Timken Super Precision Bearings for Machine Tool Applications

Back-To-Back Versus Face-To-Face Mountings
Mountings having bearings applied in any of the face-to-face (DF)
arrangements are objectionable because they provide the least
rigidity. Furthermore, when the operating speeds are comparatively
high, such mountings may buildup bearing preload excessively
because of the temperature gradient between the housings,
bearings and shafts. As this gradient increases, the bearing preload
builds up, starting a detrimental cycle that may lead to premature
spindle damage.
In spindle mountings, the shaft temperature usually changes at
a faster rate than the housing, creating temperature differentials
between the two members. These are due to their difference in mass
and their respective abilities to act as heat sinks. Thus, the shaft
and the inner-ring spacer expand at a faster rate than the housing
and the outer-ring spacer. As the shaft expands longitudinally and
the inner-ring spacer lengthens, an axial load builds up on each
bearing and continues to increase until the equilibrium temperature
is reached. This occurs when the temperature at the housing levels
off and the heat transferred from the bearings balances the heat
generated within the system. Therefore, if the housing attains an
excessively high temperature, the initial bearing temperature is
built up considerably.
In a face-to-face mounting, Figure 87, the shaft expands radially and
longitudinally and the inner-ring spacer lengthens, but at a faster
rate than the outer-ring spacer. This thermal expansion causes an
additional axial load to be imposed on both inner rings, increasing
the preload of the bearings. Conversely, in back-to-back mounting,
Figure 88, the longitudinal expansion of the inner-ring spacer tends
to relieve, rather than build up, the bearing preload.
The two back-to-back pairs, shown in Figure 89, are mounted
so that the two middle bearings are face-to-face. As previously
observed, temperature differentials cause the preload of these inner
bearings to increase during operation. This mounting operation is
not suggested. In bearing mountings of the system seen in Figure
90, undue axial loads are put on the two outer bearings as the
temperature along the shaft becomes higher than at the housing.
The two inner bearings unload, starting a vicious cycle of increasing
temperature, preload buildup and lubricant breakdown. This also
is an unacceptable mounting arrangement and is not suggested.
The same bearings are shown correctly mounted in tandem and
arranged back-to-back in Figure 91. Lateral expansion of the shaft
and inner-ring spacer of such mountings increases neither axial
loading nor bearing preload.
Therefore, to prevent increases in preload due to the thermal
expansion, back-to-back mountings are preferred for bearings on
machine tool spindles. When two pairs are used, each pair should
be mounted in tandem, but the combination should be arranged
back-to-back as in Figure 91.
FACE-TO-FACE
MOUNTING
Fig. 87. DF Mounting, fixed (not suggested).
BACK-TO-BACK
MOUNTING
Fig. 88. DB Mounting, fixed (suggested).
TWO
BACK-TO-BACK
PAIRS
Fig. 89. DB-DB Mounting, fixed (not suggested).
TWO
FACE-TO-FACE
PAIRS
Fig. 90. DF-DF Mounting, fixed (not suggested).
TWO
TANDEM PAIRS
MOUNTED DB
Fig. 91. DT-DB Mounting, fixed (suggested)
ENGINEERING
A
TIMKEN MACHINE TOOL CATALOG 99