Glossary of Spring Terms
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 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 slenderness ratio (L/D).
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.
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 (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 cycle per second) with ends restrained.
Gradient: see Rate (R)
Heat setting: Fixturing a spring atelevated temperature to minimize loss of load at operating temperature.
Helix: The spiral for (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: 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: A force that tends to keep coils of a close-wound extension spring closed and which must be overcome before the coils start to open.
Loops: Formed ends with minimal gaps at the ends of extension springs.
Mean Diameter (D): 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 G) Coefficient of stiffness used for compression and extension springs.
Modulus in Tension or Bending (E): (Young Modulus E) Coefficient or 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 (n): Lowest inherent rate of free vibration of a spring vibrating between its own ends.
Patenting: The process of heating carbon steel above its critical temperature and cooling at a controlled rate to achieve a fine pearlitic 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.
Rate (R): Spring gradient, or change in load per unit of deflection.
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 the 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 (LS): 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 (C): 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.
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): See Moment.
Total Number of Coils (N): The sum of the number of active and inactive coils in a spring body.