Page 104 - Rollingbearings

B.3 Bearing size
Lubrication condition • κ = 4 indicates a regimen for which the Where κ < 1, EP/AW additives are
– the viscosity ratio, κ rolling contact load is carried by the lubri- recommended. m
The speed factor nd is used to charac-
cant ilm – i.e. full ilm lubrication.
• κ > 4 (i.e. better than full ilm lubrication) terize the speed condition of the bearing.
When a bearing has reached its normal will not further increase the rating of the
speed and operating temperature, the lubri- bearing. However, κ > 4 may be useful in • If the nd of the bearing is lower than
m
cation condition of the bearing is: applications where the bearing tempera- 10 000, the application is operating under
ture rise is small and additional lubrication low-speed conditions (diagram 14,
n
n
κ = — condition reliability is desirable. This would page 101). This regimen requires high oil
1
apply, for example, to bearing applications viscosity to ensure that the rolling element
where with frequent start-stop running condi- load is carried by the lubricant ilm.
κ = lubrication condition of the bearing, i.e. tions or occasional temperature variations. • High-speed conditions are characterized
viscosity ratio • κ < 0,1 indicates a regimen for which the by nd > 500 000 for d values up to
m
m
ν = actual operating viscosity of the oil or rolling element load is carried by the con- 200 mm, and > 400 000 for larger d m
the grease base oil [mm 2 /s] tact of the asperities between rolling ele- values (diagram 14). At very high speeds,
ν = rated viscosity, function of the mean ment and raceway – i.e. boundary lubrica- the rated viscosity drops to very low values.
1
bearing diameter and rotational speed tion. The use of fatigue life rating for Lubrication conditions and κ values are
[mm 2 /s] lubrication conditions below 0,1 is not generally high.
appropriate as it is beyond the applicability
The actual operating viscosity, ν, of the lubri- limits of the life rating model. Where
cant can be determined from the ISO viscos- κ < 0,1, select the bearing size on the basis EP (extreme pressure) and
ity grade of the oil, or the grease base oil, of static loading criteria by means of the AW (anti-wear) additives
and the operating temperature of the bear- static safety factor, s (Size selection based
0
ing (diagram 13, page 100) . on static load, page 104). EP/AW additives in the lubricant are used to
You can determine the rated viscosity, ν , improve the lubrication condition of the
1
from diagram 14, page 101, using the bear- bearing in situations where small κ values
ing mean diameter d = 0,5 (d + D) [mm] κ value below 1 are in use. Furthermore, EP/AW additives
m
and the rotational speed of the bearing, are also used to prevent smearing between
n [r/min]. Alternatively, you can use the SKF For lubrication conditions with 0,1 < κ < 1, lightly loaded rollers and raceway, for
Bearing Calculator (skf.com/bearingcalculator). take into account the following: example, when especially heavy rollers enter
Viscosity grades, in accordance with • If the κ value is low because of very low a loaded zone at a reduced speed.
Bearing size the viscosity range for each grade at 40 °C speed, base the bearing size selection on 80 °C (175 °F), EP/AW additives in the lubri-
For operating temperatures lower than
ISO 3448, are listed in table 5, along with
the static safety factor s (Size selection
cant may extend bearing service life when κ
(105 °F).
0
The higher the κ value, the better the
based on static load, page 104).
is lower than 1 and the factor for the con-
c
factor is lower than 3.
the resulting a
expected rated life. This must be judged
ity, counteract this by selecting a higher
B.3 lubrication condition of the bearing and its • If the κ value is low because of low viscos- tamination level, η , is higher than 0,2 and
SKF
Under those conditions, a value of κ = 1
against the possible friction increase because
viscosity oil or by improving the cooling.
EP
of the higher oil viscosity. Therefore, most bear- Under these lubrication conditions, it is not can be applied, in place of the actual κ value,
ing applications are designed for a lubrication appropriate to calculate the basic rating in the calculation of a SKF for a maximum
condition ranging from κ 1 to 4 (diagram 15). life L only, because it does not take into advantage of up to a SKF = 3.
10
Alternatively, you can use the SKF Bearing account the detrimental effects of inad- EP/AW additives containing sulphur-
Calculator (skf.com/bearingcalculator) to equate lubrication of the bearing. Instead, phosphorus can reduce bearing life. Gener-
calculate the lubrication condition. to estimate the rolling contact fatigue life ally, SKF recommends testing chemical
of the bearing, use the SKF rating life reactivity of EP/AW for operating tempera-
method. tures above 80 °C (175 °F).

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