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Spring Maker's Glossary

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Active coils (n)
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.

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

Closed ends
See Squared Ends.

Closed and ground ends
See Squared and Ground Ends.

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).

Direction of coiling - or Helix
Right or left hand as in screw system. Should be optional, unless specifically required.

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.

Free angle
Angle between the arms of torsion spring when the spring is not loaded.

Free length (L)
The overall length of a spring in the unloaded position.

See Rate (R).

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.

See Spring Index.

Initial Tension
The force tending to keep the coils of an extension spring closed which must be overcome before the coils start to open. A design consideration generally specified only as a refrence dimension.

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 used 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
Ends 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 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 permanent set.

Pitch (P)
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).

The process of compressing a spring to solid height to remove any permanent deformation due to exceeding the elastic limit of the material, with an apparent increase in the elastic limit

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

Relative position
See Angular relationship of ends.

Remove Set
See Preset

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.

Hurtling small steel shot against springs to clean surface and trap beneficial surface stresses, thereby increasing 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
D/d, ratio of mean diameter to wire diameter. Best ratio is between 7 and 13. Troublesome if less than 4 or over 16.

Squared and ground ends
End finish for compression springs where pitch of the end coils is reduced so that the end coils touch and the end is then ground to provide a flat end.

Squared ends
As above except not ground.

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

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

Stress relieve
Low-temperature heat treatment to relieve residual stresses induced during coiling or forming so that springs will be more stable, more capable of withstanding higher stresses and have longer fatigue life.

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

Total number of coils (N)
In compression springs, the sum of the active coils plus the dead coils forming the ends.  In extension springs, it is equal to the active coils, and does not include the coils or material used for forming the ends.