Spring Terminology Glossary
The terms engineers use when designing and specifying springs – from active coils to wave springs – explained in plain language.
About This Glossary
This glossary collects the terms most frequently used on spring drawings, datasheets and in conversations between engineers and spring manufacturers. Where possible, definitions follow the vocabulary of ISO 26909 (Springs – Vocabulary).
Browse by group: geometry, mechanics and behaviour, manufacturing and finishing, and spring types.
Geometry
Core Dimensions
Active coils: The coils that actually deflect when the spring is loaded – total coils minus the end coils. They determine the spring rate.
Total coils: All coils on the finished spring, including the end coils.
Free length: The length of an unloaded spring; the reference dimension for every load measurement.
Solid height: The length when all coils are touching – the absolute limit of compression travel.
Pitch: The centre-to-centre distance between adjacent coils; it controls free length and solid height.
Spring index: Mean coil diameter divided by wire diameter. A good design range is roughly 4 to 12; very low or very high indices are hard to make and stress badly.
Outer / inner diameter: The largest and smallest diameters of the coil; with wire diameter they define the spring envelope.
Ends (closed / ground): Closed ends are flattened so the spring stands square; ground ends are additionally ground flat for a stable bearing surface.
Direction of wind: Whether the helix runs right-hand or left-hand; must be specified for torsion springs and nested springs.
Attachments
Hook / loop: The formed end of an extension spring used to attach it to the mechanism.
Leg: The straight arm of a torsion spring that transmits the torque.
Mechanics and Behaviour
Spring rate (stiffness): Force per unit deflection, for example N/mm or lbf/in.
Deflection: The change in length from the free length under load.
Stress: Internal force per unit area in the wire; highest on the inner fibre of the coil.
Wahl factor: A correction factor for curvature and direct shear used in helical spring stress calculations.
Initial tension: The built-in force in an unloaded extension spring that must be overcome before it starts to extend.
Set: Permanent deformation after loading, which shortens the spring and reduces free length.
Preset: A factory operation that loads the spring to solid height once so dimensions stabilise before delivery.
Stress relieving: A low-temperature heat treatment after forming that removes residual stresses.
Buckling: Lateral bowing of a slender compression spring under load.
Fatigue life: The number of load cycles a spring survives at a given stress range.
Surging: Resonance of a spring at high operating speed, which can break coils or cause erratic forces.
Natural frequency: The frequency at which a spring-mass system vibrates freely; used to avoid resonance.
Hysteresis: Energy lost per cycle due to internal friction, seen as a gap between the loading and unloading curves.
Relaxation: Gradual loss of force over time under sustained load, especially at elevated temperature.
Elastic limit: The maximum stress a material can carry without permanent deformation.
Modulus of rigidity (G): The shear modulus used in spring rate calculations for helical springs.
Manufacturing and Finishing
Cold coiling: Forming wire at room temperature; the standard process for most wire springs.
Hot coiling: Forming heated wire or bar into large, heavy springs that cannot be coiled cold.
Shot peening: Blasting the surface with small shot to create compressive residual stress and improve fatigue life.
Grinding: Squaring the ends of compression springs on a grinding machine for a flat bearing surface.
Heat treatment / tempering: Controlled heating that sets the strength and toughness of alloy spring steels.
Hydrogen embrittlement: Brittleness caused by hydrogen absorbed during acid cleaning or plating; removed by baking or avoided with zinc-flake finishes.
Passivation: A chemical treatment that improves the natural corrosion resistance of stainless steel.
Dacromet: Trade name for a chromate-free zinc-aluminium flake coating with very high corrosion resistance.
Spring Types
Compression spring: A helical spring that resists compression; the most common spring type.
Extension spring: A helical spring with hooks or loops that resists being pulled apart.
Torsion spring: A helical spring with legs that resists twisting about its axis.
Disc spring: A conical washer-shaped (Belleville) spring that carries high load in a small space; made to EN 16983, formerly DIN 2093.
Wave spring: A thin annular spring formed into waves; offers low installed height with moderate load.
Wire form: A custom-shaped wire component with a spring-like function, such as a clip or retainer.
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