Constant Force Springs

Constant Force Springs and Roll Springs for Long Travel

Constant force springs, also known as roll springs, are made from a prestressed flat spring strip that coils into a tightly wound roll when unloaded. When the free end is extended, the spring resists the unrolling movement with an approximately constant force over its usable travel.

Unlike conventional extension or compression springs, the force does not increase proportionally with the spring travel. After a short initial extension, the constant force spring reaches its specified nominal force and maintains it over most of the working range.

Roll springs are therefore particularly suitable for counterbalancing, retracting, tensioning and positioning movable components. Typical applications include drawers, covers, sliding doors, cable retractors, measuring equipment, vending machines, household appliances, medical devices and industrial guide systems.

Meaning of the Specifications in the Product Table

Material length L0 and maximum working length Lmax must not be confused. Part of the spring strip must remain on the drum or within the coiled spring even at maximum extension.

How to Select the Correct Constant Force Spring

Proceed in the following order:

1. Determine the required force: Establish the pulling force needed to retract, tension or counterbalance the movable component.

2. Define the working travel: The required stroke must be between L1 and Lmax.

3. Consider the service life: Select a version whose specified number of cycles suits the expected frequency of operation.

4. Check the installation space: Consider the strip width, coil diameter, drum diameter and space required by the extended strip.

5. Define the mounting method: The free strip end and spring coil must be securely connected to the intended components.

6. Provide suitable guidance: The spring strip must not twist, tilt or buckle sideways during extension and retraction.

7. Assess the operating environment: Humidity, temperature, cleaning agents and other media must be compatible with the material.

For counterbalancing applications, the spring force should be as close as possible to the effective weight force. Several constant force springs may be installed in parallel where a higher total force is required. Two springs rated at 10 N each produce a combined force of approximately 20 N when loaded evenly.

Installation Options

In the usual installation arrangement, the spring is wrapped closely around a drum. The coil is supported or attached to a fixed component, while the free strip end is connected to the movable component.

Alternatively, the free end may be fixed while the spring coil moves with the component. The most suitable arrangement depends on the available space, direction of movement and mounting options.

The drum diameter Do stated in the table should be observed. A significantly smaller drum increases the bending stress in the spring strip and may reduce service life. An unsuitable or excessively large drum may result in the spring coil not being retained securely.

Recommendations for Reliable Operation

The spring strip should be guided as straight as possible and without lateral deflection. For long travel, suitable guidance is recommended to prevent the thin strip from kinking, twisting or moving sideways.

Deflection rollers must be sufficiently large and must not bend the spring strip against its natural curvature. Sharp-edged guides, screws or housing components may damage the strip.

Sufficient coils must remain on the drum at maximum working length. As a design guideline, at least approximately one and a half turns should remain wound. A mechanical end stop prevents the spring from being extended beyond Lmax.

The spring strip must not be:

• kinked or folded,

• guided over sharp edges,

• twisted sideways,

• extended until completely unrolled,

• shortened, drilled or heated without technical assessment.

The strip edges may be sharp. Suitable protective gloves and eye protection should therefore be worn during installation and testing. The spring coil must be held securely because it may rewind rapidly if released unintentionally.

Service Life and Inspection

The specified service life describes the intended number of cycles when the spring is installed correctly and operated within the stated working length. Misalignment, an undersized drum, contamination, corrosion or extension beyond Lmax may significantly reduce the achievable service life.

The spring should be inspected regularly for kinks, cracks, damaged edges, abrasion, corrosion and uneven winding. Damaged constant force springs must not be straightened or reused.

Stainless Spring Steel 1.4310

The standard versions are made from stainless spring strip steel 1.4310. This material provides good spring properties and increased corrosion resistance for many technical applications.

Due to cold working, the material may become slightly magnetic. Suitability must be assessed separately for permanent exposure to seawater, chlorides, acids or aggressive cleaning agents.

Frequently Asked Questions About Constant Force Springs

Is the force exactly the same over the entire travel?

No. The force remains approximately constant within the intended working range. Deviations may occur during initial extension and close to the operating limits.

May the spring be completely unrolled?

No. Several turns must remain on the drum even at maximum extension.

Can several roll springs be combined?

Yes. Springs installed in parallel can add their forces, provided that they are guided and loaded evenly.

What does service life in cycles mean?

One cycle consists of one complete extension and retraction movement within the intended working range.

Can the spring strip be shortened?

Shortening the strip changes the working travel, mounting arrangement and function. It should therefore not be carried out without technical assessment.

Further standard and custom-made springs can be found in our spring elements overview.