Lubrication condition – the viscosity ratio, κ
When a bearing has reached its normal speed and operating temperature, the lubrication condition of the bearing is:
||lubrication condition of the bearing, i.e. viscosity ratio
|ν||actual operating viscosity of the oil or the grease base oil [mm2/s]|
|ν1||rated viscosity, function of the mean bearing diameter and rotational speed [mm2/s]|
The actual operating viscosity, ν, of the lubricant can be determined from the ISO viscosity grade of the oil, or the grease base oil, and the operating temperature of the bearing (diagram 1).
Table 1 lists the viscosity grades in accordance with ISO 3448, and shows the viscosity range for each grade at 40 °C (105 °F).
The higher the κ value, the better the lubrication condition of the bearing and its expected rated life. This must be judged against the possible friction increase because of the higher oil viscosity. Therefore, most bearing applications are designed for a lubrication condition ranging from κ 1 to 4 (diagram 3). Alternatively, you can use the SKF Bearing Calculator to calculate the lubrication condition.
- κ = 4 indicates a regimen for which the rolling contact load is carried by the lubricant film – i.e. full film lubrication.
- κ > 4 (i.e. better than full film lubrication) will not further increase the rating of the bearing. However, κ > 4 may be useful in applications where the bearing temperature rise is small and additional lubrication condition reliability is desirable. This would apply, for example, to bearing applications with frequent start-stop running conditions or occasional temperature variations.
- κ < 0,1 indicates a regimen for which the rolling element load is carried by the contact of the asperities between rolling element and raceway – i.e. boundary lubrication. The use of fatigue life rating for lubrication conditions below 0,1 is not appropriate as it is beyond the applicability limits of the life rating model. Where κ < 0,1 , select the bearing size on the basis of static loading criteria by means of the static safety factor, s0 (→ Size selection based on static load).
κ value below 1
For lubrication conditions with 0,1 < κ < 1, take into account the following:
- If the κ value is low because of very low speed, base the bearing size selection on the static safety factor s0 (→ Size selection based on static load).
- If the κ value is low because of low viscosity, counteract this by selecting a higher viscosity oil or by improving the cooling. Under these lubrication conditions, it is not appropriate to calculate the basic rating life L10 only, because it does not take into account the detrimental effects of inadequate lubrication of the bearing. Instead, to estimate the rolling contact fatigue life of the bearing, use the SKF rating life method.
Where κ < 1, EP/AW additives are recommended → Extreme pressure (EP) and anti-wear (AW) additives (below).
The speed factor ndm is used to characterize the speed condition of the bearing.
- If the ndm of the bearing is lower than 10 000, the application is operating under low-speed conditions (diagram 2). This regimen requires high oil viscosity to ensure that the rolling element load is carried by the lubricant film.
- Operating conditions leading to ndm > 500 000 for dm values up to 200 mm, and > 400 000 for larger dm values, are typical of bearings operating at high speeds (diagram 2). At very high speeds, the rated viscosity drops to very low values. Lubrication conditions and κ values are generally high.
Extreme pressure (EP) and anti-wear (AW) additives
EP/AW additives in the lubricant are used to improve the lubrication condition of the bearing in situations where small κ values are in use, e.g. when κ = 0,5. Furthermore, EP/AW additives are also used to prevent smearing between lightly loaded rollers and raceway, for example, when especially heavy rollers enter a loaded zone at a reduced speed.
For operating temperatures lower than 80 °C (175 °F), EP/AW additives in the lubricant may extend bearing service life when κ is lower than 1 and the factor for the contamination level, ηc, is higher than 0,2 and the resulting aSKF factor is lower than 3. Under those conditions, a value of κEP=1 can be applied, in place of the actual κ value, in the calculation of aSKF for a maximum advantage of up to aSKF = 3.
Some modern EP/AW additives containing sulphur-phosphorus, which are most commonly used today, can reduce bearing life. Generally, SKF recommends testing chemical reactivity of EP/AW for operating temperatures above 80 °C (175 °F).