Spring technical terms
Every spring term explained in plain language — from wire diameter to spring rate.
Understanding the terminology is the first step to specifying the right spring. We've gathered the main terms used when designing and buying springs, grouped by topic. Click any term to read its full definition.
Spring types
Compression springA helical spring designed to resist axial loads that compress it and return to its original length when the force is removed.Extension springA helical spring that works under loads that stretch it, storing energy as it is pulled and returning to its original length when released.Torsion springA helical spring that stores energy by twisting and applies an angular torque through two legs.Conical springA compression spring whose diameter varies along its length, letting the coils “telescope” and reduce the solid height.Helical springA spring made from wire coiled into a helix — the basis of compression, extension and torsion types.
Dimensions & geometry
Wire diameterWDThe thickness of the wire the spring is made from — the parameter that most influences the spring's strength and stiffness.Outer diameterODThe overall width of the spring, measured from the outer edge of one coil to the outer edge of the opposite coil.Inner diameterIDThe width of the open space inside the spring, measured from the inner edge of one coil to the inner edge of the opposite coil.Mean diameterDmThe diameter measured at the center of the wire, equal to the outer diameter minus one wire diameter (Dm = OD − d).Free lengthFLThe overall length of the spring when no load is applied.Solid heightSHThe length of a compression spring when all coils are fully compressed and touching.PitchThe distance between the centers of two adjacent coils in a spring.Total coilsTCThe complete number of coils in the spring, including both the active coils and the inactive end coils.Active coilsACThe coils that actually deform under load and contribute to storing and releasing energy.Spring indexThe ratio of the mean diameter to the wire diameter (C = Dm / d), indicating how “tight” or “open” the spring is.
Forces & performance
Spring rateThe force required to compress or extend the spring by one unit of length — the spring's stiffness (N/mm).Angular rateThe torque required to rotate a torsion spring by one degree (N·mm/°) — the spring's angular stiffness.LoadThe external force applied to a spring, causing compression or extension (measured in newtons).Deflection (travel)The distance a spring compresses or extends under load, measured from the free length.Maximum deflectionThe greatest deflection a spring can undergo before reaching its elastic limit or solid height.Initial tensionThe force that holds the coils of an extension spring closed when there is no external load.StressThe internal force per unit area within the spring material resulting from the applied load.Wahl factorA coefficient that corrects for the stress concentration on the inner face of the coil, where the wire is most loaded.Natural frequencyThe frequency at which the spring oscillates freely when disturbed.DampingThe dissipation of energy that reduces the oscillations or vibrations of a spring system.BucklingAn instability in which a compression spring bends sideways under load, moving off-axis.FatigueThe progressive weakening of a spring caused by repeated cycles of stress over time.SetThe permanent loss of length or shape when a spring is loaded beyond its elastic limit.Spring backThe tendency of the material to return to its original shape after being compressed, extended or twisted.Shear modulusA material property (G) that measures its stiffness in shear, essential when computing the spring rate.Poisson's ratioThe ratio of lateral strain to axial strain in a material under load.Residual stressThe stress that remains in a spring after manufacturing, before any external load is applied.
Ends & configuration
End typeHow the ends of a compression spring are finished: closed & ground (CG), closed & squared (C) or open (O).Wind directionThe direction the wire is coiled: right-hand (RH) or left-hand (LH).Hook typeThe shape of an extension spring's ends: machine hook, crossover hook, side hook or extended hook.Hook lengthThe measurement of the hook, from where it starts at the spring body to its end point.Free positionThe natural angular position of a torsion spring's legs when no external force is applied.Leg lengthThe length of the straight sections extending from the coiled body of a torsion spring to the centerline.
Materials & treatments
Material typeThe alloy the spring is made from, chosen by strength, environment and cost.Heat treatmentA controlled heating and cooling process that improves the spring's mechanical properties.Surface treatmentFinishes such as painting, zinc plating, nickel plating or coating that increase corrosion resistance and durability.