Kevin Robertson Posted October 31, 2003 Share Posted October 31, 2003 Dear Flymen/Technical Managers of venues with "hemp" flying. Just doing a quick survey if you dont mind... What do you consider to be the SWL of your hemp bars?How may lines are there on your hemp bars?What is material of the hemp bar?How did you arrive at the SWL stated above? Thanks in advance for any input Kev Link to comment Share on other sites More sharing options...
Tomo Posted November 2, 2003 Share Posted November 2, 2003 Untested, understaffed, 'gut instinct' answer: SWL - About 1/2 the weight of the guy on the rope.3 hemps per barBars are aluminium scaff pole SWL is a guess based on "What can one person pull without breaking themselves?" We have monstrously overloaded our flybars before and although they bent alarmingly they didn't appear to be close to breaking. It took about five people to fly the thing out, so we probably had almost 100kg sat on it.Not recommended! Apart from the hemps themselves, you need to be aware of the load on your grid and tieoff points. Link to comment Share on other sites More sharing options...
robloxley Posted November 2, 2003 Share Posted November 2, 2003 What we could concievably rate the system for if it were being used to dead hang pieces (given pulley, rope and cleat SWLs) is a lot more than a sensible safe working load for operation by a single flyman. Our general policy is that if you're flying anything over 50kg or so, you should be thinking about using the counterweight sets rather than the hemp sets.Ours mostly have 4 hemp lines, dropped from the same girders as the counterweight wires. Bars are aluminium scaff tube but custom lengths to be full-stage width of ~9m. Link to comment Share on other sites More sharing options...
peter Posted November 3, 2003 Share Posted November 3, 2003 It took about five people to fly the thing out, so we probably had almost 100kg sat on it.I remember being told once that two people doesnt necessarily mean you can lift double the load - its something like 4 people to achieve double. Link to comment Share on other sites More sharing options...
robloxley Posted November 3, 2003 Share Posted November 3, 2003 Not sure exactly how you get 4 people hauling on one hemp set?? Link to comment Share on other sites More sharing options...
Glyn Edwards Posted November 3, 2003 Share Posted November 3, 2003 Not sure exactly how you get 4 people hauling on one hemp set?? Carefully?, On several levels, Stage, Fly Floor and possibly loading gallery, if space permits. I know of at least one venue where Flymen 3 & 4 work while stood on the fly rail itself (suitably harnessed), while Flys 1 & 2 stand on the floor. HTH Glyn Link to comment Share on other sites More sharing options...
Tomo Posted November 4, 2003 Share Posted November 4, 2003 It took about five people to fly the thing out, so we probably had almost 100kg sat on it.I remember being told once that two people doesnt necessarily mean you can lift double the load - its something like 4 people to achieve double.Yes, that's about correct.The HSE stuff on manual handling says that two people can lift about 150% of what one person can lift, but I don't think they mention three-man or four-man lifts in the guidelines. Your average bloke can lift about 40kg when pulling down without any problems - he CANNOT fly it smoothly or particularly safely though.(Based on Robot Wars experience) The flying was more of a lift-lock-lift sequence as you might use to trim a sail, but as I mentioned before it's not a good idea and we wouldn't do it again, especially as we now know how cheap proper chain hoists are. If you don't have counterweights, hire in some electric hoists for heavy fkying. They're very slow, but much safer than running the risk of sending someone into the grid. At the end of the day, if the setpiece weighs more than the flyguy, DO NOT FLY with hemps, no matter how strong they are. Link to comment Share on other sites More sharing options...
Steve Macluskie Posted November 4, 2003 Share Posted November 4, 2003 It seems that the common way of thinking is to look at the lines in the system and manual handling issues to determine the SWL. I know the SWL of my lines and my own capabilities but it's the aluminium bar I'm worried about. I tried contacting the Aluminium federation to ask for a number regarding the stregth of 48mm Ali tube but they were unable to provide one. Chris Higgs new book "Rigging for Entertainment: Regulations and Practice" states on pg 76 that "Alloy barrels are not as strong as many people expect them to be.An engineer will calculate the maximum load allowing a safety factor of 2:1 as being 50kg on a 3m span of NEW tubewith no joint in he span" So on my 3 line hemp set, the lines are approx. 5m apart. Anyone care to guesstimate the ali bars strength properties at this distance? or preferably point me in the direction of where this type of information can be found. Steve Steve Link to comment Share on other sites More sharing options...
Tomo Posted November 4, 2003 Share Posted November 4, 2003 What's the wall thickness of the aluminium tube?That's just as important as the diameter. A lot does depend on whather it's point loads or area loads of course. In my experience with pressure vessels aluminium lasts longer than steel anyway, but you do need to visually inspect the bar frequently for deep scratches, signs of buckling and tears.(If you find a buckle or tear, retire the bar immediately!) www.matls.com has materials information, and you could Google for a University website with "Beam in Bending" calcs. Link to comment Share on other sites More sharing options...
Brian Posted November 4, 2003 Share Posted November 4, 2003 Ironically, beams with round cross-sections are very poor at resisting deflection. Why are they the most common beam in theater?...pipe resists shear very effectively for the same reason that it is an inefficient beam: the majority of its material is concentrated around the shear plane...Both quotes from 'Structural Design for the Stage' by Holden and Sammler Link to comment Share on other sites More sharing options...
Brian Posted November 4, 2003 Share Posted November 4, 2003 I tried contacting the Aluminium federation to ask for a number regarding the stregth of 48mm Ali tube but they were unable to provide one.I suspect that they were playing safe and trying not to answer what is in fact a very difficult question.A lot does depend on whather it's point loads or area loads of courseAnd much more besides. You can work it all out from first principles if you need to, the only figure you really need is E (Young's modulus) for the grade of ali you're using. Generally Al is about 1/3 the strength of steel. As an example (and a frightening one at that)... 1. Take a 5m long 48.3mm dia tube with 4.47mm walls in 6082-T6 Al (standard scaff tube).2. Hang it from it's ends (or support it on blocks, it doesn't matter for the maths)3. Set a maximum deflection of l/240 ie 5000mm/240 = 21mm.4. Works out the weight you need to add to the centre of the tube, as a point load, to give this deflection. Answer = 20LB I'm sure this figure is correct (I've checked the maths a couple of times) but it seems frighteningly low. Link to comment Share on other sites More sharing options...
Ellis Posted November 12, 2003 Share Posted November 12, 2003 At the Palace Theatre in Redditch, their hemp sets are rated at SWL 100kg, uniform load across the amuminium bar. The sets have three lines each. The limit was arrived at by the specialist contractors that the council have ine annually to certify the system. Any lifting equipment should be proof tested at several (can't quite remember - but that is what industy codes of practice booklets and course notes are for) times the SWL, moving it through its normal range of movement. This places a limit on the SWL of a bar of a fraction of the maximum weight that the proof testers can actually haul themselves. (assuming that this is less than 1/8 of the minimum breaking strain of the weakest element). Link to comment Share on other sites More sharing options...
MarcT Posted November 12, 2003 Share Posted November 12, 2003 As an example (and a frightening one at that)... 1. Take a 5m long 48.3mm dia tube with 4.47mm walls in 6082-T6 Al (standard scaff tube).2. Hang it from it's ends (or support it on blocks, it doesn't matter for the maths)3. Set a maximum deflection of l/240 ie 5000mm/240 = 21mm.4. Works out the weight you need to add to the centre of the tube, as a point load, to give this deflection. Answer = 20LB I'm sure this figure is correct (I've checked the maths a couple of times) but it seems frighteningly low.Well, after a quick refresher of beam bending theory, I repeated the analysis and came up with a figure of an 8.6kg. That is an 8.6kg load will produce a 21mm deflection at the midpoint of a 5m "standard" aluminium scaff bar supported or hung (not clamped) at its ends. Now, I believe 8.6kg is 18.9lbs in old money and thus confirms your result.EDIT: This ignores the weight of the bar itself - see later post. As I wasn't able to quickly find a Young's Modulus value for 6082-T6 Al, I've assumed it to be 70GPa, a quoted value for elemental aluminium. Marc Link to comment Share on other sites More sharing options...
Brian Posted November 12, 2003 Share Posted November 12, 2003 Thanks Marc When I did the sums the figure seemed so low that I thought I must be wrong but it's nice to know I wasn't. What I guess it shows is that, in structural applications, Ali tube is useless. My maximum deflection figure of 1/240 is about as large as you'd ever go in a building (if it was an upper floor, with that deflection your downstairs ceiling would crack). Link to comment Share on other sites More sharing options...
Tom Posted November 12, 2003 Share Posted November 12, 2003 I find those figures quite disturbing. :D Link to comment Share on other sites More sharing options...
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