Technical Glossary
Definitive engineering glossary of mechanical spring terminology, formulas, dimensional parameters, and manufacturing processes.
Free Length (L₀ / Lf)
The overall length of a spring when not subjected to any external load or deflection. It is a foundational dimensional specification for mechanical envelope and assembly design.
Wire Diameter (d)
The cross-sectional thickness of the spring wire, measured in millimeters (mm) or inches. Because spring stiffness varies with the fourth power of wire diameter (k ∝ d⁴), minor changes in wire diameter significantly impact spring rate.
Outer Diameter (OD)
The overall diameter of the outside envelope of the spring coils. Essential for clearance checks when the spring operates inside a bore, cavity, or cylinder tube.
Inner Diameter (ID)
The diameter of the internal cylindrical opening formed by the coils (ID = OD − 2d). Critical for verifying clearance when the spring operates over a guide rod, arbor, or locating pin.
Mean Diameter (D)
The average diameter of the spring body measured from wire centerline to wire centerline: D = (OD + ID) / 2. This is the primary diameter value used in stress, deflection, and stiffness calculations.
Spring Rate / Stiffness (k / R)
The change in load per unit deflection, expressed in N/mm or lbf/in. For helical compression springs: k = (G · d⁴) / (8 · D³ · nₐ), where G represents the material shear modulus.
Solid Height (Hs / Ls)
The total compressed length of a helical compression spring when all adjacent active and dead coils make solid metal-to-metal contact, preventing any further compression.
Initial Tension (Pi / F₀)
The internal preload force wound into close-wound helical extension springs during coil winding, holding adjacent coils tightly together prior to the application of external tensile force.
Active Coils (nₐ)
The number of working coils that elastically deform under load to store and release mechanical energy, excluding dead or closed seat coils.
Total Coils (nt)
The complete number of coils from tip to tip, comprising both active deflection coils and closed, ground, or squared end seating coils.
Pitch (p)
The center-to-center distance between adjacent active coils in the uncompressed state: p = d + (L₀ − Ls) / nₐ. Uniform pitch ensures even stress distribution along the spring axis.
Spring Index (C)
The dimensionless ratio of mean coil diameter to wire diameter (C = D / d). Optimum manufacturing range is 4 to 12. Values below 4 cause high residual stress and tooling breakage; values above 12 risk buckling and tangling.
Maximum Shear Stress (τₘₐₓ)
The maximum torsional surface shear stress developed in the spring wire under full working deflection, adjusted by the Wahl curvature correction factor (Kw) to account for wire curvature and direct shear.
Fatigue Life & Endurance Limit
The number of cyclic stress reversals a spring endures before failure or permanent load degradation. Industrial specifications generally range from 10⁵ to 10⁷+ cycles depending on dynamic fatigue requirements.
Natural Resonance Frequency (fn)
The fundamental harmonic vibration frequency of the spring body. Operating stroke frequencies should remain well below fn (typically fn / 13) to avoid destructive harmonic surging and premature failure.
Load Loss & Relaxation
The gradual reduction in load capacity over time while the spring is maintained under constant deflection, particularly accelerated in high-temperature environments.
Creep
The slow, time-dependent plastic strain and dimensional change of spring material subjected to constant mechanical load over extended periods under thermal stress.
Cold Formed Springs
Springs coiled and shaped at ambient room temperature from pre-hardened or hard-drawn spring wire, standard for wire diameters up to 16 mm (5/8 in).
Hot Formed Springs
Heavy-duty helical springs wound from alloy steel heated to austenitic forging temperatures (>850°C), then immediately quenched and tempered, typical for heavy rail, mining, and large machinery.
Presetting / Scragging (Set Removal)
A manufacturing step where a compression spring is compressed solid multiple times to introduce favorable residual compressive stresses, stabilizing free length and eliminating in-service settling.
Shot Peening
A surface cold-working process where spherical metallic media bombard the spring surface at high velocity, inducing beneficial compressive stresses that delay fatigue crack initiation and double service life.
Dead / End Coils
The inactive terminal coils on compression springs that are closed and ground square to provide flat, perpendicular seating surfaces against mating components.
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