Located just outside Chicago

Helical Extension Spring Design Resources

The section below gives some technical advice on extension springs, 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

Designing and manufacturing helical extension springs 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 by calling us at 708-202-9545 today

Contact us today for a free quote.

Small Stainless Steel Extension Spring

Helical Extension Spring Dimensions

The diagram to the right illustrates the dimensions of an extension spring. The notes below list abbreviations for the dimensions and calculations for some of the dimensions. The diagram and notes are from the Spring Manufacturers Institute Handbook of Spring Design.

  • d = wire diameter
  • I.D. = Inside Diameter
  • O.D. = Outside Diameter
  • D. = 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 Lbody = d(N+1).  N = Number of Coils.

Extension Springs Dimensions Drawing:

Helical Extension Spring Dimensions Diagram

Extension Spring Wire Diameter

Need an extension spring custom built to your exacting needs? Master Spring has the design and manufacturing capabilities to match. We have a wide range of CNC spring forming machines that can manufacturer 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 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.

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!

You can learn about the suitable uses of each material type in the Properties of Materials chart.


Common Extension Spring Ends

Common helical spring ends are twist, cross center, side loops, German hooks and extended hooks. We also manufacture swivel hooks, flat stock hooks, and more for extension springs (see the figure of Common End Configurations for Helical Extension Springs per Spring Manufacturers Institute Handbook of Spring Design to the right).

Types of Extension Spring Ends


Download the Extension Spring Checklist (PDF)

Extension Spring Design Checklist

Spring Dimensions and Specification Checklist

Specify the critical dimensions of your extension springs

What information does Master Spring need to quote your extension spring?  Download the checklist by clicking on the image to the left to make sure you specify the critical dimensions of your extension springs.  For help with your extension spring design, please call us at 708-453-2570.

Download the PDF file of the checklist here.

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

Master Spring has been manufacturing custom helical extension springs for OEMs since 1945.  Request a quote today for extension springs engineered to meet your requirements.

 Extension Springs Video Gallery

At Master Spring and Wire Form, 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 read our extension springs product page to learn about our capabilities.


An Extension Spring Success Story

Manufacturing Springs for Outdoor Power Equipment

Learn how a suggested design change increased the spring's life cycle and reduced the customer’s cost. This customer orders 55,000 of these extension springs per year.



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    © Master Spring and Wire Form Co.

    1340 Ardmore Ave., Itasca, IL 60143

    Phone: 708.453.2570 | Fax: 708-453-6420