Machinery's Handbook, 31st Edition
326 Spring Design extension springs only. Hot-wound springs and springs made from annealed steel are hardened and tempered after coiling, and therefore initial tension cannot be produced. It is possible to make a spring having initial tension only when a high tensile strength, obtained by cold drawing or by heat treatment, is possessed by the material as it is being wound into springs. Materials that possess the required characteristics for the manufac- ture of such springs include hard-drawn wire, music wire, pre-tempered wire, 18-8 stain- less steel, phosphor-bronze, and many of the hard-drawn copper-nickel, and nonferrous alloys. Permissible torsional stresses resulting from initial tension for different spring indexes are shown in Fig. 16.
44 42 40 38 36 34 32 30 28 26 24 22 20 18 16 14 12 10
The values in the curves in the chart are for springs made from spring steel. They should be reduced 15 percent for stainless steel. 20 percent for copper-nickel alloys and 50 percent for phosphor bronze.
Initial tension in this area is readily obtainable. Use whenever possible.
Maximum initial tension
8 6 4 3 4 5 6 7 8 9 10 Spring Index Inital tension in this area is difficult to maintain with accurate and uniform results.
11 12 13 14 15 16
Fig. 16. Permissible Torsional Stress Caused by Initial Tension in Coiled Extension Springs for Different Spring Indexes
Hook failure: The great majority of breakages in extension springs occurs in the hooks. Hooks are subjected to both bending and torsional stresses and have higher stresses than the coils in the spring. Stresses in regular hooks: The calculations for the stresses in hooks are quite complicated and lengthy. Also, the radii of the bends are difficult to determine and frequently vary be- tween specifications and actual production samples. However, regular hooks are more highly stressed than the coils in the body and are subjected to a bending stress at section B
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