What kind of tolerances can be imposed on a design?
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Tolerances should allow spring makers to fabricate quality parts by ordinary production means in the most economical way. Therefore, the critical dimensions should be determined early in the engineering process to allow the spring maker to adjust or compensate for allowable variations in both wire size and mechanical properties.
Product designers often impose the standard tolerance box for machined dimensions on spring designs&#;which proves impractical based on history and the nature of spring materials. Such boxes must be reviewed, and more realistic tolerances applied.
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The suggested maximum-allowable stress value for an extension spring&#;s body wire ranges between 30 and 45 percent of the material&#;s minimum tensile strength (MTS). The suggested percentage values of the MTS to be used vary with the material type, and the MTS values for a given material vary with the wire diameter. Stresses in the spring hooks are normally higher than in the spring body because there is a bending stress in addition to the wire-torsion stress, specifically in the transition region between the last body coil and the hook. Therefore, an over-stressed extension spring can be expected to fail at the hook first. The suggested allowable hook stress in torsion is 30-45 percent (depending on the material) of its MTS, while that of bending is 75 percent of its MTS.
An estimate of the total stress of a common extension spring hook or loop can be determined with the information shown on the right.
There is an increment of stress which is due to the initial tension that adds to the stress complexity; this should be accounted for in many cases. The initial tension causes the spring body to be under constant stress, which roughly varies with the spring mean diameter to wire diameter ratio. Reference the figures below for a visual representation of stress due to initial tension as well as a chart showing the Wahl curvature stress correction.
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