Spiral Spring Load Curve: Why Constant Force Is Never Constant

A constant force spiral spring is bought on a single number: the load it should produce. On the bench it is measured at one point of travel, and if that point passes, the lot ships. In service, however, the mechanism sees the whole curve, and the curve is where most complaints originate.

This article explains what shapes the load curve of a stainless spiral spring, which tolerances move it, and how to write an acceptance test that catches the variation instead of hiding it.

Where the Curve Comes From

A spiral spring is a strip or wire coiled on itself and pulled from the free end. Load is generated by bending that material to a smaller radius than its natural one. Because load is proportional to the cube of strip thickness (or the fourth power of wire diameter in a coil spring), dimensional tolerance dominates everything else.

A practical figure: plus or minus 1% on strip thickness produces roughly plus or minus 3% on load. Two suppliers working to the same nominal thickness but opposite ends of the tolerance band will deliver springs that differ by several percent on the same drawing.

Three Sources of Load Variation

1. Strip or wire thickness

The first and largest lever. For constant force applications we recommend fixing the thickness band rather than relying on general mill tolerance, and keeping one mill lot per production run. Mixing lots inside a batch is the most common cause of unit-to-unit scatter on an otherwise controlled drawing.

2. Inter-coil friction

Adjacent coils rub as the spring extends. Friction typically adds 5-15% to measured load and, more importantly, it adds hysteresis: the load going out is higher than the load coming back. Dry coils in a dusty environment can double that spread. Surface finish, cleanliness and a controlled lubricant film all change the number, so the test method must state whether the spring is tested dry or lubricated.

3. The inner end peak

At the first 10-20% of travel the material at the inner fixing is bent to the tightest radius it will ever see. This produces a load peak at the start of the stroke, typically 10-20% above the plateau. Mechanisms that are sensitive at the start of travel feel this as a jerk. A longer inner transition radius, or a preset operation, flattens the peak.

What Preset Does to the Curve

Presetting means winding the spring beyond its working range before delivery, so that the outer fibres yield in a controlled way and the spring settles into a stable shape. A correctly preset spring has a flatter curve, less relaxation in service and a repeatable free position. Over-presetting is equally real: the spring becomes shorter, the plateau load drops, and the part may no longer reach its rated travel.

How to Specify the Test

Do not accept a single point measurement. Ask for load at three positions, for example 10%, 50% and 90% of working travel, with a stated tolerance band on each and a stated direction of travel. A curve with three points shows thickness error, friction and the inner end peak separately, which a single number never can.

Specification Window

Applications

• Cable retractors and take-up reels
• Counterbalance and constant force lift assists
• Seal and gasket energising
• Medical device and diagnostic mechanisms
• Appliance cord management

What We Need to Quote

Working travel, load at the start and end of travel, maximum outer diameter, available housing width, whether the spring runs dry or lubricated, and the direction of winding. With these five items the curve can be designed rather than discovered.

TOPONE has produced stainless steel wire since 1993 and supplies wire and formed spiral springs from 0.15 mm to 12 mm, with load verification at three points of travel on request.