Closed Ends - Ends of compression springs where pitch of the end coils is reduced so that the end coils touch. Technical Article.
Closed & Ground Ends - As with closed ends, except that the end is ground to provide a flat plane. Technical Article.
Deflection (F) - Motion of spring ends or arms under the application or removal of an external load (P). Technical Article.
Free Angle - Angle between the arms of a torsion spring when the spring is not loaded. Technical Article.
Free Length (L) - The overall length of a spring in the unloaded position. Technical Article.
Hooks - Open ends or hooks of extension springs. See examples.
Initial Tension (Pi) - The force that tends to keep the coil of an extension spring closed and which must be overcome before the coils start to open. Technical Article.
Mean Coil Diameter (D) - Outside spring diameter (O.D.) minus one wire diameter (d).
Open Ends, Not Ground - Ends of a compression spring with a constant pitch for each coil. Technical Article.
Open Ends Ground - "Open ends, not ground" followed by an end grinding operation. Technical Article.
Passivating - Acid treatment of stainless steel 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 a "permanent set". Technical Article.
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). Technical Article.
Rate (R) - Change in load per unit deflection, generally given in pounds per inch (N/mm). Technical Article.
Remove Set -The process of closing to solid height a compression spring which has been coiled longer than the desired finished length, so as to increase the apparent elastic limit. Technical Article.
Residual Stress - Stress induced by set removal, shot peening, cold working,forming or other means. These stresses may or may not be beneficial, depending on the application.
Set - Permanent distortion which occurs when a spring is stressed beyond the elastic limit of the material. Technical Article.
Shot Peening - A cold-working process in which the material surface is peened to induce compressive stresses and thereby improve fatigue life.
Slenderness Ratio - Ratio of spring length (L) to mean diameter (D). Technical Article.
Solid Height (H) - Length of a compression spring when under sufficient load to bring all coils into contact with adjacent coils. Technical Article.
Spring Index - Ratio of mean coil diameter (D) to wire diameter (d). Technical Article.
Squareness of Ends - Angular deviation between the axis of a compression spring and a normal to the plane to the ends. Technical Article.
Stress Range - The difference in operating stresses at minimum and maximum loads.
Stress Relieve - To subject spring to low-temperature heat treatment so as to relieve residual stresses.
Total Number of Coils (Nt) - Number of active coils (n) plus the coils forming the ends.
Wahl Factor - A factor to correct stress in helical springs effects of curvation and direct shear.
