Extension Springs Characteristics

Extension springs are designed to resist a pulling force, returning to a tightly wound coil when the force is removed.


The highest grades of spring wire are used in manufacturing Century Spring products. Certifications of conformance for geometric tolerances for our stock springs are available upon request. We only offer material certifications for custom springs.

"Spring steel" is a term that includes music wire, hard-drawn wire and oil-tempered wire. Alloy steels for stock extension springs are offered only in 300 series stainless.


Tolerances for extension spring rates depend upon the body-diameter to wire-diameter but are usually +/- 10% and +/- 5% on the diameter. The initial tension is much more difficult to control and is offered as a reference value only.


The ends on the extension springs offered in our stock inventory are the common machine-made hook and full-loop configurations. The hooks or loops may have an across center transition of the last coil before forming the hook or loop. The angle between hook/loop planes (end position) is random. If you need a specific end configuration, it is best to contact our custom spring department.


Our stock extension springs contain a selection of commonly requested finishes:

  • Zinc
  • Gold Irridite
  • Black Oxide
  • Passivated (upon request)
  • None (can be plated upon request)