Question 1

How much interference can an annular snap-fit take?

Accepted Answer

The permissible diametral interference is simply y = ε · d, where ε is the permissible strain and d the diameter at the joint. That assumes one part is rigid. If both parts are equally flexible the strain is shared between them, so the interference can be twice as large for the same strain in each.

Question 2

Why is the assembly force only an estimate?

Accepted Answer

Because the bead does not stretch the whole ring uniformly — it expands a local length of the tube, and how much length depends on the wall thickness and the depth of the undercut. Both source design guides say so explicitly and fall back on a "beam resting on an elastic foundation" approximation. Treat the force as an indication of magnitude, and confirm anything critical on real mouldings.

Question 3

Should I design an inseparable annular snap?

Accepted Answer

Only if it genuinely must never come apart. An inseparable design needs a return angle at or beyond the self-locking angle, and moulding the resulting undercut usually requires a split-cavity tool — a real cost. It also means the only way into the assembly is to destroy it, so service and repair are gone.

Annular Snap-Fit