Timken Super Precision Bearings for Machine Tool Applications

A
ENGINEERING
BALL BEARINGS
Even though ball bearings have the least amount of friction of any
of the common rolling bearings, lubrication is required to minimize
rolling resistance due to deformation of the balls in the raceways
under load, and to minimize any sliding friction that occurs between
the balls, the raceway and the cage. Lubrication also serves to
protect the accurately ground and polished surfaces from corrosion.
In addition, lubrication in general dissipates generated heat and can
help protect the bearing from the entry of foreign matter.
Regardless of the method of lubrication or type of lubricant, it is
important that quality lubricants be used to minimize oxidation,
gumming or sludging and that the lubricant be clean and free of
moisture to minimize wear.
Only enough lubrication to accomplish these purposes should be
used since another source of heat may become present, namely
friction between the lubricant and the moving parts, in the form of
churning or internal shear of the lubricant itself.
In the lubrication of ball bearings it is important to realize that a small
quantity of oil or grease will, if constantly present in the bearing,
suffice for its requirements. It should be noted that trouble can
result from too much lubrication just as it can from too little. Both
conditions should be avoided. Excessive oil or grease will result
in high temperature and possible damage. When grease is used,
it is necessary to take into consideration the maximum operating
temperature. Particular attention must be given to the housing
design relating to the proximity of the grease to the bearing, to
assure adequate purge room and grease retention.
Depending upon operating speeds, loads and temperatures,
machine tool ball bearings are lubricated with grease, oil or oil mist.
In general, oils are required when bearings operate at high speeds
as they provide greater cooling than is possible with grease.
The following relationship may be used to estimate the effect
of preload and lubrication method on the Permissible Operating
Speed (S P).
Where:
S P = F L x F P x F B x N G
F L = lubrication factor
F P = preload factor
F B = ball material factor
N G is Permissible Speed for a single, grease-lubricated bearing
with inner ring rotation. This value is found with the part number
tables.
Factors are as follows:
Lubrication factor (F L)
Grease F L = 1.00
Oil bath F L = 1.50
Oil mist F L = 1.70
Oil jet or
metered oil
F L = 2.00
Table 11. Lubrication factor = (F L).
Bearing mounting
arrangement
Bearing preload
L M H
0.85 0.70 0.50
0.80 0.60 0.40
0.65 0.50 0.30
0.65 0.50 0.30
0.70 0.60 0.35
0.60 0.40 0.20
0.65 0.45 0.25
Table 12. Bearing preload factor = (F p).
Ball material factor (F B)
Steel balls F B = 1.00
Ceramic balls (1) F B = 1.20
(1) Ceramic balls allow 20 percent increase to speed factor.
Table 13. Ball material factor (F B).
58 TIMKEN MACHINE TOOL CATALOG