Torsion Snap Fit — Eurisco Labs

A snap-fit that uses twisting deflection instead of bending — higher strain capacity and more compact than cantilever designs.

How it works

Instead of bending a beam, a torsion snap-fit twists a bar or arm around its axis. The hook rotates into position rather than deflecting linearly. Less common than cantilever snap-fits but useful when space is limited.

Top view:

  ████████████
  █          █
  █   ┌──────█──── arm with hook
  █   │pivot █
  █   └──────█──── twists around pivot
  █          █
  ████████████

Key design parameters

Parameter	Guideline
Torsion bar length	Longer = lower stress for same deflection
Cross section	Round or square — round distributes stress better
Max shear strain	~50% higher capacity than bending for same material

Design tips

Round cross-section torsion bars distribute shear stress more evenly than square ones.
Torsion allows more deflection in less space than a cantilever — useful for compact designs.
The pivot must be constrained — the torsion bar needs to be anchored at both ends or supported to prevent bending.

FDM considerations

Torsion bars are risky on FDM — twisting loads peel layers apart regardless of orientation. Print with high infill (80%+) or solid.
Minimum cross-section ~3mm for FDM — smaller and the layer adhesion can’t handle torsion loads.

When to use

Compact enclosures where cantilever beam length is limited
Rotating latches and catches
Parts that need high deflection in a small space

Common failures

Torsion bar shear fracture — bar snaps from twisting. Cause: too short, cross-section too small.
Layer delamination (FDM) — twisting peels layers apart. Cause: low infill, poor layer adhesion.