Assembly Considerations
| Tight Tolerances |
Hoop stress experienced by the hub in a press-fit assembly can sometimes impose quite tight tolerance limits on the molded components driving the molding cost up. Figure 1 shows interference versus shaft/hub diameter relationship for Makrolon polycarbonate. As an example, take a 0.25" diameter shaft made of Makrolon polycarbonate. When used with a Makrolon polycarbonate hub of 0.45" outside diameter, the maximum diametrical interference allowed, as per the graph, should be 0.0093"/inch of shaft diameter. As the shaft diameter is one fourth of an inch, the maximum absolute diametrical interference allowed will be one fourth of 0.0093", which is approximately 0.002".
What this means for the molder is that the shaft diameter and the inside diameter of the hub have to be molded within a very close tolerance limit. If the interference turns out to be less than 0.002" after molding, the assembly may not have the designed pull-out strength or the torque resistance. If it is more than 0.002" the hoop stresses might be high enough to cause cracking or crazing of the hub either immediately or in the long term. For this reason, press fits should not be considered as primary joining methods for components made of Makrolon polycarbonate. Consult Bayer engineers regarding the suitability of other Bayer materials in press fits.
If resistance to rotation is not critical but the axial pull- out strength is important, a recessed shaft, as shown in figure 2, can be used for the assembly. To reduce the short term hoop stress when the hub is pushed over the large diameter of the shaft, shrink-fit technique can be applied by heating (within allowed limits) the hub and/or freezing the shaft. Because the high torque resistance is not critical, the shaft diameter can be designed smaller and the tolerance limits more liberal. The hub will be located axially in the recessed area and the larger diameter will provide the resistance to axial pull.
