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Glossary of Spring Terminology

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Active Coils (Na) – The coils which are free to deflect under a force.

Alloy – A metal composed of two or more elements bound together.

Arbor – A round rod on a spring coiler over which wire is coiled to form a spring.


Baking – Heating electroplated springs to remove hydrogen.

Brass – A copper-base alloy of copper and zinc.

Bronze – A copper-base alloy of copper and tin.

Buckling – Bowing or lateral deflection of compression springs when compressed, related to the slenderness ratio.


Deflection (d) – Displacement of the ends or arms of a spring on the application of an external force (P).


Elastic Limit – Maximum stress to which a spring can be stressed without taking a permanent set.

Endurance Limit – Maximum stress which can be repeatedly applied without incurring spring failure.


Fatigue Failure – When a spring is deflected continually, the metal becomes fatigued and failure may occur at a stress level far below the elastic limit.

Fatigue Life – Number of deflection cycles until failure occurs at a predetermined stress.

Fatigue Strength (Endurance Strength) – Stress at which failure occurs after a specific number of deflections.

Force (P) – Active power applied to a spring to cause a deflection. Often erroneously called load.

Free Angle – Angle between the arms of a torsion spring when the spring is not loaded.

Free Height (Hf) – The overall height of a compression spring in the unloaded position.

Free Length (Lf) – The overall length of an extension spring in the unloaded position.

Frequency (Natural) – The number of cycles per minute at which a spring will vibrate freely once it has been excited.


Gauge – Standards for measuring material thickness or diameter for non-metric units of measure.

Gradient (Rate) (k) – Change in force per incremental unit of deflection e.g. gm per mm, Newton per mm, gm-mm per degree.


Heat Setting – Deflecting a spring in a fixture at elevated temperatures in order to reduce loss of load at operating temperatures.

HelicalSprings of spiral or cylindrical shape such as compression, extension or torsion springs.

Hydrogen Embrittlement – All high carbon and high strength steel parts with hardness greater than Rockwell C35 are susceptible to embrittlement caused by absorption of hydrogen during the processing of the steel during pickling, cleaning or electroplating. Zinc plating can cause this embrittlement on high strength steels used to make springs, etc.

There is no assured way on the processing cycle for cleaning and plating such steels with zinc that reliably avoid the embrittlement. Precautionary measures recommends baking the parts for 3 to 4 hours at 190 to 205°C, ideally within 4 hours after electroplating, reduces the danger of such failure for susceptible steel parts.

Hysteresis – The difference in forces at various deflections when loading or unloading a spring.


Initial Tension (Pi) – A force wound into an extension spring during coiling to hold the coils together.


Load (Force) (P) – Active power applied to a spring to cause a deflection (F).


Mean Coil Diameter (D) – Outside spring diameter (O.D.) minus wire diameter (d).

Modules in Shear or Torsion (G) – Coefficient of stiffness for extension and compression springs. (Modulus of Rigidity).

Modules in Tension or Bending (E) – Coefficient of stiffness for torsion and flat springs. (Young’s Modulus).


Passivation – An acid dipping treatment applied to stainless steel to remove contaminants and improve corrosion resistance.

Permanent Set – The difference in length of a highly stressed spring upon release of a force.

Pitch (p) – The distance from center of the wire in two adjacent coils.

Preset – The process of closing to solid height a compression spring which has been coiled longer than the desired finish length, so as to increase the apparent elastic limit.

Proportional Limit – Maximum load at which strain or deformation is directly proportional to stress, at zero percent offset.


Rate (k) (Gradient) – Change in force per incremental unit of deflection e.g. gm per mm, Newton per mm, gm-mm per degree.

Residual Stress – Stresses incorporated within a spring by coiling, compressing to remove set, shotpeening, cold working or induced by heat treatment.

Resilience – Elastic ability of a material to rebound to its original shape after deflection.


Set – Permanent distortion which occurs when a spring is stressed beyond the material’s elastic limits.

Slenderness Ratio – Ratio of spring free height (Hf) to the mean coil diameter (D). If the ratio is over 4.0 compression springs may buckle depending upon the amount of deflection.

Solid Height (Hs) – Length of compression spring when force causes coil contact with all adjacent coils.

Spring Index – Ratio of mean coil diameter (D) to wire diameter (d). Best ratio is between 7 – 14.

Strain – The deformation produced by a stress as compared to the original shape.

Stress – The force divided by the area over which it acts.

Stress Relieve – The process of heating the spring to and holding it at a suitably low temperature followed by cooling at an appropriate rate to relieve residual stresses induced during coiling. It is cooled slowly to minimize development of new stresses.


Tempering – Heating a quench-hardened or normalized steel wire to a temperature below the transformation range and then cooling it at a desired rate. It is primarily to reduce brittleness and internal strains.

Tensile Strength – Maximum force which a material is capable of sustaining. It equals force reached divided by original cross-sectional area.

Torque (Moment) – A twisting force in torsion springs which tends to produce rotation, equal to the force multiplied by the distance from the force perpendicular to the axis of the spring body.

Total Coils (Nt) – For compression springs, number of active coils (Na) add to the number of dead coils forming the ends.


Wahl Correction Factor – A factor to correct stress in helical springs effects of curvature and direct shear.


Yield Point – Stress point which will cause an elongation to the original length of the spring.