Loads at length define the force a tension spring produces when stretched to a specific length. Unlike compression springs, tension springs are designed to work in extension and include an initial preload that must be overcome before any movement occurs.

Loads at length provide a clear and practical way to specify real working performance.

What Are Loads at Length

A load at length is the force measured when a tension spring is extended to a defined overall length.

For example, a spring may be specified as producing 15 N at 60 mm length and 40 N at 90 mm length. These values show how the spring behaves across its operating range.

The Role of Initial Tension

Tension springs are manufactured with built in initial tension. This is the force required to start extending the spring from its free length.

Loads at length include this initial tension, which means the force at the first working length is never zero. This is critical for applications that rely on immediate force, such as return mechanisms, latches and linkages.

Why Loads at Length Are Used

Spring rate alone is not sufficient to describe a tension spring. Two springs with the same rate can have very different behaviour depending on their initial tension.

Loads at length account for both spring rate and initial tension, making them far more useful for real assemblies.

Working Range and Limits

The working range is the span between the minimum and maximum operating lengths.

Tension springs should never be extended close to their maximum extension, as this increases stress in the coils and hooks and dramatically reduces fatigue life. Loads at length help define safe working forces within this range.

Accuracy and Tolerances

Loads at length are affected by variation in wire diameter, coil diameter, hook geometry and heat treatment.

For precision applications, load tolerances should be clearly specified and verified through testing.

Best Practice

Always specify at least two loads at defined lengths. Clearly define the free length and maximum extension. Avoid designing close to the spring’s extension limit and ensure hooks are strong enough to handle the applied loads without yielding.

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