Safely Unloading Cradles

[Glyn Edwards]

Afternoon all,

 

I have a bit of a query about the best way to empty a cradle in a slightly unusual situation...

 

Our counterweight sets are single purchase,

 

The set for our latest show consists of "trees" that are clamped to our bars. These trees are rigid boxes of ply on a 3x1 frame, standing approx 7.2m tall. At the top of each is a scaff pole with two 90 deg clamps that are clamped to the two rails of our ladder bar so that the trees don't wobble etc. Our grid is 13m abouve the floor.

 

To get the set up, the crew loaded the cradles on the loading gallery, then slipped them through the brakes / clips until the bar was far enough out to fasten the scaff to. Next they moved along the bar to fasten the next tree etc. The total weight of a bar full of trees is in the region of 350kg.

 

My concern is how we can safely get the trees out again. We could remove some of the weight at fly gallery height, but it is unlikely we could reduce the load to much below 280kg. We can't treat the trees like a big frenchman since they are rigidly attached to the bar. The stage suggestion was to basically work in reverse, remove the trees then haul the bar in to the deck so the cradle ends up in the grid for unloading. I'm not keen on this idea since it strikes me that the chances of controlling the set when it is so far out of balance is minimal. Even if this were possible I'd be concerned about damaging the tube of the bars. The other suggestion they made was to slip the bars out once they were empty and then unload the cradles at stage level. If we do this then obviously we need to get the weight back up to the gallery, which will involve a lot of hauling. (around 130okg total weight in 30kg loads doesn't strike me as a fun way to spend an evening.)

 

My current thinking is that the safest way to achieve this is to rig a multiple purchase tackle between the header beam and the cradle so we can haul the cradle out once the trees are removed.

 

Does anyone have any other thoughts, suggestions or observations?

[themadhippy]

coouldnt you take off 1 of the scaff clips,and loosen the other so it slips ,bring the bar in slowly whilst pulling the trees away,deck the bar and unload

[Glyn Edwards]

Sadly (and for reasons unknown to me) our ladder beam is 51mm dia, but the workshop chose to use 48mm scaff fittings, which only just fit the ladder. Consequently any attempt to spin the clamp round the bar is likely to completely muller the tube :(

 

Apologies, I should have mentioned this in the original post.

[indyld]

Have to say, I like the idea of rigging up some kind of random x-purchase block to haul the cradle out even less than a conventional bar overhaul. If you wanted to assist the cradle, a motor at least, but of course the rigging of that would need to be done properly.

 

I'm not clear which you are concerned most about: How to strike the trees, or how to deal with the imbalance.

 

If the latter, I'm struggling to see how this differs from any other assisted overhaul, with a stout two or three doubled lines over the bar, haul it in with assistance from stage and hold it there while they go bananas in the loading gallery.

 

Some of the guys in one of our local No1 house can de-weight at a phenomenal rate, you can hear weights coming off faster than you can imagine.

 

I personally dislike relying on brake slippage and usually try to get some deck assistance in this direction also.

 

E2A: Have to say, having started out in theatre flying and then spent time in the event rigging world, some practices that still survive in theatre give me the wobbles. Slipping overweighted cradles through the brake designed to simply meet the needs of keeping the *control* line of a normally balanced set fast is one of those. Hence my requirement for plenty of deck assistance.

[OllieDuff]

Do you need the set again after this out? If not, is there any scope for partially deconstructing the set in the air? I'm thinking remove 4ft of structure, take out that amount of weight, fly in by 4ft, repeat until set is light enough to haul in.

 

Ollie

[indyld]

Do you need the set again after this out? If not, is there any scope for partially deconstructing the set in the air? I'm thinking remove 4ft of structure, take out that amount of weight, fly in by 4ft, repeat until set is light enough to haul in.

 

Ollie

 

The issue with that, is without an intermediate gallery, how to do you get to the cradle? Plenty of theatres only have two levels: fly and loading. With a single purchase lineset, some of the stopping off points are likely to be below fly floor level.

 

E2A: (One day I'll make a post without having to edit it for typos or an additional idea)

 

The issue of getting weights from deck level, assuming you can actually access the cradles and they aren't in a cage, can be facilitated using a loaded bar with a few weights short that will fly the full travel. This might be a disconnected LX bar or a header/border. OK, so if you do a few at a time, it's a few trips but it may be preferred to a heavy haul.

[Seano]

If you wanted to assist the cradle, a motor at least, but of course the rigging of that would need to be done properly.

 

I'm thinking motors too, but unless access to the cradle is unusually straightforward I think I'd be more inclined to overhaul the bar itself.

 

Is there space between 'trees' on the bar?

If so, I think I'd look at hiring in some CM Prostars. (Four seems like a good number - its not unusual for them to travel 4 to a flightcase, and of course 4 way controllers are also common.)

 

Suggested sequence of events:

• 
   
  
• Bring trees in to the deck.
  
• Rig 4x prostars on the bar at suitable intervals (they're effectively going to become the payload when the trees come off so it'd be good to distribute the load along the bar).
  
• Float motors. Bring 100kg ballast into position beneath each hoist. (eg: a suitably slung stack of stageweights, or several brought together at a master link)
  
• Run slack out of hoists, release counterweight brake and carefully take up tension.
  
• De-rig trees (paying attention to any movement as clamps are released and adjusting motors accordingly as required).
  
• When trees are clear run motors to haul bar in to deck and unload cradle.
  
• Run the motors all the way back out again before packing them away.
  (This bit added on behalf of the supplier - or their next client if they're one of those outfits too shabby to fettle the motors between hires, but I'm sure you wouldn't be doing business with one of those companies. ;))
   
  

If you have any doubts at all, you should probably hire a rigger. (In which case I'd suggest giving UK Rigging a call with a view to hiring rigger + kit.)

 

Another option might be to use some 1/2t manual blocks in place of motors.

On the downside they're slightly more painful to rig, its a bit more labour intensive and can be difficult to keep them nicely synchronised as you haul.

On the upside they're likely to be a bit cheaper, and individually you can adjust them as little, and as slowly, as you like.

 

I personally dislike relying on brake slippage and usually try to get some deck assistance in this direction also.

Me too. Its a bit smelly.

 

Though the problem with getting plenty of bodies around it on the deck is that it conflicts with the need - from an 'elf'n'safety' point of view - to minimise the number of bodies around (and especially underneath) work taking place on the bar. Better, perhaps, to use a method similar to what I've described above.

[indyld]

I'd pretty much discounted the idea of a top rigged motor because as Seano says, access and point availability above the cradle is never good for temporary lifts (a bit different to counterweight assist winch systems which don't need room for spansets, wraps, bridles etc. and - the motor. Using the motor/bar system (which I'd also poo poo'ed due to not being able to think of a suitable ground anchor point) you could dismantle piece meal at leisure, with the knowledge that no one had to rush and let the motors take the effort out of it. The weak point here for me is proper anchorage at deck level and the motor chain not fouling things you don't want it to foul ie. the trees.

 

Though the problem with getting plenty of bodies around it on the deck is that it conflicts with the need - from an 'elf'n'safety' point of view - to minimise the number of bodies around (and especially underneath) work taking place on the bar. Better, perhaps, to use a method similar to what I've described above.

 

Indeed, another issue that one could have with standard theatre practice, not mitigating risks to personnel by distancing them from the lifting operation where possible. Although a bit of a best practice fan, I'm kinda ok with traditional overhauls done by well rehearsed teams. But I take the point.

 

Have to say, the old Rock n Roll addage of "remembering the 'out" during the build seems to have gone unheeded in this case generally ** laughs out loud **.

[paulears]

My venue is always ar5e about face. Our problem would have been getting this one out! Our cradles are loaded from stage level, and to load/unload when the cradle is fully out is impossible, but by the time the bar is about 3m above the stage, you can reach the cradle by standing on the fly rail. This isn't so scary as it sounds because the fly rail is off-stage of the rear of the cradles. So you can't fall to the stage because the steelwork is in the way. However, loading can only be done with one hand so is pretty unpleasant. At our venue we spend far too much time loading a bar with a couple of movers, heaving it right out to add over weight, then hauling it back in again - sometimes 3 times for a bar near the 250Kg load limit.

[indyld]

My venue is always ar5e about face. Our problem would have been getting this one out! Our cradles are loaded from stage level, and to load/unload when the cradle is fully out is impossible...

 

Wow, great. A counterweight system that you have to keep hauling all over the place. So, you can load gridded empty bars up to the max, or get to haul out whatever load you've just slung on the bar, until the cradle reaches you, gridded and way past it's final dead. At which point does the concept of counter, er, weighting, become useful in this system?

 

It sounds like a hemp house only made especially harder with the addition of extra weights, should the user not feel like going to the gym that week. Special mention the designers of that, eh, FFS.

 

One other point to add to Seano's suggestions, added for the benefit of others not just the OP. I really wouldn't skimp and try this with manual chain blocks.

 

The likelihood of the whole cradle weight sitting on one point is just too much for me. I can't believe some of the lifts I've seen done on chain blocks with a coctail of slack chains, centre points taking all the weight plus the cantilever, sagging, hogging, dogging (OK, not that last one), I'm gonna have to stop thinking about it as I need to sleep tonight.

 

I think that the key point to any of these operations, is that they need to be undertaken properly by experienced peeps and that perhaps the method the crew are used to, presumably overhauling in the traditional manner, might be preferred if no rigger can be found. Set against the notion of 4 stacks of stageweights, 4 Pro Stars and associated cabling plus the motor controller and crew clinging onto the remote, all dangling from a speedily gridded Bar 15....

[Giles Favell]

Some counterweight cradles and systems are designed with lifting points at the top for motor assist - so if yours have this, it is an indication that it could be a reasonable route to persue. Or not (without specialist help)

Rope locks are designed to hold - from stationary - 30% of the max cradle load, before slipping. however, such is the nature of things, once you start slipping, you won't be able to stop the same 30% - therefore slipping through the brake as a technique is hazardous to say the least. (incidentally, I started my professional life flying, so I'm familiar with the practice as well as the theory!). The requirement for the rope lock holding 30% of the cradle's capacity is in BS 7905.

 

[indyld]

Rope locks are designed to hold - from stationary - 30% of the max cradle load, before slipping. however, such is the nature of things, once you start slipping, you won't be able to stop the same 30% - therefore slipping through the brake as a technique is hazardous to say the least. (incidentally, I started my professional life flying, so I'm familiar with the practice as well as the theory!). The requirement for the rope lock holding 30% of the cradle's capacity is in BS 7905.

 

Quite, from stationary. So as soon as you start to increase the acceleration, down the capacity! By the time your hand has slipped and the load is starting to go out of control, the likelihood of regaining it slims pretty quick. 30% isn't much either really is it, whatever the max cradle weight is (presumably we are talking about actually total cradle weight and not just payload). It's enough to secure something that's a a bit out of balance, maybe even a bit overweight, and to break small dynamic loads - if anyone actually does that. But not actually a serious overhaul which is why various rope brakes are also employed for a temporarily unbalanced lineset. (BTW, Giles, I'm not trying to tell you all this, just mentioning it for BR next generation type peeps)

 

I'd be interested to know if, as the brakes are reasonably symmetrical, they are good for that 30% static load in either direction, as would be the case if you underloaded for some reason during the process. The rail system would also need to be good for such a down pull, as well as an upward one.

[paulears]

It's fairly standard for us to need two people to get a part weighted bat down to stage level to add things. Add a third person to work the brake. The system is fine for cloths, which once up can go in and out easily. The problems for us are simply people bringing in maybe half a dozen movers, which is getting close to capacity. It means to fly the six on an empty bar, it takes 3 separate loadings - 2 at a time, then more weight, then the same again and so on. I've just had the second lot of surgery to fix my shoulder which I damaged a few years ago when I went up with the hauling rope. My own stupid fault for not engaging brain before ripping off the brake. However careful you are, they bite back!

[Seano]

Set against the notion of 4 stacks of stageweights, 4 Pro Stars and associated cabling plus the motor controller and crew clinging onto the remote, all dangling from a speedily gridded Bar 15....

 

That's not going to happen, since the weight of all that lot exceeds the weight of the fully loaded cradle. But yes, obviously, bad things could happen if someone does something really stupid. The same is true of all rigging and flying, all the time.

 

Some counterweight cradles and systems are designed with lifting points at the top for motor assist ...

 

Or at the bottom, possibly.

[Giles Favell]

1332624464[/url]' post='433737']

I'd be interested to know if, as the brakes are reasonably symmetrical, they are good for that 30% static load in either direction, as would be the case if you underloaded for some reason during the process. The rail system would also need to be good for such a down pull, as well as an upward one.

 

absolutely. The brake should be rated in either direction, and the mounting rail likewise.

NOTE: this 30% is when the locks are new, adjusted up properly, clean, and working on new, appropriate hauling ropes. As any of these factors degrade, so will the performance - probably quite significantly!