How to Choose the Right Extension Spring | AxiSpring
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Extension springs are designed to resist pulling force and return to their free length when released. They are used wherever two parts need to be held together, pulled back into position, or where a controlled tensile force is required. Selecting the right extension spring requires understanding a few parameters that differ from compression spring selection - most importantly, end type and initial tension.
How extension springs work
An extension spring is wound with coils in contact under no load. When pulled, the coils separate and the spring resists the extension force. When the load is removed, the spring returns to its free length.
Unlike compression springs, extension springs are wound with an initial tension - a pre-load force built into the spring during manufacture. This means a small pull force is required before the spring begins to extend at all. Above that threshold, force increases linearly with extension according to the spring rate.
The key dimensions
Free length (L0) - the spring's length in the unloaded state. Note that in your application the spring will be longer than this when under load. Design your assembly so the spring is extended - not compressed - during normal operation.
Outer diameter (D) - the spring's outer diameter. Ensure there is adequate clearance if the spring runs inside a bore or over a rod. For springs over a rod, the inner diameter must be larger than the rod.
Wire diameter (d) - the thickness of the wire. Thicker wire gives a higher spring rate and initial tension.
Spring rate - R (N/mm)
Spring rate tells you how much additional force is required for each millimetre of extension beyond the initial tension threshold. A spring with R = 5 N/mm and an initial tension of 20 N will require 70 N to extend it 10 mm (20 N initial tension + 5 N/mm x 10 mm).
To select the right rate, calculate the total force you need at your maximum extension, subtract the initial tension, and divide by the extension distance. This gives the minimum rate required.
Initial tension
Initial tension (also called pre-load) is the built-in force that holds the coils closed. It is always present before the spring begins to extend. For most standard catalog springs, initial tension is specified in the product data.
In applications where the spring must begin extending under very low force - for example, in a return mechanism with light loads - choose a spring with low initial tension. For applications where the spring should stay closed under vibration or minor disturbances, a higher initial tension is better.
End types
Extension springs are available with different hook or end configurations that determine how the spring attaches to your assembly. The most common types are:
English hook (side hook) - the most common end type. The hook lies in the same plane as the spring body. Easy to attach but the hook opening may reduce the maximum load the spring can carry before the hook opens.
German hook (over-centre hook) - the hook curves over the centre of the spring. Stronger than the English hook and less likely to open under high loads.
Straight end (plain end) - no hook; the wire ends straight. Used where the spring is attached by other means - for example, pinned through a hole in the end coil.
For most industrial applications, the German hook gives the best combination of strength and ease of attachment. Check that the hook opening fits the pin or bolt diameter in your assembly.
Material
AxiSpring extension springs are available in two main materials:
Carbon steel (1.1200/1.0500) - the standard choice for most industrial applications. High strength and long service life in normal environments.
Stainless steel (1.4310) - choose this where corrosion resistance is required: humid environments, the food industry, or outdoor use. Note that stainless steel extension springs typically have lower initial tension for the same dimensions compared to carbon steel.
A note on fatigue life
Extension springs are more fatigue-sensitive than compression springs because the hook is a stress concentration point. If your application involves a high number of load cycles, check the load against the spring's maximum extension force and keep peak loads well within the rated limit. Avoid sharp bends or misalignment at the hook attachment - these accelerate fatigue failure at the hook.
Find the right spring
AxiSpring stocks extension springs across a wide range of outer diameters, wire diameters, free lengths and spring rates for immediate delivery. Use the filter table in our collection to search by outer diameter, free length and spring rate.