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 build up bearing preload excessively because the temperature gradient between the housings, bearings, and shafts. As this gradient increases, the bearing preload builds up, starting a vicious cycle which may lead to premature spindle failure.
In spindle mountings, the shaft temperature usually changes at a faster rate than the housing, creating tem- perature 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 rather than the housing and the outer-ring spacer. As the shaft expands longitudinally and the inner-ring spacer lengthens, a thrust load builds up on each bearing and continues to increase until the equilib- rium temperature is reached. This occurs when the tem- perature at the housing levels off and the heat transferred from the bearings balances the heat generated in them. Therefore, if the housing attains an excessively high tem- perature, the initial bearing is built up considerably.
In a face-to-face mounting, Figure 5, the shaft expands radially and longitudinally and the inner-ring spacer length- ens, but at a faster rate than the outer-ring spacer. This thermal expansion causes an additional thrust to be im- posed on both inner rings, increasing the preload of the bearings. Conversely, in back-to-back mounting, Figure 6, 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 7, are mounted so that the two middle bearings are face-to-face. As previously observed, temperature differentials cause the preload of these inside bearings to increase during opera- tion. This mounting operation is not recommended. In bearing mountings of the type seen in Figure 8, undue thrust loads are put on the two outside bearings as the temperature along the shaft becomes higher than at the housing. The two inside bearings unload, starting a vicious cycle of increasing temperature, preload build-up, and lubricant depletion. This is also an unacceptable mounting arrangement, and is not recommended. The same bear- ings are shown correctly mounted in tandem and arranged back-to-back in Figure 9. Lateral expansion of the shaft and inner-ring spacer of such mountings increase neither thrust loading nor bearing preload.
Therefore, in order 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 tandom but the combination should be arranged back-to-back as in Figure 9.
Figure 5 – DF Mounting, Fixed
Figure 6 – DB Mounting, Fixed
TWO BACK-TO-BACK PAIRS
Figure 7 – DB-DB Mounting, Fixed (Not Recommended)
TWO FACE-TO-FACE PAIRS
Figure 8 – DF-DF Mounting, Fixed (Not Recommended)
TWO TANDEM PAIRS MOUNTED DB
Figure 9 – DT-DB Mounting, Fixed