[Edited 7 Dec 2014: revised sketch/image to clarify gap, G.]
Needed to compute the belt length for a three leadscrew + driver motor setup.
While I did this for the leadscrews at the vertices of an equilateral triangle,
it's trivial to generalize the math to other configurations (e.g., the 3*l1 term
becomes l1 + l2 + l3).
The diameters of the pulleys and idlers ideally should be pitch diameters: the
diameter of the belt center line when mounted on the pulley or idler. SDP/SI
provides that diameter with their pulley specs. Likely other major suppliers