I’m going to run Harry’s numbers on a point that I felt was overloaded, and just see what comes up. Center point on an upstage video wall. Three one ton motors. All hung on 3/8″ steel. The truss itself is massive 30″ truss and weights much more than the regular 30″ truss we all know. Total calculated weight for the truss and the video wall is 3,750. The center point should see 1,875 lbs, and each end should see about 938 lbs (not counting motor weight and cables.
Everything is hung on 3/8″ cable (14,400 lb breaking strength) and hung right to the basket hitch. The hitch is wrapped tight around the beam, so I figure it’s good to about 150% of capacity (less than 90 degrees, more than 60). So the breaking strength of the basket hitch is round about 21,000 lbs. ( I will check the safety factor on the shackles next, but they should be good) .
A load on a 16fpm motor going down will see as much as a 50% increase in dynamic load. I’m not 100% sure that 50% is correct, but it’s the larger of the 2 numbers I have seen, so I’m running this with the larger number. The 1875 lbs center point would see a dynamic load of about 2,800 lbs. That number is over rated for the motor, but the motor will still pick it and not slip and is still safe because it’s well with in the SWL of the motor.
We take the 21,000 lbs rated hitch, and divide by the 2,800 lbs dynamic load and we see a safety factor of 7.5.
Just for fun if we take that 21,000 lbs and divide by where the load protector kicks in (say if the motor gets behind and takes full weight) then the safety factor was still 5.6.
So it looks like I was wrong, and the point was within a safety factor of 5. I did not think it was within the safety factor. Still, next time this comes up, I want to do it with 1/2″ steel. That was a bunch of weight on 3/8″ and I still don’t like it.
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