You are right, the arbors are set to limit the load. To quote Wikipedia "
Fly system infrastructure consists of the relatively permanent load-bearing and load-transferring structures of a
stage house. The infrastructure, generally fabricated of structural steel members, is sized by a
structural engineer during the design of a new
theatre, or
theatre renovation. Rigging
system infrastructure ultimately limits a fly
system's capacity." To know if you can add more weight, you must know the total load and how it is distributed. If a
point load is on one cable past the
loft block to the
arbor, what is the safe working limit? In other words, there are specific limits on every component, from the bottom of the piece being flown along each pick
point all the way to the
arbor. I would suggest to first check the manufacture of the
arbor system for the load limits. Then find the safe working limit based on your
layout. If you or your boss do not know how to calculate this, bring in a professional rigger.
Ask this question, if this fails what will happen? That overweight load will come crashing down in about 4 seconds, accelerating at 32 ft/sec.