Why Torsion Spring Legs Are Essential
The terminal wire segments extending from the coiled body of a torsion spring are designated as legs (or arms). They anchor the spring to its mounting chassis and transmit rotational torque to the mating lever, hinge flap, or valve stem. The geometry of the legs directly governs moment arm length ($L$), torque transmission efficiency, spatial packaging, and local stress concentration.
Unlike compression springs which seat flat on their coil faces, a torsion spring cannot function without legs. Sizing leg bend radii and moment arms properly ensures that the spring achieves its full theoretical fatigue life.
5 Primary Leg Geometries
1. Straight Tangential Legs (Tangent)
The wire extends straight along the tangent line of the outer coil without secondary bending operations. Provides the lowest manufacturing cost, minimal stress risers, and maximum flexibility in setting radial lever arms.
Best For: Envelopes with ample radial clearance, flat push tabs, or sliding slotted cam interfaces.
2. Axially Bent (90° Axial Bend)
The leg turns 90° to run parallel to the longitudinal central axis of the spring coil. Allows direct insertion into machined dowel holes, hinge leaves, or retention slots.
Best For: Door and window hinges, vehicle glovebox latches, and tight coaxial shaft housings.
3. Formed Loops (180° Bend / Eyelet)
The terminal wire curves back 180° to form a complete eyelet or half-loop hook. Accommodates retention pins, cotter pins, or tension wire cable linkages without sharp sliding friction.
Best For: Counterbalance mechanisms, automotive tailgate cables, and electrical disconnect switches.
4. Radial Bends (Inward or Outward)
The legs bend 90° perpendicularly toward the center axis (short radial inward) or away from the axis (short radial outward). Provides solid positive stops against internal shafts or external casing walls.
Best For: Motor commutator brush holders, rotary switches, and ratcheting mechanisms.
5. Multi-Planar Custom Formed Legs
Engineered with compound 3D bends, swaged steps, offset forks, or integrated tabs manufactured on CNC multi-axis wire forming machines.
Best For: Complex robotic end-effectors, medical surgical instruments, and aerospace throttles.
Engineering Design Rules for Leg Bends
- Inside Bend Radius ($R \ge 2d$): Always maintain an internal bend radius of at least 2 times wire diameter ($R \ge 2d$). Sharp inside corners create acute stress concentration factors exceeding 2.5, causing premature leg fatigue fractures.
- Virtual Active Coil Consideration: Deflection along long legs contributes to total angular rotation! Add virtual coils to the analytical calculation: N_virtual ≈ (L₁ + L₂) / (3πD).
- Shot-Peening at Bend Radii: In cyclic applications exceeding 500,000 cycles, specify secondary shot-peening across leg bend zones to induce compressive residual stress.
Require Custom Torsion Spring Leg Tooling?
TOSTA multi-axis CNC wire formers support any custom 3D leg profile. Submit your STEP/IGS CAD models for an instant DFM analysis and prototypes within 1–3 business days.
Frequently Asked Questions (FAQ)
Is leg breakage typically a manufacturing defect or a design issue?
Over 80% of leg fractures stem from mechanical design flaws—insufficient bend radii (sharp corners), excessive angular deflection, or cyclic overload. TOSTA conducts FEA stress simulations and dual-stage shot peening to prevent bend fractures. Raw material defects account for less than 5%.
Can standard stock torsion spring legs be manually bent in the field?
Manual bending is strongly discouraged. Re-bending post-tempered spring wire induces severe localized residual tensile stress and micro-fissures, reducing fatigue life by more than 50%. Custom leg shapes should always be formed prior to stress-relief heat treatment.
What is the cost difference between straight tangential legs and 90° bent legs?
Straight tangential legs carry the lowest cost (zero secondary forming). 90° bends add single-step tooling (+10%–20%); 180° loops add two forming steps (+25%–40%); complex custom formed legs are quoted per CAD drawing.
Is surface shot-peening necessary at leg bends?
For high-cycle dynamic applications (> 500,000 cycles), shot-peening the bend transitions is strongly recommended to boost fatigue endurance by 30%–50%. For low-cycle static holding, shot peening may be omitted provided the inside bend radius R ≥ 2d.
How complex can custom torsion spring legs be manufactured?
TOSTA multi-axis CNC wire formers support hooks, loops, T-shapes, forked pins, offset steps, and multi-planar compound bends. We accept STEP/IGS CAD models and provide complete DFM reviews within 1–3 days.