Assembly Considerations
| Assembly Force |
Excessive undercut may cause the cantilever snap arm to break when deflected during the assembly. Too much force required for the assembly may render the joining operation impractical for use. It is therefore equally important to design the joint features so that the assembly force is kept within reasonable limits.
Assembly force (W) is the force required to overcome the deflection force P and the friction force F during a snap fit assembly operation. The deflection force P required to bend a cantilever arm can be calculated as a function of the geometrical rigidity (cross X-sectional shape), the permitted strain, and the rigidity of the material (Young's modulus).
Since the strains that occur during the joining operation are frequently outside the proportionality range of the stress-strain curve, the so-called "secant modulus" (=strain -dependent Young's modulus) should be used instead of the Young's modulus, since this will allow the deflection force to be determined more accurately. Figure 1 shows how this is established. The strain determined for the requisite deflection must always be entered here.
