Tension Springs: Engineering Selection Guide 2026
What Is a Tension Spring and How Does It Function?
Tension springs store energy by resisting axial pulling forces, with a coiled body that returns to its original length when released.
Unlike compression springs that store energy by being squashed, tension springs are engineered to be pulled apart. The coils are wound tightly together in a ‘closed’ state, creating a built-in force known as initial tension.
This initial tension must be overcome before the coils begin to separate. Once the force exceeds this internal threshold, the spring rate becomes linear, meaning the force required to stretch the spring increases at a constant rate per unit of deflection.
How Is Initial Tension Calculated for Industrial Design?
Initial tension is the internal force holding coils together, measured by the pull required to just begin separating the spring coils.
Initial tension is a function of the spring’s mean coil diameter and the wire diameter. It is not an arbitrary variable but a product of the manufacturing process during the coiling phase.
- Initial tension increases as the spring index (D/d) decreases.
- It is independent of the total number of active coils.
- It allows the spring to maintain a ‘closed’ state without external load.
- Precision design requires this value to be defined at the specification stage.
What Is the Relationship Between Spring Rate and Material?
Spring rate is defined as the load per unit of deflection, determined by wire diameter, mean diameter, and the material shear modulus.
The rate of a tension spring is calculated using the standard helical spring formula, accounting for the shear modulus of the material. Whether you use carbon steel, stainless steel, or phosphor bronze, the modulus significantly impacts performance.
In 2026, we see high-cycle applications moving toward specialized alloys to prevent premature fatigue. For standard industrial use, ASTM A228 music wire remains the industry benchmark for consistent rate performance.
How Do I Choose the Correct End Fitting for My Application?
End fittings are the attachment points that transfer load to the spring, including machine loops, side loops, and crossover loops.
The failure of a tension spring most frequently occurs at the transition point where the hook meets the coil body. Selecting the right geometry is critical to preventing stress concentration at this junction.
Why Use Crossover Loops Instead of Machine Loops?
Crossover loops offer lower stress concentrations at the loop-to-body transition, improving fatigue life in high-vibration environments.
Machine loops are the most economical choice for mass-produced hardware, but they create a tight bend radius. Crossover loops wrap the end wire over the centre of the spring, distributing the load more effectively across the final coil.
Can I Use Swivel Hooks for Rotating Load Attachments?
Swivel hooks allow the spring to rotate independently of the load, preventing the torsional stress that leads to premature failure.
If your application involves a load that rotates, a fixed hook will introduce unwanted torque into the spring body. This torque forces the coils to tighten or loosen, which alters the spring rate and risks catastrophic failure.
What Are the Common Failure Modes in Tension Springs?
Tension springs fail primarily through fatigue from cyclical loading, stress corrosion, or exceeding the elastic limit of the wire.
Engineering for longevity requires anticipating how the environment interacts with the metallurgy. Even high-grade springs will fail if the design does not account for operating cycles.
How Does Cyclical Loading Affect Fatigue Life?
Fatigue life is the number of cycles a spring can endure before failure, calculated using the stress amplitude and mean stress levels.
Every extension cycle introduces stress. If the operational stress exceeds the endurance limit of the material, micro-cracks form at the inner diameter of the coils. For more information on spring endurance, consult our force rating resources to understand load limits.
Why Do Corrosion and Temperature Limit Performance?
Corrosion creates stress risers that accelerate crack propagation, while high temperatures reduce the material shear modulus significantly.
In environments with salt spray or chemicals, standard carbon steel will degrade rapidly. We recommend 316 stainless steel for these conditions. Temperature is equally critical, as prolonged heat exposure causes permanent set, or ‘creep,’ where the spring fails to return to its original length. See our marine-grade component analysis for further insights on material selection.
Frequently Asked Questions About Tension Springs
Can I cut a tension spring to change the rate?
Cutting a tension spring reduces the number of active coils, which effectively increases the spring rate and requires new end loops.
What is the difference between open and closed coils?
Closed coils maintain initial tension, while open coils allow for specific gap spacing between each winding for unique flow properties.
Do tension springs lose force over time?
Tension springs lose force if they are stretched beyond their elastic limit or exposed to temperatures that cause material stress relaxation.
Where can I source custom-engineered tension springs?
Custom springs are sourced through industrial manufacturers using your specific load, stroke, and environmental performance requirements.
Proper specification is the difference between a reliable assembly and a failed component. If your project requires precise force application, consider reviewing our online force calculator to determine if a gas spring or a mechanical tension spring better serves your geometry.
