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Glossary of Spring Terminology Active Coils 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 Length of a compression spring when deflected under sufficient load to bring all adjacent coils into contact. 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 Length of a compression spring when deflected under sufficient load to bring all adjacent coils into contact. Coils Per Inch Distance from center to center of wire in adjacent coils in an open-wound spring. 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. 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 Overall length of a spring which is not under load. Gradient Spring gradient, or change in load per unit of deflection. 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 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 (Modulus of Rigidity G) Coefficient of stiffness used for compression and extension 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. Patenting The process of heating carbon steel above its critical temperature and cooling at a controlled rate to achieve a final pearlitic microstructure. 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 ends not square. Plain Ends, Ground Same as Plain Ends, except 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 materials elastic limit. Set Point Stress at which some arbitrary 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 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 Length of a compression spring when deflected under sufficient load 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 Same as Closed Ends, except the first and last coils are ground to provide a flat bearing surface. Squared Ends Compression spring ends with coil pitch angle reduced so they are square with the spring axis and touch the adjacent coils. 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 A product of the distance from the spring axis to the point of load application, and the force component normal to the distance line. Total Number of Coils The sum of the number of active and inactive coils in a spring body. Bibliographical References
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