Spring Glossary

A | B | C | D | E | F | G | H | I | L | M | O | P | R | S | T | W

A

Active Coil (na): spring coils which are free to deflect under load.

Angular Relationship of Ends: the relative position to one another of the plane of the Hooks (see below) or Loops (see below) of extension springs.

B

Baking: heating of electroplated springs to relieve hydrogen embrittlement caused by the plating process.

Bend Radius: (neutral axis). The common term for this is the “K-factor.”

Buckling: bowing or lateral deflection of compression springs when compressed; related to the Slenderness Ratio (L/D) (see below).

C

Closed Ends: ends of compression springs where pitch of the end coils touch.

Closed and Ground Ends: similar to closed ends, except that the ends are ground to provide a flat plane.

Closed Length: length of a compression spring when under sufficient load to bring all coils into contact with adjacent coils; also known as Solid Height (see below).

Close-Wound: coil springs that are coiled in such a way that adjacent coils touch.

Coils Per Inch: the distance from center to center of the wire in adjacent Active Coils (see above); recommended practice for specifying coil springs is to specify the number of active coils; also known as Pitch (see below).

Compression Spring – Helical compression springs have applications to resist applied compression forces or in the push mode, store energy to provide the "push". Different forms of compression springs are produced. There are conical, barrel, hourglass, or straight conical compression springs. These compression springs can be made with or without variable spacing between coils. Round wire springs can store more energy than rectangular wire compression springs.

D

Deflection (F): motion of spring ends or arms under the application or removal of an external load (P).

E

Elastic Limit: maximum stress to which a spring material may be subjected without permanent set.

Endurance Limit: maximum stress at which any given spring material will operate indefinitely without failure for a given stress.

Extension Spring: Extension springs exert a pulling force or energy. They are usually close wound with initial tension and are generally manufactured from round wire. Extension springs may be produced with a wide variety of end types: hooks, loops, bends, crossbars, etc.

F

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

Free Length (H): the overall length of a spring in the unloaded position.

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

G

Gradient: change in load per unit deflection, generally given in pounds per inch (lbs./in.) or Newtons per millimeter (N/mm); also known as Rate (see below).

H

Heat Setting: fixturing a spring at elevated temperature to minimize loss of Load (see below) at operating temperature. See also Hot Pressing (below)

Helix: the spiral form (open or closed) of compression springs, extension springs, and torsion springs.

Hooks: open loops or ends of extension springs.

Hooke's Law: Load (see above) is proportional to displacement.

Hot Pressing: fixturing a spring at elevated temperatures to minimize loss of Load (see below) at operating temperature. See also Heat Setting (above).

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. Immediate post-process Baking (see above) is required for hydrogen embrittlement relief.

Hysteresis: the mechanical energy loss that always occurs under cyclic loading and unloading of a spring, proportional to the area between loading and unloading load-deflection curves within the elastic range of the spring.

I

Initial Tension (Pi): the force that tends to keep the coils of extension springs closed and which must be overcome before the coils will open.

L

Load (P): the force applied to a spring that causes Deflection (see above).

Loops: coil-like wire shapes at the ends of extension springs that allow for attachment and force application.

M

Mean Coil Diameter (D): outside spring diameter (OD) minus one wire diameter (d).

Modulus in Shear of Torsion (G): coefficient of stiffness for extension springs and compression springs.

Modulus in Tension or Bending (E): coefficient of stiffness used for torsion springs or flat springs (a.k.a. Young’s Modulus).

Moment (M): a twisting action in torsion springs which produces rotation, equal to the Load (see above) multiplied by the distance (or moment arm) from the load to the axis of the spring body. Usually expressed in inch-pounds (in.-lbs.) or Newton-millimeters (N-mm). See also Torque (below).

O

Open Ends Not Ground: end of a compression spring with a constant pitch for each coil.

Open Ends Ground: “Open Ends Not Ground” followed by an end grinding operation.

P

Passivating: acid treatment of stainless steel spring materials to remove contaminants and improve corrosion resistance.

Permanent Set: a material that is deflected (see Deflection, above) so far that its elastic properties have been exceeded and it does not return to its original condition upon release of Load (see above) is said to have taken a “permanent set”.

Pitch (p): the distance from center to center of the wire in adjacent Active Coils (see above); recommended practice for specifying coil springs is to specify the number of active coils; also known as Coils Per Inch (see above).

Preset: the process of closing a compression spring to Solid Height (see below); compression springs which have been coiled longer than the desired finish length so as to increase the apparent elastic limit. Also known as Remove Set (see below).

R

Rate (R or K): change in load per unit deflection, generally given in pounds per inch (lbs./in.) or Newtons per millimeter (N/mm); also known as Gradient (see above).

Remove Set: the process of closing a compression spring to Solid Height (see below); compression springs which have been coiled longer than the desired finish length so as to increase the apparent elastic limit. Also known as Preset (see above).

Residual Stress: stress induced by set removal (see Set, below), shot peening (see Shot Peening, below), cold working, forming or other means.  These stresses may or may not be beneficial, depending on the application of the coil spring.

S

Set: permanent distortion which occurs when a spring is stressed beyond the Elastic Limit (see above) of the material.

Shot Peening: a cold working process in which the material surfaces of coil springs are peened to include compressive stresses and thereby improve fatigue life.

Slenderness Ratio: ratio of spring length (L) to mean coil diameter (D). See also Buckling (above).

Solid Height (h): length of a compression spring when under sufficient Load (see above) to bring all coils into contact with adjacent coils; also known as Closed Height (see above).

Spring Back: Springback is the change made to a metal part at the end of the forming process. When the part has been released from the forming tool residual stresses will cause the sheet metal to spring back slightly.

Spring Index: ratio of mean coil diameter (D) to wire diameter (d).

Squared and Ground Ends: refer to Closed and Ground Ends (see above).

Stress Range: the difference in a material’s operating stresses at minimum and maximum load.

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 (see above), except with the spring under load.

T

Torque (M): a twisting action in torsion springs which produces 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-pounds (in.-lbs.) or Newton-millimeters (N-mm). See also Moment (above).

Torsion Spring: a torsion spring provides rotational energy or Torque (see above). Torsion springs can be single bodied or double bodied. Torsion springs require three points of support, with the body usually resting on a shaft or arbor. The design possibilities for the ends (or legs) of torsion springs are limitless. Stresses in torsion springs results in bending. Round wire is generally the preferred material for torsion springs, due to the cost of rectangular wire; however, though rectangular is more efficient in bending.

Total Number of Coils (Nt): number of active coils (n) plus the coils forming the ends of a coil spring.

W

Wahl Factor: a factor to correct the stress effects of curvature and direct shear on a helical spring.

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