Active Coils

Those coils which are free to deflect under load.


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

Closed Ends

Ends of compression springs where pitch of the end coils is reduced so that the end coils touch.

Closed and Ground Ends

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


Coiled with adjacent coils touching.

Coils Per Inch

See Pitch.


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

Endurance Limit

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

Free Angle

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

Free Length

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.


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


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.


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

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.


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

Mean Coil Diameter

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

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”.


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


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

Remove Set

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

Residual Stress

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


See Remove Set.


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

Solid Height

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 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.


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.

Total Number of Coils

Number of active coils plus the coils forming the ends.