Assembly Considerations
Press fits or shrink fits allow easy assembly of parts made of similar or dissimilar materials without the help of additional fasteners or adhesives. They are most commonly used with round components even though other geometries are possible and are sometimes used. Unlike snap fits, the components in press fits remain under stress until disassembled. To avoid stress cracking or stress relaxation that can cause the press fit to fail over time, the press fit design must not exceed the long-term loading limits for the thermoplastics material. Because of the potential for stress cracking over time, press-fits should be avoided in polycarbonate blends. Press-fit assemblies must have adequate resistance to both torsional and axial movement, while maintaining stress levels in the components within acceptable limits. As opposed to press fits, the insertion force required in a shrink fit is minimal because the hub is heated or the shaft is cooled just prior to the assembly. The stress levels generated in a shrink fit, however, will be similar to those in a press fit with similar amount of interference between the hub and the shaft. The insertion force required for a press-fit assembly depends on the interference between the components, their frictional characteristics and the surface area of engagement. When using press fits, clean the components to ensure that they are free of contaminants like lubricants, mold release agents etc. Internal lubricants in the resin and/or compatible external lubricants may help reduce the required insertion force. However, this will also result in reduction of torsional resistance (transmitting torque) of the assembly. Reduction in insertion force can be accomplished, without sacrificing the torsional strength, by freezing the shaft prior to assembly as in shrink fits. Short-term, datasheet properties are not adequate for evaluating the long-term performance of press fits. Consult long-term data such as isochronous stress-strain curves or follow material specific guidelines for stress or stain based on long-term loading. The long-term limits are often a small fraction of the short-term load limits. |