations were sampled and stored in the memory.
¡For the 2000 data values, difference of twoconsecutive data values was calculated to produce
1,999 non-repetitive run-out data.
Non-repetitive run-out measured in this fashion of
the air spindle unit was typically ± several nm. In
contrast, non-repetitive run-out of the conventional
spindle shown in Fig. 14 was ±33nm. This isbecause the air spindle used for this study had adummy clamp protruding significantly from the bearing
and a steel ball was secured at the tip of this clamp.
As compared to the conventional spindle, nonrepetitive run-out of the prototype unit (Fig. 15) was
reduced to about 1/2 or ±17nm. This improvement isbelieved to have been made by the improvement in
the damping factor of the journal bearing with thetriple-row feed hole configuration.

6. Measurement of run-out on a
prototype spindle
Based on the above study results, bearing c wasselected for prototype spindle, and repetitive run-out
and non-repetitive run-out were measured andcompared to those of the conventional spindle using
bearing a0. The structure of the prototype is the sameas one shown in Fig. 2 except that the total length of
the spindle is longer than that of the conventional
spindle by 30mm because the width of the journalbearing was extended by 15mm. The longer support
portion of the journal bearing will increase the angular
stiffness at the bearing. Except for the journalbearing, everything is the same between the prototype
and the conventional spindle.
Fig. 13 shows the repetitive run-out of the prototype
unit measured by the 3-point method. The
conventional spindle (Fig. 4) showed the peaks at the
5th, 9th, and the 17th order, but the prototype unit does
not exhibit any apparent peaks. The staggered feedholes on the prototype unit apparently reduced run-outof the 5th order that is caused by the profile error andthe feed hole layout. Furthermore, since the triple row
layout of the feed holes increased the damping factor,
run-out at the 9th and the 17th orders apparently
decreased.

Non-repetitive run-out nm

10
5
0
2

5

10

15

20

25

30

Rotational orderFig.13 Measured repetitive radial runout of the
improved spindle

50
40
30
20
10
0
-10
-20
-30
-40
-50
0

200

400

600

800

1000 1200 1400

1600 1800 2000

Measurement point

Fig.14 Measured non-repetitive runout of theconventional spindle
Non-repetitive run-out nm

Amplitude nm

15

50
40
30
20
10
0
-10
-20
-30
-40
-50
0

200

400

600

800

1000 1200 1400

1600 1800 2000

Measurement point

Fig.15 Measured non-repetitive runout of the
improved spindle-76-Improvement of the Accuracy of an Air Spindle of the Nano-meter Order

7. Conclusion
Calculations and experiments have verified theeffects of improvement in the run-out accuracy of a
new aerostatic journal bearing that has adopted triplerow feed holes with a staggered layout.
Run-out accuracy of air spindles is subject to manyother factors other than the bearing performance,
which was the focus of this study. All other factors
should be reviewed comprehensively. The accuracy
in the rotating speed also needs to be further
improved. NTN will continue to resolve these issues
so that we may provide the market with higher level ofprecision to meet the market needs.

Reference
1) Japanese Published Patent Application 2002-334498
KAWAGUCHI, YAMAZAKI, KUSAKAYA
2) AOKI, OZONO, J_JSPE, 32, 12 (1966), p831
3) YABE, Trans, JSME(c), 58, 548. (1992), p1177

Photos of authors

Yoshio FUJIKAWA

Tahashi HARAGUCHI

Kazuyuki AONO

Teruyoshi HORIUCHI

Precision Equipment Division
Product Engineering Dept.

Precision Equipment Division
Product Engineering Dept.

Precision Equipment Division
Product Engineering Dept.

Precision Equipment Division
Product Engineering Dept.

-77-

NTN TECHNICAL REVIEW No.72（2004）[ Technical Article ]Nanometer accuracy positioning system

Akio NAKAJIMA*This paper describes two types of very high precision positioning systems. One is the linear positioning stage that is
capable of one nanometer accuracy. This stage consists of an Air Slide, Linear Motor, and Laserscale. The other isthe rotational positioning Air Spindle that has a resolution of 151-million increments per revolution. We recently
completed development of an exclusive controller system, the Nanoscale Controller.

1. Introduction

2. Higher precision in positioning
technology

The production equipment for semiconductors,
optical disks, hard disks, and other information
devices requires highly accurate precision positioning
technology. Lately, accuracy as little as a nanometeris required of such equipment. This report describes
the development and commercialization of a precision
positioning system that is capable of stopping at
nanometer accuracy and has reduced speed
fluctuation at low speed movement. The mechanism
consists of an air slide that is driven by a voice coiltype linear motor. The amplifier is a newly developed
low-noise linear amplifier and the measurement
section uses a glass scale. As a result, the system
has exceeded the original expectations.
In the manufacture of optical disks and hard disks,
not only linear positioning but also angular positioning
may be required at times. NTN applied thispositioning technology to an air spindle that utilized anaerostatic bearing to accomplish angular resolution of
151.2 million pulses per circumference or nanometer
accuracy resolution. This paper also explains thisproduct.

*Precision Equipment Division

One of the applications of a high precision
positioning system is the optical disk mastering device(L.B.R.).
Photo 1 is the main portion of a L.B.R. This device
uses a spindle equipped with a rotating glass plate
and an optical head that moves in the radial directionof the glass plate, and sends short-wave laser tocreate spiral grooves.
For such a device, it is important for its positioning
system to accurately determine not only the final
target position, but also the positions in the path.
Therefore, the device must have accurate dynamic
performance. The accuracy requirements are
determined by the track pitch or the movement of the
optical head per rotation of the spindle. The track
pitch of a CD is 1.6μm and that for a DVD is 0.74μm.
However, the latest Blu-ray disk requires 0.32μm, 1/5
that of a CD.
One of the most important elements about theaccuracy of positioning systems is a position
measurement scale. L.B.R. typically requires 1~3%

Product Engineering Department-78-

Nanometer accuracy positioning system

normal track pitch.
Naturally, NTN's precision positioning systems have
improved the position measuring resolution every year
as shown in Fig. 1.
A laser interferometer is used for resolution of 10nm
and a glass scale for all other resolutions.
Generally, laser interferometers and capacitancetype sensors are often used for the measuring units ofprecision positioning systems. Presently at NTN, theglass scales are used more often. Capacitance typesensors have short range of measuring distance andexhibit big temperature drift, but they show excellent
frequency response characteristics. Laserinterferometers, on the other hand, are highly
accurate, but vulnerable to air disturbances.
The glass scale, though its cumulative error at long
distances is greater than that of a laser interferometer,is less prone to temperature changes and shows
relatively good accuracy within the track pitch
distance. The lattice pitch is typically the minimum of

about 1μm, and accuracy at the lower levels is
obtained by electrically dividing the sin/cos signals
produced by the scale. The division error is theinterpolative error. The interpolative error of the scaleoccurs at about 1% the lattice pitch. With theseissues taken into consideration, NTN engaged in ajoint development of a high precision scale with ascale manufacturer. As a result, we learned that inorder to increase accuracy and achieve it at or below
1nm within a narrow range, it was necessary tominimize the lattice pitch and select a smaller
interpolative error with respect to the lattice pitch.1) 2)

Photo 1 Main part of L.B.R.

Photo 2 Photograph of the positioning system

3. Nanometer positioning system
Photo 2 and Fig. 2 show the positioning table,
which is the mechanism for the nano-scale positioning
system. The system specifications are as follows.
This device is an application of optical heterodyne
FFT3). The performance requirements for this device
is the same as those for L.B.R.

12

10

Aerostatic table
10

Voice coil motor

190

6

430
130

4
2.7
130

2.7

Linear scale

2
1
'94

'95

'96

'97

'98

'99

0.86
'00

0.27

0.07

'01

'02

80

Resolution nm

8.6
8

0

Guide

10

Year

Fig.1 Trend of the positioning system resolution

Fig.2 Schematic of the positioning system-79-

NTN TECHNICAL REVIEW No.72（2004）

The table specifications are as follows.
The slide stroke, resolution, etc. can be changed
depending on applications.

As shown in Fig. 4, transient movement
accompanied by linear acceleration of about 0.04
second turned out well. If better acceleration method
or control method is used, better characteristics
should be achieved.
In order to maintain performance at the nano-scalelevel as one uses such a high precision positioning
system, various precautions must be taken. A damper
bench to eliminate impacts of floor vibration andstiffness of the components must be carefully
evaluated. Lack of stiffness will cause unnecessary
vibration which adversely affects the control system.
Connecting cables and hoses to moving portions of
the system must also be carefully installed to prevent
them from interfering with the operation of the system.4)

Guide ⋯⋯⋯⋯⋯⋯⋯⋯⋯⋯Air slide
Linear motor ⋯⋯⋯⋯⋯⋯Voice coil type
Materials ⋯⋯⋯⋯⋯⋯⋯⋯Aluminum alloy
Rated load ⋯⋯⋯⋯⋯⋯⋯30N
Stroke ⋯⋯⋯⋯⋯⋯⋯⋯⋯100mm
Resolution of scale ⋯⋯⋯About 0.2759nm
Thrust constant ⋯⋯⋯⋯⋯5.7N/A
Mass of moving portions ⋯4.02kg
Maximum current ⋯⋯⋯⋯4.0A
Control type⋯⋯⋯⋯⋯⋯⋯PID
Servo sample ⋯⋯⋯⋯⋯⋯50kHz
Amplifier ⋯⋯⋯⋯⋯⋯⋯⋯Linear amplifier
Actual measurement of straightness of the table
under rated load was as follows.
Up/down ⋯⋯⋯⋯⋯⋯⋯⋯0.09μm/100mm
Side to side ⋯⋯⋯⋯⋯⋯⋯0.01μm/100mm
The table and the scale are normally off position,
causing Abbe's error. Therefore, it is extremely
important for the table to possess straightness in theside to side direction.
As shown in Fig. 3, the speed fluctuation at or
below 1mm/s turned out to be ±1nm or less inposition conversion, which was good. Deviation in thestop positions was also within 1nm. Due to theabsence of external means that can easily measure
the table position by the units of 1nm or smaller,
signals of deviation between the controlling scale
signals and the command position signals are usedinstead. Thus, changes in the speed are indicated aschanges in the positions (deviation from the ideal
position). Accuracy of this system has been fully
verified indirectly by actually producing products using
this system.

Speed mm/s

1

Fluctuation nm

-1
-2
-3
0.08

0

0.1

0.2

0.05

0.3

Fig.4 Transient response of speed

0

0.06

0.35
0.02

Elapsed time s

1

0.04

1

0.4

0

2

0.02

1.05

0.6

0.2

3

0

0.8

0.1

Elapsed time s

Fig.3 Positioning error while moving at 1mm/s

-80-

0.08

0.4

Nanometer accuracy positioning system

until a minimum deviation of 1 pulse is produced.
Consequently, high-resolution rotary encoders of 1
million pulses or greater per turn may be used. Under
this method, accuracy of a rotary encoder is extremely
critical and is most closely related to the stopping
accuracy of the spindle. To increase the angularresolution, an appropriate rotary encoder must be
selected. The rotary encoder6) developed for use on
the hard disk servo track writer (STW), shown in Fig.
6, was used to develop a spindle unit that has
resolution up to 151.2 million pulses. The spindle
body is a standard spec unit of NTN. The encoder
head uses a reflective type head for detection of the
STW arm head position, and for a reflective typescale, an improved donut shape scale of 12.04mm in
effective radius for detection of the STW clock is used.
Fig. 7 shows the schematic of the encoder. The
spindle specifications are as follows.

4. Spindle unit for high-resolution
positioning
An air spindle supports a rotating shaft by supplying
air or other pressurized gases in between the rotating
and fixed portions, thereby forming a non-contact
structure.
Fig. 5 shows NTN's standard air spindle. The
structure of this air spindle consists of two journalbearings and two thrust bearings that support a
rotating shaft, an AC servo motor, and a non-contact
type optical rotary encoder directly connected to theshaft. Since the rotating portions are of a totally noncontact structure, there is very little friction. Also, theaveraging effect of aerostatic bearings improves
accuracy. For these characteristics, NTN's standard
air spindles are used in broad applications, especiallyin high precision fabrication and inspection. 5)

Spindle: NTN standard spindle(HRA3M equivalent 7)
Encoder: SONY P.T. BH10
Multiplying Equipment : SONY P.T. BB100
Pulse count: 302,400×500 Multiplying
Total pulse count: 151,200,000 pulses/turnResolution: 0.00857 second
Cumulative accuracy :±12 seconds(actual measurement for encoder)
Controller: Nano-scale controller
Made by NTN (linear drive)
Maximum rpm: 300rpm (NTN controller)
Stopping accuracy:±5~10 pulses(with NTN controller)

Thrust bearing

Journal bearing
Rotating shaft
Thrust bearing

AC servo motor

Rotary encoder

Fig.5 Construction of air spindle

For continuous rotation, the PLL control method issuitable. Rotational fluctuation (deviation in therotating frequency) can be controlled at or below
1PPM depending on the operating conditions. With
the PLL control method, phase difference between thecommand and the feedback pulses determines
deviation, and therefore it is possible to introduce
small deviations of less than 1/2 pulse into the control
system. On the contrary, deviation equal to or greaterthan 1/2 pulse can't be detected. Consequently, NTN
primarily uses those air spindles of which the rotary
encoders have low resolution of 1024 or 4096 pulses
per turn. With this system, rotation can't be stopped.
The deviation pulse method is used to determine
the stop position. This method can't detect deviation

Fig.6 Schematic of high resolution air spindle

Head

Laser beam
Scale

Fig.7 Schematic of rotary encoder-81-

NTN TECHNICAL REVIEW No.72（2004）

Error s

The resolution at the outer most circumference,
r=60mm, of a CD or a DVD, for example, is 2.5nm in
distance conversion. In reality, there is a positioning
error on NTN's controller.
The positioning accuracy was ±40 seconds. A 12sided polygon mirror was used for the measurement
and the expected measuring error is about±1 second.
Fig. 8 shows the measurement data.
The errors shown occurred in the form of asinusoidal wave, and therefore the rotational
synchronization was the main component. When thiscomponent was removed, the remaining 2nd orderand higher were about±5 seconds. The primarycause of the 1st order of this error was believed to be
misalignment of installation of the donut shaped scale.
However, since repeatability was within±1 second,
calibration should improve the absolute accuracy
markedly.
Due to the absence of means that can dynamicallymeasure fluctuation in the rotational speed, signals of
deviation between the controlling scale signals andthe command position signals are used instead.
Thus, changes in the indexing speed are indicated aschanges in the indexing positions (deviation from theideal position). As shown in Fig. 9, deviation was
more or less±0.1 second or less at low speeds.

60
50
40
30
20
10
-0
-10
-20
-30
-40
-50
-60

0

60

120

180

240

300

6. Control method
The control method for the servo unit is the popular
PID control for both the spindle and the slide. The
controller uses the same specifications of thepreviously mentioned nano-scale controller for superprecision positioning slides that is capable of stopping
at 1nm. Like the trends in the recent controllers, thescale directly receives the scale signals without
multiplying equipment, hence achieving high-speed
operation even though it accomplished high accuracy.
With switching elements becoming more popular inrecent years, PWM (Pulse Width Modulation)
amplifiers are often used to power motors. However,these amplifiers produce much noise and torquefluctuation among other issues. They also use
constant voltage, and therefore they are not suitablefor powering fine movement. The newly developed
device uses a sinusoidal wave linear amplifier to avoid
harming the hard disk head that handles very small
signals, and it has proven to show good results. Fig.
10 illustrates the main components of the amplifier.
In the positioning controller, the direction of micro
current reverses at high speeds when the device isstationary. For this reason, the amplifier demands
close attention to the crossover distortion, which, in
continuous rotation, would not become an issue. Fig.
11 shows an example of crossover. In general,occurrence of crossover distortion creates deadzones, affecting the servo unit and impairing accuracy.
For this reason, proper bias voltage is applied to the
FET drive or the final stage to prevent crossoverdistortion from occurring. Photo 3 is the developed
special controller and the driver.

360

+V

Angle ˚Current
detection

Fig.8 Cumulative positioning error
U

CommandV
current

0.3

W

-V

U-Channel amplifier

Deviation s

0.2

V-Channel amplifier

0.1

W-Channel amplifier

0.0

Fig.10 Diagram of amplifier

-0.1
-0.2
-0.3
0

0.02

0.04

0.06

0.08

0.1

Elapsed time s

Fig.9 Indexing error while rotating at 1 degree/s

-82-

Motor

Nanometer accuracy positioning system

10

7. Conclusion

8

A positioning system that can perform uniform
motion within 1nm deviation has been developed
along with its evaluation method. This system has
proven to be production-worthy. This paper alsopresented the structure of and the measurement
results using a super precision positioning spindle
based on the same technology. NTN will continue toresearch the technologies in an effort to develop
positioning systems that meet the market demands forhigher precision and accuracy.

6

Output A

420
Dead zone

-2
-4
-6
-8
-10
-1

-0.8

-0.6 -0.4

-0.2

0

0.2

0.4

0.6

0.8

1

Input V

Fig.11 Example of crossover distortion

Photo 3

Reference
1) Motohiro Takaya, Kayoko Taniguchi and Tadahiko
Shiman, Nanometetr Controlled Stage, digitalscaleinternational workshop, Tsukuba, (2001-11)
2) Hiroshi Saitou, high precision , high resolution Linear
encoder, Fuji, Technosystem, highly accuracy
presision positioning technology, (2000) 419
3) Akiko Hirai, Lijiang ZENG and Hirokazu Matumoto,
Heterogyne Fourier Transform Spectroscopy using
Moving Diffraction Grating, Jpn. J. Appl. Phys. Vol.40(2001) 6138 Part 1, No.10 Oct. 2001
4) Akio Nakajima, Nanometer accuracy positioning
technology to practical, optical disk's mejor field atapplication, engineering society, high accuracy
precision positioning special committees, No.2002-4(4) 27 2002.11.15. NTN corp.
5) Yoshimi Fujikawa, The Tribology 3 (2003), 44
6) Yukihiro Uematu, Masanori Fukushi, and Kayoko
Taniguchi，Development of the Pushpin free STWIEEE TRANSACTIONS ON MAGNETICS 37, NO.2,
MARCH 200
7) Air Spindle Unit NTN Catalog No.5403-III/J

Nanoscale Controller and amplifier

Photos of author

Akio NAKAJIMA
Precision Equipment Division
Product Engineering Dept.

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NTN TECHNICAL REVIEW No.72（2004）[ New Products Information ]FA Tapered Roller Bearings
Achieve long life through grain refinement and carbonitriding technologies for bearing steel
Fine Austenite Strengthening: Grain refining and carbonitriding technologies for bearing steel

Uniform refined grain
structure throughout
materials and carbonitride() reinforce
surfaces

0.05mmFA treated

0.05mmNormal quenching

The prior austenite grain boundaries

Performance (compared to NTN’s 4Top series)

Compact body¡Compact structure by FA treatment anddesign optimization

φ60

FA tapered roller bearing

φ30

φ72

4Top

Weight
400g ⇒180g（▲55％）
20.75

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φ30

¡Nearly 3 times longer life under lubrication
with clean oil¡Nearly 14 times longer life under lubrication
with contaminated oil¡Lower torque by 10%, or greater, within
normal rotational range¡Improved seizure resistance by 25% inrotating speed and 200% in surface pressure¡Reduced preload loss by half
¡Reduced assembly width settling rpm by half
¡Improved indentation resistance by about 1.5
times

15.5

New Products Information

Double-Row Thrust Needle Bearings
for automobile air-conditioners and compressors

Achieve long life, low torque, and low noise
through double-row rolling elements

Resin cage
Light-weight and improved
self-lubrication

Rolling elements
Two rolling elements
in one pocket

Features

Applications¡12 times longer life (from damage originating on surface)¡50% reduced rotating torque¡8dBA reduced noise¡Lightweight resin cage

¡Automobile air-conditioners and compressors

Performance (compared to current models)Rotating torque

Noise

80

90

50

10

85

60

Noise leve dBA

Current model New model

Current model
New model

Current model
New model

95

Rotating torque N･cm

Cumulative probability of damage %Life before damage on surface

40

20

100

Life h

1000

75

70

0
10

80

65
0.05

0.1

0.15

0.2

0.25

Load/Basic rated dynamic load

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1800

3600

Rotating speed min-1

NTN TECHNICAL REVIEW No.72（2004）

FA Needle Bearings for rocker arms
Achieve longer life for rocker arm bearings through adoption of FA treatment
Fine Austenite Strengthening: Grain refining and carbonitriding technologies for bearing steel

Features

Applications¡4 times longer life
¡20% lighter weight by size reduction(through improved allowable surface pressure)¡Engines and rocker arms

Performance
Correlation of surface contact pressure and life

95
90

50

4 times10
FA treated

Current model
5
10

100

Surface contact pressure GPa

Cumulative probability of damage %Comparison of life

1000

2.9

4 times
2.1

Current model

1.7
0Life in hours h-86-

FA treated

20％
UP

2.5

50

100

150

寿命時間

h

200

250

New Products Information

Ultra-Light GEN3 Hub Bearings
Extremely light, “Mass weight of 1.0kg”, hub bearing
for sub-compact automobiles

ld’s
Wor estt
ligh

Features

Applications¡Lightweight¡Compact design¡High-strength carbon steel(newly developed material)¡Long-life grease¡Long service life

¡Wheel bearings for sub-compact
automobiles

Improved performance andfuel consumption
by reduced unspring mass weight-87-

NTN TECHNICAL REVIEW No.72（2004）

GEN4 Hub Joints
Integrated new constant-velocity joint and GEN3 hub bearing,
Offering compact, lightweight, and easy-to-assemble GEN4 hub bearing with
Unique expansion crimp

Features

Structure¡Lightweight
Integration of the CVJ and bearing, adoption of ahollow joint shaft, and use of the new CVJ bearing
tightening method reduces the weight by 10% or
more.

¡Smaller axial dimension
Integration of the new constant-velocity joint andbearing reduces the axial dimension by 20% or
more.

¡High efficiency and low heat generation
Adoption of the new E-series constant-velocity joint
reduces torque loss by 30% during power
conveyance. Heat generation has also been
reduced by 20˚C compared to the conventional
type.

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New Products Information

Integrated Sensor Bearing with Absolute Encoder(Hall IC type)
Integrated sensor bearing with built-in absolute type encoder (hall IC type)and high precision home position

Structure
Magnetic encoder
Hall IC for home position

Hall IC for angles

Hall IC
2-Phase analog

Hall IC
1-Phase rectangular

AD converter
CPU

AD control module
External angle detection circuit

Angle

FeaturesHome position

Specifications¡Detects absolute angles¡Detects home position with high accuracy¡Lightweight/Compact¡Bearing number:#6811¡Sensor type:
Magnetic encoder + Hall sensor¡Angular accuracy: ±6˚C

Applications¡Home position repeatability:
0.05˚ or smaller¡Operating temperature range:
〜50˚C
0〜

¡Joints for robots¡Motors¡Locations of which absolute angles need
detection-89-

NTN TECHNICAL REVIEW No.72（2004）

Zero-Speed Compatible Active Type
Wireless ABS Sensor Unit
The wireless ABS sensor unit detects zero-speed and the direction of rotationof wheels by transmitting wireless feed to the active magnetic sensor andthe sensor signal transmitter that are built in the hub bearing.

Structure

Features¡Wireless feed
¡Detects rotating speed of ultra low speed(zero-speed compatible)¡Detects direction of rotation

MagnetOutputs rotating signals of different phases by 90˚(A/B phases)¡Lightweight/Compact¡Reduced assembly time
¡Improved design flexibility
Magnetic field sensor
Transmitter

Applications

Receiver¡Automobile axle units-90-

New Products Information“The First Mono-Zukuri Product Creation Awards” Innovative Product Award

Fluid Dynamic Bearing Unit
Hydrodynamic pressure bearing utilizing oil-impregnated sintered alloy is unitized.
“FDB Unit” realized low NRRO by radial/thrust hydrodynamic pressure effects.

Features

Applications¡Non-contact spindle rotation due tohydrodynamic pressure effects¡HDD spindle motors(PC's, HDD video cameras, automotive navigation
systems, etc.)

High running accuracy
Extremely low noise¡Spindle motors for high-density optical
disks¡Polygon scanner motors, etc.

¡Sintered alloy bearing sleeve with pressformed dynamic grooves
Superior manufacturability¡Sintered alloy impregnated withlubrication oil
Anti-seize features ensure long service life

¡Lead-free brass housing thrust busing
Environmentally sound design

Thanks to NTN’s unique product concept and the motor technology of Nippon Densan Co., NTN’s “FDB
Unit” won the “Innovative Product Award” at the “The First Mono-Zukuri Product Creation Awards”(sponsored by Nikkan Kogyo Shimbun, Ltd.).

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NTN TECHNICAL REVIEW No.72（2004）

Winner of ADY2004 <Display Testing Equipment Category> Grand Prix Award!

Multi-Repair System for LCD Color Filters
First in the industry, the Multi-Repair System consolidates all functions necessary torepair LCD color filters in one unit. This system dramatically improves productivity
and quality in the manufacturing processes for LCD color filters.

Laser cutting torepair black defects

Ink coating toeliminated white defects

This system has been developed jointly with Takano Co., Ltd.

Tape polishing tofix particle defects

Main functions¡Laser cutting function
Uses the second and third high-order YAG laser(equipped with the slit q function) to repair defects.

¡Ink coating function

FeaturesCoats ink on white defects with the coating needle(patented) (needle tip diameter: φ30, φ50, φ70μm).

¡Provides high quality repairs for a varietyof color filter defects¡Greatly reduces equipment costs,
required floor space, and overall repair
time¡Can be built with an integrated inspection
unit¡Can handle large glass color filter
substrates up to seventh generation(1870 x 2200mm) display panels¡Tape polishing function
Controls the polishing pressure while polishing particle
defects to a uniform height (accuracy: ±0.5mm).

¡Height measurement function
Measures particle height, using probe pressure of
1mN max., 0.1mm resolution, and measuring rangeof ±35μm.

¡Review function
Uses a glass surface plate to emit reflected andtransmitted light to make it possible to observe thestate of defects.
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