High Precision Ball Bearings - Kraftmek

Service life of grease – Lubrication intervals
The lubricating interval is defined on principle as the
value for a 10 to 20% probability of failure of the service
life of the grease.
The service life of the grease is essentially dependent
on the influencing parameters
• Grease
• Operating conditions
• Design
Therefore, the selection of the grease is of decisive
importance. In the opposite graph 1 (reproduced from
the Recommendations 2.4.1 of the Society of Tribology),
the lubricating interval t f is plotted against the operating
speed and the limiting speed of grease lubrication. It
provides guiding values for the service life and applies
to lithium soap grease up to an operating temperature
of +70 °C (measured on the outer ring) and moderate
conditions of loading (P/C < 0.1).
Special environmental factors, such as humidity and
vibration will decrease the lubricating interval to 1/5 of
the initial value.
Unusual conditions of operation
e.g. extreme temperatures and high loading (P/C > 0.1)
call for special greases. These greases will enable longer
lubricating intervals to be achieved than those obtained
from the graph.
Lubricating intervals of more than 5 years are possible
only under very favourable environmental conditions.
Reduction in lubricating intervals at high
temperatures
The degradation of the lubricating greases increases
considerably at higher temperatures. A temperature
increase by 15 K, starting at 70 °C, will decrease the
lubricating interval to half the initial value, particularly
with lithium greases. Guidance is provided by the
graph 2.
Lubricating interval t f [h]
Reduction factor
50 000
30 000
20 000
10 000
5 000
3 000
2 000
1 000
500
300
200
100
Graph 1
1.0
0.5
0.25
0.1
0.05
0.025
0.2 0.3 0.5 1 2 3 5 10 20 30 50
n fett
Speed ratio
n
· K L
KL = 1.6 Coefficient for spindle bearings
KL = 1.8 Coefficient for deep groove
bearings
... for bearing calculation
0.01
0.75 1.0 1.25 1.5 1.75 2.0
Graph 2
t/tR
t = Bearing temperature
t R = Limiting temperature where
lubricating interval reduction
starts (for lithium greases
for instance 70° C)
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