What is spring rate (stiffness)?

Spring rate k is the force required to compress the spring by one unit of length, calculated as k = (G × d⁴) / (8 × D³ × Na). A higher G or wire diameter increases stiffness; more active coils reduce it.

What is the spring index and why does it matter?

Spring index C = D / d. Values below 4 are difficult to manufacture; above 12 the spring may buckle or tangle. Indices between 6 and 9 are typical for good manufacturability.

What is the Wahl correction factor?

The Wahl factor Kw accounts for stress concentration at the inner coil surface and wire curvature. It amplifies the nominal shear stress and gives a more realistic peak stress value.

How is natural frequency calculated?

Natural frequency fn = (d / (π × D² × Na)) × √(G / (2ρ)), where ρ is the material density. The spring should operate below one-third of its natural frequency to avoid resonance.

What does solid height mean?

Solid height Ls = Nt × d (total coils × wire diameter) is the length when all coils are fully compressed. The maximum working deflection must leave a small gap above solid height.

Which material should I select?

Music wire (ASTM A228) is the default for high-strength general applications. Hard-drawn wire is lower cost for static loads. Stainless 302 suits corrosive environments. Chrome-vanadium is preferred for elevated temperatures.

Spring Design Calculator

Name: Spring Design Calculator
Availability: InStock
Author: Nham Vu

Compute spring rate, shear stress, deflection, and natural frequency for helical compression springs.

Spring Parameters

Material

Wire Diameter d (mm)

Mean Coil Diameter D (mm)

= outer diameter − wire diameter (approx.)

Active Coils Nₐ

Total Coils Nₜ (for solid height)

Typically active coils + 2 (closed ends)

Free Length L₀ (mm)

Applied Load F (N) — optional

Spring Properties

Spring Rate k

—

N/mm

Spring Index C

—

D / d

Solid Height Ls

—

Max Deflection (to solid)

—

Max Load (to solid)

—

Natural Frequency

—

Summary