# Springs

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.

 Beam fixed at each end, load in centre where E = Youngs modulus [Nm-2] I = area moment of inertia [m4] l = length [m]

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

 Cantilever beam with load at end where E = Youngs modulus [Nm-2] I = area moment of inertia [m4] l = length [m]

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

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.

 Helical spring where k = stiffness [Nm-1] G = shear modulus [Nm-2] d = wire diameter [m] D = mean coil diameter [m] n = number of turns

Hysteresis

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).

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

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 centre to centre of the wire in adjacent active coils.

 Quarter-elliptic laminated leaf spring where E = Youngs modulus [Nm-2] n = number of leaves b = width of leaves [m] t = thickness of leaves [m] l = span [m]

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.

 Rod under axial load where E = Youngs modulus [Nm-2] A = cross sectional area [m2] l = length [m]

 Semi-elliptic laminated leaf spring where E = Youngs modulus [Nm-2] n = number of leaves b = width of leaves [m] t = thickness of leaves [m] l = span [m]

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

 Simply supported beam Load applied at the centre. where E = Youngs modulus [Nm-2] I = area moment of inertia [m4] l = length [m]

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 of a compression spring and a normal to the plane of the other ends.