Hysteresis. The lagging of the effect behind the cause of the effect. A measure of hysteresis in a spring is represented by the area between the loading and unloading curves produced when the spring is stressed within the elastic range.
Index. The ratio of the mean coil diameter of a spring to the material diameter for circular sections or radial width of cross section for rectangular or trapezoidal sections.
Initial tension. The part of the force exerted, when a close coiled spring is axially extended, that is not attributable to the product of the theoretical rate and the measured deflection.
Inside coil diameter of a spring. The diameter of the cylindrical envelope formed by the inside surface of the coils of a spring.
Loop (eye, hook). The formed anchoring point of a helical spring or wire form. When applied to an extension spring, it is usually called a loop. If closed, it may be termed an eye and if partially open may be termed a hook.
Modulus of elasticity. The ratio of stress to strain within the elastic range.
NOTE. The modulus of elasticity in tension or compression is also known as Young's modulus and that in shear as the modulus of rigidity.
Open end. The end of an open coiled helical spring in which the helix angle of the end coil has not been progressively reduced.
Outside coil diameter. The diameter of the cylindrical envelope formed by the outside surface of the coils of a spring.
Permanent set (set). The permanent deformation of a spring after the application and removal of a force.
Pitch. The distance from any point in the section of any one coil to the corresponding point in the next coil when measured parallel to the axis of the spring.
Prestressing (scragging). A process during which internal stresses are induced into a spring.
NOTE. It is achieved by subjecting the spring to a stress greater than that to which it is subjected under working conditions and higher than the elastic limit of the material. The plastically deformed areas resulting from this stress cause an advantageous redistribution of the stresses within the spring. Prestressing can only be performed in the direction of applied force.
Rate (stiffness). The force that has to be applied in order to produce unit deflection.
Relaxation. Loss of force of a spring with time when deflected to a fixed position.
NOTE. The degree of relaxation is dependent upon, and increases with, the magnitude of stress, temperature and time.
Safe deflection. The maximum deflection that can be applied to a spring without exceeding the elastic limit of the material.
Screw insert. A plug screwed into the ends of a helical extension spring as a means of attaching a spring to another component. The plug has an external thread, the diameter, pitch and form of which match those of the spring.
Shot peening. A cold working process in which shot is impacted on to the surfaces of springs thereby inducing residual stresses in the outside fibres of the material.
NOTE. The effect of this is that the algebraic sum of the residual and applied stresses in the outside fibres of the material is lower than the applied stress, resulting in improved fatigue life of the component.
Solid length. The overall length of a helical spring when each and every coil is in contact with the next.
Solid force. The theoretical force of a spring when compressed to its solid length.
Space (gap). The distance between one coil and the next coil in an open coiled helical spring measured parallel to the axis of the spring.
Spring seat. The part of a mechanism that receives the ends of a spring and which may include a bore or spigot to centralise the spring.
Stress (bonding stress, shear stress). The force divided by the area over which it acts. This is applied to the material of the spring, and for compression and extension springs is in torsion or shear, and for torsion springs is in tension or bending.
Stress correction factor. A factor that is introduced to make allowance for the fact that the distribution of shear stress across the wire diameter is not symmetrical.
NOTE. This stress is higher on the inside of the coil than it is on the outside.
Stress relieving. A low temperature heat treatment carried out at temperatures where there is no apparent range in the metallurgical structure of the material. The purpose of the treatment is to relieve stresses induced during manufacturing processes.
Variable pitch spring. A helical spring in which the pitch of the active coils is not constant.
Spring Terminology
Age Hardening Also known as precipitation hardening. A hardening method for certain alloys which are held at an elevated temperature for between 1 hour and several hours, causing a precipitation of a constituent from a supersaturated solution. Examples are beryllium copper and 17-7 stainless steel.
Allow for Set Spring is supplied longer than specified to compensate for length loss when fully compressed in assembly by customer.
Austempering A hardening method where steel is quenched from above the transformation point to a temperature slightly above the upper limit of martinsite formation and held there until austenite is transformed to bainite.
Baking Heating of electroplated springs to remove hydrogen and relieve hydrogen embrittlement.
Blueing Heating until forming a thin blue film of oxide on ferrous alloys.
Buckling Bowing or lateral deflection of compression springs when compressed, related to the slenderness ratio.
Closed Ends Compression spring ends with pitch reduced on the last coil so that end is squared up and tip touches last coil.
Closed and Ground Ends Same as with closed ends, except that the end is ground to provide a flat end.
Closed Wound Coiled with all coils in contact with each other.
Creep See relaxation.
Decarburization Loss of carbon from the surface of steel during a high temperature process.
Deflection Motion of spring ends or arms under the application of an external load.
Ductility Plastic deformation of a material before fracture.
Elastic Limit Maximum stress to which a material may be subjected without permanent set.
Endurance Limit Maximum stress at which a material will operate for a stated number of cycles without failure.
Fatigue Life The life of a spring in number of cycles when it is loaded numerous times.
Free Angle Angle between the arms of a torsion spring when the spring is not loaded.
Free Length The overall length of a spring when unloaded.
Goodman Diagram Derived from S-N curves and used to predict cycle life.
Hand The direction in which a spring is coiled. Right hand wound would fit over a right hand thread and left hand would fit on a left hand thread.
Heat Setting or Hot pressing Fixturing a spring at elevated temperature during manufacturing in order to improve stress relaxation during service.
Helical Springs Springs coiled in a helical form such as compression, extension and torsion springs.
Hydrogen Embrittlement Brittleness in a material caused by retained hydrogen that has been absorbed during pickling or electroplating.
Hysteresis The difference in load at a particular point between loading and unloading. Caused by friction, and is most evident when the coils rub on the mating part, such as a torsion spring working over a pin.
Initial Tension The force that keeps the coils of an extension spring closed and which must be overcome before the coils start to open.
Load The force applied to a spring that causes a deflection .
Loops Coil-like wire shapes at the end of extension springs that provide for attachment and force application.
Mean Coil Diameter Outside spring diameter (O.D.) minus one wire diameter.
Modulus of Elasticity in Shear or Torsion (Modulus of rigidity, G) Coefficient of stiffness for extension and compression springs.
Modulus of Elasticity in Tension or Bending (Young's modulus, E)Coefficient of stiffness used for torsion and flat springs.
Moment See Torque.
Natural Frequency The speed at which a spring will inherently vibrate.
Open Ends, Not Ground End of a compression spring with a constant pitch for each coil.
Open and Ground Ends constant pitch of the coils but ends ground.
Passivating Acid treatment of stainless steel to remove contaminates and to restore the oxide film thereby improving its corrosion resistance.
Patenting A process of heating carbon steel above its critical temperature and then cooling slowly, resulting in a fine pearlitic microstructure with good ductility and conducive to drawing.
Permanent Set The difference between the original free length of a spring and its length after being deflected to a certain height and released.
Pitch 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).
Precipitation hardening
Preset Removal of set by deflecting a spring to a high stress condition and releasing.
Rate Change of load per unit deflection.
Relaxation Plastic deformation of a material over a period of time, usually at elevated temperatures.
Remove Set
Residual Stress Stresses induced by preset, shot peening, cold working, forming etc. Residual stresses may sometimes be beneficial as in the case of shot peening and preset.
Scragging Pressing to solid to remove set.
Set Length change cased by deflection beyond the materials elastic limit.
Shot Peen Bombarding the wire surface with small steel shot to induce compressive stresses and thereby improve fatigue life.
Slenderness Ratio Ratio of spring length to mean coil diameter. The major factor in determining if a spring will buckle.
Solid Height Height of a compression spring when under sufficient load to bring all the coils into contact with adjacent coils.
Spring Index Ratio of the mean coil diameter to wire diameter.
Squared and Ground Ends See Closed and Ground Ends.
Squared Ends See Closed Ends.
Squareness of Ends Angular deviation from vertical of a compression spring when standing upright.
Squareness Under Load Same as Squareness of Ends, except with the spring under load.
Stress Relieve A low-temperature heat treatment used to relieve residual stresses.
Tensile Strength The maximum tensile stress that a material is able to take. It is equal to the maximum force required to break the material divided by the original cross-sectional area of the material.
Torque The product of the distance from a torsion spring's axis to the point where a load is applied and the component of force perpendicular to that lever arm. Torque is usually expressed in inch-pounds or in foot-pounds.
Total Number of Coils Number of active coils plus the inactive coils forming the ends.
Wahl factor A stress correction factor used to account for the effects of curvature and shear.
Yield strength The stress at which a material begins to experience "plastic deformation" or permanent deformation. Usually defined as the point at which .2% permanent elongation is experienced by the material.
Glossary of Spring Terminology
Active coils
Those coils which are free to deflect under load.
Angular relationship of ends
The relative position of the plane of the books or loops of extension springs to each other.
Baking
Heating of electroplated springs to receive hydrogen embrittlement.
Closed ends
Ends of compression springs where 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.
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.
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 a 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 torsion and flat springs (Youngís modulus).
Moment (M)
See Torque.
Open ends, not ground
End of compression spring with a constant pitch for each coil.
Open ends ground
"Open ends, not ground" followed by an end grinding operation.
Passiviating
Acid treatment of stainless steel to remove contaminants and improve corrosion resistance.
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 local is said to have taken a ìpermanent set.î
Pitch (p)
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).
Poissonís 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 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).
Squared and grounds ends
See Closed and ground ends.
Squared ends
See Closed ends.
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.
Active Coils (na) those coils which are free to deflect under load.
Angular Relationship of Ends Relative position of hooks or loops of an extension spring (or ends of a torsion spring) to each other.
Baking Heating of electroplated springs to relieve hydrogen embrittlement.
Block See Solid Height.
Buckling Bowing or lateral displacement of a compression spring. This effect is related to slenderness ratio L/D.
Close Wound Adjacent coils are touching.
Closed and Ground Ends Same as Closed Ends, except the first and last coils are ground to provide a flat bearing surface.
Closed Ends Compression spring ends with coil pitch angle reduced so they are square with the spring axis and touch the adjacent coils.
Closed Length See Solid Height.
Coils Per Inch The number of coils in one inch.
Deflection (F) Motion imparted to a spring by 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 a given stress ratio, at which material will operate in a given environment for a stated number of cycles without failure.
Fixture Tempering Restraining parts during tempering to improve dimensional control.
Free Angle Angular relationship between arms of a helical torsion spring which is not under load.
Free Length (L) Overall length of a spring which is not under load.
Gradient See Rate
Heat Setting A process to pre-relax a spring in order to improve stress relaxation resistance in service.
Helical Springs Springs made of bar stock or wire coiled into a helical form. This category includes compression, extension and torsion springs.
Hooks Open loops or ends of extension springs that are generally longer than a standard loops.
Hysteresis Mechanical energy loss occurring during loading and unloading of a spring within the elastic range. It is illustrated by the area between load-deflection curves.
Initial Tension (Pi) The force that tends to keep coils of a closewound extension spring closed and which must be overcome before the coils start to open.
Loops Circular formed ends with at the ends of extension springs that provide a means for attachment.
Mean Diameter (M) The average diameter of the mass of spring material, equal to one-half the sum of the outside and inside diameters. In a helical spring, this is the equivalent to the outside diameter minus one wire diameter.
Modulus in Shear or Torsion (G) (Modulus of Rigidity) The coefficient of stiffness used for compression and extension springs.
Modulus in Tension or Bending (E) (Young’s Modulus) The coefficient of stiffness used for torsion or flat springs.
Moment (M) A product of the distance from the spring axis to the point of load application, and the force component normal to the distance line.
Natural Frequency The lowest inherent rate of free vibration of a spring vibrating between its own ends.
Passivation An acid treatment for stainless steel which removes iron deposits and improves corrosion resistance.
Patenting The process of heating carbon steel above its critical temperature and cooling at a controlled rate to achieve a fine paralytic microstructure.
Pitch (p) Distance from center to center of wire in adjacent coils in an open-wound spring.
Plain Ends End coils of a helical spring having a constant pitch and ends not squared.
Plain Ends, Ground Same as Plain Ends, except wire ends are ground square with the axis.
Preset See Set Removal
Rate (R) The change in load per unit of deflection. Generally expressed as Lbs/in or N/mm
Residual Stress Stress mechanically induced by such means as set removal, shot-peening, cold working, or forming. It may be beneficial or not, depending on the spring application.
Set Permanent Change of length, height or position after a spring is stressed beyond material’s elastic limit.
Set Point Stress at which some arbitrarily chosen amount of set (usually 2%) occurs. Set percentage is the set divided by the deflection which produced it.
Set Removal An operation which causes a permanent loss of length or height due to spring deflection.
Shot-Peening Blasting the surfaces of spring material with steel or glass pellets to induce compressive stresses that improve fatigue life.
Slenderness Ratio Ratio of spring length to mean diameter L/D in helical springs.
Solid Height (H) Length of a compression spring when deflected under sufficient load to bring all adjacent coils into contact - no additional deflection is possible.
Spiral Springs Springs formed from flat strip or wire wound in the form of a spiral, loaded by torque about an axis normal to the plane of the spiral.
Spring Index Ratio of mean diameter to wire diameter.
Squared and Ground Ends See Closed and Ground Ends.
Squared Ends See Closed Ends.
Squareness Angular deviation, between the axis of a compression spring in a free state and a line normal to the end planes.
Squareness Under Load same as Squareness but measured while there is a load applied to the spring.
Stress Range Difference in operating stresses at minimum and maximum loads.
Stress Ratio Minimum stress divided by maximum stress.
Stress Relief A low temperature heat treatment given springs to relieve residual stresses produced by prior cold forming.
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. Generally expressed as in-lbs or N-mm. also see Moment
Total Number of Coils (Nt) The sum of the number of active and inactive coils in a spring body.
Wahl Factor A factor to correct stress in helical springs effects of curvatures and direct shear.
