Spring Manufacturing Terminology

Active Coils

Coils which are free to deflect under load. 

Angular relationship of ends

The relative position of the loops or ends of an extension spring or torsion spring to each other.


Process of heating the material so that it becomes soft.

Stress relieve

Heating of electroplated springs to relieve hydrogen embrittlement.


The lateral displacement of a compression spring due to loading. This effect is a function of slenderness ratio L/D.

Close coiled

Generally applies to tension & torsion springs where adjacent coils touch.

Closed and ground ends

The end coils of a compression spring are closed and ground flat square to the axis.

Closed ends

The end coils of compression spring are closed by reducing the coil pitch angle until they are square with the spring axis and touch the adjacent coil.

Closed length (mm)

Compression springs, see solid height.

Coils per mm

Number of coils per length, see pitch.

Dead coils

The inactive coils of a compression spring.

Deflection (mm)

The distance moved due to an external load.

Elastic limit

The stress point at which the wire material, if subjected to higher stress, will no longer return to its original shape. Applied to a spring, the maximum stress at a given stress ratio at which material will operate in a given environment for a stated number of cycles without failure.

Free angle

The angle between the legs of a torsion spring when not under load.

Free length (mm)

The overall length of a compression or extension spring when not under load.

Rate N/mm

Ratio of load [P] to deflection [mm].

Hot setting

A process to improve spring relaxation when operating at elevated temperatures.

Helical springs

Wire or bar wound to form a helical coil.


The loss of mechanical energy during loading and unloading whilst operating in the elastic range. Ilustrated by the area between deflection curves.

Initial tension

The amount of force that is required to separate the closely wound coils of an extension spring.


The end coil of an extension spring which is lifted to form a loop.

Mean diameter (mm)

The average diameter of the mass of the spring diameter.

Modulus of rigidity

Coefficient of stiffness.

Youngs modulus

Ratio of Stress/Strain.

Moment N/m

Product of the distance (m) and load (N), from the spring axis to the point of load application. The force component is at 90 degrees to the distance line.

Natural frequency Hz

Frequency is the ratio of oscillations per unit of time. The natural frequency is the lowest inherent rate of free vibration of a spring oscillating between its own ends.

Pull to pull length

The length measured from the inside of one loop or hook to the inside of  the other on an extension spring.


Heat treatment process where carbon steel is heated above its critical temperature followed by a controlled rate of cooling to produce a fine pearlitic microstructure.

Pitch (mm)

The distance from wire centres of adjacent coils in an open-wound spring.

Open ends

The ends of a compression spring where the end coils have a constant pitch and not squared.

Open and ground ends

The same as open ends, except wire ends are ground square to the axis.

Rate N/mm

Ratio of change of Load (P) / Deflection (mm).


For a compression spring, it is the permanent reduction in length and is a function of material, stress and temperature. The longer the time period a spring is exposed to higher stresses and higher temperature the greater the relaxation. Springs held at a constant stress relax more than if they are cycled between that stress and a lower stress.

Residual stress N/mm2

Mechanically induced Stress by processes such set removal, shot-peening, cold working, or forming. It may be beneficial or not, depending on the spring application.


Permanent change of length, height or position after a spring is stressed beyond material’s elastic limit.

Set point

Stress at which some arbitrarily chosen amount of set (usually 2%) occurs. Set percentage is the set divided by the deflection which produced it.

Shot peen

Blasting the spring wire material surface with steel or glass beads to induce compressive stresses to improve fatigue life.

Slenderness ration

The ratio of spring length to mean diameter L/D.

Soil height (mm)

When a compression spring is loaded to bring all adjacent coils into contact and no further deflection is possible.

Spring index

The ratio of mean diameter to wire diameter.

Squared ends

See closed ends.

Squared and ground ends

See closed and ground ends.


The angular measurement of the deviation between the axis in free state and a line at right angles to the end planes of a compression spring.

Stress range N/mm2

The difference between the operating stresses at minimum and maximum loads.

Stress ration

The ratio of Minimum stress / Maximum stress.

Stress relief

A low temperature heat treatment process to relieve residual stresses produced when the spring is cold formed.

Torque N/m

See moment.

Total number of coils

The sum of active coils and dead coils.

Ashfield Springs Ltd
Nunn Brook Rise
NG17 2PD