Glossary of Spring Terms

Active coils
Those coils which are free to deflect under load.
Angular relationship of ends
The relative position of the plane of the hooks or loops of extension springs from one end to the other.
Baking
Heating of electroplated springs to relieve hydrogen embrittlement.
Buckling
Bowing or lateral deflection of compression springs when compressed, related to the slenderness ratio of the spring.
Closed ends
Ends of compression springs where pitch of the end coils is reduced so that the end coils touch; the end coils are then considered as dead coils.
Closed and ground ends
As with closed ends, except that the end is ground to provide a flat plane.
Closed length
See Solid height
Close-wound
Coiled with adjacent coils touching.
Coils per inch
See Pitch
Deflection
Motion of spring ends or arms under the application or removal of an external load.
Elastic limit
Maximum stress to which a material may be subjected without incurring permanent set or deformation.
Endurance limit
Maximum stress at which any given material will operate during its cycle life without failure.
Free angle
Angle between the arms of a torsion spring when the spring is not loaded or is in its free position.
Free length
The overall length of a spring in its unloaded position.
Gradient
See Rate
Heat setting
Fixturing a spring at a given temperature to minimize loss of load at operating temperature.
Helix
The spiral form (open or closed) of compression, extension, and torsion springs; sometimes referred to as the direction of coiling (left hand or right hand helix)
Hooks
Open loops or ends of an extension spring
Hydrogen embrittlement
Hydrogen absorbed in electroplating or picking of carbon steels, tending to make the spring material brittle and susceptible to cracking and failure, particularly under sustained loads.
Hysteresis
The mechanical energy loss that always occurs during the loading and unloading (cycling) of a spring, proportional to the area between the loading and unloading load-deflection curves within the elastic range of a spring.
Initial tension
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
The force applied to a spring that causes a deflection
Loops
The coil-like wire shape configurations at the ends of extension springs which provide for attaching the spring.
Mean coil diameter
The outside spring diameter minus one wire diameter, or the inside spring diameter plus one wire diameter.
Modulus in shear or torsion
Coefficient of stiffness for extension and compression springs
Modulus in tension or bending
Coefficient of stiffness used for torsion and flat springs (Young’s Modulus)
Moment
See Torque
Open ends, not ground
End of a compression spring with a constant pitch for each coil.
Open ends ground
End of a compression spring with a constant pitch for each coil, but the end coil has a grinding operation.
Passivating
Acid treatment of stainless steel to remove contaminants and improve corrosive resistance.
Permanent set
A material that is deflected so far that its elastic properties have been exceeded and it does not return to its original condition upon release of load; this can result in deformation of a spring
Pitch
The distance from center to center of the wire in adjacent coils; typically calculated in a spring that has space between its coils.
Preset
See Remove set
Rate
Change in load per unit deflection, generally given in pounds per inch (N/mm).
Remove set
The process of compressing a compression spring to solid height (all coils touching) in order to increase the apparent elastic limit, and prevent free length loss during operation.
Residual stress
Stresses induced by set removal, shot peening, cold working, forming or other means; these stresses may or may not be beneficial, depending on the application.
Set
Permanent distortion to a compression or extension spring’s length or a torsion springs free position which occurs when the spring is stressed beyond its elastic limit.
Shot peening
A cold-working process in which the material surface is peened with a media (steel, plastic, or glass pellets) to induce compressive stresses that improve fatigue life.
Slenderness ratio
Ratio of spring length to mean coil diameter.
Solid height
Length of a compression spring when under sufficient load, forces all coils into contact with one another.
Spring index
The ratio of the mean coil diameter to the wire diameter; typically a spring index of 6 – 9 is ideal for manufacturing.
Squared and ground ends
See Closed and ground ends
Squared ends
See Closed ends
Squareness of ends
The angular deviation between the axis of a compression spring and a normal to the plane of the ends.
Stress range
The difference in operating stresses at minimum and maximum loads.
Stress relieve or relief
A low temperature heat treatment given to springs (or spring wire product) for the purpose of relieving residual stresses caused during cold forming. This process causes a restructuring of the molecular content of the material, which gives the material its spring properties.
Torque
A twisting or rotating action in torsion springs which produces a load (force) multiplied by the distance (or moment arm) from the load to the axis of the spring body. Usually expressed in oz.-in., lb.-in., lb.-ft., or in N-mm.
Total number of coils
The number of active coils plus the inactive (dead) coils forming the ends.