Power springs

The traditional form of the strip spring is the power spring.  

The special feature of the MAXIMO power spring is that the combination of winding forward and then back increases the power density, making maximum use of the available material properties.

Service life

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

Depending on the application, the requirements in terms of service life can be very high. Because the spaces in which they are installed are small in most cases, power springs are generally exposed to high bending stresses, which have an adverse impact on their service life.

The decisive factors for their service life are as follows:

1. Ratio of material thickness to axis diameter

        The higher the ratio, the longer the service life


Formula symbol


Material thickness



Spring axis diameter



Core thickness ratio





2. Ratio of working range to total revolutions

If this ratio is low and both the pretension and reserve are high,
a very high number of load cycles can be achieved.


Formula symbol


Total revolutions



Working range



Revolution ratio



If the revolution ratio qu is less than 60% and the pretension is greater than 0.2 Uges,
good service life values can be achieved.

Other influencing factors that affect the service life of a power spring:

- Homogeneity of the material
- Edge quality
- Pretension and working stroke
- Test and working speed
- Relaxation phenomena/setting properties
- Spring dimensioning
- Mounting of the spring ends
- Friction/lubrication
- Environmental and installation influences


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 the Wöhler diagram, the power spring is found mainly in the finite life fatigue strength (high cycle fatigue) range.

In the Wöhler diagram, this is marked as section II. 

Because of the many factors that can influence it, the number of load changes (cycles) a power spring can perform is checked separately for each spring.
The two ratios give only an initial indication of the service life that can be achieved. 

KERN-LIEBERS tests the maximum service life of power springs on service life test benches developed in-house.