What this calculator does
Extension springs store energy in tension and are usually pre-loaded with initial tension so the coils stay closed until a threshold load is reached. This calculator separates initial tension from the rate, so the load at any travel is predicted accurately. Hook stress is checked separately because the end forms, not the body, are the usual weak point.
Inputs and outputs
Inputs
- Wire diameter d (mm)
- Mean coil diameter D (mm)
- Number of body coils
- Free body length (mm)
- Initial tension (N)
- Working travel (mm)
- Hook type and material
Results
- Spring rate k (N/mm)
- Initial tension and load at travel
- Body shear stress
- Bending and torsion stress at the hooks
- Maximum travel before the coils separate
- Load–deflection curve
Typical applications
- Trampolines and sports equipment
- Garage doors and gates
- Agricultural machinery
- Retail fixtures and displays
- Appliance lids and latches
Design notes
Keep the stress at the hooks below the allowable value for the selected hook type. For high cycle counts, specify rounded and well-blended hooks, or move to a movable-hook design where the end can rotate.
The interactive calculator interface is currently in Chinese. Enter your values there, or send us the load, travel and space constraints and our engineers will return a checked calculation sheet in English.
Frequently asked questions
What is initial tension?
Initial tension is the force that holds the coils closed before the spring starts to extend. It is created during winding and can be controlled within a practical range; the calculator reports the force at which the coils begin to separate.
How much travel is safe?
The body should not extend beyond the point where the coils separate into a uniform pitch with a measurable gap. The calculator warns when the requested travel approaches that limit.
Do you manufacture to the calculated size?
Yes. Send the drawing or the parameter list and we will review manufacturability, wire availability and hook tooling before quoting.
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Need this spring manufactured?
Send a drawing, a sample or the parameters from this calculator. We will review manufacturability and reply with a quote and a sample lead time.