Spring Terminology

Active Coils (n) (also known as effective coils or working coils) – The number of coils which are
free to move when the spring is loaded.

Angle of Grind – Angle subtended by the ground end surface of the spring.

Angular relationship of ends – The relative position of the hooks or loops of tension springs, or the legs of a torsion spring, to each other.

Buckling – The tendency of a compression spring to bow or to deflect laterally, when loaded.

Closed and unground ends (also known as Closed and Square ends, or Squared Ends) – The ends of a compression spring where the pitch of the end coils is reduced to zero.

Closed and Ground Ends (also known as Squared and Ground) – The ends of a compression spring where the pitch of the end coils is reduced to zero and the ends are ground square with the spring axis.

Closed length – See Solid Height.

Close-wound or Close-Coiled – Coiled with adjacent coils touching.

Coils per inch – see Pitch.

Creep – The change in length of a spring when subjected to a constant force.

Compression Spring – A spring whose dimension, in the direction of the applied force, reduces under the action of that force.

Dead Coils – The coils of a spring that do not affect the spring rate.

Deflection (s) – Relative displacement of the ends of a spring under the application of a force.

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

Endurance Limit – (See Fatigue Strength)

End Fixation Factor – A factor used in the calculation of buckling to take account of the method of locating the ends of the spring.

Extension Spring (also known as Tension Spring) – A spring whose length, in the direction of the applied force, increases under the application of that force.

Fatigue – A phenomenon giving rise to a type of failure which takes place under conditions involving repeated or fluctuating stresses below the elastic limit of the material.

Fatigue Strength – (also known as Endurance Limit). Stress condition under which a material will have a life of a given number of cycles.

Free length (LO) – The length of a spring when it is not loaded.

Gradient – (See Rate)

Heat Stabilization (also known as Hot Setting) – The process of removing primary creep and inducing beneficial stresses into a spring, so that, when the spring is subjected to an operating stress and temperature, it will exhibit improved stress temperature relaxation properties.

Helix Angle – The angle of the helix of a helical coil spring.

Hooke’s Law – A  principle of physics that states that the force needed to extend or compress a spring by some distance is proportional to that distance.

Hydrogen Embrittlement – Hydrogen absorption during electroplating or pickling of carbon steels, which makes the spring material brittle and susceptible to cracking.  Appropriate post-process heat treatment is essential to minimise the risk of failure.

Hysteresis – The lagging of the effect behind the cause of the effect.  A measure of hysteresis in a spring is represented by the area between the loading and unloading load-deflection curves produced when the spring is stressed within the elastic range.

Index – (see Spring Index).

Initial Tension (F0) – 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 (F) – See Force. 

Loops (also known as hooks or eyes) – Formed anchoring points at the ends of extension springs that provide for attachment and force application. (The term ‘hook’ is generally used when it is partially open, a ‘loop’ is generally fully closed).

Mean Coil Diameter (D) – The mean value of the outside and inside diameters.

Modulus of Elasticity (E) also known as ‘Young’s Modulus’ – Coefficient of stiffness used for torsion and flat springs; the ratio of stress over strain.

Modulus Of Rigidity (G) – Coefficient of stiffness for extension and compression springs; the modulus in shear.

Moment  – (See Torque).

Natural Frequency (fe) – The frequency at which a spring will freely vibrate once it has been excited.

Open End – The end of an open coiled helical spring in which the helix angle of the end coil has not been progressively reduced.

Parallelism – The degree to which the two ground ends of a spring are parallel to each other.

Passivating – Acid treatment of stainless steels to remove contaminants and encourage the formation of a protective oxide layer.

Permanent Set – A material that is deflected so far that its elastic properties have been exceeded and it does not fully return to its original condition upon release of load has taken a “permanent set.”

Pitch –The distance from any point in the section of one coil to the corresponding point in the next coil when measured parallel to the axis of the spring.

Plain Ends – End coils of a tension spring having no loops or other end forms.

Prestressing, also known as Scragging, or Removing Set – The process of raising the elastic limit of a spring by subjecting it to a stress greater than that to which it is subjected under working conditions and higher than the elastic limit of the material.  The plastically deformed state resulting from this stress causes a beneficial redistribution of the stresses within the spring.

Rate (S) – Change in force per unit deflection, generally given in N/mm.

Residual Range – Deflection of a spring available beyond the maximum working position up to the solid position.

Set – Permanent distortion in length or position which occurs when a spring is stressed beyond the elastic limit of the material.

Shot Peening – Impacting the surfaces of the spring with pellets to induce compressive stresses and thereby improve fatigue life.

Slenderness Ratio – Ratio of spring length (LO) to mean coil diameter (D).

Solid Height (LS) also known as Solid Length and Closed Height – Length of a compression spring when compressed to the extent that each and every coil is in contact with the next.

Spring Index (w) – Ratio of mean coil diameter (D) to wire diameter (d).

Squared And Ground Ends – See Closed and Ground Ends.

Squared Ends – See Closed Ends.

Stress Correction Factor (k) – Factor that is introduced to make allowance for the fact that the distribution of shear stress across the wire diameter is not symmetrical.

Stress Range – The difference in operating stresses under minimum and maximum forces.

Stress Relieving – Low-temperature heat treatment designed to relieve the detrimental stresses induced by the manufacturing processes.

Stroke – The distance between the minimum and maximum working positions of a spring.

Torque (M) also known as Moment – The product of the distance from the spring axis to the point of load application, and the force component normal to the distance line. Usually expressed in N.mm

Torsion Spring – A helical spring which provides rotational energy or torque. The material in such a spring is stressed in bending.  A torsion spring can have two separate sets of coils in which case it is known as a double torsion spring.

Total Number of Coils (nt) – The total of all active and inactive coils.


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