Knowledge Hub

Glossary – The definitions of key spring terms and technologies

• 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 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 has 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.
• Pre-stressing, 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.

By Neil Fletcher