Truss and motor leveling

[ambientlight816]

Alright, again I work for a theatre that has less than qualified staff.. My boss is arguing that truss should only need to be leveled at working height then raised and it will stay level. I have always been taught to level the truss once in its final position because some motors run faster/slower than others, Plus it's a good safety check. Am I correct in saying this?! Why wouldn't you want to recheck the truss level?! I always laser to every point to make sure every motor has tension and is level. She puts a level on the truss and calls it good even though it doesn't account for the middle motor... Can anyone give me fancy documentation that states how it should be done? Anything will help! Thanks!

 

[BillConnerFASTC]

If you have just two motors on the truss, it's probably OK. More than two - just about impossible and always unsafe IMHO - as better explained here: http://www.eilon-engineering.com/code/stage-safety.html. So over two picks unless every motor is rated for 100% of the load, I'd recommend (insist on) load cells.

 

[porkchop]

I can assure you that motors run at different speeds so you should be concerned with level at working height and at trim. Small differences in the motors will cause measurable differences in speed and if you have uneven loads on a motor system (like having the cabling come off only one side) the motors with more load will raise slower and lower faster. I'm talking about a difference of a few inches as you raise a complex system 30 feet, but it is enough to unload a motor in the system. If you're raising something close to motor capacity and going a ways with it, a half way level check isn't a bad idea.
IMHO you can make a level work to check your truss in a few very specific situations, but normally I'd prefer a laser disto. Fabric tape measures attached with velco to important parts of the truss are also not unheard of but not a very reliable if you ask me.
You do have to consider how much you're lifting vs. what the motors are rated for. You'll see cable picks on touring shows at all kinds of wonky angles and that works because (generally) the hoists are each individually rated for more than 100% of the total load. Also those rigs are generally engineered before hand and measured with load cells once built so they have a lot better idea whats going on up there.

 

[Footer]

That and a difference in cable length/line loss to each motor can effect how fast each spin. Always best to check each point, especially when your working with 3 points.

 

[soundman]

Its a good start to level at working height but you still need to level at trim., unless your motors are encoded. Motors run out of sync and if there is any tow with the chain it will effect the speed, as will how quickly the brake releases and closes. I am a fan of tape measure because I find them faster than laser distos. 1/2" tolerance is fine for most things I do anyway. Trying to get much closer than that with out a rotatory head laser is pointless as a floor can be out of level by that much or more. Like all things rigging use your head, have someone spot the truss as it is going up. If it is a long lift you might have to stop and level it a few times. The more you do it the better you get, after a while you can see and hear subtle things that give clues about how much load is on a motor. Not as accurate as load cells but most operations a visual on the truss and a quick run down the tapes will keep everything in line.

Load cells on every truss over three points might be a bit much. If the weight is that close I would add another point, I like to keep about 20% of the motor capacity as head room just for this sort of thing.