302 Soap-Coated Spring Wire - Spring Index and Load

Two springs wound from the same nominal wire can carry different loads at the same deflection. The reason is usually not the grade but geometry: the spring index, and the actual wire diameter inside its tolerance band. Because spring rate depends on the wire diameter raised to the fourth power, a diameter that is one percent heavy is four percent stiff. This page covers the index range that works in practice, how diameter tolerance feeds through into load, and why the soap coating on the wire matters during coiling rather than only during storage.

Product Specifications

The Spring Index and Why It Sets the Design

Definition and Practical Range

The spring index is the mean coil diameter divided by the wire diameter. A spring wound from 2 mm wire to a mean diameter of 16 mm has an index of eight. The practical working range runs from four to twelve, with six to ten being the comfortable band. Below four the wire is bent so tightly that the inside of the coil carries very high stress and the tooling needs to be very precise; above twelve the spring becomes laterally unstable and the coils tend to tangle.

How Index Changes the Stress

Coiling puts the wire into torsion, and the stress is not uniform across the section. The Wahl factor corrects the nominal shear stress for the curvature and the direct shear at the inside of the coil, and it grows quickly as the index falls. At an index of ten the correction is around 1.14; at an index of five it is around 1.31. That difference is why a spring design that is comfortable at index eight can fail at index five, even with the same wire and the same load.

Where Diameter Tolerance Enters the Load

The Load Formula in One Line

Spring rate is proportional to the wire diameter to the fourth power, divided by the cube of the mean diameter and by the number of active coils. The fourth power is the important part. A wire specified at 1.00 mm that arrives at 1.01 mm is four percent stiffer than the nominal design; at 1.02 mm it is eight percent stiffer. Where a spring is designed to a load window of plus or minus five percent, the wire tolerance alone can consume most of that window.

Tolerance Stacking in Practice

Three tolerances add up: the wire diameter, the mandrel or coiling tool wear, and the number of active coils after grinding. If the wire is bought to a plus or minus 0.02 mm band at 1 mm, the spring rate itself can move by about eight percent before any tooling variation is considered. Setting the wire tolerance tighter, or accepting the load spread and sorting at the end of the line, is a commercial decision that is much cheaper to make at the purchase order stage.

Coiling and the Soap Coating

Friction During Coiling

Soap coating on spring wire is a drawing lubricant carrier that is left in place deliberately. During coiling it lowers friction between the wire and the feed rolls and between adjacent coils, which reduces scratching and helps the coils settle at an even pitch. Wire that has been degreased for a clean appearance often coils with more scuffing and a wider pitch scatter than the same wire with the coating intact.

Coating and Post-Processing

The coating has to be removed before electroplating, passivation or welding, and that removal is an extra process step with its own waste stream. Where the spring will be plated, the coating choice should be agreed with the plater rather than assumed. Where the spring stays bare and dry, the coating can remain, and it also gives short term protection in storage.

Applications

Compression and Extension Springs

302 spring temper wire at 0.3 mm to 6 mm covers most general purpose springs. Static applications can run at the higher end of the tensile range, while springs that cycle continuously need the lower end with better fatigue reserve at the same index.

Wire Forms and Clips

Clips and wire forms use the same index logic even though they are not springs in the strict sense: the bend radius relative to the wire diameter decides whether the part cracks or survives. A bend radius below three times the diameter is the practical limit for spring temper wire without annealing.

Instrument and Appliance Springs

Small springs in instruments and appliances are usually wound from 0.15 mm to 1 mm wire, where diameter tolerance has the largest relative effect on load. In this range a wire tolerance of plus or minus 0.005 mm is sometimes worth paying for, because the load window cannot be met any other way.

Product Advantages

• Soap coating retained as a coiling lubricant, reducing scuffing and pitch scatter on the winder.
• Diameter tolerance of ±0.01 mm on fine sizes, keeping the spring rate close to the nominal design.
• Spring temper tensile strength of 1400-1800 MPa with size related control, so the same grade behaves consistently across diameters.
• Controlled cast and helix, so the wire feeds without twisting on high speed coilers.
• Surface free of seams and laps, avoiding the notch effect that starts fatigue cracks at the inside of the coil.

Quality Control

Diameter is measured at the coil ends and at intermediate points, and tensile strength and elongation are tested per lot. Surface is inspected for seams, laps and die marks, since a longitudinal defect on the inside of a coil becomes a fatigue initiation site. Coil set is checked against the agreed value. Test results are recorded against the heat number so that a load deviation found during winding can be traced back to a specific delivery.

Selection and Ordering Notes

1. State the wire diameter, the mean coil diameter and the number of active coils, so the index and the expected rate can both be checked.
2. Give the load window the spring must meet. If it is tighter than plus or minus five percent, tighten the wire diameter tolerance at the order stage.
3. Say whether the spring will be plated, passivated or welded, so the coating can be chosen to suit the next process.
4. For cycling applications, state the cycle count and the stress ratio, as fatigue reserve depends on both the temper and the index.
5. Confirm whether cast and helix need to be controlled, as free coils and level wound coils are packed and priced differently.

Spring performance is set by geometry first and material second. Send the wire diameter, the coil diameter and the load window, and the wire specification can be matched to the spring rather than to a general temper description.