Bearing LifeBearing life calculations are reported to be open for improvement and standardized under ISO 281, ISO 16281, and ISO 76.
Tolerance ClassesDepending on the application requirements, different tolerance classes are available. Bearing precision becomes essential for applications with smaller tolerances and higher speed requirements (such as bench housings). For non-demanding applications such as conveyor applications, tighter tolerance classes are not necessary. Bearing component tolerances depend on the housing and shaft production tolerances.
Bearing tolerances are defined in detail under ISO 492 and TS 6269 standards:
P0 (Normal) tolerance class : Normal operating and tolerance class (not shown)
P6 tolerance class : Tighter operating and tolerance class than P0
P5 tolerance class : Tighter operating and tolerance class than P6
P4 tolerance class : Tighter operating and tolerance class than P5
P2 tolerance class : Tighter operating and tolerance class than P4
Furthermore, bearing standards are available for chamfer dimensions; however, changes apply for special applications.
Bearing ClearanceBearing clearance is the free play in between the inner and the outer races of the bearing in radial and axial directions. Bearing clearance before mounting and after mounting might be different due to thermal expansion of the bearing rings and associated components, and compression or expansion due to different fits. Therefore, initial internal clearance and the operational clearance of a bearing might be different. In most applications, the initial internal clearance is greater than the operational clearance. Different clearances are available to choose the appropriate bearing clearance for the application requirements and working conditions.
Bearing clearances are defined in detail under ISO 5753-1, ISO 5753-2, and TS 6269 standards:
C2 clearance : Smaller radial clearance than C0
C0 (Normal) clearance : Normal clearance (not shown)
C3 clearance : Larger radial clearance than C0
4 clearance : Larger radial clearance than C3
C5 clearance : Larger radial clearance than C4
A set of rules while choosing the appropriate radial clearance is as follows:
1 - Normal clearance (C0) is chosen for normal working conditions and light press fit to one of the rings.
2 - If transition fit is needed, C2 clearance should be used. Fits should not be too tight.
3 - Bearings of increased clearance groups C3, C4 and C5 should be used when special operation coditions exists, tighter fits in case of high loads, press fits on both inner ring and outer ring of the bearing, large temperature difference between inner and outer ring, cooling of the housing and temperature increases in the shaft, etc.
Heat TreatmentORS bearings are generally heat-treated to retain dimensional stability up to 150 °C (302 °F). Bearings with a requirement of over 150 °C (302 °F) are specially heat-treated.
S0 (Normal): Operating temperature up to max. 150 °C (302 °F) (designation not shown on the bearing). Ring hardness: HRc 57-61
S1 : Operating temperature up to max. 200 °C (392 °F). Ring hardness: HRc 57-61
S2 : Operating temperature up to max. 250 °C (482 °F). Ring hardness: HRc 53-57
S3 : Operating temperature up to max. 300 °C (572 °F). Ring hardness: HRc 51-55
S4 heat treatment group : Operating temperature up to max. 350 °C (662 °F). Ring hardness: HRc 50-54
MaterialMaterial plays an important role in bearing performance and life. Repeated high stress, hardness, high stress at contact points, operating temperature, wear, impact loads, vibration, and stability affect are some of the factors to be considered during selection of the material.
Low-alloy, high-carbon, special steel is used for rings and rolling elements based on DIN 1.3505-100Cr6 norm.
Depending on the application some other materials such as other bearing steels, cage hardening steels, or any other special steels are used in ring and rolling element production.