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Glossary of Spring Terminology:

Active coils (na)
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 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).

Closed ends (Squared ends)
Ends of compression springs where pitch of the end coils is reduced to that the end coils touch.

Closed and ground ends (Squared and ground ends)
As with closed ends, except that the end is ground to provide a flat plane.

Coiled with adjacent coils touching.

Deflection (F)
Motion 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.

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)
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.

Heat setting
Fixturing a spring at an 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.

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.

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

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

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

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.

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 the pitch).

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

Remove set
The process of closing to solid height a compression spring which has been coiled larger than the desired finished length, so as 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 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 a spring length (L) to mean coil diameter (D).

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).

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 so as 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., N•mm.

Total number of coils
Number of active coils (na) plus the coils forming the ends.

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