When it comes to a torsional spring, its deflection is rotational. This means that when one of the torsion spring’s legs are in motion, the torsion spring’s body is being tightened. Therefore, since the spring is being torqued and tightened, the torsional spring diameter shrinks. Because torsion springs will usually be placed over a shaft, you want to make sure that the spring’s inner diameter isn’t gonna shrink to the point where it grips (bites down) on the mandrel or shaft. An example of torsion spring deflection is shown to the right as well.
Loaded ID = N ÷ (N + REVs) x ID
N = Number of Coils
REVs = Leg Revolutions
Loaded ID = Loaded Inner Diameter
I.D. = Unloaded Inner Diameter