Factor ηc for contamination level

This factor was introduced to consider the contamination level of the lubricant in the bearing life calculation. The influence of contamination on bearing fatigue depends on a number of parameters including bearing size, relative lubricant film thickness, size and distribution of solid contaminant particles, types of contamination (soft, hard etc.) The influence of these parameters on bearing life is complex and many of the parameters are difficult to quantify. It is therefore not possible to allocate precise values to ηc that would have general validity. However, some guideline values are provided in table.
If the bearing is used in an application with a satisfactory record in the field and past life calculations were based on the use of the old adjustment factor a23, then a corresponding (implicit value) ηc factor can be derived to give an aSKF equivalent to the a23 adjustment as explained in the section A special case - the adjustment factor a23.
Note that this approach will probably indicate only an approximate value of the effective ηc for the contamination level of the application. A second method to obtain a value for the factor ηc that is representative for an application is by quantifying the contamination level of the lubricant as input for the evaluation of the value for the factor ηc.

ISO contamination classification and filter rating

The standard method for classifying the contamination level in a lubrication system is according to ISO 4406:1999. In this classification system the result of the solid particles counting is converted into a code using a scale number, see table and diagram 1.
One method for checking the contamination level of the bearing oil is the microscope counting method. With this counting method two scale numbers, relating to the number of particles ≥ 5 μm and ≥ 15 μm are used. Another method refers to automatic particle counters, where three scale numbers are used relating to the number of particles ≥ 4 μm, ≥ 6 μm and ≥ 14 μm. The classification of the contamination level comprises three scale numbers.
Typical examples of contamination level classifications for lubricating oil are –/15/12 (A) or 22/18/13 (B) as shown in diagram 2.
Example A means that the oil contains between 160 and 320 particles ≥ 5 μm and between 20 and 40 particles ≥ 15 μm per millilitre oil. Though it would be ideal if lubricating oils were continuously filtered, the viability of a filtration system would depend on the optimization between increased costs and increased service performance of the bearing.
A filter rating is an indication of filter efficiency. The efficiency of filters is defined as the filter rating or reduction factor β, which is related to a given particle size. The higher the β value, the more efficient the filter is for the specified particle size. Therefore both the β value and the specified particle size have to be considered. The filter rating β is expressed as the relationship between the number of specified particles before and after filtering. This can be calculated as follows:

βx = n1/n2


filter rating related to a specified particle size x
particle size [μm]
number of particles per volume unit (100 ml) larger than x, upstream the filter
number of particles per volume unit (100 ml) larger than x, downstream the filter


The filter rating β only relates to one particle size in μm, which is shown as the index e.g. β3, β6, β12, etc. For example, a complete rating "β6 = 75" means that only 1 of 75 particles of 6 μm or larger will pass through the filter.

Determination of ηc when oil contamination level is known

For oil lubrication, once the oil contamination level is known, either from microscopic counting or from an automatic particle counter analysis described in ISO 4406:1999, or indirectly as a result of the filtration ratio that is applied in an oil circulation system, this information can be used to determine the factor ηc for the contamination level. Note that the factor ηc cannot be derived solely from only the measure of oil contamination. It depends strongly on the lubrication condition, i.e. κ and the size of the bearing. A simplified method according to DIN ISO 281 Addendum 4:2003 is presented here to obtain the ηc factor for a given application. From the oil contamination code, (or filtration ratio of the application) the contamination factor ηc is obtained, using the bearing mean diameter dm = 0,5 (d + D), mm, and the viscosity ratio κ of that bearing (diagrams 3 and 4).
Diagrams 5 and 6 provide typical values for the factor ηc for circulating oil lubrication with different degrees of oil filtration and oil contamination codes. Similar contamination factors can be applied in applications where an oil bath shows virtually no increase in the contamination particles present in the system. On the other hand, if the number of particles in an oil bath continues to increase over time, due to excessive wear particles or the introduction of contaminants, this must be reflected in the choice of the factor ηc used for the oil bath system as indicated in DIN ISO 281 Addendum 4:2003.
For grease lubrication ηc can also be determined in a similar way, although the contamination may be difficult to measure and is therefore defined in a simple, qualitative manner.

Diagrams 7 and 8 provide typical values for the factor ηc for grease lubrication for operating conditions of extreme cleanliness and normal cleanliness

For other degrees of contamination for circulating oil, oil bath and grease lubrication, please refer to DIN ISO 281 Addendum 4:2003 or consult the SKF application engineering service.
An indication of the strong effect of contamination on fatigue life can be obtained from the following example. Several 6305 deep groove ball bearings with and without seals were tested in a highly contaminated environment (a gearbox with a considerable number of wear particles). No failures of the sealed bearings occurred and the tests were discontinued for practical reasons after the sealed bearings had run for periods which were at least 30 times longer than the experimental lives of the unsealed bearings. The lives of unsealed bearings equalled 0,1 of the calculated L10 life, which corresponds to a factor ηc = 0 as indicated in table.
Diagrams 9, 10, 11 and 12 indicate the importance of cleanliness in lubrication by the rapid reduction in the values for the factor aSKF with a diminishing ηc. Using integral seals is a very good and economical way to provide high cleanliness in the bearings.
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