Constant force springs

At KERN-LIEBERS, we classify our constant force springs into 3 groups:

MIGRA Flat characteristic curve, i.e MInimal GRAdient
NEGRA Falling characteristic curve, i.e NEgative GRAdient
POGRA Rising characteristic curve, i.e POsitive GRAdient

Service life of constant force springs

The service life refers to the number of extractions and retractions that a spring can perform within the working range without breakage.

Constant force springs are most commonly used in applications which, depending on the installation space and the required
spring forces, subject the spring steel strip to high bending stresses.

The service life of the spring is then defined in the lower range of the fatigue strength in the order of 5,000 to 20,000 load cycles.
If higher requirements need to be met, the effort that goes into design and the installation space must be significantly increased.

Schematic representation of the influence of bending stress on the service life of constant force springs — Influence of bending stress on the service life of constant force springs

In general, the expected service life is divided into three categories based on the Wöhler diagram: static, finite life fatigue strength and fatigue limit.
In practical designs, the load-bending stress is often set well above the tensile strength of the material.

Wöhler diagram of a constant force spring — Wöhler diagram