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Lighting Bar Hangers - seeking


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This website has good explainers on forces in rope systems. https://www.ropebook...-vector-forces/

 

As has been mentioned, it's important to understand the difference between:

Minimum Breaking Load (MBL) - the load the manufacturer has stated it will withstand and above which it will probably fail.

 

Working Load Limit (WLL) - the maximum load the manufacturer allows you to impose. This is in relation to a single component, it includes a safety factor they have determined but generally does not take into account the way you are using it. Some components, like bow shackles for example and petzl climbing pulley, do indicate a WLL for different configurations or angles of use. But in our case the brackets WLL can not take into account the pulley system or dynamic loads. It just states the load you may apply.

 

Safe Working Load (SWL). - this is established by a competent person and applies to the system as a whole. It will consider all the components, how they are being used and what they are rigged from. (Actually you quite often just need to consider the weakest component).

 

 

So for the bar, you can establish a SWL taking into account the beam it's rigged from, the fixings used and the bar it's self. If there was a permanently installed pulley system you could establish the maximum load it was safe to lift on it, taking into account the additional dynamic forces. This SWL would be different from the SWL of the bar itself.

 

If, as is more likely, you will be using a temporary pulley system, or just a line chucked over the bar, the person doing that needs to have an understanding of the above, or have been instructed in the loads they can lift. And that is why we have Method Statements. Someone else can do the maths.

 

 

A note on Safety Factors - you should not really be relying on these to to take up any of these incidental forces discussed. Nor to allow for obvious wear and tear.

 

 

Tom I has referenced the Ringling Brothers Circus hair hanging incident. There were in fact (to my understanding) two main factors. As well as the extremely experienced riggers failure to understand the difference between a MBL and. WLL, the karabiner was also used incorrectly - it had been tri- axelly loaded. This put a side loading on it that it was not designed to withstand. The riggers defence was that he had not loaded it beyond its marked loading limit (I am not sure if he had taken the dynamic loading into account) But it was a combination of the two factors that caused the failure. My point is, the manufacturer may rate a piece of equipment but they have no way of controlling how you use it and it is important that you understand what limitations they have set and the implications of what you are doing.

 

 

T

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The thing that bends my brain slightly is if you use a 2:1 purchase, so a pulley on the load as well, that reduces the loading on the bar as you are only putting half the down force on the rope. That seems totally counter-intuitive to me.

You are halving the effort of the grunt on the rope, but aren't you just spitting the same down force between 2 lengths of rope? If you add in friction doesn't 2:1 slightly increase the total down force?

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Surely if the downforce was the same there would be no mechanical advantage in using a 2:1 pulley system? Something has to give and that must be speed. The trade off it seems to me is less force applied to the rope set against increased time to do the job. Wouldn't the down force be the load plus the weight of the second pulley plus the half force needed on the rope plus any friction. If the load was 100 Kg over one pulley the total downforce would be 200Kg, in the case of a 2:1 system the downforce would be 100kg plus 50Kg plus the weight of the additional pulley and rope. I seem to recall from physics that the speed ratio in any ideal machine matches the mechanical advantage so in the case of a ideal 2:1 pulley system it would take twice as long to do the same amount of work as a simple pulley but it would be easier.

 

Or have I missed something?

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Surely if the downforce was the same there would be no mechanical advantage in using a 2:1 pulley system? Something has to give and that must be speed. The trade off it seems to me is less force applied to the rope set against increased time to do the job. Wouldn't the down force be the load plus the weight of the second pulley plus the half force needed on the rope plus any friction. If the load was 100 Kg over one pulley the total downforce would be 200Kg, in the case of a 2:1 system the downforce would be 100kg plus 50Kg plus the weight of the additional pulley and rope. I seem to recall from physics that the speed ratio in any ideal machine matches the mechanical advantage so in the case of a ideal 2:1 pulley system it would take twice as long to do the same amount of work as a simple pulley but it would be easier.

 

Or have I missed something?

I wish I'd concentrated more in O-Level Physics classes!!

 

I think Sunray has it right. Surely the total down force has to remain the same, regardless of how many pulleys the rope goes round? Multiple purchasing merely reduces the effort for the guy or gal on the haul rope, at the expense of, as you say, speed. If you manually haul a 100Kg load it still weighs100Kg, regardless of the path the haul rope takes. Using 2:1 purchasing gives you much the same mechanical advantage as using a bit of gas-barrel to extend your wheel-spanner after you've had a new tyre fitted, or using a longer handle on a jack. Or am I missing something too?

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Think of it in simple terms. In a balanced system, the force applied to the fixing of the top pulley is the combination of the load weight (x gravity) plus the force you are applying when you pull down.

 

If you employ some form of mechanical advantage, that reduces the amount of force required to balance the system and therefore the amount of load on the top point.

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As far as I can work out if you have a 2:1 purchase the loading on the bar is less.

 

If you were lifting 100kg using a 2:1 purchase, and you weighed 50kg, you would be able to lift the load by hanging your whole weight on the rope. Total bar load 150kg.

 

If using a single pulley, you'd have to hang 100kg on the rope to balance the load. Total bar load 200kg.

 

Like I say it is not intuitive but I think that's right.

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Can't fault your logic, but if you follow that argument through then an infinity:1 purchase would give you zero down force, which surely can't be right. Isn't a high purchase ratio akin to a very low geared winch? Very easy to turn, but its fixings are still supporting the full weight of the load. Isn't adding extra pulleys the equivalent of changing the number of teeth on your cogs?

 

Any structural engineers out there who can put us out of our misery?

Edited by sandall
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30+30 Is the static load when you have taken the strain and are ready to lift the moving light, when you actually move the moving light you will apply more that its mass to get it moving, two chaps and a 2:6 heave could add 10kG or 20kG to the pull rope AND an equal amount to the lift side, so we now have 20kG or 40kG extra load, but as above the rope over the bar will be inefficient so to get 20kG of lift the pull side might be pulled 80kG, total now 100kG, now what was the SWL on the bar, and how soon until the rope pullers are wearing the bar in Casualty?

 

If there is a better point to hang the pulley, perhaps a grid or girder with enough SWL

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Some food for thought if anyone wishes.

I have done a very simple experiment with a 5/8ths hemp rope (or 5mm paracord in brackets), 4 pint milk bottles and water.

Initially I hung the rope over a shiny ali scaff with a full bottle of water each end, I added an additional empty bottle on one side and gradually added water, it took nearly 3 (nearly 2) pints of water to make it start moving and 3.5 (just over 2) pints to make it keep going to the end of the rope.

With a rusty steel scaff it took over 9 (6.5) pints extra to make it move but then it plummeted.

With a 6mm shiny mild steel round rod 12 pints wasn't enough to make it move, thats 4 pints Vs 16 pints being held by the bumps in the rope. (6 pints made it start moving but 8 pints needed to keep it going)

I then put a 80mm long piece of ali scaff round the 6mm rod to form a crude pulley and both ropes made the bottle drop with only 0.5 pint.

 

None of this is very scientific or accurate, even the rope or bottle was rubbing against the opposite ends bottle.

 

As far as I can work out if you have a 2:1 purchase the loading on the bar is less.

 

If you were lifting 100kg using a 2:1 purchase, and you weighed 50kg, you would be able to lift the load by hanging your whole weight on the rope. Total bar load 150kg.

 

If using a single pulley, you'd have to hang 100kg on the rope to balance the load. Total bar load 200kg.

 

Like I say it is not intuitive but I think that's right.

This is correct.
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It's like the Grand Hyatt walkway collapse

, you need to think of all the forces you are placing on the system as a whole, as well as how those forces work as part of that system

There was a vaguely similar arrangement to this on a plantroom unistrut frame where the initial vertical supports needed to be extended higher but instead they were bolted to the existing top horizontal and an additional horizontal about a foot lower.

 

A lot of weight was added to both sides of the frame which was being held by bolts & Zebs (Rather than down to the floor) and just like the walkway they gave way, some the Zebs slipped but mostly the M10 bolts sheared.

 

Whole thing was caught on CCTV. it was really lucky that it happened a few minutes after knocking off time and the room was empty as there had been loads of people working in there.

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Ooh, ooh, pulleys! Me please!

 

I think theres a bit of confusion upthread between 'load'(/force) and 'mechanical advantage'...Tom and Tim had it right, and after a bit of trawling, I've found this page that seems to explain it quite well:

https://www.educatedclimber.com/mechanical-advantage-explained/

 

Essentially, when you start introducing purchase systems, the 'load' exerted on the bar will, if anything, start to decrease, on the basis that you are using the mechanical advantage of the pulley systems such that you have to exert less force to lift the load (that is not changing in mass, and so will ultimately 'weigh' the same to the fixings in the ceiling).

 

 

Of course if you spread your top pulleys across separate bars/hanging points, you will exert even less (downward) force on the bars, but that's for another diagram...!

 

Ian

Edited by IRW
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I think theres a bit of confusion upthread between 'load'(/force) and 'mechanical advantage'.............after a bit of trawling, I've found this page that seems to explain it quite well:

https://www.educated...tage-explained/

Ian

In reality a heavy light is probably going to be rigged off a MWP, tower, tallescope or ladder, rather than hauled up on a rope, in which case all this is a bit academic, but to keep the discussion going -

 

Try forgetting the rope for a moment (no pun intended) & think of the bar as a fulcrum, with say a 100Kg lump of metal on one side & a long arm on the other. If you are the same distance away from the fulcrum you will need a force of 100Kg to balance the load, giving a down-force on the fulcrum of 200Kg. If you now move to 10x the distance you only need to apply 10Kg to balance the load, but the down-force on the fulcrum hasn't changed - it is still 200Kg, so while mechanical advantage has eased the job of lifting your lump of metal off the deck it hasn't changed the total force on your side of the fulcrum.

 

When sailors hoist a sail by hand the tension on the first pulley the haul-rope goes over must stay the same regardless of how many subsequent pulley sheaves are involved. The diagrams in Ian's link show the force in EACH length of rope as the total weight divided by the number of lengths, i.e. the total load on the top anchor pulley hasn't changed, but the effort needed for lifting has reduced. I believe they call it Conservation of Energy.

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i.e. the total load on the top anchor pulley hasn't changed, but the effort needed for lifting has reduced. I believe they call it Conservation of Energy.

 

That isn't so though. The total load on the fixing of the top pulley goes down the more purchase you put on it. A 2 pulley system lifting 100Kg only puts 150kg load on the top fixing; a 1 pulley system puts 200kg load on the top fixing.

I am not sure you are right about your lever/fulcrum point either, I think if you put 10kg at 10x the distance the down force on the fulcrum is now only 110Kg.

 

 

(sorry for measuring force in kg but you know what I mean)

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Try forgetting the rope for a moment (no pun intended) & think of the bar as a fulcrum, with say a 100Kg lump of metal on one side & a long arm on the other. If you are the same distance away from the fulcrum you will need a force of 100Kg to balance the load, giving a down-force on the fulcrum of 200Kg. If you now move to 10x the distance you only need to apply 10Kg to balance the load, but the down-force on the fulcrum hasn't changed - it is still 200Kg.......

 

 

Wrong.

 

If you have 100kg on each side then yes, you fulcrum is supporting 200kg.

 

But when you have 100kg and 10kg its only supporting 110kg

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