active coils: those coils which are free to deflect under load.
baking: heating of electroplated springs to relieve hydrogen embrittlement.
buckling: bowing or lateral displacement of a compression spring; this effect is related to slenderness ratio L/D.
closed and ground ends: same as closed ends, except that the first and last coils are ground to provide a flat bearing surface.
closed ends: compression spring ends with coil pitch angle reduced so that they are square with the spring axis and touch the adjacent coils.
close-wound: wound so that adjacent coils are touching.
deflection: 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.
free angle: angular relationship between arms of a helical torsion spring which is not under load.
free length: overall length of a spring which is not under load.
gradient: see rate.
heat setting: a process to prerelax 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.
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: in a helical spring, the outside diameter minus one wire diameter.
modulus in shear or torsion (modulus of rigidity G): coefficient of stiffness used for compression and extension springs.
modulus in tension or bending (Young’s modulus E): coefficient of stiffness used for torsion or flat springs.
moment: 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: lowest inherent rate of free vibration of a spring vibrating between its own ends.
pitch: 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 with the ends not squared.
plain ends, ground: same as plain ends, except that wire ends are ground square with the axis.
rate: 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 material’s elastic limit.
set point: stress at which some arbitrarily chosen amount of set (usually 2 percent)
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 because of spring deflection.
solid height: length of a compression spring when deflected under load sufficient to bring all adjacent coils into contact.
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.
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: see moment.
total number of coils: the sum of the number of active and inactive coils in a spring
body.
Robert E. Joerres
Applications Engineering Manager
Associated Spring, Barnes Group, Inc.
Bristol, Connecticut
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