Engineering tool
Extension spring calculator
Rate and body stress for a close-wound tension spring. Hooks add a little flexibility; they also fail first.
Results
Enter a wire diameter greater than 0.
Method
Mean diameter D = OD − d. Active coils Na = Nb + G / E, which credits the hooks with a little extra flexibility. Rate uses the same formula as compression:k = G · d⁴ / (8 · D³ · Na).
Load at extension x is F = Fi + k · x. Body length is (Nb + 1) · d. Body shear stress uses the Wahl factor. Hook stress is higher than body stress — the hook usually fails first — so treat the body figure as a lower bound.
G and E are typical published values. Wire outside Ø 0.40–2.50 mm is flagged because that is our standard range.
Questions
Why are active coils more than body coils?
Why do you warn about hook stress?
What is initial tension?
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