Assembly Considerations
Because the components are constantly under strain for a long time, the long-term performance of press-fit joints is influenced by the stress relaxation properties of the material. Stress relaxation assumes uniform and constant strain. Over time, the stress on the components decays while the strain remains at the same level. If the components are separated immediately after assembly, the pull-out force will be equal to the push-in force used to assemble the components. With time, the pull-out force reduces due to the stress relaxation effect. The press-fit assembly suffers a performance failure when the disassembly force drops below the expected level. Given enough time, the stress could relax even to the point that the components could slide or rotate with respect to each other with minimal force. Design your long-term applications such that the stress on the hubs is low enough to minimize stress relaxation and the potential for premature failure due to slippage or cracking. Assemblies that are designed with consideration only to the short-term properties of the material may not show failure soon after assembly but may develop cracks or crazes months or years as the stresses relax. You must ensure adequate retention of your press-fit assembly for the designed functional life of the assembly by selecting the right material with acceptable stress relaxation properties, and designing the joint only after careful engineering calculations. |