Timken Super Precision Bearings for Machine Tool Applications

A
ENGINEERING
BALL BEARINGS
Heat Generation
Low operating temperatures, combined with adequate spindle
rigidity, are important and highly desirable for precision machine
tools. This is particularly true for high-speed grinding spindles where
the preload of the bearings is the principal load imposed upon them.
Some of the benefits derived from low operating temperatures are
better dimensional stability of the processed work, less need for
bearing lubrication, prevention of objectionable heat at the external
surfaces of the spindle housing, and elimination of troubles due to
thermal effects on mounting fits and preloads.
Preload and Heat Generation
The heat developed at the bearings under load is a function of the
operating speed and the bearing preload. Preloading is necessary
for maximum axial and radial rigidity. Unfortunately, if speeds are
increased, the bearing preload may have to be lessened to maintain
proper operating temperatures at the bearing.
For high-speed operation, the bearing preload should be sufficient
to maintain proper rolling friction for the balls, but not so high as to
generate excessive heat. In cases where lower operating speeds
are desired, bearing preloads may be increased to obtain additional
bearing rigidity, provided the proper operating temperatures
are maintained. Thus, a balance between heat generation and
spindle rigidity dictates the amount of bearing preload that is
used, commensurate with the operational speed and the bearing
life required.
How bearing preload affects the operating temperature is
illustrated in Figure 69. This graph applies to 207-size, angular
contact, duplexed super precision ball bearings, mounted backto-back.
Curve A is a plot of operating temperature at the bearing
outside diameter for the speeds indicated, using bearings with
Approximate Temperature Rise Above Room
100˚ F
90˚ F
80˚ F
70˚ F
60˚ F
50˚ F
40˚ F
30˚ F
20˚ F
10˚ F
0
A
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Speed in RPM 1 = 1000
A High Preload
B Low Preload
Fig. 69. Effect of preload on temperature rise.
B
50˚ C
40˚ C
30˚ C
20˚ C
10˚ C
a high built-in preload. Curve B is for bearings having a low
preload. The slope of Curve A is much steeper than that of Curve
B. Using bearings with a high preload, the temperature rise at the
bearing outside diameter is 34˚ C (60° F) when operating at 3600 RPM.
For the same temperature rise using bearings with low preload, an
operating speed of 15300 RPM is indicated. Therefore, it is evident
that for higher-speed operation, the bearing preload should be kept
to the minimum necessary to ensure sufficient bearing rigidity.
For workhead spindles, the operating speeds are generally low and
the loading conditions heavy. Maximum radial and axial spindle
rigidity is required under these loads, making increased bearing
preload mandatory.
Bearing Geometry and Heat Generation
It should be noted that a bearing's internal geometry has a major
impact on heat generation. High-speed designs, such as the Timken
HX Series, incorporate “optimized” internal geometries that balance
load-carrying capacity, stiffness and heat generation.
Heat Dissipation
When ball bearing spindles are grease lubricated, the heat
generated is removed only by conduction through the surrounding
parts. With jet or circulating oil lubrication, generated heat is
dissipated by the oil passing through the bearings as well as by
conduction through the shaft and housing. Both means of removing
heat from the bearings are important, but generally, dissipation
through conduction is less obvious.
As an example, in an oil mist-lubricated grinding spindle, the nose
or wheel-end bearings are fixed and close to the grinding coolant.
The pulley-end or rear bearings are secured axially on the shaft,
but permitted to float laterally in the housing to compensate for size
variations due to thermal changes. Heat is conducted away from
the front bearings at a faster rate because of the thermal mass of
the spindle nose and the intimate contact of the outer rings with
the housing shoulder, the end cover and the housing bore. This
condition, coupled with oil mist lubrication and the proximity of the
grinding coolant, takes away generated heat efficiently.
The rear or floating pair of bearings is not so favored. Usually, the
thermal mass of the shaft at the drive-end is not so great. The driveend
possesses some heat-conduction ability, but also receives heat
generated by belt friction. The absence of grinding coolant and
the reduced area of conduction usually results in a slightly higher
operating temperature.
66 TIMKEN MACHINE TOOL CATALOG