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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => LAB Lounge => Topic started by: Al Rettich on July 16, 2018, 02:46:30 pm

Title: Please be aware of your rigging points.
Post by: Al Rettich on July 16, 2018, 02:46:30 pm
Hey everyone.  Iím hoping by attaching this photo when you are at your gig you wonít rig your PA this way.  This is a K1/K2 array by my bare eye guess Iím saying 1900-2000 pounds.  To get a front and back point they took a five foot piece of truss and clamped from the top.  Again itís been awhile, but I think the last time I checked the heavy duty clamps were good for 1100lbs. Looking at this photo front and back is good for 2200lbs.  Yet if youíve been rigging for awhile you know the load we worry about is shock. I ran into a similar situation like this and the clamps were rated for 600lbs each.  Please be safe out there. 
Title: Re: Please be aware of your rigging points.
Post by: Mike Pyle on July 17, 2018, 11:04:56 am
It would appear the load is resting on top of the clamps, not suspended by them.
Title: Re: Please be aware of your rigging points.
Post by: Al Rettich on July 17, 2018, 10:17:55 pm
Correct, but as the Pa goes up in the air itís pulling from the back to the front or vice Versa.  What happens if a problem with the front motor, and the full amount of the PA is resting on either the front or back point.  Think those clamps would hold 2000lbs of weight??
It would appear the load is resting on top of the clamps, not suspended by them.
Title: Re: Please be aware of your rigging points.
Post by: Milt Hathaway on July 17, 2018, 10:26:48 pm
Think those clamps would hold 2000lbs of weight??

In compression? Certainly. And much more than the thinner aluminum pipes they are sitting between.

I'm betting all ratings for double-cheeseboroughs are either in tension parallel to the connecting pin or the shear strength of the pin itself, whichever is lowest. Either of these is certainly lower then straight compression.
Title: Re: Please be aware of your rigging points.
Post by: Jason Glass on July 17, 2018, 11:11:24 pm
In compression? Certainly. And much more than the thinner aluminum pipes they are sitting between.

I'm betting all ratings for double-cheeseboroughs are either in tension parallel to the connecting pin or the shear strength of the pin itself, whichever is lowest. Either of these is certainly lower then straight compression.
If either one of those motors or chains lets go, one pair of clamps instantly becomes a fulcrum and the other pair goes into tension, with the mass on a cantilever, multiplying its load on the tensioning clamps.  Think see-saw.  Same situation if strong wind starts swinging the PA.

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Title: Re: Please be aware of your rigging points.
Post by: brian maddox on July 17, 2018, 11:25:03 pm
If either one of those motors or chains lets go, one pair of clamps instantly becomes a fulcrum and the other pair goes into tension, with the mass on a cantilever, multiplying its load on the tensioning clamps.  Think see-saw.

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I went back and looked at the picture and then imagined the downstage point giving way.  The extra leverage and the resulting torsional force on that upper truss.  The PA Swinging down and back and the centripetal forces involved.

*shudder.... 

Yeah, that would break things and come down hard, I donít care what the rating of the cheesboroughs is.

It bears repeating.  Safe rigging isnít figuring out what will be safe if everything holds.  Itís figuring out what will still be safe when something fails.