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Active Coils
Those coils which are free to deflect under load.

Bowing or lateral deflection of compression springs when compressed, related to the slenderness ratio.

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

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 permanent set.

Endurance Limit
Maximum stress at which any given material will 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.

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

The spiral form (open or closed) of compression, extension, and torsion springs.

Open loops or ends of extension springs.

The mechanical energy loss that always occurs under cyclic loading and unloading 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.

The force applied to a spring that causes a deflection.

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

Mean Coil Diameter
Outside spring diameter minus one wire diameter.

Modulus in Shear
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).

The distance from center to center of the wire in adjacent active coils (recommended practice is to specify number of active coils rather than pitch).

Poisson's Ratio
The ratio of the strain in the transverse direction to the strain in the longitudinal direction.

Change in load per unit deflection.

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.

Stress Relieve
To subject springs to low-temperature heat treatment so as to relieve residual stresses.

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.

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