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Life Span


The life span of a rolling bearing is usually estimated according to Palmgren’s life span theory. According to DIN ISO 281 can the life span be calculated with the formula

 

Lh = (C/P)p.

 

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It is implied that

 

Lh the nominal life span with 10% breakdown,

C the basic dynamic load rating (according to DIN ISO 281, catalog data of the manufacturer)

P equivalent load.

 

 

The formula for the nominal life span holds also true for standard rolling bearings in good operating conditions. In cases where the conditions of lubrication are not ideal, dirt comes in the bearings or higher temperatures exist can the extending life span be calculated according to

 

L = a1 a2 a3 Lh.

 

a1  Matching coefficient fort life span (depending on failure probability) 
a2  Matching coefficient for steel materials and heat treatment 
a3  Matching coefficient for lubrication 


This formula holds only true under limited deviation of the ideal operating conditions. The formula is not applicable in cases of frequent deficiencies in lubrication or absence of adequate lubrication.

 

Test stand trials together with experiences gained through praxis demonstrate a multiple increase of the life span when using ceramic. This effect, which is especially a trybological characteristic of ceramic, is not considered by the norm. Hybrid rolling bearings outlast steel rolling bearings a100 times more under conditions where a shorter life span exists due to bad lubrication.

Especially in areas of extreme operating conditions, where steel bearings cannot be used or fail, are hybrid rolling bearings often interference free. In this case an estimation of the life span can only occur through calculating the hertzian pressure in the bearing. Further a comparison of results which are under the same stress needs to be made. For ceramic and hybrid rolling bearings do the lubricated media recommend the following:

 

at 46 mm²/s (Oil, grease)      Po < 2.400 MPa 
at 0,4 mm²/s (Water, 80°C)     Po < 1.500 MPa 
at 0 (=Dry running condition)     Po < 1.000 MPa 

 
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