 # 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.

## Spring calculation

Power springs are components subjected to fatigue stresses from bending, similar to torsion springs.

The power spring obtains its torque via the profile cross-section of the strip material.
The most influential variable here is the material thickness via the area moment of inertia.

### Calculation of the area moment of inertia:

The material thickness s influences the torque to the third power, whereas the width b has only a linear influence on the torque.

### This gives the following formula for the torque:

Along the strip material, the spring leaf is loaded with maximum tensile and compressive stresses only in the outer layers.
A neutral axis is created in the middle of the material.

Only a part of the longitudinal cross-section is used to store the spring’s energy. Because of this, and because of the friction that occurs between the layers, the power spring therefore has a utilization factor (volume utilization value) of ~0.25 in relation to the spring volume.

### Calculation of the utilization factor

The traditional tension rod, because it uses the entire cross-section for storage, has a better utilization factor than the power spring.

In contrast to springs subjected to torsional stress, the utilization factor is significantly higher, as they have a utilization factor of only ~0.17.

This makes  power springs a compact energy storage device with a high degree of efficiency.

The calculation of power springs is usually done with appropriate software, because the software stores many influencing parameters that can describe the torque and its progression better than rigid formulas.

The values provide a starting point for the design of the power spring and the required installation space.

### Formulas for calculating the traditional power spring:

When designing the spring, the fill level must be taken into account in the calculation.
A power spring has its maximum utilization of installation space with a fill level of approx. 50%.
With this optimum fill factor, however, the material input is disproportionately large and a lot of friction is to be expected as a result of its own weight.

These formulas enable an initial theoretical calculation of the spring.
However, the final spring design must always be tested and confirmed using samples.

The KERN-LIEBERS group of companies calculates the optimum spring dimension on the basis of the data provided by the prospective customer with a spring calculation program developed specifically for this purpose. Our aim is to offer the optimum spring type and spring dimensions in terms of both functionality and cost.