[SML] Rigging calculation question...

[Kristi R-C]

This is a great engineer vs mathematician question1

Yes, we could be a mathematician and figure out the exact weight - and if it's critical to know, load cells would confirm or adjust the answer, but the person asking just needs to be close enough to engineer it to make it work safely. We always use a safety factor to derate the stated capacity of each item in the rig and chain motors only come in certain sizes, so it's fairly easy to assume a constant load (which we know it isn't but for the purpose of our task, it's good enough) and provide an abundant safety factor to cover the fact the weight calculation is not exact. And as you noted, there will be some actual differences in what the motor is carrying vs. what the math says it is because of the slight differences in how each motor runs, so a more generous safety factor, again, covers the inaccuracies in the calculations. 

If the question was "how much can my roof/grid support?" then a phone call to a structural engineer is certainly in order. I don't think we have one here.

"Do I need half ton, one ton, or two ton motors if I'm hanging X pounds over Y equal points on Z kind of truss?" is something many of us can do the math to figure out. I'd hate for someone to not ask a question because they are "not qualified." Being smart enough to ask when they are not 100% confident puts them into the qualified category in my book. 

Kristi R-C

Bill Conner said:


Well, not trying to be mean but if you have to ask, you're not qualified to do it.  If the screen is wider than the middle 4 points, then the center two are carrying a majority and the next one each side less, and the long and short nearly nothing. That all assumes the controls are perfect.
A licensed structural engineer would be a good idea.



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