ACTIVE COILS
Those coils which are free to deflect under load. 

CLOSED ENDS
Ends of compression springs where pitch of the end coils is reduced so that the end coils touch.

CLOSED & GROUND ENDS
As with closed ends, except that the end is ground to provide a flat plane. 

DEFLECTION (F)
Motion of spring ends or arms under the application or removal of an external load (P). 

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. 

HOOKS
Open ends or hooks of extension springs. 

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. 

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. 

OPEN ENDS GROUND
“Open ends, not ground” followed by an end grinding operation. 

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 (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). 

RATE (R)
Change in load per unit deflection, generally given in pounds per inch (N/mm). 

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. 

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.

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). 

SOLID HEIGHT (H)
Length of a compression spring when under sufficient load to bring all coils into contact with adjacent coils. 

SPRING INDEX
Ratio of mean coil diameter (D) to wire diameter (d). 

SQUARENESS OF ENDS 
Angular deviation between the axis of a compression spring and a normal to the plane to the ends.

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.