Active Coils – Coils that are free to deflect under load.Air Spring– A spring that uses the elasticity of the air, one fluid spring.

Buckling – Bowing or lateral deflection of compression springs. The effect related to slenderness ratio L/D.

Closed Ends – The ends of compression springs where the pitch of the end coils are reduced to touch the adjacent coils.

Closed and Ground Ends – Identical as closed ends, except the first and last coils are ground to provide a flat bearing surface.

Deflection – Motion of the spring ends or arms under the application or removal of an external load.

Elastic Limit – The maximum stress a material may be subjected to without permanent set.

Endurance Limit – The maximum stress, at a given stress ratio, at which any given material may operate indefinitely in an environment.

Fixture Tempering – The process of restraining parts during tempering. This process improves dimensional control.

Free Angle – The angular relationship between arms of a helical torsion spring which is not under load.

Free Length – The length of a spring which is not loaded.

Heat Setting – The process of fixturing a spring in order to improve stress relaxation resistance in service.

Helical Springs – Springs made into helical form from bar stock or coiled wire. Including compression, extension and torsion springs.

Hysteresis – The term applied to mechanical energy loss during loading and unloading of a spring within the elastic range.

Initial Tension – The force that keep coils of closewound extension springs closed.

Mean Coil Diameter – Outside spring diameter minus one wire diameter.

Modulus in shear or torsion (Modulus of Rigidity G) – Coefficient of stiffness used for compression and extension springs

Modulus in tension or bending (Young’s Modulus E) – Coefficient of stiffness used for torsion or flat springs.

Natural Frequency – The lowest inherent rate of free vibration from a spring vibrating between its own ends.

Pitch – Distance from center to center of wire in adjacent active coils.

Residual Stress – The mechanical stress produced by set removal, shot-peening, cold working, or forming.

Set Point – The stress at which some arbitrarily chosen amount of set occurs.

Set Removal – A procedure that causes loss of length or height due to spring deflection.

Shot-Peening – Blasting spring material with steel or glass pellets to produce compressive stresses improving fatigue life.

Slenderness Ratio – The ratio of spring length to diameter in helical springs.

Spiral Springs – Springs formed in a spiral fashion from a flat strip or wire wound.

Stress Relief – A heat treatment, typically low temperature, applied to springs to relieve residual stresses.