Spring design guidelines

Keep the spring index, mean coil diameter divided by wire diameter, between 4 and 12. Use no more than about 80–85% of the travel to solid, keep free length under about 4× mean diameter unless the spring is guided, and give bends an inside radius of at least 1× wire diameter. Specify load at one or two heights, with ±10% load tolerance as the commercial default.

Try the spring calculator Check a part in the builder

Spring index (D/d): 4–12
Usable travel: 80–85% to solid
Load tolerance: ±10% commercial
Wire bend radius: At least 1× wire dia

SSG MFG · design guideDG-06 · 13 rules

What are the springs & wire forms design rules?

13 rules, in the order you meet them on a model. Every value is typical; the principal quoting your part confirms its own limits.

Design rules and typical values
#	Rule	Typical value
01	Keep the spring index in rangeMean diameter divided by wire diameter (D/d) between 4 and 12; 6–9 is easiest	Mean diameter divided by wire diameter (D/d) between 4 and 12; 6–9 is easiest
02	Check stress with the Wahl factorK = (4C − 1)/(4C − 4) + 0.615/C; about 1.40 at C = 4, 1.18 at C = 8	K = (4C − 1)/(4C − 4) + 0.615/C; about 1.40 at C = 4, 1.18 at C = 8
03	Keep solid stress under the allowableCorrected stress at solid under about 45% of minimum tensile strength; about 60% with set removal	Corrected stress at solid under about 45% of minimum tensile strength; about 60% with set removal
04	Leave travel before solidUse no more than about 80–85% of available deflection	Use no more than about 80–85% of available deflection
05	Check compression springs for bucklingFree length no more than about 4× mean diameter unless guided	Free length no more than about 4× mean diameter unless guided
06	Clear the bore and the rodBore at least about 10% over spring OD; rod at least about 10% under spring ID	Bore at least about 10% over spring OD; rod at least about 10% under spring ID
07	Specify loads, not everythingLoad at one or two working heights, with free length and rate as reference	Load at one or two working heights, with free length and rate as reference
08	Call out ground ends only when neededClosed and ground ends give about 3° squareness; ground ends typical above about 0.020 in (0.5 mm) wire	Closed and ground ends give about 3° squareness; ground ends typical above about 0.020 in (0.5 mm) wire
09	Size extension spring hooksHook bend inside radius at least 2× wire diameter	Hook bend inside radius at least 2× wire diameter
10	Allow for torsion spring ID reductionWound ID ≈ D × N / (N + θ/360) − d; mandrel about 10% under that ID	Wound ID ≈ D × N / (N + θ/360) − d; mandrel about 10% under that ID
11	Bend wire at 1× diameter or moreInside radius at least 1× wire diameter; 1.5–2× for music wire and hard-drawn stainless	Inside radius at least 1× wire diameter; 1.5–2× for music wire and hard-drawn stainless
12	Leave straight wire between bendsAt least about 2× wire diameter of straight between bends	At least about 2× wire diameter of straight between bends
13	Bake after platingHydrogen embrittlement relief per ASTM B850, typically within 4 hours of plating	Hydrogen embrittlement relief per ASTM B850, typically within 4 hours of plating

These build on the short list on the springs & wire forms process page, with the failure each rule prevents and a drawing for each.

Which spring index and stress will work?

Change the wire diameter against the mean coil diameter to see how the spring index moves the Wahl correction on stress, and where a spring becomes too tight to coil or too loose to hold its size.

WIRE 0.047 IN · MEAN ⌀0.375 IN · INDEX 8.0

Start from

Wire diameter, music wire

Mean coil diameter

Working load

Spring index C = D/d: 8.0target 4–12
Wahl factor K: 1.185+18% stress
Corrected shear stress: 54 ksi(376 MPa)
Share of min. tensile: 17%of 313 ksi, limit 45%

Works: coils well, stress in range

Index 8.0 coils consistently, and 17% of minimum tensile leaves margin under the typical 45% static limit.

Static check for ASTM A228 music wire, typical values. Fatigue loading needs a lower limit. Wire 0.047 in, minimum tensile about 313 ksi by size.

Why does each rule matter?

Each rule with its typical value, the reason behind it and what happens on the floor when it's ignored. Switch any drawing between Do and Avoid; point at a callout to pick it out.

Rule 01

Keep the spring index in range

Mean diameter divided by wire diameter (D/d) between 4 and 12; 6–9 is easiest

Why: Low index wire is bent too hard to coil consistently. High index springs are floppy and vary in diameter.
If you ignore it: Below 4 the wire cracks or the coiler cannot hold size; above 12 springs tangle and diameters wander.

Fig. 1Spring index

Do. An index of 4–12 coils consistently, holds load tolerance and keeps stress correction modest.

Avoid. Below about 4 the wire is bent so tightly it cracks or takes a set in coiling; above about 12 the spring tangles and won't hold size.

Rule 02

Check stress with the Wahl factor

K = (4C − 1)/(4C − 4) + 0.615/C; about 1.40 at C = 4, 1.18 at C = 8

Why: Torsional stress is highest on the inside of the coil, and the correction grows as the index drops.
If you ignore it: A spring sized on uncorrected stress takes a permanent set or breaks early at the inside of the coil.

Standard: SMI Handbook of Spring DesignSee Fig. 1, spring index

Rule 03

Keep solid stress under the allowable

Corrected stress at solid under about 45% of minimum tensile strength; about 60% with set removal

Why: A spring that can be compressed solid in assembly or service must survive that stress without yielding.
If you ignore it: The spring takes a permanent set the first time it goes solid and loses load.

Standard: SMI Handbook of Spring Design

Rule 04

Leave travel before solid

Use no more than about 80–85% of available deflection

Why: Coils close unevenly near solid, so rate turns nonlinear and stress climbs fast.
If you ignore it: Load at the working height scatters and coils clash, which shortens fatigue life.

Fig. 2Free, working and solid height

Do. Specify load at a working height that uses no more than about 80–85% of the travel to solid, and give free length as reference.

Avoid. Run to solid, the coils clash and the stress spikes: the spring takes a permanent set and loses load.

Rule 05

Check compression springs for buckling

Free length no more than about 4× mean diameter unless guided

Why: Slender springs bow sideways under load unless they run in a bore or over a rod.
If you ignore it: The spring bows, rubs the bore wall and loses load at the working height.

Fig. 3Buckling

Do. Keep free length under about 4× the mean diameter, or run the spring on a rod or in a bore.

Avoid. A slender, unguided spring bows sideways under load: force drops, coils rub and it can jump out of its seat.

Rule 06

Clear the bore and the rod

Bore at least about 10% over spring OD; rod at least about 10% under spring ID

Why: Compression springs grow in diameter as they compress, and coil diameter carries a tolerance.
If you ignore it: The spring binds in the bore or on the rod, and load drops from friction.

Rule 07

Specify loads, not everything

Load at one or two working heights, with free length and rate as reference

Why: The maker adjusts coils and free length to hit load. Fixing every dimension leaves nothing to adjust.
If you ignore it: An over-constrained drawing cannot be met in production and parts go to 100% sorting.

Fig. 4A complete drawing package

Do. Every requirement is on the drawing: general tolerance, material by specification, finish by specification, critical dimensions and revision.

Avoid. Whatever the drawing leaves blank, the supplier decides, usually with the cheapest local equivalent.

Rule 08

Call out ground ends only when needed

Closed and ground ends give about 3° squareness; ground ends typical above about 0.020 in (0.5 mm) wire

Why: Ground ends seat flat and load evenly but add an operation and a burr to remove.
If you ignore it: Ungrounded ends tip the spring off axis, while needless grinding adds cost to every part.

Rule 09

Size extension spring hooks

Hook bend inside radius at least 2× wire diameter

Why: The bend where the hook leaves the body combines bending and torsional stress.
If you ignore it: Hooks break at the bend long before the body of the spring fatigues.

Fig. 5Extension spring hooks

Do. A hook bend with an inside radius of at least 2× the wire keeps the peak stress near the body's own.

Avoid. A sharp bend at the hook is where extension springs usually break, often long before the body would.

Rule 10

Allow for torsion spring ID reduction

Wound ID ≈ D × N / (N + θ/360) − d; mandrel about 10% under that ID

Why: A torsion spring adds coils and shrinks in diameter as it winds up.
If you ignore it: The spring clamps onto its arbor, binds and breaks at the legs.

Rule 11

Bend wire at 1× diameter or more

Inside radius at least 1× wire diameter; 1.5–2× for music wire and hard-drawn stainless

Why: Spring wire is cold drawn to high strength and has limited ductility left for tight bends.
If you ignore it: Tight bends crack on the outside or leave a stress riser that fails in service.

Fig. 6Wire bend radius

Do. An inside radius of at least one wire diameter forms without cracking and leaves a smaller stress riser in service.

Avoid. A tight bend overstrains the outside of the wire; hard-drawn and music wire crack there, in forming or later in fatigue.

Rule 12

Leave straight wire between bends

At least about 2× wire diameter of straight between bends

Why: The bending head needs straight wire to grip and form each bend.
If you ignore it: Back-to-back bends need custom tooling or come out with the angle and plane off.

See Fig. 6, wire bend radius

Rule 13

Bake after plating

Hydrogen embrittlement relief per ASTM B850, typically within 4 hours of plating

Why: Electroplating drives hydrogen into hardened spring steel.
If you ignore it: Springs crack hours or days after assembly with no warning.

Standard: ASTM B850

What tolerances can springs & wire forms hold?

Spring tolerances depend on spring index, wire size and active coils. Load is the controlling tolerance, and dimensions that the maker adjusts to hit load should be reference.

Typical and tight tolerances with the cost of going tight
Feature	Typical	Tight	Cost of tight
Load at a working heightTight usually means 100% testing or sorting	±10%	±5%	Moderate
Spring rate	±10%	±5%	Moderate
Free lengthReference when load is specified	±1.5–3% of length	±1% of length	Moderate
Coil outside diameterLooser at high index	±1–2% of diameter	±0.5–1% of diameter	Moderate
End squareness, ground ends	3°	1.5–2°	Moderate
Torsion spring free angle	±5–15° by coil count	±3–5°	High
Wire form bend angle	±2°	±1°	Moderate
Wire form leg length	±0.015 in (±0.4 mm)	±0.005 in (±0.13 mm)	Moderate

Basis: SMI Handbook of Spring Design tolerance tables. Put a general tolerance note on the drawing and tighten only the features that need it; every tight callout is priced. See the tolerance guide.

How does the material change the rules?

The same springs & wire forms rules shift with the alloy or compound.

Music wire (ASTM A228)
Highest strength of common spring wires and the lowest cost, so it allows the smallest spring for a load. It rusts, tops out near 250 °F (120 °C), and plated parts need an ASTM B850 bake.
Stainless steel 302 (spring temper)
ASTM A313 wire resists corrosion and runs to about 550 °F (290 °C), at roughly 15% lower strength than music wire. Plan a slightly larger wire or lower stress for the same load.
Stainless steel 316
Chosen for chloride and marine exposure. It is weaker than 302 in spring tempers, so allowable stress drops and wire size goes up.
Stainless steel 304
Common for wire forms and light springs where 302 is not stocked. Its strength is close to 302, and it becomes slightly magnetic after cold working.
Phosphor bronze C510 (spring temper)
Conductive and nonmagnetic, used for electrical contacts. Tensile strength is well below steel wire, so springs need a larger wire or lower stress for the same load.
Steel 1008 cold-rolled (CRS)
Low-carbon wire suits formed hooks, guards and handles that do not flex in service. It bends tight and costs little, but it takes a set and is not a spring material.

Springs & wire forms design questions

What is a good spring index?

A spring index, mean coil diameter divided by wire diameter, of 6–9 is easiest to make and holds size well. The workable range is 4–12. Below 4, the wire is bent so tightly that it is hard to coil and stress rises sharply. Above 12, springs are flimsy, vary in diameter and tangle in bulk packaging.

What is the Wahl correction factor?

The Wahl factor corrects torsional stress for coil curvature and direct shear. It is K = (4C − 1)/(4C − 4) + 0.615/C, where C is spring index. At an index of 4, K is about 1.40, so peak stress is 40% above the uncorrected value. At an index of 8, K is about 1.18.

How much should a compression spring deflect?

Design the working deflection to use no more than about 80–85% of the travel from free length to solid height. The last coils close unevenly, which makes the rate nonlinear and raises stress. If the spring can be forced solid in assembly, check solid stress against the allowable for the wire.

When does a compression spring buckle?

A compression spring with a free length more than about 4× its mean diameter tends to buckle unless it runs in a bore or over a rod. Squared and ground ends seated on parallel plates raise the limit somewhat. If the spring must be slender, guide it and leave about 10% diametral clearance.

What should a spring drawing specify?

Specify wire material and diameter, the load at one or two working heights with a tolerance, the maximum OD or minimum ID the spring must fit, end type and finish. Mark free length, rate and coil count as reference. The spring maker adjusts those to hit the loads, which avoids sorting.

What is initial tension in an extension spring?

Initial tension is the force that holds the coils of a close-wound extension spring together before it starts to stretch. It is wound in during coiling and depends on spring index. The spring does not extend until the load exceeds it. Specify load at an extended length rather than initial tension alone.

Check your part against these rules.

Drop a STEP file in the builder for an instant DFM check and a budgetary estimate, then send it to principals who run springs & wire forms every day.

Build a part Send a drawing

Spring design guidelines

Rule

Keep the spring index in rangeMean diameter divided by wire diameter (D/d) between 4 and 12; 6–9 is easiest

Check stress with the Wahl factorK = (4C − 1)/(4C − 4) + 0.615/C; about 1.40 at C = 4, 1.18 at C = 8

Keep solid stress under the allowableCorrected stress at solid under about 45% of minimum tensile strength; about 60% with set removal

Leave travel before solidUse no more than about 80–85% of available deflection

Check compression springs for bucklingFree length no more than about 4× mean diameter unless guided

Clear the bore and the rodBore at least about 10% over spring OD; rod at least about 10% under spring ID

Specify loads, not everythingLoad at one or two working heights, with free length and rate as reference

Call out ground ends only when neededClosed and ground ends give about 3° squareness; ground ends typical above about 0.020 in (0.5 mm) wire

Size extension spring hooksHook bend inside radius at least 2× wire diameter

Allow for torsion spring ID reductionWound ID ≈ D × N / (N + θ/360) − d; mandrel about 10% under that ID

Bend wire at 1× diameter or moreInside radius at least 1× wire diameter; 1.5–2× for music wire and hard-drawn stainless

Leave straight wire between bendsAt least about 2× wire diameter of straight between bends

Bake after platingHydrogen embrittlement relief per ASTM B850, typically within 4 hours of plating

Feature

Typical

Tight

Cost of tight

Load at a working heightTight usually means 100% testing or sorting

±10%

±5%

Moderate

Spring rate

±10%

±5%

Moderate

Free lengthReference when load is specified

±1.5–3% of length

±1% of length

Moderate

Coil outside diameterLooser at high index

±1–2% of diameter

±0.5–1% of diameter

Moderate

End squareness, ground ends

3°

1.5–2°

Moderate

Torsion spring free angle

±5–15° by coil count

±3–5°

High

Wire form bend angle

±2°

±1°

Moderate

Wire form leg length

±0.015 in (±0.4 mm)

±0.005 in (±0.13 mm)

Moderate

What are the springs & wire forms design rules?

Which spring index and stress will work?