Extension Springs

Helical Extension Springs for Return, Tensioning and Retaining Functions

Helical extension springs are cylindrical springs that generate an opposing tensile force when pulled apart. Once the load is removed, the spring attempts to return to its original length. The force is transmitted through loops or other attachment elements at both ends of the spring.

Helical extension springs are used to pull components together, return moving parts to their initial position or generate a defined initial tension. Typical applications can be found in mechanical engineering, the automotive industry, electrical engineering, precision mechanics, building technology, medical devices, locking systems, lever mechanisms and return mechanisms.

Use the product table above to select suitable helical extension springs by outside diameter, wire diameter, free length, initial tension, test length, spring force, spring rate and material.

Meaning of the Specifications in the Product Table

The following values will help you select a suitable helical extension spring:

Calculating the Force of a Helical Extension Spring

The coils of a helical extension spring lie closely together in the unloaded state. This manufacturing process creates an initial tension F0. The spring body only begins to extend once the externally applied force exceeds this initial tension.

Within the approximately linear working range, the spring force can be estimated using:

F = F0 + c × s

where:

• F = spring force in N

• F0 = initial tension in N

• c = spring rate in N/mm

• s = extension beyond the free length in mm

The extension is calculated as:

s = L − L0

where L is the current length of the extended spring.

For the test length L1 specified in the table:

F1 ≈ F0 + c × (L1 − L0)

Due to manufacturing and measurement tolerances, the calculated value may differ slightly from the stated test force F1.

How to Select the Right Helical Extension Spring

Proceed in the following order:

1. Check the installation space: Determine the maximum permissible outside diameter Da and the available installation length.

2. Define the attachment points: Determine the distance between the mounting points in both the unloaded and operating positions.

3. Determine the required spring travel: Calculate the difference between the free length L0 and the intended operating length.

4. Determine the required spring force: Take both the initial tension F0 and the additional force generated by the extension into account.

5. Check the test length: The intended operating length should not exceed the specified test length L1 without additional technical verification.

6. Select the material: Consider humidity, corrosion exposure, temperature and other environmental conditions.

7. Check the loops and attachments: Make sure that the loops fit the existing mounting points and cannot become twisted or misaligned during movement.

The test length L1 must not be confused with the normal installation dimension. It is the defined length at which the spring force F1 is specified.

Materials for Helical Extension Springs

Depending on the version, the helical extension springs in the Febrotec standard range are available in spring steel 1.1200 or stainless spring steel 1.4310.

Spring Steel 1.1200

Spring steel 1.1200 offers high strength and is suitable for a wide range of applications in dry indoor environments. Without an additional coating or surface treatment, the material is not corrosion-resistant.

Stainless Spring Steel 1.4310

Material 1.4310, also known as X10CrNi18-8, offers good corrosion resistance for many industrial applications and humid environments. It has limited suitability for permanent contact with seawater or media containing high levels of chloride.

Due to cold forming during spring manufacture, 1.4310 may be slightly magnetic.

At elevated or very low temperatures, the required spring force, load duration and number of load cycles must be considered in addition to the general temperature range of the material.

Recommendations for a Long Service Life

The highest stresses in a helical extension spring often occur at the transition between the spring body and the loops. This area is particularly important for service life in applications involving frequent movement.

For reliable operation, we recommend:

• avoiding unnecessary extension of the spring,

• maintaining an adequate safety margin below the test length,

• avoiding lateral loads and misalignment of the loops,

• applying the load as straight as possible in the direction of pull,

• using suitable pins or mountings with rounded edges,

• protecting the spring surface against corrosion and mechanical damage.

The actual service life depends on spring travel, spring force, number of load cycles, installation conditions, material and environmental conditions.

Typical Applications

Helical extension springs are commonly used in:

• return mechanisms,

• flaps and locking systems,

• lever and operating mechanisms,

• electrical and electronic assemblies,

• precision mechanical devices,

• automotive components,

• building and door technology,

• machines and fixtures,

• medical devices,

• sports and leisure equipment.

Pre-galvanised trampoline springs and other special versions are available on request.

Custom Helical Extension Springs

In addition to the standard dimensions available from stock, helical extension springs can be manufactured according to individual requirements.

Customisable characteristics include:

• outside diameter and wire diameter,

• free length,

• initial tension,

• spring rate and spring force,

• spring travel,

• loop and hook designs,

• material and surface finish,

• winding direction.

For a technical enquiry, please provide information about the available installation space, attachment points, shortest and longest operating length, required spring force and operating conditions.

Frequently Asked Questions About Helical Extension Springs

What does initial tension F0 mean?

Initial tension is the force already present between the closely wound coils. The spring body only begins to visibly extend once the external tensile force exceeds F0.

What is the difference between initial tension and spring rate?

Initial tension is the force initially required to separate the coils. The spring rate indicates how many newtons the spring force increases by for each additional millimetre of extension.

What does the test length L1 mean?

The test length L1 is a defined length of the extended spring. At this length, the spring generates the force F1 stated in the product table.

Can a helical extension spring be extended beyond L1?

Extension beyond the test length should only take place after technical verification. Excessive extension may cause permanent deformation, reduced initial tension or damage to the loops.

Which length is decisive when selecting a spring?

Both the free length L0 and the maximum operating length must be considered. It must also be verified that the spring force at both operating points is suitable for the application.