What is a viscosity ratio

Modified service life

The modified service life, taking into account the state of lubrication, contamination of the lubricant and the material fatigue limit, is calculated in accordance with ISO 281: 2007 (E).

The extended modified life Lnm Taking into account special bearing properties and special operating conditions, the following equation is given for ball bearings:



L.nmExtended modified service life taking into account lubrication, contamination and material fatigue limit in 106 Revolutions (the index n indicates the deviation between the actual and the 100% probability of occurrence) 
a1Service life coefficient for a probability of experience deviating from 90%
aISOService life factor taking into account the operating conditions

In the case of a 90% probability of experience, formula 4 takes the following form:



In the case of a constant bearing speed, the bearing service life in operating hours is very often desired.
The following applies to ball bearings:



L.nmhextended modified service life in operating hours

Service life coefficient for the probability of experience a1

The service life coefficient for the probability of experiencea1 is required to have a life other than the fatigue life L10m to determine d. H. a service life that reaches or exceeds a 90% probability of being experienced. Values ​​for a1 are shown in Table 3 below.

Table 3: Service life coefficient for the probability of experience a1


Variable bearing loads and speeds

If the load and / or the speed of dynamically stressed ball bearings change over a period of time, this must be taken into account when calculating the equivalent bearing service life. The bearing life is typically made up of a large number of individual loads [P1,2, ... t] and speeds [n1,2, ... t] of a certain duration [q1,2, ... t] together. This means that the bearing has an equivalent bearing load P1 is loaded and meanwhile with a speed n1 rotates and so on. For such composite operating conditions, the extended modified life can be calculated as follows:





L.10m1, L10m2, L10m3 ..... can be calculated according to formula 5 for ball bearings.

n1,2, ... tSpeed ​​during the partial operating state in revolutions per minute
nmAverage operating speed in revolutions per minute
q1,2, ... tProportional operating status in relation to the total operating time in percent
D.PWPitch diameter of the ball set in millimeters
dBearing bore diameter (see product tables)
D.Bearing outside diameter (see product tables)

Service life factor aISO

The service life coefficient aISO takes into account, among other things, the influence
  • the material fatigue limit of the rolling bearing components through the fatigue limit load Cu
  • the degree of contamination by the factor ec
  • the lubrication condition through the viscosity ratio

Fatigue limit load Cu

The stress in the raceway that is decisive for fatigue ultimately depends on the internal stress distribution in the rolling contact of a bearing that is subjected to the highest stress. To simplify this calculation, the fatigue limit load Cu introduced. The fatigue limit load is defined as the load below which no material fatigue occurs under laboratory conditions.

This factor takes into account:
  • the type, size and internal geometry of the bearing
  • the profiling of rolling elements and raceways
  • the manufacturing quality of the bearing
  • the fatigue limit of the bearing material
The fatigue limit load Cu of our single and double row deep groove ball bearings as well as that of our single and double row angular contact ball bearings is compiled in the product tables for all of our bearing types.

Contamination coefficient ec

The coefficient ec takes into account the influence of impurities such as B. solid particles in the lubrication gap, on the service life. The reduction in service life due to solid particles depends on their size in relation to the height of the lubricant film, their hardness and their quantity.

Table 4: Contamination coefficient ec


More detailed information about the pollution coefficient ec depending on the viscosity ratio and the bearing size can be found in ISO 281: 2007 (E).

Illustration 1: Viscosity as a function of temperature for various mineral oils


Figure 2:Reference viscosity


Viscosity ratio

The viscosity ratio serves as a measure of the quality of the lubricating film formation. is the ratio of the kinematic viscosity of the lubricant at operating temperature to the reference viscosity and can be calculated using the following equation:



Viscosity ratio
Operating viscosity of the lubricant at operating temperature in mm2/ s (approximate values ​​for mineral oils and fats with mineral base oil from Fig. 1)
Reference viscosity in mm2/ s

The operating viscosity of the base oil is decisive for greases. The reference viscosity, which depends on the bearing speed n and the pitch circle diameter DPW of the ball bearing can be seen in Figure 2:

Service life coefficient aISO

The service life coefficient aISO can be seen in Figure 3.


Figure 3: Service life coefficient aISO for radial ball bearings according to ISO 281: 2007 (E)


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