Tension Springs: Engineering, Specification, and Selection Guide 2026

What Are Tension Springs and How Do They Function?

Tension springs are helical coils designed to store energy by resisting axial pulling forces, characterized by tightly wound coils under preload.

Unlike compression springs, tension springs operate by exerting a restoring force when stretched from their resting state. The spring maintains its shape through a combination of wire diameter, coil diameter, and the initial tension built into the winding process during manufacturing. These components serve as essential energy storage units in mechanisms requiring predictable return forces, such as garage door counterbalances, industrial linkage returns, and medical equipment actuators.

What Is Initial Tension in a Spring?

Initial tension is the internal force holding coils together, which must be overcome before the spring begins to extend under a load.

Initial tension is a defining characteristic of tension springs that differentiates them from those wound with a gap. It acts as a pre-load, ensuring the coils remain tightly closed until the applied force exceeds the internal stress. Engineers specify this to keep assemblies compact and to prevent rattling in low-force applications.

Why Does the Spring Rate Matter?

Spring rate defines the force required to stretch the spring per unit of length and is calculated using the wire and coil diameter.

The spring rate, or ‘k’ factor, dictates the linearity of the force curve. If you require a consistent tension through the entire stroke, you must ensure your operating range falls within the elastic limit of the spring. Always review our gas spring force calculator concepts when planning your system to understand how loading curves impact performance.

Which End Fitting Configuration Should You Choose?

End fittings are the connection points, including machine loops, side hooks, and swivel eyes, chosen based on stress and mounting.

How Do Machine Loops Affect Performance?

Machine loops provide the highest strength-to-weight ratio and are the industry standard for most heavy-duty industrial applications.

Machine loops are formed by bending the end of the spring wire toward the center of the coil. This design reduces stress concentrations at the transition point. When specifying these for machinery, ensure the loop orientation matches your mounting orientation to avoid twisting the spring body.

What Are the Benefits of Swivel Hooks?

Swivel hooks allow for 360-degree rotation, preventing the spring from twisting or binding during high-cycle industrial operations.

Swivel fittings are critical in applications where the load path might shift during operation. By allowing the hook to rotate freely, you prevent torsional stress that would otherwise cause premature metal fatigue. Refer to our gas strut end fittings guide to compare these mechanical couplings with pressurized strut counterparts.

When Are Side Hooks Necessary?

Side hooks are custom-formed to clear obstacles in confined spaces where standard center-aligned loops cannot provide necessary access.

Side hooks increase the stress on the final coil due to the offset geometry of the bend. If you use this configuration, we recommend an uprating of the wire gauge to compensate for the higher stress intensity. These are common in furniture and cabinetry mechanisms where space constraints dictate atypical mounting paths.

How to Diagnose Tension Spring Failure

Failure is typically caused by hook fatigue, stress corrosion cracking, or exceeding the elastic limit through over-extension.

Why Do Hook Points Snap?

Hook points snap because they are the most significant point of stress concentration and frequently fail due to bending cycle fatigue.

The transition from the coil body to the hook creates a ‘stress riser’. Over time, the repeated opening and closing of the hook during extension causes microscopic cracks. Once a crack propagates, the hook will snap, often resulting in a complete release of the retained load.

What Are the Signs of Over-Extension?

Over-extension occurs when the spring is stretched beyond its design limits, leading to permanent coil deformation and loss of load.

• Coils no longer return to a closed position when relaxed.
• Visible gaps appearing between the coils at rest.
• Noticeable drop in return force during the operating cycle.
• Permanent change in the free length of the component.

How Does Environmental Corrosion Impact Life?

Corrosion pits act as crack initiators, significantly reducing the fatigue life of high-carbon steel springs in humid environments.

For applications in marine or high-moisture settings, surface protection is vital. We advocate for stainless steel grades to minimize rust-induced failure. If you are operating in such conditions, always assess your hardware for signs of hydrogen embrittlement or surface pitting.

Frequently Asked Questions

Can I cut a tension spring to length?

Cutting a tension spring destroys the factory-wound end hook, renders the initial tension void, and creates an unusable mounting point.

What material is best for high temperatures?

Inconel or high-temperature alloys are required for springs operating above 150 degrees Celsius to prevent permanent load loss.

How do I calculate the required length?

Measure the free length plus the operating stroke, then account for initial tension to ensure the spring never exceeds elastic limits.