Constant force spring as a torque generator

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

Constant force springs in a biaxial arrangement – work roller and storage roller – are used as a generator of constant torque.
In retractors, the special torque characteristic ensure winding with constant force over many revolutions.


Cable retractors for electrical appliances and constant force spring drives also take
advantage of the fact that the characteristic curve of the spring can be adjusted to the application coil ratios.

MIGRA 2B constant force springs

These springs are used in applications that require the most consistent torque possible and many revolutions.
The characteristic curve is flat, and this where the name comes from – MInimal GRAdient. 

The typical characteristics of a MIGRA 2B constant force spring are:

- Very small hysteresis

- High number of revolutions

- Torque characteristic curve rises only slightly

- Service life depending on design up to approx. 20,000 load cycles (typical applications require 5,000 – 15,000 cycles)   

- Width from 5 – 80 mm / thickness from 0.1 – 1.0 mm (guide values)



The MIGRA 2B constant force spring can be provided with a variable curvature of the spring strip during manufacture.
The spring is wound from the supply roller onto the work roller when wound.
In the return motion, the spring runs back onto the supply roller and generates an almost constant torque
that can be used for drives of all kinds.

As a guideline, the supply roller should have a diameter of 120 times the strip thickness and the work roller a diameter of at least 200 times the strip thickness.

NEGRA 2B constant force springs

NEGRA 2B constant force springs are used where a negative characteristic curve is required.
The characteristic curve is falling. The name NEGRA 2B constant force spring comes from the falling characteristic curve it produces,
i.e. from the NEgative GRAdient.

The typical characteristics of a NEGRA 2B constant force spring are:   

- Very small hysteresis

- High number of revolutions

- Falling torque characteristic curve

- Width 5 - 80 mm / thickness 0.10 – 1.0 mm


MIGRA 2B and NEGRA 2B constant force springs are relatively easy to install, and the spring end used to mount the constant force spring to the work roller must therefore be as simple as possible.


Outer end

Designs with a hole or T-shaped cut at the outer end are preferred
as they allow easy mounting of the spring.

For the manufacture of other hook shapes, the spring end usually has to be annealed first, and
the hook then often needs to be bent in a separate operation.



Inner end

It is not always necessary to fix the inner end of the spring to the supply roller. The spring length should be designed to ensure that
after unwinding, one to two coils remain fixed to the supply roller.

If the spring end is punched to allow internal fixing to the roller, assembly becomes more difficult. This is not recommended.

Examples of installation of constant force springs as torque generators

Where angles of twist are small and retraction forces are high, the constant force springs can be arranged in parallel, side by side or one above the other: