Extension Springs

Extension Springs for Return, Tensioning and Retaining Functions

Extension springs absorb force when they are pulled apart and then attempt to return to their original length. They are used to pull components together, return moving parts to their initial position or generate a defined tensile force.

The Febrotec range includes conventional helical extension springs with loops as well as garter springs that can be joined to form a closed ring. Both spring types operate under tensile load, but differ significantly in their design and application.

Extension Springs or Garter Springs?

Helical Extension Springs

Helical extension springs have a cylindrical, closely wound spring body and attachment elements at both ends. These are usually loops that connect the spring to the adjacent components.

As the spring is extended, the spring force increases with the spring travel. Helical extension springs are suitable for applications such as:

• return mechanisms,

• flaps and locking systems,

• lever and operating mechanisms,

• electrical and precision mechanical assemblies,

• machines and fixtures,

• automotive and building technology.

Select a helical extension spring when a force must be transmitted axially between two attachment points.

Garter Springs

Garter springs are long extension springs without conventional end loops. Their ends can be joined together to form a closed spring ring. When the diameter of the ring is increased, the spring generates an inward radial force.

Garter springs are particularly suitable for:

• pressing sealing lips against shafts,

• compensating for seal wear,

• applying radial tension to flexible components,

• compensating for temperature-related and dimensional changes,

• retaining or preloading ring-shaped components.

Select a garter spring when a uniform radial tensioning force is required in a circular application.

Selecting a Helical Extension Spring

The following values are particularly important when selecting a suitable helical extension spring:

For an extension spring with initial tension, the spring force within the approximately linear working range can be calculated as follows:

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

The spring must not be extended beyond its technically permissible working travel. In dynamic applications, particular attention must be paid to the loops and the transitions between the spring body and the loops.

Selecting a Garter Spring

The most important values when selecting a garter spring are the outside diameter of the spring body, the wire diameter, the installed ring diameter, the active spring length and the internal initial tension.

The active length is the length of the spring before it is joined into a ring. When cutting a garter spring to length, it must be taken into account that the selected length affects both the final ring diameter and the resulting radial tensioning force.

The internal initial tension describes the force already present within the spring body. The required installed diameter, radial force and type of end connection must also be considered when designing a garter spring.

Materials for Extension Springs

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

Spring steel 1.1200 provides high strength and is particularly suitable for applications in dry environments. Without an additional surface treatment, the material is not corrosion-resistant.

Stainless spring steel 1.4310 is suitable for a wide range of industrial applications and for use in humid environments. It has limited suitability for permanent contact with seawater or media with a high chloride content. Due to cold forming, 1.4310 may be slightly magnetic.

The standard garter springs are manufactured from stainless spring steel 1.4310.

Custom Extension Springs

In addition to the available standard springs, extension springs can be manufactured according to individual requirements. Customisable characteristics include:

• wire and spring diameters,

• free length,

• spring force and spring rate,

• initial tension,

• loop and attachment designs,

• material and surface finish,

• installed diameter for garter springs.

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

Frequently Asked Questions About Extension Springs

What is the initial tension of an extension spring?

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 this initial tension.

What is the difference between spring force and spring rate?

Spring force is the force currently generated by the spring. The spring rate indicates how many newtons the spring force increases by for each additional millimetre of extension.

Can an extension spring be extended by any distance?

No. Every extension spring has a limited permissible working travel. Excessive extension can cause permanent deformation or damage to the loops and spring body.

Can garter springs be cut to the required length?

Yes. Depending on the design, garter springs can be cut to length and then joined to form a ring. However, the selected length affects both the installed diameter and the resulting radial tensioning force.