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Common Spring and Wire Form Terminology

Looking for technical specs, dimension guides, or a glossary of terms? Familiarize yourself with common spring and wire terminology today.

Spring and Wire Form Glossary of Terms

We at Master Spring are dedicated to the custom spring and wire form industry. We’ve been making industry-leading custom wire forms and springs since 1945. Our experts are excited to solve any puzzles our clients send our way.

We know that not everyone lives in the world of industrial spring manufacturing—or has our passion for it. If you are wondering what technical spring jargon means, look no further. Below is a handy glossary of terms and definitions.

Still have questions? Let’s talk, or take a look at our technical resources.

Active coils (na)

Those coils which are free to deflect under load. For compression springs with squared ends, the total number of coils minus two is a good approximation of the number of active coils. For extension springs, the number of active coils is approximately equal to the number of coils in its body. For extension springs with threaded inserts or swivel hooks, the number of active coils is less than the total coils in the body. Hooks and loops add to the number of active coils.

Examples of Wire Forms

Angular relationship of ends

The relative position of the plane of the hooks or loops of extension springs to each other


Heating of electroplated springs to relieve hydrogen embrittlement


Bowing or lateral deflection of compression springs when compressed, related to the slenderness ratio (L/D)

Examples of Compression Springs:

Closed ends

Location on extension springs where pitch of the end coils is reduced so that the end coils touch

Examples of Extension Springs

Closed and ground ends

As with closed ends, except that the end is ground to provide a flat plane

Closed length

See Solid Height


Coiled with adjacent coils touching

Coils per inch

See Pitch

Deflection (F)

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

Single and Double Torsion Springs from Master Spring

Examples of Torsion Springs

Elastic limit

Maximum stress to which a material may be subjected without permanent set

Endurance limit

Maximum stress to 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)

The overall length of a spring in the unloaded position

Frequency (natural)

The lowest inherent rate of free vibration of a spring itself (usually in cycles per second) with ends restrained


See Rate (R)

Heat Setting

Fixturing a spring at elevated temperature to minimize loss of load at operating temperature


The spiral form (open or closed) of compression, extension, and torsion springs

Hooke’s Law

Load is proportional to displacement


Open loops or ends of extension springs

Hot pressing

See Heat setting

Hydrogen embrittlement

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


The mechanical energy loss that always occurs under cyclic loading and unloading of a spring, proportional to the area between the loading and unloading load-deflection curves within the elastic range of a spring

Initial tension (Pi)

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)


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

Mean coil diameter (D)

Outside spring diameter (O.D.) minus one wire diameter (d)

Modulus in shear or torsion (G)

Coefficient of stiffness for extension and compression springs

Modulus in tension or bending (E)

Coefficient of stiffness used for torsion and flat springs (Young’s Modulus)

Moment (M)

See Torque

Open ends, not ground

End of a compression spring with a constant pitch for each coil

Open ends ground

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


Acid treatment of stainless steel to remove contaminants and improve corrosion resistance

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 taken a “permanent set”

Pitch (p)

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

Poisson’s Ratio

The ratio of the strain in the transverse direction to the strain in the longitudinal direction


See Remove set

Rate (R)

Change in load per unit deflection, generally given in pounds per inch (or in metric, N/mm)

Remove set

The process of closing to solid height a compression spring which has been coiled longer than the desired finished length, to increase the apparent elastic limit

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.


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

Shot peening

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

Slenderness ratio

Ratio of spring length (L) to mean coil diameter (D)

Solid height (H)

Length of 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)

Squared and ground ends

See Closed and ground ends

Squared ends

See Closed ends

Squareness of ends

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

Squareness under load

As in Squareness of ends, except with the spring under load

Stress range

The difference in operating stresses at minimum and maximum loads

Stress relieve

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

Torque (M)

A twisting action in torsion springs which tends to produce rotation, equal to the load multiplied by the distance (or moment arm) from the load to the axis of the spring body. Usually expressed in oz * in., lb. * in., lb. * ft., or in N * mm.

Total number of coils (Nt)

Number of active coils (na) plus the coils forming the ends

Wahl Factor

A factor to correct stress in helical spring’s effects of curvature and direct shear

Glossary reprinted with permission from the Spring Manufacturers Institute (SMI) Handbook of Spring Design.

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