Assembly Considerations
| Hoop Stress |
In most cases, the compressive stress developed on the shaft will be of little concern but the tensile stress on the hub can contribute to failure if poorly designed or subjected to adverse environment. Be sure to properly evaluate the press fit application by calculating the tensile/hoop stresses on the hub either by using standard text book formulas or with the help of "Interference vs. Shaft/Hub Diameter" graphs if available from the material suppliers. See details under the heading," Tight Tolerances".
Exposure of the press-fit assembly to a harsh environment or chemicals during assembly, transportation or end use; will accelerate the crazing or cracking of the hub under assembly stress. If the hub has a weld line in it, which will generally be the case if it is injection molded, the hoop strength of the hub will be reduced. The hub may have molded-in tensile stress on the surface. The assembly stress will be superimposed on the existing molded-in tensile stress thus further reducing the tensile strength of the hub. Use of regrind in the material may also have a similar detrimental effect on the hub strength.
Injection-molded hubs have draft along the internal diameter to facilitate ejection. Unfortunately, this results in non-uniform stress loading over the length of the hub. The location of the narrowest diameter of the hub can be moved somewhere in the middle, instead of at the end, by using split core pins as shown in figure 1(a). Also, because the coring is done from two ends, the difference between the large and the small diameters can be effectively cut in half using the same draft angle. If the draft is not at all acceptable, the holes can be reamed after molding. However, this added operation would increase cost. If the shaft is also injection molded, a corresponding taper, equal to the draft angle in the hub, may be incorporated in it for maximum surface contact.
If the assembly will be subjected to thermal cycling at any time, avoid using press-fitting components made of different materials. Thermal cycling may induce additional stresses in the hub due to different thermal expansion properties of the components.
