Question 1

How do I calculate the spring rate of a compression coil spring?

Accepted Answer

Use k = G·d⁴ / (8·D³·Na), where G is the material shear modulus, d is the wire diameter, D is the mean coil diameter and Na is the number of active coils. For example, d = 2 mm, D = 16 mm, Na = 5 and steel G = 80000 MPa give k = 80000 × 16 / (8 × 4096 × 5) = 7.81 N/mm.

Question 2

What is the spring index C and why does it matter?

Accepted Answer

The spring index is C = D / d, the ratio of mean coil diameter to wire diameter. A practical range is about 4 to 12. Below 4 the spring is hard to coil and stresses concentrate sharply; above 12 it becomes flimsy and prone to tangling and buckling.

Question 3

What is the Wahl correction factor?

Accepted Answer

The Wahl factor Kw = (4C − 1)/(4C − 4) + 0.615/C corrects the shear stress for the extra stress on the inner side of the coil caused by curvature and the direct shear. The corrected stress is τ = Kw × 8·F·D / (π·d³).

Question 4

Which shear modulus G should I use?

Accepted Answer

Typical transverse (shear) modulus values are about 80000 MPa for piano/hard-drawn steel wire, 69000 MPa for SUS304-type stainless, 41000 MPa for phosphor bronze and 39000 MPa for brass. Use the exact value from your wire datasheet for accurate results.

Question 5

Does this calculator account for buckling and fatigue?

Accepted Answer

No. It gives a theoretical estimate for the elastic range assuming closed and ground ends. It does not check buckling, surging, fatigue life or set, so always verify a real design against the relevant spring standard and the manufacturer's data.

Compression SpringCalculator

Compression Spring
Calculator