Active coils (na)
Those coils which are free to deflect under load. For
compression springs
with squared ends, the total number of coils minus two is a good approximation
of the number of active coils. For
extension springs,
the number of active coils is approximately equal to the number of coils in its
body. For
extension
springs
with threaded inserts or swivel hooks, the number of active coils is less than the
total coils in the body. Hooks and loops add to the number of active coils.
Angular relationship of ends
The relative position of the plane of the hooks or loops of
extension springs
to each 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 (L/D).
Closed ends
Ends of compression springs
where pitch of the end coils is reduced so that the end coils touch. [Compression Spring | End Treatment]
Closed and ground ends
As with closed ends, except that the end is ground to provide a flat plane. [Compression Spring | End Treatment]
Closed length
See Solid Height.
Close-wound
Coiled with adjacent coils touching.
Coils per inch
See Pitch.
Deflection (F)
Motion of spring ends or arms under the application or removal of an external load
(P).
Elastic limit
Maximum stress to which a
material
may be subjected without permanent set.
Endurance limit
Maximum stress to 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 (L)
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.
Gradient
See Rate (R).
Heat Setting
Fixturing a spring at elevated temperature to minimize loss of load at operating
temperature.
Helix
The spiral form (open or closed) of
compression,
extension
and
torsion springs.
Hooke’s Law
Load is proportional to displacement.
Hooks
Open loops or ends of
extension springs.
Hot pressing
See Heat setting.
Hydrogen embrittlement
Hydrogen absorbed in electroplating or pickling 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 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 (Pi)
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 (P)
The force applied to a spring that causes a deflection (F).
Loops
Coil-like wire shapes at the ends of
extension springs
that provide for attachment and force application.
Mean coil diameter (D)
Outside spring diameter (O.D.) minus one wire diameter (d).
Modulus in shear or torsion (G)
Coefficient of stiffness for extension and
compression springs.
Modulus in tension or bending (E)
Coefficient of stiffness used for flat and
torsion springs
(Young’s Modulus).
Moment (M)
See Torque.
Open ends, not ground
End of a
compression spring
with a constant pitch for each coil. [Compression Spring | End Treatment]
Open ends ground
"Open ends, not ground" followed by an end grinding operation. [Compression Spring | End Treatment]
Passivating
Acid treatment of
Stainless steel
to remove contaminants and improve corrosion resistance. Passivation is a post-fabrication method of maximizing the inherent corrosion resistance of the stainless alloy from which the workpiece was produced. It is not a scale removal treatment, nor is it like a coat of paint.
[Spring Passivation]
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 (p)
The distance from center to center of the wire in adjacent active coils. Recommended
practice is to specify the 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.
Preset
See Remove set.
Rate (R)
Change in load per unit deflection, generally given in pounds per inch (N/mm).
Remove set
The process of closing to solid height a
compression spring
which has been coiled
longer than the desired finished length, so as to increase the apparent elastic
limit.
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 which occurs when a spring is stressed beyond the elastic limit
of the
material.
Shot peening
A cold-working process in which the
material
surface is peened to induce compressive stresses and thereby improve fatigue life.
Slenderness ratio
Ratio of spring length (L) to mean coil diameter (D).
Solid height (H)
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 (D) to wire diameter (d). An index of 6 to 9 is ideal for spring manufacturing.
Springs with indices below above the ideal range of 6 to 9, are more difficult to produce because more stress is generated during coiling and the risk of breaking the wire guides and coiling tools increases. The smallest spring index that is considered safe to produce is around 3.
Springs with indices above the ideal range of 6 to 9, are more difficult to produce while maintaining a constant geometry and load within strict tolerances. The largest spring index that is considered safe to produce is aroung 30.
Squared and ground ends
See Closed and ground ends. [Compression Spring | End Treatment]
Squared ends
See Closed ends.[Compression Spring | End Treatment]
Squareness of ends
Angular deviation between the axis of a
compression spring
and a normal to the plane
of the ends.
Squareness under load
As in Squareness of ends, except with the spring under load.
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.
Torque (M)
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 oz
* in., lb. * in., lb. * ft., or in N * mm.
Total number of coils (Nt)
Number of active coils (na) plus the coils forming the ends.
Wahl Factor
A factor to correct stress in helical springs effects of curvature and direct shear.
