Active Coils - those coils which are free to deflect under load.
Allow for Set - spring is supplied longer than specified to compensate for length loss when fully compressed in assembly by customer; recommended for large quantity orders to reduce cost.
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.
Blue - a thin blue film of oxide on ferrous alloys, sometimes used to indicate that the material has been stress relieved.
Buckling - bowing or lateral deflection of compression springs when compressed, related to the slenderness ratio (L/D).
Closed and Ground Ends - same as with closed ends, except that the end is ground to provide a flat plane.
Closed Ends - ends of compression springs where pitch of the end coils is reduced so that the end coils touch.
Close-Wound - coiled with adjacent coils in contact.
Deflection Motion - of spring ends or arms under the application or removal of an external load.
Elastic Limit - maximum stress to which a material may be subjected without producing permanent set.
Endurance Limit - maximum stress at which any given material will operate indefinitely without failure for a given minimum stress.
Free Angle - angle between the arms of a torsion spring when the spring is not loaded.
Free Length - the overall length of a spring in the unloaded position.
Frequency (natural) - the lowest inherent rate of free vibration of a spring itself (usually in cycles per second) with ends restrained.
Heat Setting - fixturing a spring at elevated temperature to minimize loss of load at operating temperature; sometimes referred as hot pressing
Helix - the spiral form (open or closed) of compression, extension and torsion springs.
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 - the mechanical energy loss that always occurs under cyclic loading and unloading of a spring, proportional to the area between the loading and unloading load-deflection curves within the elastic range to a spring.
Initial Tension - the force that keeps the coils of an extension spring closed and which must be overcome before the coils start to open.
Load - the force applied to a spring that causes a deflection.
Loops - coil-like wire shapes at the ends of extension springs that provide for attachment and force application.
Mean Coil Diameter - outside spring diameter minus one wire diameter.
Modulus in Shear or Torsion - coefficient of stiffness for extension and compression springs.
Modulus in Tension or Bending - coefficient of stiffness used for torsion and flat springs (Young’s Modulus).
Open and Ground End - "open ends, not ground" followed by an end grinding operation.
Open Ends, Not Ground - end of a compression spring with a constant pitch for each coil.
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”.
Pitch - 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).
Preset or Remove Set - full compression of a spring to solid state by manufacturer when needed to prevent length loss in operation.
Rate - change in load per unit deflection, generally given in pounds per inch.
Residual Stress - stresses 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 - length loss in operation due to the high stress condition of the spring.
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 to mean coil diameter.
Solid Height - height of a compression spring when under sufficient load to bring all coils into contact with adjacent coils; sometimes referred as closed length
Spring Index - ratio of the mean coil diameter to wire diameter
Squared and Ground Ends - see Closed and Ground Ends.
Squared Ends - see Closed Ends.
Squareness of Ends - angular deviation between the axis of a compression spring and a normal to the plane of the ends.
Squareness Under Load - same as in Squareness of Ends, except with the spring under load.
Stress Range - the difference in operating stresses at minimum and maximum loads.
Stress Relieve - to subject springs to low-temperature heat treatment so as to relieve residual stresses.
Torque or Moment - a twisting action in torsion springs which tends to produce rotation, equal to the load multiplied by the distance (or moment arm) from the load to the axis of the spring body. Usually expressed in inch-oz., inch-pounds or in foot-pounds.
Total Number of Coils - number of active coils plus the coils forming the ends.
Trapped Stress - see Residual Stress.