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ISO 281 defines the fatigue limit, , for a bearing as the load below which metal fatigue does not occur.

With poor lubrication, or contamination of the lubricant in particular, the bearing can fatigue at loads which are significantly below the fatigue limit,.

For the fatigue limit to be a valid value, the lubricant film must fully separate the rolling elements from the raceways and that dents from contaminants or from handling do not exist on the rolling surfaces.

The contamination factor, 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.

• means perfectly clean conditions without any indentations.

•  means severely contaminated conditions resulting in pronounced indentations.

In the SKF rating life model, contamination, designated by the contamination factor, , acts as a stress raiser, thereby reducing the fatigue load limit to .

We then compare the reduced fatigue load limit, ,  to the actual bearing load, , to give a fatigue resistance value of

•Clean conditions and a load that is less than the fatigue load limit results in a high fatigue resistance value.

•Contaminated conditions and a load that is more than the fatigue load limit results in a low fatigue resistance value.

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 that would have general validity.

If a catalog does not list the Fatigue Load Limit, and the mean bearing diameter is less than . (nearly all Follower-Roller Bearings are less than ), then you can use this approximation:

Roller and needle bearings, with a Mean Diameter < 100mm.

Ball bearings, with a Mean Diameter < 100mm

Mean Diameter = (Outside Diameter + Inside Diameter) / 2

The Viscosity Ratio, , indicates the quality of the lubricant film formation.

The lubricant-film separates the raceway and rolling-elements. It is expressed as:

Note 1: Viscosity Ratio, , assumes that the surface finish and hardness are for Follower-Roller bearings.

Note 2: An approximate relationship between Film Thickness Ratio and Viscosity Ratio, ,  is:

Notes:

A Viscosity-Ratio less than 0.1 is outside of the limits of ISO 281. It is near to metal-to-metal contact.

A Viscosity-Ratio greater than 4 is the maximum that can be used by ISO 281. However, ISO 281 simply states that the Viscosity-Ratio is equal to 4 if it is actually greater than 4. A viscosity-ratio greater than 4 is getting too high for bearings. The needles or balls may slide and refuse to roll in the 'thick-oil', or the oil may churn and increase the oil and bearing temperatures.

A Viscosity-Ratio in the range of 1.5 - 2.5 is approximately ideal.

The Reference Kinematic-Viscosity, (sometimes called the Rated, or Required Viscosity) is the viscosity that is required to separate the surfaces, with combined Cam-Profile and Follower-Roller Roughness values : .

It assumes that the oil is a mineral oil, with a Viscosity-Index of approximately 100.

Synthetic Hydrocarbon type oils can be used.

if

if

- inside diameter of Follower-Roller

- outside diameter of Follower-Roller

We calculate for you the Viscosity, , at the Operating Temperature, from the Viscosity at two other temperatures.

Nearly always, the two temperatures are and , but not necessarily.

The lubricant's data-sheet usually specifies the Viscosity at ( ) and ( )

You must enter all four parameters in the dialog:  

•2 x temperatures, and , and

•2 x viscosity at those two temperatures, and

You must also enter the Operating-Temperature, .

We calculate the Viscosity at the Operating-Temperature, .

If a contaminant particle moves to the inside of a bearing the rollers (or balls), outer-race, and inner-race are prone to dent because of the small internal bearing clearances and the small rolling radii of the rollers (or balls). An indent leads to localized stress, which will decrease the life of the bearing.

The contamination may even prevent the rollers (or balls) rotating.

The contamination factors that reduce the lifetime of a Follower-Roller bearing are a function of the:

•diameter of the Follower-Roller

•lubricant film thickness (viscosity ratio, )

•size, type, and hardness of the particle contaminant.

Guide values for the contamination factor are in the table below. They are typical levels of contamination for well lubricated bearings.

Contamination and Lubrication Method.

We can find for you the contamination factors with these lubrication methods:

Circulating oil lubrication with On-Line Filtration, before being supplied to the bearings.

Oil bath Lubrication, or Recirculating Oil Lubrication, with Off-Line Filtration.

NOTE

Research concludes that it is difficult to accurately find the oil cleanliness especially if you analyze very clean oils. It is easy to pollute an oil-sample, with oil additives that precipitate into the oil and particle calculation.

Grease Lubrication, Contamination Factors

Simplified Values of Oil Contamination Factor  ,

Contamination Level for Grease Lubrication

The table below provides scale values as a function of particle concentration (particles/ml) - it is from ISO 4406

A three number code defines the amount of contamination for three particle sizes: 4, 6, and 14 μm. Each time a  number increases by 1, the quantity of particles is doubled for a particular particle size.

Example: ISO code = 21 / 19 / 17

This Contamination Class describes a fluid containing:

•between 10,000 and 20 ,000 particles of ≥ 4 μm(c) per 1 ml sample

•between 2,500 and 5 ,000 particles of ≥ 6 μm(c) per 1 ml sample

•between 640 and 1 300 particles of ≥ 14 μm(c) per 1 ml sample

If the leading number is missing, then that size of particle is not counted.