Glossary of Terms

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 (-Iends of a torsion spring) to each other.
Baking
Heating of electroplated springs to relieve hydrogen embrittlement.
Block
Length of a compression spring when deflected under sufficient load to bring all adjacent coils into contact - no additional deflection is possible.
Buckling
Bowing or lateral displacement of a compression spring. This effect is related to slenderness ratio L/D.
Closed and ground ends
Same as Closed Ends, except the first and last coils are ground to provide a flat bearing surface.
Closed length
Length of a compression spring when deflected under sufficient load to bring all adjacent coils into contact - no additional deflection is possible.
Close wound
Adjacent coils are touching.
Coils per inch
Distance from center to center of wire in adjacent coils in an open wound spring.
Deflection
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.
Gradient
Spring gradient, or change in load per unit of deflection.
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.
Hooks
Open loops or ends of extension springs.
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
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 tension or bending
(Young's Modulus E) Coefficient of stiffness used for torsion or flat springs. Moment A product of the distance from the spring axis to the point of load application, and the force component normal to the distance line.
Modulus in shear or torsion
(Modulus of Rigidity G) Coefficient of stiffness used for compression and extension springs.
NASCAR
NASCAR is an abbreviation which stands for: National Association for Stock Car Auto Racing
Natural frequency
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
Distance from center to center of 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
Spring gradient, or change in load per unit of deflection.
Rear-end locker springs
Rear-end locker springs maintain a critical constant force on the ratcheting mechanism to ensure a smooth and consistent transfer of power to the rear drive line.
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 a spring is stressed beyond material's elastic limit.
Set point
Stress at which some arbitrarily chosen amount of set (usually 2%) occurs. 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
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
Ratio of mean diameter to wire diameter.

Squared and ground ends
See Closed and Ground Ends.

Squared 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 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.
Torque
A product of the distance from the spring axis to the point of load application, and the force component normal to the distance line.
Total number of coils
The sum of the number of active and inactive coils in a spring body.

All terms are © 2000 Spring Manufacturers Institute. All rights reserved. Used here with the express written permission of the Spring Manufacturer's Institute.