Glossary of Spring Terms

SSSspring industrial has composed this short list of Industry Terms.  This language is used commonly when designing or manufacturing springs and other wire forms.

Glossary of Spring Terms

Active coils: Those coils which are free to deflect under load.

The angular relationship of ends: The relative position of the plane of the hooks or loops of extension springs to each other.
Baking: Heating of electroplated springs to relieve hydrogen embrittlement.
Buckling: Bowing or lateral deflection of compression springs when compressed, related to slenderness ratio (L/D).
Closed ends: Ends of compression springs where the 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.
Close-wound: 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 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 spring itself (usually in cycle per second) with ends restrained.
Heat setting: Fixturing a spring at elevated temperature to minimize loss of load at operating temperature.
Helix: The spiral for (open or closed) of compression, extension, and torsion springs.
Hooke's Law: Load is proportional to displacement.
Hooks: 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.
Hysteresis: Mechanical energy loss occurring during loading and unloading of a spring within the elastic range. It is 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.
Loops:   Formed ends with minimal gaps at the ends of extension springs.
Mean Diameter (D):  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 the equivalent to the outside diameter minus one wire diameter.
Modulus in Shear or Torsion (G):  (Modulus of Rigidity G) Coefficient of stiffness used for compression and extension springs.
Modulus in Tension or Bending (E):  (Young Modulus E) Coefficient or stiffness used for torsion or flat springs.
Moment (M):  A product of the distance from the spring axis to the point of load application, and the force component normal to the distance line.
Natural Frequency (n):  Lowest inherent rate of free vibration of a spring vibrating between its own ends.
Patenting:  The process of heating carbon steel above its critical temperature and cooling at a controlled rate to achieve a fine pearlitic microstructure.

Pitch (p):  Distance from center to center of the wire in adjacent coils in an open-wound spring.
Plain Ends:  End coils of a helical spring having a constant pitch and ends not squared.
Plain Ends, Ground:  Same as Plain Ends, except wire ends are ground square with the axis.
Rate (R):  Spring gradient, or change in load per unit of deflection.
Residual Stress:  Stress mechanically induced by such means as set removal, shot-peening, cold working, or forming. It may be beneficial or not, depending on the spring application.
Set:  Permanent change of length, height, or position after spring is stressed beyond the material’s elastic limit.
Set Point:  Stress at which some arbitrarily chosen amount of set (usually 2%) occurs. The set percentage is the set divided by the deflection which produced it.
Set Removal:  An operation which causes a permanent loss of length or height 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 (LS):  Length of a compression spring when deflected under sufficient load to bring all adjacent coils into contact – no additional deflection is possible.
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 (C):  Ratio of mean diameter to wire diameter.
Squared and Ground Ends:  See Closed and Ground Ends.
Squareness:  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 the 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.
Total Number of Coils (N):  The sum of the number of active and inactive coils in a spring body.