The number of active turns in a torsion spring is equal to the number of body turns, plus a contribution from the ends. For straight torsion ends, this contribution is equal to 1/3 of the moment arms and is usually expressed as an equivalent number of turns:
Ne = (L1 + L2) / 3∏D
L1 = length of the moment arm of the first end
L2 = length of the moment arm of the second end
Na = Nb + Ne
Nb = number of body turns
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