Page 333 - The Tribology Handbook

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Rolling bearing lubrication C8

SELECTION OF THE LUBRICANT

Table 8.7 General guide for choosing between grease and oil lubrication

Factor affecting the choice 1Jse grease Use oil
Temperature Up to 120°C-with special greases or short Up to bulk oil temperature of 90°C OF bearing
relubrication intervals up to 20O/22O0C temperature of 200°C-these temperatures may
be exceeded with special oils
Speed factor. IJp to dn factors of 300 000/350 000 (depending Up to dn factors of 450 000/500 000
on design) (drpending on type of bearing)
Load Low to moderate All loads up to maximum
Bearing design Not for asymmetrical spherical roller thrust All types
bearings
Housing design Relatively simple More complex seals and feeding devices necessary

Long periods without Yes, depends on operating conditions, especially No
attention temperature
Central oil supply for No-cannot transfer heat efficiently or operate Yes
other machine elements hydraulic systems
Lowest torque When properly packed can be lower than oil on For lowest torques use a circulating system with
which the grease is based scavenge pumps or oil mist
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Dirty conditiom Yes-proper design prevents entry of con- Yes, if circulating system with filtration
taminants

* dn factor (bearing bore (mni) x speed (revimin)).
Note: for large bearings (> 65 mm bore) use nd, (d, is the arithmetic mean of outer diameter and bore (mm) )

G R EASE LUI B RICATION
Table 8.2 The effect of the method of appli-
Grease sellection cation on the choice of a suitable grade
of grease
The principle factors governing the selection of greases
for rolling bearings are speed, temperature, load, environ- System NLGI grade no.
ment and method of application. Guides to the selection
of a suitable grease taking account of the above factors are Air pressure 0 to 2
given in Tables 8.2 and 8.3. depending on type
The appropriate maximum speeds for grease lubrication
of a given bearing type are given in Figure 8.1. The life Pressure-guns or mechanical up to 3
lubricators
required from the grease is also obviously important and
Figure 8.2 gives a guide to the variation of grease Compression cups up to 5
operating life with percentage speed rating and tempera-
ture for a high-quality lithium hydroxystearate grease as Centralised lubrication 2 or below
derived from ]Figure 8.1. (These greases give the highest (a) Systems with separate metering Normally 1 or 2
speed ratings.) valves
When shock loading and/or high operating temperatures
tend to shake the grease out of the covers into the bearing, (b) Spring return systems I
a grease of a harder consistency should be chosen, e.g. a (c) Systems with multi-delivery 3
no. 3 grease instead of a no. 2 grease. pumps

Note: it should be recognised that the curves in Figures 8.1 and 8.2
can only be a guide. Considerable variations in life are possible
depending on precise details of the application, e.g. vibration, air
flow across the bcearing, clearances, etc.

C8.1