●Bearing Materials

13. Bearing Materials

steel which have undergone standard heat treatment are

13.1 Raceway and rolling element materials used at temperatures above 120˚C for long durations,
While the contact surfaces of a bearing's raceways and unacceptably large dimensional changes can occur. For this
rolling elements are subjected to repeated heavy stress, they reason, a dimension stabilizing treatment (TS treatment) has
still must maintain high precision and rotational accuracy. To been devised for very high temperature applications. This
accomplish this, the raceways and rolling elements must be treatment however reduces hardness of the material, thereby
made of a material that has high hardness, is resistant to reducing rolling fatigue life. (See item 3.3.2 on page A-18.)
rolling fatigue, is wear resistant, and has good dimensional
stability. The most common cause of fatigue in bearings is For standard high temperature bearings used at
the inclusion of non-metallic impurities in the steel. Non- temperatures from 150˚C – 200˚C, the addition of silicone to
metallic inclusion includes hard oxides that can cause fatigue the steel improves heat resistance and results in a bearing
crack. Clean steel with minimal non-metallic inclusion must with excellent rolling fatigue life with minimal dimensional
therefore be used. change or softening at high temperatures.

For all NTN bearings, steel low in oxygen content and non- A variety of heat resistant steels are also incorporated in
metallic impurities, then refined by a vacuum degassing bearings to minimize softening and dimensional changes
process as well as outside hearth smelting, is used. For when used at high temperatures. Two of these are high
bearings requiring especially high reliability and long life, speed molybdenum steel and high speed tungsten steel. For
steels of even higher in purity, such as vacuum melted steel bearings requiring heat resistance in high speed
(VIM, VAR) and electro-slag melted steel (ESR), are used. applications, there is also heat resistant case hardening
molybdenum steel. (refer to Table 13.3)
1) High/mid carbon alloy steel
In general, steel varieties which can be hardened not just 4) Corrosion resistant bearing steel
on the surface but also deep hardened by the so-called For applications requiring high corrosion resistance,
"through hardening method" are used for the raceways and stainless steel is used. To achieve this corrosion resistance
rolling elements of bearings. Foremost among these is high a large proportion of the alloying element chrome is added to
carbon chromium bearing steel, which is widely used. For martensite stainless steel. (Table 13.4)
large type bearings and bearings with large cross sectional
dimensions, induction hardened bearing steel incorporating 5) Induction hardened steel
manganese or molybdenum is used. Also in use is mid- Besides the use of surface hardening steel, induction
carbon chromium steel incorporating silicone and hardening is also utilized for bearing raceway surfaces, and
manganese, which gives it hardening properties comparable for this purpose mid-carbon steel is used for its lower carbon
to high carbon chromium steel. content instead of through hardened steel. For induction
Table 13.1 gives chemical composition of representative hardening of the deep layers required for larger bearings and
high carbon chrome bearing steel that meets JIS standards. bearings with large surface dimensions, mid-carbon steel is
SUJ2 is frequently used. SUJ3 with enhanced hardening fortified with chrome and molybdenum.
characteristics containing a large quantity of Mn is used for
large bearings. SUJ5 is SUJ3 to which Mo has been added
6) Other bearing materials
to further enhance hardening characteristics, and is used for
For ultra high speed applications and applications requiring
oversized bearings or bearings with thick walls.
very high level corrosion resistance, ceramic bearing
The chemical composition of SUJ2 is equivalent to AISI
materials such as Si3N4 are also available.
52100 (US) and DIN 100Cr6 (Germany).
13.2 Cage materials
2) Case hardened (carburizing) steel Bearing cage materials must have the strength to
Carburizing hardens the steel from the surface to the withstand rotational vibrations and shock loads. These
proper depth, forming a relatively soft core. This provides materials must also have a low friction coefficient, be light
hardness and toughness, making the material suitable for weight, and be able to withstand bearing operation
impact loads. NTN uses case hardened steel for almost all of temperatures.
its tapered roller bearings. In terms of case hardened steel
for NTN's other bearings, chromium steel and chrome For small and medium sized bearings, pressed cages of
molybdenum steel are used for small to medium sized cold or hot rolled steel with a low carbon content of approx.
bearings, and nickel chrome molybdenum steel is used for 0.1% are used. However, depending on the application,
large sized bearings. austenitic stainless steel is also used.
Table 13.2 gives the chemical composition of
representative JIS case hardened steel. Machined cages are generally used for large bearings.
Carbon steel for machine structures or high-strength cast
3) Heat resistant bearing steel brass is frequently used for the cages, but other materials
When bearings made of ordinary high carbon chromium such as aluminum alloy are also used.

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●Bearing Materials

Tables 13.5 and 13.6 give the chemical composition for also widely used for cages. Polyamide resin reinforced with
these representative cage materials. glass fibers is generally used. Cages made of high-polymer
materials are lightweight and corrosion resistant. They also
Besides high-strength brass, medium carbon nickel, have superior damping and characteristics and lubrication
chrome and molybdenum that has been hardened and performance. Heat resistant polyimide resins now enable
tempered at high temperatures are also used for bearings the production of cages that perform well in
used in aircraft. The materials are often plated with silver to applications ranging between -40˚C – 120˚C. However,
enhance lubrication characteristics. they are not recommended for use at temperatures
exceeding 120˚C.
High polymer materials that can be injection molded are

Table 13.1 Chemical composition of representative high carbon chrome bearing steels
Chemical composition (%)
Standard Symbol Remarks
C Si Mn P S Cr Mo
SUJ2 0.95∼1.10 0.15∼0.35 Max. 0.50 Max. 0.025 Max. 0.025 1.30∼1.60 Max. 0.08
JIS G 4805 SUJ3 0.95∼1.10 0.40∼0.70 0.90∼1.15 Max. 0.025 Max. 0.025 0.90∼1.20 Max. 0.08
SUJ5 0.95∼1.10 0.40∼0.70 0.90∼1.15 Max. 0.025 Max. 0.025 0.90∼1.20 0.10∼0.25
ASTM A295 52100 0.98∼1.10 0.15∼0.35 0.25∼0.45 Max. 0.025 Max. 0.025 1.30∼1.60 Max. 0.10 SUJ2 equivalent
Grade 1 0.90∼1.05 0.45∼0.75 0.95∼1.25 Max. 0.025 Max. 0.025 0.90∼1.20 Max. 0.10 SUJ3 equivalent
ASTM A485
Grade 3 0.95∼1.10 0.15∼0.35 0.65∼0.90 Max. 0.025 Max. 0.025 1.10∼1.50 0.20∼0.30 SUJ5 equivalent

Table 13.2 Chemical composition of representative case hardened steel (carburizing steel)
Chemical composition (%)
Standard Symbol
C Si Mn P S Ni Cr Mo
JIS G 4104 SCr420 0.18∼0.23 0.15∼0.35 0.60∼0.85 Max. 0.030 Max. 0.030 ― 0.90∼1.20 ―
JIS G 4105 SCM420 0.18∼0.23 0.15∼0.35 0.60∼0.85 Max. 0.030 Max. 0.030 ― 0.90∼1.20 0.15∼0.30
SNCM220 0.17∼0.23 0.15∼0.35 0.60∼0.90 Max. 0.030 Max. 0.030 0.40∼0.70 0.40∼0.65 0.15∼0.30
JIS G 4103 SNCM420 0.17∼0.23 0.15∼0.35 0.40∼0.70 Max. 0.030 Max. 0.030 1.60∼2.00 0.40∼0.65 0.15∼0.30
SNCM815 0.12∼0.18 0.15∼0.35 0.30∼0.60 Max. 0.030 Max. 0.030 4.00∼4.50 0.70∼1.00 0.15∼0.30
5120 0.17∼0.22 0.15∼0.35 0.70∼0.90 Max. 0.030 Max. 0.040 ― 0.70∼0.90 ―
4118 0.18∼0.23 0.15∼0.35 0.70∼0.90 Max. 0.030 Max. 0.040 ― 0.40∼0.60 0.08∼0.15
ASTM A534 8620 0.18∼0.23 0.15∼0.35 0.70∼0.90 Max. 0.030 Max. 0.040 0.40∼0.70 0.40∼0.60 0.15∼0.25
4320 0.17∼0.22 0.15∼0.35 0.45∼0.65 Max. 0.030 Max. 0.040 1.65∼2.00 0.40∼0.60 0.20∼0.30
9310 0.08∼0.13 0.15∼0.35 0.45∼0.65 Max. 0.025 Max. 0.025 3.00∼3.50 1.00∼1.40 0.08∼0.15

Table 13.3 Chemical composition of high-speed steel

Chemical composition (%)
Standard
C Si Mn P S Cr Mo V Ni Cu Co W
6491 (M50) 0.77∼0.85 Max. 0.25 Max. 0.35 Max. 0.015 Max. 0.015 3.75∼4.25 4.00∼4.50 0.90∼1.10 Max. 0.15 Max. 0.10 Max. 0.25 Max. 0.25
AMS 5626 0.65∼0.80 0.20∼0.40 0.20∼0.40 Max. 0.030 Max. 0.030 3.75∼4.50 Max. 1.00 0.90∼1.30 ― ― ― 17.25∼18.25
2315 (M50NiL) 0.11∼0.15 0.10∼0.25 0.15∼035 Max. 0.015 Max. 0.010 4.00∼4.25 4.00∼4.50 1.13∼1.33 3.20∼3.60 Max. 0.10 Max. 0.25 Max. 0.25

Table 13.4 Chemical composition of stainless steel

Chemical composition (%)
Standard Symbol
C Si Mn P S Cr Mo
JIS G 4303 SUS440C 0.95∼1.20 Max. 1.00 Max. 1.00 Max. 0.040 Max. 0.030 16.00∼18.00 Max. 0.75
AISI 440C 0.95∼1.20 Max. 1.00 Max. 1.00 Max. 0.040 Max. 0.030 16.00∼18.00 Max. 0.75

Table 13.5 Chemical composition of steel plate for pressed cages and carbon steel for machined cages
Chemical composition (%)
Standard Symbol
C Si Mn P S Ni Cr
JIS G 3141 SPCC ― ― ― ― ― ― ―
Pressed JIS G 3131 SPHC ― ― ― Max. 0.050 Max. 0.050 ― ―
retainer BAS 361 SPB2 0.13∼0.20 Max. 0.04 0.25∼0.60 Max. 0.030 Max. 0.030 ― ―
JIS G 4305 SUS304 Max. 0.08 Max. 1.00 Max. 2.00 Max. 0.045 Max. 0.030 8.00∼10.50 18.00∼20.00
Machined retainer JIS G 4051 S25C 0.22∼0.28 0.15∼0.35 0.30∼0.60 Max. 0.030 Max. 0.035 ― ―

Table 13.6 Chemical composition of high-strength cast brass for machined cages
Chemical composition (%) Impurities
Standard Symbol
Cu Zn Mn Fe Al Sn Ni Pb Si
JIS H 5120 CAC301 55.0∼60.0 33.0∼42.0 0.1∼1.5 0.5∼1.5 0.5∼1.5 Max. 1.0 Max. 1.0 Max. 0.4 Max. 0.1

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