Double-purchase fly system question

Caroline

Member
I just got a gig working fly (solo) for a ballet, but when I mentioned the venue to my TD at school he said that the venue has a double purchase system and that I wouldn't be able to operate it alone. He worked a show there last week and they had to hire a second fly op because he couldn't do it on his own. He said that if he jumped and pulled on it with all his body weight (easily 200lbs) that it would still barely move, and he's definitely stronger than I am.
I've only ever operated single-purchase and I can do that no problem. While I've heard that double purchase can be more difficult to operate, it sounds like there's something very wrong with their system if a grown man can't operate it single handedly. It must be weighted wrong, right? But load-in is in a week (and the ballet company is not allowed to be in the theatre until then) and there's no way I'll have time to re-weight their entire system (and I'm unsure if I'd even be allowed to, since I'm a contractor for a company renting space from this venue). I'm strongly considering backing out of this gig, partially because I'm clearly not strong enough to operate their system solo, and I'm concerned that their system is improperly weighted and unsafe. Thoughts?
 
Hmm, if the system is weighted properly and the brakes are properly adjusted a 10 year old should be able to move the battens. Surely they are not loading in a show with only you on the rail though are they? at a minimum you should have two people ion the rail for a load-in.
 
Double purchase should not inherently be a lot more difficult to operate, not like you describe at least. The harder part is having to load or unload double the weight. Not sure how you load in without loading. Sounds like a lot more wrong than just being double purchased. Also sounds like a friction problem rather than out of balance. Something about his is yet to be revealed.
 
Sounds like they aren't removing the pig irons after a load out and just leaving whatever weight was on their from previous performances.
 
I'm definitely the only one operating the fly during the show, I honestly have no clue if they plan on having more people on the rail for load-in.
For any sane load in / out, I'd expect at least one person on the operating floor AND at least one person on the loading / unloading floor. Please enlighten these folks and invite them out of the dark ages. If their arbors are extremely long, so long that their system includes two loading / unloading floors, then I'd expect to have even one more person on hand on the second loading / unloading floor.
Toodleoo!
Ron Hebbard.
 
We need to know if it was hard to operate when balanced or because it was not balanced. Easy to tell - equally hard in both directions or one way only, and easy the other. I sensed it hard both ways, but that is a guess based on original post.
 
I just got a gig working fly (solo) for a ballet, but when I mentioned the venue to my TD at school he said that the venue has a double purchase system and that I wouldn't be able to operate it alone. He worked a show there last week and they had to hire a second fly op because he couldn't do it on his own. He said that if he jumped and pulled on it with all his body weight (easily 200lbs) that it would still barely move, and he's definitely stronger than I am.
I've only ever operated single-purchase and I can do that no problem. While I've heard that double purchase can be more difficult to operate, it sounds like there's something very wrong with their system if a grown man can't operate it single handedly. It must be weighted wrong, right? But load-in is in a week (and the ballet company is not allowed to be in the theatre until then) and there's no way I'll have time to re-weight their entire system (and I'm unsure if I'd even be allowed to, since I'm a contractor for a company renting space from this venue). I'm strongly considering backing out of this gig, partially because I'm clearly not strong enough to operate their system solo, and I'm concerned that their system is improperly weighted and unsafe. Thoughts?
Just to belabor this a bit:
With a normal, single purchase, counter-weighted fly system; if you have a 1,000 pound load, you add 1,000 pounds of counter-weight and the load travels the same distance as the counter-weight.
With a normal, double purchase, counter-weighted fly system; if you have a 1,000 pound load, you add 2,000 pounds of counter-weight (which is why the arbors are often twice as long and they have two loading floors) and the counter-weight only travels half as far as the load.
Ignoring friction and losses from sheave bearings and the flexing of galvanized cables, in the case of a single purchase system, if you have a 1,000 pound load you're moving and stopping 2,000 pounds while in the case of a double purchase system, if you have a 1,000 pound load you're moving and stopping 3,000 pounds. Appreciate there are a few irrefutable laws of physics in play here. Ignoring all losses, 2,000 pounds represents a sizeable chunk of inertia at rest to put into motion and a sizeable chunk of inertia in motion to bring to a halt. In the case of the example cited above, 3,000 pounds is clearly a larger lump of inertia at rest, or in motion, to have to deal with. Balanced or not, these are still pretty ugly amounts of weight to move, accelerate, decellerate and halt plus there are still additional amounts of friction and such to be taken into account.
I'm not sure I want to meet that '10 year old' a previous poster was referencing.
I'll relinquish the lectern and watch the further comments from the comfort of my desk chair.
Toodleoo!
Ron Hebbard.
 
Just to belabor this a bit:
With a normal, single purchase, counter-weighted fly system; if you have a 1,000 pound load, you add 1,000 pounds of counter-weight and the load travels the same distance as the counter-weight.
With a normal, double purchase, counter-weighted fly system; if you have a 1,000 pound load, you add 2,000 pounds of counter-weight (which is why the arbors are often twice as long and they have two loading floors) and the counter-weight only travels half as far as the load.
Ignoring friction and losses from sheave bearings and the flexing of galvanized cables, in the case of a single purchase system, if you have a 1,000 pound load you're moving and stopping 2,000 pounds while in the case of a double purchase system, if you have a 1,000 pound load you're moving and stopping 3,000 pounds. Appreciate there are a few irrefutable laws of physics in play here. Ignoring all losses, 2,000 pounds represents a sizeable chunk of inertia at rest to put into motion and a sizeable chunk of inertia in motion to bring to a halt. In the case of the example cited above, 3,000 pounds is clearly a larger lump of inertia at rest, or in motion, to have to deal with. Balanced or not, these are still pretty ugly amounts of weight to move, accelerate, decellerate and halt plus there are still additional amounts of friction and such to be taken into account.
I'm not sure I want to meet that '10 year old' a previous poster was referencing.
I'll relinquish the lectern and watch the further comments from the comfort of my desk chair.
Toodleoo!
Ron Hebbard.
As far as starting and stopping the load and inertia, don't overlook the fact that when you move the handline, the arbor is moving at half the speed and distance. The operator is doing the same amount of work. The same amount of weight is moved the same distance, albeit one part is twice as heavy but moving half the distance. The added friction of the additional sheaves on the arbor and additional flexing of both lift lines and handline us the difference in operating. All in all, about like a back muled set.
 
And of course, if it's a long pipe with lots of lift lines, as well as a heavy load, it's sometimes a bear to get moving.
 

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