Hanging Iron/Centre of Gravity

[shackle]

is there anybody that has found an alternative method to the traditional way of fixing a lipped hanging iron to a pice of flown scenery to try and get the hanging point closet to the centre of gravity. Standard practice is to bolt the hanging iron to the bottom of the scenery piece with the iron to the back of the flat. Our flats are usually constructed by 75mm x 30mm timber framing with a 6mm ply facia either covered in canvas/ cloth/ or polibond texture and painted, When hung the the piece always wants to leans forward as the pick up point is not the centre of gravity (ie the back of the flat). Im trying to convince our carpenters to build the hanging iron into the piece of scenery and to bore a hole down through the framwork to allow the eye of the steel cable to be fed trhough the piece so try and get a pick up point closer to where all the weight is generated. Has anybody used this method before and is there an easier way to calculate where the centre of gravity is.

 

Thanks for any help in advance, Pat

[Jivemaster]

The CofG will be forward of the mid point of the 75 x 30 member by the effect of the ply layer. However the effect of the cloth and (repeated?) paintings will prevent accurate calculations.

 

Pick a point, say the front face of the board of the flat. For the board there is a CofG at the centre (it is a uniform solid) and it's mass gives a turning moment about the front face of mass x half the thickness. For the frame the Cof G will be half way through the thickness, and it's turning moment about the front face of the flat will be the frame mass X (board thickness + half the frame thickness) Add the moments about the front face of the flat and divide by the mass of the flat should give the position of the CofG as distance behind the front face.

 

As a guess probably the support should be one third of the thickness of the flat fron the front.

 

Everything will change when it get's painted!

 

Now whether this converts into theatre practise........

 

I used to have to find the CofG of light aircraft

[andy_s]

I think the lipped hanging irons mentioned are really only effective on 3 by 1 flats where the timber framing is constructed with the larger dimension vertical - as the piece has less "thickness", the iron is nearer the c of g.

 

Don't know many set builders who make flats like this any more....so a new way of hanging them should definitely be found! boring through the framework of a flat made "3 by 1 on edge" is something I've certainly seen, but the hole wpuld need to be nearer the front than in the middle, to compensate for the weight of the texture and paint....

[shackle]

Thanks Andy and Jive, I guess im looking for solution to a few problems with trying to get the hanging iron closet to the Centre of Gravity, the hole I mentioned should really be a slot in the timber to feed the eye of the cable through rather than maybe boring a 30mm hole right through the structure of the flat creating a weakness, I now have access to a metel work shop so I might see if I can get a fitting made up with some sort of adjustable fixing to move the cable to a suitable hanging point along the width of the flat. Once I've cracked it I'll send them out and get a test cert and a SWL stampped on them. any ideas would be welcomed .. Cheers Pat

[andy_s]

I know some people don't like them, but it might be worth looking at bullets , then you could feed the wire rope through a small hole in the timber and then make it off at the bottom with the bullet. It's what unusual rigging used to attach our counterweight bars when they did the installation.

[DaveBeattie]

Just a thought that occured to me: take a 2mm steel plate 75mm square. Weld to it a piece of 10mm steel rod 200mm long so that it sticks out evenly each side. Weld an eye to each end and then bend the rod so that the eyes are behind the plate. Screw the plate to the top section of the flat and pass the flying line through both eyes. This should act as a divertor, moving the effective hanging point toward the centre of gravity of the flat, dependant upon the amount of bend that you have put in the rod. The reason for using two eyes is to remove any twisting effect on the plate, which might make it more inclined to rip out of the wood.

 

Any thoughts?

 

Regards,

 

Dave

[Droycewalker]

I don't know if I'm missing the point here, but surely all you need is a grummet at the top for the SWR to run through? Something like this... http://www.flints.co.uk/acatalog/copy_of_Grummet.html

Attach the hanging irons as you normally would, at the base of the flat, run the SWR (or, indeed, whatever you are using to hang the set piece!) through the grummet, and up to the bar. I think there are even Opera grummets available that allow an eye through. This way, the downward weight of the flat counteracts the flat's urge to lean forward as it is attached top and bottom. Place a bottle jack in line and you can even get the tensions balanced on the wires.

Hope this helps, and apologies if I've misunderstood the OP... :)

 

DW

[andy_s]

I don't know if I'm missing the point here, but surely all you need is a grummet at the top for the SWR to run through? Something like this... http://www.flints.co.uk/acatalog/copy_of_Grummet.html

Attach the hanging irons as you normally would, at the base of the flat, run the SWR (or, indeed, whatever you are using to hang the set piece!) through the grummet, and up to the bar. I think there are even Opera grummets available that allow an eye through. This way, the downward weight of the flat counteracts the flat's urge to lean forward as it is attached top and bottom. Place a bottle jack in line and you can even get the tensions balanced on the wires.

Hope this helps, and apologies if I've misunderstood the OP... :)

 

DW

 

the grummet is exactly the thing for holding the steel wire rope to the back of the structure, but won't unfortunately help with the front-to-back centre of gravity question - the flat will still tend to "lean" forward if the front face has any weight to it.

[DaveBeattie]

Please excuse the crude drawing, but would something like this work?

 

http://www.beattiesnet.co.uk/Divertor.gif

 

Regards,

 

Dave

[GRisdale]

Flints supply an "eye in" hanging iron. If you fitted these to the front of the frame-work (with a bit of chiselling to get them flush) before the ply layer goes on that should provide your bottom fixing. Wouldn't give you exact positioning, but they are available off the shelf and already certified...

 

Gareth.

[gyro_gearloose]

If the flats are to covered in ply, why not just use an 'eye-in' hanging iron on the top rail of the frame?

[andy_s]

flying flats are traditionally supported from the bottom so that all the weight is on the flying line, rather than being transfered to the lines through the joints in the flats, which were seen as potential weak points - specially in the old days of "proper" carpentry with mortice and tenon joints, the tightness of which might be affected by climatic conditions.

 

At least that's what I recall from carpentry classes at drama school 25 years ago....

this predates many current scenic carpentry techniques and materials, and certainly predates the idea of rated flying fittings, so I don't suggest it's necessarily the right method these days, just a reason for the traditional approach.

[gyro_gearloose]

I understand why canvas flats are suspended from their bottom rails, but the OP will making the flats out of 6mm ply. Since the flats will made with ply they will be much stronger than a more traditional canvas flat so it matters less wether they are hung from the top o bottom rails.

[andy_s]

I understand why canvas flats are suspended from their bottom rails, but the OP will making the flats out of 6mm ply. Since the flats will made with ply they will be much stronger than a more traditional canvas flat so it matters less wether they are hung from the top o bottom rails.

 

I quite agree -I didn't mean to imply your suggestion was for some reason incorrect! That's why I "owned up" to presenting an old fashioned view - however, traditional methods often survive without people necessarily knowing why, or realising that there are alternative modern techniques available - and "traditional" was referred to in the OP. So long as the hanging method is inconspicuous (assuming we'd like to see as little of the flying mechanics as possible) and is demonstrably robust enough ("fit for purpose") then I wouldn't argue against it...