Contamination factor, ηc
The contamination factor, ηc, takes into account how the level of solid particle contamination of the lubricant influences the calculated bearing fatigue life. The particles cause indentations in the rolling surfaces of the bearing, and these indentations increase the local contact stress, which reduces the expected fatigue life (fig. 1).
- ηc = 1 means perfectly clean conditions without any indentations.
- ηc → 0 means severely contaminated conditions resulting in pronounced indentations.
In the SKF rating life model, the contamination factor for a certain bearing acts as a stress raiser, by reducing the bearing fatigue load limit Pu (i.e. multiplying it by the contamination factor ηc).
Comparing the reduced fatigue load limit to the actual bearing load, the fatigue resistance value (ηc x Pu/P) takes both the relative bearing load and the local stress field into account (diagram 1).
- Clean conditions (large contamination factor ηc) and a bearing load lower than the fatigue load limit results in a high resistance to fatigue.
- Contaminated conditions and a bearing load larger than the fatigue load limit results in a lower resistance to fatigue.
The stress-raising influence of contamination on bearing fatigue depends on a number of parameters, including: bearing size, relative lubricant condition, size and distribution of solid contaminant particles and types of contaminants (soft, hard, etc.). Therefore, it is not meaningful to specify precise values for the contamination factor ηc that would have general validity. However, guideline values in accordance with ISO 281 are listed in table 1.
To simplify calculation of the contamination factor ηc, use the SKF Bearing Calculator.
A more detailed method for estimating the contamination factor ηc is described in a separate paper → Method for estimating contamination factor, ηc, based on lubricant cleanliness [PDF]