Active Coils

Those coils which are free to deflect under load. *

Angular Relationship of Ends

Relative position of hooks or loops of an extension spring (ends of a torsion spring) to each other. *


Heating of electroplated springs to relieve hydrogen embrittlement. *


Bowing or lateral displacement of a compression spring. Related to slenderness ratio. *

Closed Ends

Compression spring ends with coil pitch angle reduced so they are square with the spring axis and touch the adjacent coils. *

Closed and Ground Ends

Closed End spring in which the first and last coil are grounf to provide a flat bearing surface. *

Close Wound

Adjacent coils are touching. *


Motion imparted to a spring by application or removal of an external load. *

Elastic Limit

Maximum stress to which a material may be subjected without permanent set. *

Endurance Limit

Maximum stress, at a given stress ratio, at which material will operate in a given environment for a stated number of cycles without failure. *

Fixture Tempering

Restraining parts during tempering to improve dimensional control. *

Free Angle

Angular relationship between arms of a helical torsion spring which is not under load. *

Free Length

Overall length of a spring which is not under load. *

Heat Setting

A process to pre-relax a spring in order to improve stress relaxation resistance in service. *

Helical Springs

Springs made of bar stock or wire coiled into a helical form. This category includes compression, extension and torsion springs. *


Open loops or ends of extension springs. *


Mechanical energy loss occurring during loading and unloading of a spring within the elastic range, illustrated by the area between load deflection curves. *

Initial Tension

A force that tends to keep coils of a close wound extension spring closed and which must be overcome before the coils start to open. *


Formed ends with minimal gaps at the ends of extension springs. *

Mean Diameter

The average diameter of the mass of spring material, equal to one half the sum of the outside and inside diameters. In a helical spring, this is equivalent to the outside diameter minus one wire diameter. *

Modulus in Tension or Bending

(Young's Modulus E) - Coefficient of stiffness used for torsion or flat springs. *

Modulus in Shear or Torsion

(Modulus of Rigidity G) - Coefficient of stiffness used for compression and extension springs. *


Abbreviation for the National Association for Stock Car Auto Racing.

Natural Frequency

Lowest inherent rate of free vibration of a spring vibrating between its own ends. *


Process of heating carbon steel above its critical temperature and cooling at a controlled rate to achieve a fine, pearlitic microstructure. *


The distance from center to center of wire in adjacent coils of an open wound spring. Also known as Coils per Inch. *

Plain Ends

End coils of a helical spring having a constant pitch. Ends not squared. *

Plain Ends, Ground

End coils of a helical spring having a constant pitch and wire ends ground square with the axis. *


Spring gradient, or change in the load per unit of deflection. *

Rear-end Locker Springs

Also known as Differential Locker Springs.

Residual Stress

Stress mechanically induced by such means as set removal, shot peening, cold working or forming. Benefits depend on the spring application. *


Permanent change of length, heigh or position after a spring is stressed beyond the material's elastic limit. *

Set Percentage

Amount of set divided by the deflection which produced it. *

Set Point

Stress at which some arbitrarily chosen amount of set (usually 2%) occurs. *

Set Removal

An operation which causes a permanent loss of length or heigh due to spring deflection. *

Shot Peening

Blasting the surfaces of spring material with steel or glass pellets to induce compressive stresses that improve fatigue life. *

Slenderness Ratio

Ratio of spring length to mean diameter (L/D) in helical springs. *

Solid Height

Length of a compression spring when deflected under sufficient load to bring all adjacent coils into contact, with no additional deflection possible. Also known as Block or Closed Height. *

Spiral Springs

Springs formed from flat strip or wire wound in the form of a spiral, loaded by torque about an axis normal to the plane of the spiral. *

Spring Index

Ratio of mean diameter to wire diameter. *


Angular deviation between the axis of a compression spring in a free state and a line normal to the end planes. *

Stress Range

Difference in operating stresses at minimum and maximum loads. *

Stress Ratio

Minimum stress divided by maximum stress *

Stress Relief

A low temperature heat treatment given springs to relieve residual stresses produced by prior cold forming. *


A product of the distance from the spring axis to the point of load application, and the force component normal to the distance line. Also known as Moment. *

Total Number of Coils

The sum of the number of active and inactive coils in a spring body. *


*- © 2000, Spring Manufacturers Institute. All Rights Reserved.
Used with the express written permission of the Spring Manufacturers Institute.