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Active Coils: those coils which are free to deflect under load

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

Closed Ends: ends of compression springs where pitch of the end coils is reduced so that the end coils touch

Closed and Ground Ends: as with closed ends except that the end is ground to provide a flat plane

Closed Length: see Solid Height

Close-wound: coiled with adjacent coils touching

Coils per Inch: see Pitch

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 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

Gradient: see Rate

Heat Setting: fixturing a spring at the elevated temperature to minimize loss of load at operating temperature

Helix: the spiral form (open or closed) of compression, extension and torsion springs

Hooks: Open loops or ends of extension springs

Hot Pressing: see Heat Setting

Initial Tension: the force that tends to keep 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 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 (O.D.) minus one wire diameter (D)

Open Ends, Not Ground: end of 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: 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: change in load per unit deflection, generally given in pounds per inch

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

Set: permanent distortion which occurs when a spring is stressed beyond the elastic limit of the material

Solid Height: 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

Squared and Ground Ends: refer to closed and ground ends

Squared Ends: refer to closed ends

Stress Relieve: to subject springs to low temperature heat treatment so as to relieve residual stresses

Shot Peening: a cold-working process in which the material surface is peened to induce compressive stresses and thereby improve fatigue life

Squareness of Ends: angular deviation between the axis of a compression spring and a normal to the plane of the ends

Total Number of Coils: number of active coils (n) plus the coils forming the ends