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Miniature & small ball bearings

3. Material

Ball bearing performance is significantly influenced by the selection of the proper material for components such as balls, inner and outer rings.

Bearings are exposed to severe stress. The contact area of inner and outer rings, as well as the balls, are repeatedly exposed to stress exceeding 1000 MPa.Material type, purity and hardness are important factors for long bearing life under repeated high stresses.
High carbon chromium bearing steel and high corrosion-resistant martensitic stainless steels are used for raceway rings and balls in NMB products.
High-carbon chromium bearing steel uses high-grade vacuum degassed high-carbon chromium bearing steel (JIS G 4805 SUJ 2, AISI / SAE E52100) or equivalent material.With proper heat treatment, they have high load capacity,longer life and low noise levels.
MinebeaMitsumi developed stainless steel "DD400" material.
The hardness of DD400 after heat treatment is higher than that of SUS440C. The DD400 material also has excellent service life and load resistance.
The noise level of DD400 material is close to that of chromium steel because the carbon is dispersed spherically.
From the results of tests based on ASTM-A380, the corrosion resistance of DD400 material is equal to SUS440C.

To meet longer life and low noise level requirements, ball bearings with ceramic balls are also available.
When compared to the conventional chromium bearing steel balls, improved noise level and life can be achieved by using ceramic balls (Silicon Nitride) with the use of using ceramic balls (Silicon Nitride) with the use of conventional chromium rings. The extremely low conductive characteristics (insulation properties) of silicon nitride protect bearings from electric corrosion, which is generated under a conductive environment.

Material properties

High carbpm chromium bearing steel

Standard Symbol Chemical Composition (wt%)
C Si Mn P S Cr Mo
JIS G 4805 SUJ 2 0.95~1.10 0.15~0.35 0.50 or less 0.025 or less 0.025 or less 1.30~1.60 -
AISI/SAE E52100 0.98~1.10 0.15~0.35 0.25~0.45 0.025 or less 0.025 or less 1.30~1.60 -

Stainless steel

Standard Symbol Chemical Composition (wt%)
C Si Mn P S Cr Mo
- DD400 0.60~0.75 1.00 or less 1.00 or less 0.03 or less 0.02 or less 11.50~13.50 0.30 or less

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