Timken Super Precision Bearings for Machine Tool Applications

SETTING AND PRELOADING GUIDELINES
TAPERED ROLLER BEARINGS
The optimum operating setting of a bearing system has a
direct influence on the spindle performance as far as accuracy,
dynamic stiffness, operating temperature and cutting capabilities
are concerned.
An operating setting range between zero and light preload is
generally the optimum value for simple dual TS or TSF layouts.
To reach this range, it is important to evaluate the different
parameters that will directly influence the operating setting in order
to determine the cold-mounted setting:
• Rotating speed.
• Applied loads.
• Spindle layout.
• Lubrication system.
• External sources of heat.
This evaluation occurs generally during the testing phase of the
spindle because of the complexity of each individual parameter
and the interaction of all of them during running conditions. At the
same time, it also is important to consider the bearing layout and
particularly the bearing spread to evaluate their effect on bearing
setting.
It has been demonstrated that an optimum bearing spread for
stiffness exists. In the same way, an optimum spread for thermal
stability can be determined should this be the overriding factor.
Under steady-state temperature
conditions, the spindle and
housing temperature is not
uniformly distributed. Generally,
a temperature gradient of 2° C to
5° C (4˚ F to 9˚ F) exists between
the spindle and housing. This
phenomenon is valid for any
type of bearing and has a direct
influence on the bearing setting.
In the case of pure radial
bearings, such as cylindrical
roller bearings, the radial setting
will vary proportionally to the
radial temperature gradient without any possibility for correction.
The use of tapered roller bearings allows the radial loss of end
play due to the gradient between the spindle and the housing to be
compensated by the axial expansion of the spindle with respect to
the housing through optimization of the bearing spread.
D o1
L
ENGINEERING
Figure 93 shows a graphical way to determine this optimum spread.
To define the optimum spread for thermal compensation or to
calculate the effect on setting for a given spread in a simple 2TS(F)
bearing system, the designer can use the formula below for ferrous
housings and spindles.
Fig. 93. Graphical determination of optimum thermal spread.
Fig. 94. Resulting distance between bearing geometric centerlines.
Loss of end play = 12 x 10 -6 x t x
[ ( K1
Where:
t = temperature difference between shaft /
inner ring rollers and housing /
outer ring ( θ s - θ h )
K 1 and K 2 = respective K factor of bearings 1 and 2
from bearing tables
D o1 and D o2 = respective outer race mean diameter
L = distance between bearings geometric
centerlines
D o2
x
) ( Do2)
Do1 + K2 x
]
- L 0.39 2 0.39 2
(°C)
(mm)
(mm)
A
TIMKEN MACHINE TOOL CATALOG 101