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NTN TECHNICAL REVIEW No.742006

[ New Product ]

Ultra-Clean Bearing for Clean Environments

1. Overview

Extremely low dust generation is needed for the

bearings used in the systems and machines that

manufacture semiconductor devices and liquid crystal

display panels, as well as for those used in the

vacuum evaporation systems for these products.

Recently, demand has been mounting for longer

maintenance intervals for these devices and systems.

Given these demands, the bearings used in these

situations must be capable of durable operation in

high-temperature and high-load environments. To

address these engineering challenges, NTN has

developed a unique lubrication system, and with this,

they have successfully developed unique Ultra-Clean

NC bearings that feature long life in vacuums underhigh bearing pressure and unique Ultra-Clean WB

bearings that boast long life in high-temperature

environments. This report introduces these unique

bearings.

2. Introduction to NTN Ultra-Clean NCBearings (Fig. 1)

For lubrication, the authors adopted a porous resin

cage that was developed independently by NTN. This

cage is impregnated with a low-vapor-pressure oil(Fig. 2).

*Industrial Engineering Dept. Industrial Sales headquarters

NTNmanufactures Ultra-Clean bearings for clean

environments used for a semiconductor and LCD panel

fabrication equipments, vacuum evaporation systems, etc.

NTN developed Ultra-Clean NC bearings and Ultra-Clean

WB bearings to supply the demand for a long-life under high

temperature and heavy load conditions.

Ryouichi NAKAJIMA*

Masaaki HONDA*

Fig. 1 Ultra-Clean NC bearingNCNetwork Cell

Fig. 2 Porous resin cage

Pore

400

Conventional oil-impregnated resin cages are made

of sintered resin materials, and, consequently, their

porosity, the ratio of pores to retain lubricating oil, was

20% at maximum. In contrast, NTNs newly developed

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Ultra-Clean Bearing for Clean Environments

oil-impregnated porous resin cage features 35% or

higher porosity thanks to a special manufacturing

technique, allowing this cage to retain more lubricating

oil. Compared to conventional ultra-clean bearings,

NTNs new ultra-clean bearings are capable of longer

life while maintaining low dust generation in high load

applications.

2.1 Evaluation of various characteristics

2.1.1 Evaluation of dust generation characteristics

Under the test conditions summarized in Table 1,

the test bearing was allowed to run for 150 hours, and

the amount of resultant dust generated during the

duration of the test was measured. As summarized in

Fig. 3, the dust generated with the Ultra-Clean NC

bearing was equivalent to the amount generated by

bearing types usually used for clean applications (alow-dust-generating prefilled-grease type for vacuum-

use and a special PTFE-coated type).

2.1.2 Rotating torque

NTN evaluated the rotating torque for the Ultra-

Clean NC bearing under the test conditions

summarized in Table 2. As illustrated in Fig. 4, the

Ultra-Clean NC bearing features a lower torque

compared to the conventional bearing (special PTFE-

coated type).

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Fig. 3 Results of dust generation test

1

10

100

1000

Numberofdustparticlesgenerated

Ultra-CleanNC bearing

Low-dust-generatingprefilled-grease type

for vacuum-use

SpecialPTFE-coated

type

Fig. 4 Results of rotating torque

0

10

30

40

20

50

R

otatingtorque

mN

m

Ultra-CleanNC bearing

Low-dust-generatingprefilled-grease type

for vacuum-use

SpecialPTFE-coated

type

Fig. 5 Results of endurance test

1

10

1000

100

10000

Life

h

Cessation of test run

196N 686N 981N

Ultra-Clean NC bearing

Low-dust-generating prefilled-grease type for vacuum-use

Special PTFE-coated type

Ce ss at ion o f t es t r un C es sa ti on o f t es t r un

Table 1 Test conditions of dust generation

Degree of vacuum

Test bearing

10-5Pa

50 min-1

0.2m or greater

9.8N

Ordinary temperature

150 hrs.

Size of particles being counted

Thrust load

Running speed

Temperature

Duration

Deep groove ball bearing (#608)8227 mm

Table 2 Test conditions

Degree of vacuum

Test bearing Deep groove ball bearing (#608) 8227 mm

10-5Pa

500 min-1

Ordinary temperature

196NThrust load

Running speed

Temperature

Table 3 Test conditions of endurance test

Degree of vacuum

Test bearing Deep groove ball bearing (#608) 8227 mm

10-5Pa

500 min-1

Ordinary temperature

196N686N981N

The point when a sudden increase in torque occurred

Thrust load

End of life criterion

Running speed

Temperature

2.1.3 Endurance evaluation

Each bearing type was evaluated at each of the

three thrust load levels specified in Table 3. As can be

seen from the data in Fig. 5, the Ultra-Clean NC

bearing boasts a longer life in all three loading

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NTN TECHNICAL REVIEW No.742006

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3.1 Evaluation of various performance

considerations

3.1.1 Rotating torqueUnder the test conditions summarized in Table 4,

the rotating torque of the Ultra-Clean WB bearing in a

vacuum was evaluated. As can be seen from the

Fig. 7 WS2 ball

Fig. 6 Ultra-Clean WB bearing

WBWS2 BallLubrication with tungsten disulfide ball

conditions when compared to the conventional

bearings. In particular, under a high-loading condition,

the life of the authors Ultra-Clean NC bearing is more

than 10 times as long compared to the conventional

bearings subjected to the same demanding

conditions.

3. Introduction to NTN Ultra-Clean WBBearings (Fig. 6)

Aware of the lubricating power of tungsten disulfide

(hereafter, WS2), NTN adopted one rolling element

made of this material (Fig. 7) in the Ultra-Clean WB

bearing. When the WS2 rolling element rolls in the

bearing, WS2 is transferred onto the rolling surface

and functions as a solid lubricant. Additionally, since

WS2 can withstand a maximum temperature of 400C,which is much higher heat resistance compared to

conventional special PTFE coating, the Ultra-Clean

WB bearing is an ideal choice for vacuum and high-

temperature environments such as those on vacuum

evaporation systems and sputtering systems.

WS2 rolling element

Fig. 8 Results of rotating torque

0

5

20

15

25

Rotatingtorque

mN

m

Ultra-CleanWB bearing

SpecialPTFE-coated

type

Table 4 Test conditions of rotating torque

Degree of vacuum

Test bearing Deep groove ball bearing (#608) 8227 mm

10-5Pa

500 min-1

Ordinary temperature

196NThrust load

Running speed

Temperature

results in Fig. 8, the rotating torque with the Ultra-

Clean WB bearing is much lower than that with the

conventional bearing (special PTFE-coated type).

3.1.2 Endurance

1) High-temperature (300C) endurance test

The applicable test conditions are summarized in

Table 5, while the test results are illustrated in Fig. 9.

The life of the Ultra-Clean WB bearing is more than

five times as long as that of the conventional bearing

(special PTFE-coated type).

2) High-load endurance test

The test conditions used are summarized in Table

6, while the test results obtained are illustrated in Fig.10. The life of the Ultra-Clean WB bearing is more

than nine times as long as that of the conventional

bearing (special PTFE-coated type).

4. Conclusion

Products incorporating flat panel displays, such as

personal computers and flat-panel TV sets, have

become used more commonly, and demands are

growing rapidly for bearings capable of use in the

clean-environment manufacturing systems for theseproducts. NTN believes that our Ultra-Clean NC

bearings and Ultra-Clean WB bearings, which are

enhanced variants of conventional Ultra-Clean

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Fig. 10 Results of heavy load endurance test

1

10

1000

100

10000

100000

E

ndurance

h

Ultra-CleanWB bearing

Cessation of test run

SpecialPTFE-coated

type

Table 6 Test conditions

Deep groove ball bearing (#608) 8227 mm

10-5Pa

500 min-1

Ordinary temperature

256 N

The point when a sudden increase in torque occurred

Degree of vacuum

Test bearing

Thrust load

End of life criterion

Running speed

Temperature

Table 7 Performance of Ultra-Clean bearing series

Lubrication systemSpecial

PTFE-coatingLow dust-generating

vacuum grease

Porous resin cage+

low vapor pressure oil

Tungsten disulfide(WS2)

Ultra-Cleanbearing

Ultra-Cleanbearing

Ultra-CleanNC bearing

Ultra-CleanWB bearing

Load bearing capacity

Low torque feature

Dust generation

Corrosion resistance

Temperature range tolerance (C)

Degree of vacuum tolerance (Pa)

+260 +200 +150 +400

10-6atm 10-5atm 10-5atm 10-5atm

Optimal Good Acceptable

Fig. 9 Results of high temperature endurance test

1

10

1000

100

10000

En

durance

h

Ultra-CleanWB bearing

Cessation of test run

SpecialPTFE-coated

type

Table 5 Test conditions

Deep groove ball bearing (#608) 8227 mm

10-5Pa

1,550 min-1

300C

9.8N

The point when a sudden increase in torque occurred

Degree of vacuum

Test bearing

Thrust load

End of life criterion

Running speed

Temperature

Photos of authors

Ryouichi NAKAJIMA

Industrial Engineering Dept.

Industrial Sales headquarters

Masaaki HONDA

Industrial Engineering Dept.

Industrial Sales headquarters

bearings, will satisfy the characteristics required of

these manufacturing systems: low dust generation,

tolerance of high vacuum conditions, and higher levels

of endurance and heat resistance. NTN has already

added new products to its Ultra-Clean bearing Series

(see Table 7). We will continue to develop unique

products that satisfy market needs, and we hope to

contribute to improvements in various vacuumsystems by helping extend their maintenance intervals

and reduce their energy consumption.

Ultra-Clean Bearing for Clean Environments