Spring Terminology and Definitions

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The following are common mechanical spring definitiona and 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 spring to each other.

Buckling
Bowing or lateral deflection of compression springs when compressed, related to the slenderness ration (L/D).

Closed ends
Ends of compression springs where the pitch of the end coils is reduced so that the end coils touch.

Closed and ground ends
As with closed ends, except that the end is ground to provide a flat plane.

Close-wound
Coiled with adjacent coils touching.

Coils per inch
Same as "Pitch"

Deflection
Motion of the 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 permanent set.

Endurance limit
Maximum stress at which any given material may operate indefinitely without failure for a given minimum stress.

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

Free length
The overall length of a spring in the unloaded position.

Frequency (natural)
The lowest inherent rate of free vibration of a spring itself (usually in cycles per second) with ends restrained.

Hysteresis
The mechanical energy loss that always occurs under cyclical loading and unloading of a spring, proportional to the are 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 coil starts to open.

Loops
Coil-like wire shapes at the ends of extension springs that provide for attachment and force application.

Mean coil diameter
Outside wire diameter minus 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).

Open ends, not ground
End of a compression spring with a constant pitch for each coil.

Open ends ground
"Opens ends, not ground" followed by an end grinding operation.

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 is said to have taken a "permanent set".

Pitch
The distance from center to center of the wire in adjacent active coils.

Rate
Changes in load per unit of deflection, generally given in pounds per inch (lbs/in).

Remove set
The process of closing to a solid height a compression spring which has been coiled longer than the desired finished length, so as to increase the elastic limit.

Set
Permanent distortion which occurs when a spring is stressed beyond the elastic limit of the material.

Slenderness ratio
Ratio of spring length to mean coil diameter.

Solid height
Length of a compression spring when under sufficient load to bring all coils into contact with adjacent coils.

Spring index
Ratio of mean coil diameter to wire diameter.

Stress range
The difference in operating stresses at minimum and maximum loads.

Squareness of ends
Angular deviation between the axis o a compression spring and a normal to the plane of the other ends.

Squareness under load
As in squareness of ends, except with the spring under load.

Torque
A twisting action in torsion springs which tends to produce rotation, equal to the load multiplied by the distance (or moment arm) from the load to the axis of the spring body. Usually expressed in inch-oz, inch-pounds or in foot-pounds.

Total number of coils
Number of active coils plus the coils forming the ends.

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