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Spring Definitions

Active Coils

Those coils which are free to deflect under load.

Allow for Set

Spring is supplied longer than specified to compensate for length loss when fully compressed in assembly by customer. Usually recommended for large quantity orders to reduce cost.

Angular Relationship of Ends

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


A thin blue film of oxide on ferrous alloys, sometimes used to indicate that the material has been stress relieved.


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

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

Closed and Ground Ends

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

Closed Length

See Solid Height.


Coiled with adjacent coils in contact.

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 producing 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

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.

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.


Open loops or ends of extension springs.

Hot pressing

See Heat Setting.

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 to a spring.

Initial Tension

The force that keeps 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 (F).


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

Mean Coil Diameter

Outside spring diameter (O.D.) minus 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 and Ground End

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


Acid treatment of stainless steel to remove contaminates 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 center to center of the wire in adjacent active coils (recommended practice is to specify number of active coils rather than pitch).


See Remove Set.


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

Remove Set

Full compression of a spring to solid state by manufacturer when needed to prevent length loss in operation.

Residual Stress

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


Length loss in operation due to the high stress condition of the spring.

Slenderness Ratio

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

Solid Height

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

Spring Index

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

Squared and Ground Ends

See Closed and Ground Ends.

Squared Ends

See Closed Ends.

Stress Range

The difference in operating stress at minimum and maximum loads.

Stress Relieve

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

Shot Peened

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

Squareness of Ends

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

Squareness Under Load

Same as Squareness of Ends, except with the spring under load.


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 inch-oz., inch-pounds or foot-pounds.

Total Number of Coils

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

Trapped Stress

See Residual Stress.

Wahl Factor

A factor to correct stress in helical springs effects of curvation and direct shear.