Timken Super Precision Bearings for Machine Tool Applications

PERMISSIBLE OPERATING SPEED
When determining the permissible operating speeds corresponding
to the bearing preloads used in machine tool spindles, many
influencing factors are involved. Among those considered are
spindle mass and construction, type of mounting, spindle rigidity and
accuracy requirements, spindle loads, service life, type of service
(intermittent or continuous), and method of lubrication.
Bearing temperatures, generally, vary directly with both speed and
load. However, high-speed applications must have sufficient axial
loading on the bearings to prevent heat generation due to rolling
element skidding. The amount of bearing preload is determined
primarily from these operating conditions. At lower speeds, the
operating loads are heavier and the bearing deflections are
greater. Therefore, the bearing preload must be high enough to
provide adequate bearing rigidity under the heaviest loads and still
maintain reasonable temperatures when the spindle is operated at
high speeds.
TAPERED ROLLER BEARINGS
Measuring Rib Speed
The usual measure of the speed of a tapered roller bearing is the rib
speed, which is the circumferential velocity at the midpoint of the
inner ring large end rib (Figure 54). This may be calculated as:
Rib speed:
Where:
V r =
=
D mn
60000
D mn
12
(m/s)
(ft/min)
D m = mean inner ring large rib diameter mm, in.
n = bearing speed rev/min
Effect of Lubrication on Speed Capability
ENGINEERING
The design of the tapered roller bearing results in a natural pumping
effect on the lubricant, where the lubricant is forced from the small
end of the roller end, heading toward the wider end. As speed
increases, the lubricant begins to move outward due to centrifugal
effects. At excessive speed, the contact between the roller large
ends and the cone’s rib face can become a concern. This is the
primary reason for suggestions on the use of oil jets at this large
end, ribbed-cup designs, or high-speed TSMA designs as operating
speeds increase. Refer to the following speed guidelines for more
details.
Fig. 55. Pumping effect of
a tapered roller bearing
There are no clear-cut speed limitations for tapered roller bearings
since performance depends on the bearing design and lubrication
system. The guidelines given in the table on page 54 are based on
typical industrial experiences relating to speed and temperature
for various types of lubrication systems, with bearings having low
G 1 factor.
Timken suggests that testing be performed for all new high-speed
applications.
A
Inner ring rib diameter
Fig. 54. Cone rib diameter. The inner ring rib diameter may be scaled
from a print.
TIMKEN MACHINE TOOL CATALOG 53