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As with any technical field, spring manufacturing has a number of unique terms common only to its industry. The following are some of the common terms and definitions we use in the spring manufacturing process.


  • Active Coils (NA)

    Coils which are free to deflect under load.

  • Arbor

    A round, hardened rod or shaft upon which springs are wound.


  • Closed Ends

    Not ground (ceng) ends of compression springs where pitch of the end coils is reduced so that the end coils touch.

  • Closed-wound

    Coiled with adjacent coils touching.

  • Compressed Length

    The length of the spring after operating force has been applied. The compressed length is computed by subtracting the initial compression and the operating travel from the free length.


  • Deflection (F)

    Movement of spring ends or arms under the application or removal of an external load (P).


  • Elastic Limit

    Maximum stress to which a material may be subjected without permanent set (deformation).

  • Endurance Limit

    Maximum stress at which any given material will operate indefinitely without failure for a given minimum stress.


  • Free Angle

    Angle between the arms of a torsion spring when the spring is not loaded.

  • Free Length (L) or Free Height (FH)

    The overall length of a spring in the unloaded position.


  • Helical Springs

    Springs made of bar stock or wire coiled into a helical form. This includes compression, extension and torsion springs.

  • Helix

    RHW (right hand wound) and LHW (left hand wound). The direction of spiral form (open or closed) of compression springs. If direction of coiling is not specified, springs may be coiled in either direction. Nested springs with small diametric clearances should be coiled in opposite directions.

  • Hole Diameter

    This identifies the outside diameter of the die spring. Die Springs are available in eight different hole sizes matched to standard drill sizes. Each spring is made the fit in the hole so the O.D. of the spring is actually less than the hole diameter.

  • Hooks

    Open loops or ends of extension springs.

  • Hydrogen Embrittlement

    Hydrogen absorbed in electroplating or pickling of carbon steels, tending to make the spring material brittle and susceptible to cracking and failure, particularly under sustained loads.

  • Hysteresis

    Mechanical energy loss occurring during loading and unloading of a spring within the elastic range. It is illustrated by the area between load-deflection curves.


  • Initial Tension (IT)

    The force that tends to keep the coils of an extension spring closed and which must be overcome before the coils start to open.


  • Load (P)

    The force applied to a spring that causes a deflection (F).

  • Loops

    Coil-like wire shapes at the ends of extension springs that provide for attachment and force application.


  • Mean Coil Diameter (D)

    Outside spring diameter (OD) minus one wire diameter (d).

  • Modulus in Shear or Torsion (G)

    Co-efficient of stiffness for extension and compression springs.

  • Modulus in Tension or Bending (E)

    Co-efficient of stiffness used for torsion and flat spring (Young’s Modulus).


  • Open Ends, Not Ground (OENG)

    End of a compression spring with a constant pitch or each coil.

  • Open Ends, Ground (OEG)

    “Open ends, not ground” followed by an end grinding operation.

  • Operating Travel

    The distance which is subtracted from the spring length after operating force has been applied.


  • Permanent Set

    A material that is deflected so far that its elastic properties have been exceeded and it does not return to its original condition upon release of load is said to have a “permanent set.”

  • Pitch (P)

    The distance from center to center of the wire in adjacent active coils (recommended practice is to specify number of active coils rather than pitch).

  • Preload

    The distance the free length of the die spring is reduced by the pressure of the assembled tool.


  • Rate (R)

    Change in load per unit deflection, generally given in pounds per inch.

  • Remove Set

    The manufacturing process of closing a compression spring to solid to eliminate load loss in operation.

  • Residual Stress

    Stresses induced by set removal, shot peening, cold working, forming or other means. These stresses may or may not be beneficial, depending on the application.


  • Set

    Permanent distortion which occurs when a spring is stressed beyond the elastic limit of the material.

  • Solid Height (H)

    Length of a compression spring when under sufficient load to bring all coils into contact with adjacent coils.

  • Spring Index

    Ratio of mean coil diameter (D) to wire diameter (d).

  • Stress

    In a spring this describes the internal force that resists deflection under load. This force is equal to, and in opposite direction to the external load. Stress is expressed in thousands of pounds per square inch of sectional area.

  • Stress Range

    Difference in operating stresses at minimum and maximum loads.

  • Stress Ratio

    Minimum stress divided by maximum stress.

  • Stress Relieve

    To subject springs to low-temperature heat treatment so as to relieve residual stresses.

  • Shot Peening

    A cold-working process in which the material surface is peened to induce compressive stresses and thereby improve fatigue life.

  • Squareness

    Angular deviation, between the axis of a compression spring in a free state and a line normal to the end planes.

  • Squareness of Ends

    Angular deviation between the axis of compression spring and a normal to the plane of the ends.


  • Torque (M)

    A twisting action in torsion springs which tends to produce rotation, equal to the load multiplied by the distance (or movement arm) from the load to the axis of the spring body. Usually expressed in inch-pounds or in foot-pounds.

  • Total Number of Coils (NT)

    Number of active coils (n) plus the coils forming the ends.

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