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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 and types of contaminants (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 in accordance with ISO 281:2007 are listed in table 1.

ISO contamination classification and filter rating

The standard method for classifying the contamination level in a lubrication system is described in ISO 4406:1999. In this classification system, the result of the solid particle count is converted into a code using a scale number (table 2 and diagram 1).
One method to check the contamination level of bearing oil is the microscope counting method. This method uses two particle size ranges: ≥ 5 μm and ≥ 15 μm are used. Another more modern method is to use an optical automatic particle counter in accordance with ISO 11171:2010. The calibration scale of the automatic counting method differs from that of the microscopic counting method. It uses three particle size ranges indicated by the symbol (c) e.g. ≥ 4 μm(c), ≥ 6 μm(c) and ≥ 14 μm(c). Typically, only the two larger particle size ranges are used, as the larger particles have a more significant impact on bearing fatigue.
Typical examples of contamination level classifications for lubricating oils are –/15/12 (A) or 22/18/13 (B), as shown in diagram 1.
Example A indicates that the oil contains between 160 and 320 particles ≥ 5 µm and between 20 and 40 particles ≥ 15 µm per millilitre of oil. Though it would be optimal if lubricating oils were continuously filtered, the viability of a filtration system would depend on the equipment costs versus maintenance and downtime costs.
A filter rating is an indication of filter efficiency and is expressed as a reduction factor β. The higher the β value, the more efficient the filter is for the specified particle size. The filter rating β is expressed as a ratio between the number of specified particles before and after filtering. This can be calculated using

Filter rating

filter rating related to a specified particle size x
particle size (c) [μm] based on the automatic particle counting method, calibrated in accordance with ISO 11171:2010
number of particles per volume unit larger than x, upstream the filter
number of particles per volume unit larger than x, downstream the filter
The filter rating β only relates to one particle size in μm, which is shown in the index such as β3(c), β6(c), β12(c), etc. For example, a complete rating “β6(c) = 75” means that only 1 in 75 particles, 6 μm or larger, passes through the filter.

Determining ηc when the contamination level is known

Once the oil contamination level is known, either from the microscope counting method or the automatic particle counting method, both in accordance with 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. The factor ηc cannot be derived solely from a particle count. It depends largely on the lubrication conditions, such as κ, and the size of the bearing. A simplified method in accordance with ISO 281:2007 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 for that bearing (diagram 2 and diagram 3).
Diagram 2 and diagram 3 provide typical values for the factor ηc for circulating oil lubrication systems 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 or the ingress of contaminants, this must be reflected in the choice of the factor ηc used for the oil bath system as indicated in ISO 281:2007.
For grease lubrication, ηc can be determined in a similar way using ISO values for five levels of contamination as shown in table 3.

Diagram 4 and diagram 5 provide typical values for the factor ηc for grease lubrication for high and normal cleanliness (table 3).
For other degrees of contamination or in the most general case of circulating oil, oil bath and grease lubrication, the contamination factor for a bearing arrangement can be determined using the simplified equation

Contamination factor

min (#1, #2) = use the smallest of the two values

where c1 and c2 are constants characterizing the cleanliness of the oil in accordance with ISO 4406:1999, or of the grease according to the classifications in table 3. In case of oil filtration, the corresponding level of filtration efficiency (in accordance with ISO 16689:2012) (table 4) can also be applied in place of the metrological characterization of the status of cleanliness of the oil.
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