Assembly Considerations

Snap-fit joints usually require the component with the undercut or hook to momentarily deflect during assembly. Sometimes deflection may be experienced by the mating component also, but that is usually minimal. The amount of deflection should be enough to clear the undercut for the assembly to take place.

Plastics differ in the level of stress and strain they can tolerate. Because stress and strain are interrelated, components can be designed based on either stress or strain limits of the material. The permissible deflection y for a simple cantilever beam with uniform cross-section, as shown in figure 1, is given by the equation:

y = 0.67· ε.I²h

Where ε is the total permissible short term strain for the material, l is the length of the cantilever arm, and h is the thickness. The permissible deflection depends not only on the shape and the geometry of the component, but also on the total permissible strain for the material used. Table 1 and 2 lists the permissible short-term strain limits at room temperature for various families of Bayer engineering thermoplastics.

Table 1 - Un-reinforced

Apec®	High Heat Polycarbonate	4%
Makrolon®	Polycarbonate	4 %
Makroblend®	Polycarbonate Blends	3.5 %
Bayblend®	Polycarbonate /ABS Blend	2.5 %

Table 2 - Glass-Fiber Reinforced

Makrolon®	Polycarbonate (GF 10%)	2.2 %
Makrolon®	Polycarbonate (GF 20%)	2.0 %

Use these limits as guidelines for the applications involving one-time snaps only. For frequent separation and rejoining, use 60% of these values. Always perform proper engineering calculations when designing the snap-fit joints to avoid premature failure of the joints.

Tapering the thickness of a cantilever arm from the root to the hook produces a more uniform stress distribution along the length of the arm when deflected. Tapering the width also gives similar results. Hence, for the same deflection, the strain on a tapered arm is less than that on the arm with a uniform cross section. Permissible deflection y for a cantilever arm tapering from the thickness h at the root to a thickness h/2 at the hook is given by the equation:

y = 1.09· &epsilon· l²/ h

The only change from the equation for the arm with uniform cross section is the numerical constant that changed from 0.67 to 1.09. This means that the tapered arm of the same length can have approximately 63% more deflection for the same amount of strain.

Figure 2 shows examples of a cantilever and annular snap joints. In both these type of snap-fit joints, the deflection takes place as a result of flexural load. In torsional snap-fit joints, the deflection is produced by torsional deformation of the fulcrum.

Consult Bayer's "Snap-fit Joints for Plastics" manual for the annular and torsional type of joints. The manual also gives design details for other cantilever geometries such as tapered cantilever arm and U-shaped cantilever joints.

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