Glossary
Active Coils (Na)
Those coils which are free to deflect under load.
Angular Relationship of Ends
Relative position of hooks or loops of an extension spring (or ends of a torsion spring) to each other.
Baking
Heating of electroplated springs to relieve hydrogen embrittlement.
Block
See Solid Height.
Buckling
Bowing or lateral displacement of a compression spring. This effect is related to slenderness ratio L/D.
Close Wound
Adjacent coils are touching.
Closed and Ground Ends
Same as Closed Ends, except the first and last coils are ground to provide a flat bearing surface.
Closed Ends
Compression spring ends with coil pitch angle reduced to they are square with the spring axis and touch the adjacent coils.
Closed Length (Ls)
See Solid Height.
Coils per Inch
See Pitch.
Deflection (F)
Motion imparted to a spring by application or removal of an external load.
Elastic 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 (Θf)
Angular relationship between arms of a helical torsion spring which is not under load.
Free Length (Lf)
Overall length of a spring which is not under load.
Gradient
See Rate.
Heat Setting
A process to prerelax 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.
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 closewound 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 equivalent to the outside diameter minus one wire diameter.
Modulus in Shear or Torsion (Modulus of Rigidity (G))
Coefficient of stiffness used of compression and extension springs.
Modulus in Tension or Bending (Young's Modulus (E))
Coefficient of 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 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 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 caused 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
Ration 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.
Squared Ends
See Closed Ends.
Squareness
Angular deviation between he 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 (M)
See Moment.
Total Number of Coils (N)
The sum of the number of active and inactive coils in a spring body.
Bibliographical Reference: Spring Manufacturers Institute, Inc., Handbook of Spring Design, Illinois: Spring Manufacturers Institute, Inc., 2002.