Speciality Yarns
Two strands, one spindle, and an angle that decides whether it is a ply or a lump.
— deg
Full included angle at the convergence point
The convergence geometry treats both strands as meeting at a single point in a plane, which is a good approximation for the angle and ignores the twist-driven spiralling that actually occurs above the convergence. Strand tex assumes the two rovings are equal, which is the normal arrangement but not the only one - deliberately unequal strands are used for effect yarns and this calculation does not cover them. Strand diameter uses the bulk density of the drafted strand, which is lower than the finished yarn density since the strand is untwisted at that point, so it is indicative rather than measured. The ply balance index is a comparative figure formed from twist and spacing, with no units and no absolute meaning.
Siro Spinning Strand Spacing, Convergence Angle & Twist Balance — free, with the formula and a worked example, at Textile School.