Request a Quote »

Helical Extension Spring Design Resources

The section below gives some technical advice on extension springs specifications, such as how to specify spring dimensions, wire diameter sizes that we can coil into extension springs, and the most common types of ends, loops and hooks.

Custom Extension Spring Design

Helical extension spring specifications to match your needs

At Master Spring, we’ve been helping OEMs with their needs concerning large and small extension springs for more than 70 years. Learn how we can design and manufacture custom extension springs for your next application.

Contact us online for a free quote now.

Small Stainless Steel Extension Spring

Helical Extension Spring Dimensions

Explore industry-standard abbreviations for extension spring dimensions as well as calculations for free length and mean coil diameter. These notes and the diagram in Figure 1 are sourced from the Handbook of Spring Design by the Spring Manufacturers Institute (SMI).

  • d= Wire diameter
  • D.= Inside diameter
  • D.= Outside diameter
  • = Mean coil diameter = (O.D. + I.D.)/2 or O.D. – d
  • Lf= Free length = The distance between the inner surfaces of the ends. It is equal to the spring body length plus ends, where spring body length is given by L body = d(N+1) and N = Number of c

Extension spring formulas can help you find several other values as well. Learn how to measure extension springs and their initial tension in this easy-to-read extension spring resources guide.

Extension Springs Dimensions Drawing:

Figure 1 shows helical extension spring dimensions

Figure 1: Extension spring dimensions

Extension Spring Wire Diameter

Need an extension spring custom built to your exact needs? Master Spring has the design and manufacturing capabilities to deliver. We have a wide range of CNC spring forming machines that can manufacture custom extension springs with wire diameters as small as 0.010” or as large as 0.250” (0.25 mm to 6.4 mm)—and any size wire in between!

Extension Spring Materials

Stainless steel extension springs, music wire, hard-drawn wire, and more

We manufacture custom extension springs from all types of material. The most common material types for extension springs are stainless steel (type 302, type 316 and type 17-7 PH), music wire and hard-drawn wire.

Common extension spring materials include:

  • High-carbon spring wire
  • Carbon and alloy wire
  • Hard-drawn wire
  • Alloy steel wire
  • Stainless steel wire
  • Non-ferrous alloy wire
  • And many more!

Still curious? Learn about the suitable uses for each material type in the Properties of Materials chart.

Common Extension Spring Ends

Our helical spring ends include twist, cross center, side loops, German hooks, and extended hooks. We also manufacture swivel hooks, flat stock hooks, and more for extension springs. Learn more below about common spring ends and see Figure 2 from the Spring Manufacturers Institute’s Handbook of Spring Design for details.

Swivel Hooks

When attached to a spring, swivel hooks move at their base. The swivel function gives this spring end its strength. Swivel hooks are also strong fasteners because they can move and adjust to any movements. We manufacture heavy-duty swivel hooks in all sizes for many mechanical applications.

Extended Hooks

Extended hooks are spring ends that are not enclosed. Rather, they have an extended shank. Many OEMs use extended hooks when they need a longer hook length that can provide more force throughout the spring.

German Hooks

German hooks are unenclosed, or open hooks, attached to the ends of a spring. The German hook is a versatile spring end because it can attach to many other parts used in mechanical and industrial applications.

Figure 2 shows types of dimension spring ends

Figure 2: Common end configurations for helical extension springs

Download the Extension Spring Checklist (PDF)
Extension Spring Design Checklist

Checklist: Custom Extension Spring Specifications

Ready to request a quote? Download the checklist to help you specify the critical dimensions we’ll need to quote your extension springs. For help with your extension spring design, please call us at 708-453-2570.

The extension spring specification checklist includes:

  • Material (such as music wire or stainless steel 302)
  • Maximum outside diameter of spring (how much room is available for spring?)
  • Initial tension
  • Load at a certain length
  • Rate (pounds per inch of extension)
  • Maximum extended length
  • Type of ends (such as center loops or side loops or extended hooks)
  • Position of ends relative to each other
  • Gap opening in hooks

Download the Extension Spring Checklist (PDF)


 Extension Springs Video Gallery

At Master Spring and Wire Form Co., we have the capability to design and manufacture a wide variety of extension springs. We specialize in supplying OEMs their requirements of medium to high quantities of springs and wire forms. Browse our gallery of custom extension springs or explore our extension springs product page to learn about our capabilities.


An Extension Spring Success Story

Manufacturing Springs for Outdoor Power Equipment

A Master Spring customer orders 55,000 extension springs per year—and relies on us to deliver high-quality springs without fail. Learn how a suggested design change increased an extension spring's life cycle and reduced this outdoor power equipment manufacturer’s cost.



  • Upload CAD and or PDF Files: Tips

    **CAD, STEP, and IGS files can no longer be uploaded and must be emailed to info@masterspring.com.

  • Accepted file types: step, cad, pdf, sldprt, stp, igs, jpg, jpeg, tiff, png.

  • This field is for validation purposes and should be left unchanged.
  • We respect your privacy

    © Master Spring and Wire Form Co.

    1340 Ardmore Ave., Itasca, IL 60143

    Phone: 708.453.2570 | Fax: 708-453-6420