Active coils

Those coils which are free to deflect under load.

Angular relationship of ends

The relative position of the plane of the hooks or loops of extension springs from one end to the other.


Heating of electroplated springs to relieve hydrogen embrittlement.


Bowing or lateral deflection of compression springs when compressed, related to the slenderness ratio of the spring.

Closed ends

Ends of compression springs where pitch of the end coils is reduced so that the end coils touch; the end coils are then considered as dead coils.

Closed length

See Solid height


Coiled with adjacent coils touching.

Coils per inch

See Pitch


Motion of spring ends or arms under the application or removal of an external load.

Elastic limit

Maximum stress to which a material may be subjected without incurring permanent set or deformation.

Endurance limit

Maximum stress at which any given material will operate during its cycle life without failure.

Free angle

Angle between the arms of a torsion spring when the spring is not loaded or is in its free position.

Free length

The overall length of a spring in its unloaded position.


See Pitch

Heat setting

Fixturing a spring at a given temperature to minimize loss of load at operating temperature.


The spiral form (open or closed) of compression, extension, and torsion springs; sometimes referred to as the direction of coiling (left hand or right hand helix)


Open loops or ends of an extension spring

Hydrogen embrittlement

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


The mechanical energy loss that always occurs during the loading and unloading (cycling) of a spring, proportional to the area between the loading and unloading load-deflection curves within the elastic range of a spring.

Initial tension

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


The force applied to a spring that causes a deflection


The coil-like wire shape configurations at the ends of extension springs which provide for attaching the spring.

Mean coil diameter

The outside spring diameter minus one wire diameter, or the inside spring diameter plus one wire diameter.

Modulus in shear or torsion

Coefficient of stiffness for extension and compression springs

Modulus in tension or bending

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


See Torque

Open ends, not ground

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

Open ends ground

End of a compression spring with a constant pitch for each coil, but the end coil has a grinding operation.


Acid treatment of stainless steel to remove contaminants and improve corrosive 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; this can result in deformation of a spring


The distance from center to center of the wire in adjacent coils; typically calculated in a spring that has space between its coils.


See Remove set


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

Remove set

The process of compressing a compression spring to solid height (all coils touching) in order to increase the apparent elastic limit, and prevent free length loss during 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.


Permanent distortion to a compression or extension spring’s length or a torsion springs free position which occurs when the spring is stressed beyond its elastic limit.

Shot peening

A cold-working process in which the material surface is peened with a media (steel, plastic, or glass pellets) to induce compressive stresses that improve fatigue life.

Slenderness ratio

Ratio of spring length to mean coil diameter.

Solid height

Length of a compression spring when under sufficient load, forces all coils into contact with one another.

Spring index

The ratio of the mean coil diameter to the wire diameter; typically a spring index of 6 – 9 is ideal for manufacturing.

Squared and ground ends

See Closed and ground ends

Squared ends

See Closed ends

Squareness of ends

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

Stress range

The difference in operating stresses at minimum and maximum loads.

Stress relieve or relief

A low temperature heat treatment given to springs (or spring wire product) for the purpose of relieving residual stresses caused during cold forming. This process causes a restructuring of the molecular content of the material, which gives the material its spring properties.


A twisting or rotating action in torsion springs which produces a load (force) 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

The number of active coils plus the inactive (dead) coils forming the ends.