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View Full Version : Load limits and sizes on wooden and hemp rope A-frame lifting gear



Bolero
10-06-2015, 02:02 AM
Writing a fantasy and want to lift heavy items on and off a cart - but not near any building. So the answer is an A-frame and ropes and pulleys.

So far so good. Have been googling A-frames and what I've found have almost all been aluminium. Now, that is interesting to see all the ways the ropes are rigged for different jobs, but not helpful for working out how thick the rope and how thick the wood of the frame would have to be, to lift 300lb, or 500lb or 1,000lb at a time.

Also want some idea of the weight of the A-frame itself - as they are carrying it with them.

Any other info on A-frame usage would be appreciated.

King Neptune
10-06-2015, 02:38 AM
There are many variables. In my youth I was involved in hoisting out engine on various A-frames. I would assume that the engines weighted around three hundred pounds, certainly none over four hundred. The Frames were either logs of maybe 6" diameter chained together at the top. Or they were from 2x6's or 2x8's fastener at the top. The 2x's were three pairs of them with one nailed at right angles to the other to add stiffness. I wouldn't trust a three hundred pound plus thing to sit on some twelve foot 2x8's, but uprights were steep enough that most
of the weight was longitudinal. I don't remember the ropes, or if we used a chainfall, but 3/4 fiber rope (hemp sisal, or whatever) will handle 600 lbs (if I remember correctly).

Two people could carry the A-frame, and it's easy to unchain them at the top and have three twelve foot pairs of 2x8's, which is an easy thing to carry.

BTW, I would be dubious of an aluminum A-frame. If made well it woould be fine, but aluminum sometimes bends when oone would rather it didn't.

Bolero
10-06-2015, 01:24 PM
There are many variables. In my youth I was involved in hoisting out engine on various A-frames. I would assume that the engines weighted around three hundred pounds, certainly none over four hundred. The Frames were either logs of maybe 6" diameter chained together at the top. Or they were from 2x6's or 2x8's fastener at the top. The 2x's were three pairs of them with one nailed at right angles to the other to add stiffness. I wouldn't trust a three hundred pound plus thing to sit on some twelve foot 2x8's, but uprights were steep enough that most
of the weight was longitudinal. I don't remember the ropes, or if we used a chainfall, but 3/4 fiber rope (hemp sisal, or whatever) will handle 600 lbs (if I remember correctly).

Two people could carry the A-frame, and it's easy to unchain them at the top and have three twelve foot pairs of 2x8's, which is an easy thing to carry.

BTW, I would be dubious of an aluminum A-frame. If made well it woould be fine, but aluminum sometimes bends when oone would rather it didn't.

Aluminium - yup, not for my book, just all I could see in A frame pictures (mostly mountaineers and rescuers doing fancy things) but agree with you on Aluminium.

2 X 6' - you are saying lumps of timber, 2inches by 6 inches cross section, (and 2 inches by 8 inches).
Three pairs - are you saying you nailed together 3 pieces of 2 inch by 6 inch timber to make an "H" profile? And then those were lashed into a square lifting frame, rather than an A frame?

King Neptune
10-06-2015, 04:51 PM
Aluminium - yup, not for my book, just all I could see in A frame pictures (mostly mountaineers and rescuers doing fancy things) but agree with you on Aluminium.

2 X 6' - you are saying lumps of timber, 2inches by 6 inches cross section, (and 2 inches by 8 inches).
Three pairs - are you saying you nailed together 3 pieces of 2 inch by 6 inch timber to make an "H" profile? And then those were lashed into a square lifting frame, rather than an A frame?

I was trying to describe a tripod hoisting frame https://www.google.com/search?q=tripod+frame+hoist&biw=1148&bih=740&tbm=isch&tbo=u&source=univ&sa=X&ved=0CCkQ7AlqFQoTCNirjIrtrcgCFcg9PgodMcsI2w
The problem with A-frames is the crosspiece; it has to be strong and unlikely to bend, so steel is the thing. If you want to make an A-frame for sure, then you would want something like a 6x6 crossbeam and uprights to match. The theoretical strength off most woods would say that it could work, but wood beams aren't stiff enough, assuming an 8 foot span, so laminated beams or something else would be required, if you want to be confident about a 1000 pound load.

Bolero
10-06-2015, 10:00 PM
I was trying to describe a tripod hoisting frame https://www.google.com/search?q=tripod+frame+hoist&biw=1148&bih=740&tbm=isch&tbo=u&source=univ&sa=X&ved=0CCkQ7AlqFQoTCNirjIrtrcgCFcg9PgodMcsI2w
The problem with A-frames is the crosspiece; it has to be strong and unlikely to bend, so steel is the thing. If you want to make an A-frame for sure, then you would want something like a 6x6 crossbeam and uprights to match. The theoretical strength off most woods would say that it could work, but wood beams aren't stiff enough, assuming an 8 foot span, so laminated beams or something else would be required, if you want to be confident about a 1000 pound load.

Are we talking about the same thing with A frame? I thought the load bearing was done via the apex of the A, not the cross beam that holds it together.
Just did a google search to get a picture of what I meant (could find loads yesterday) and the A-frame lifting gear search brought up modern things referred to as A frame gantries, which were not at all what I meant.
The one I mean (if only I could find a picture) is literally a capital A - two long timbers meeting, one cross bracing - and it is used more as a pivot point for the ropes. So a rope goes over the top of the A down to what you are lifting and you pull the A upright, which lifts the item. Not describing this very well, will try to find another picture later.

King Neptune
10-06-2015, 10:32 PM
Are we talking about the same thing with A frame? I thought the load bearing was done via the apex of the A, not the cross beam that holds it together.
Just did a google search to get a picture of what I meant (could find loads yesterday) and the A-frame lifting gear search brought up modern things referred to as A frame gantries, which were not at all what I meant.
The one I mean (if only I could find a picture) is literally a capital A - two long timbers meeting, one cross bracing - and it is used more as a pivot point for the ropes. So a rope goes over the top of the A down to what you are lifting and you pull the A upright, which lifts the item. Not describing this very well, will try to find another picture later.

I think you mean a tripod hoisting frame.
https://www.google.com/search?q=tripods+hoisting+frame++image&biw=1920&bih=964&tbm=isch&tbo=u&source=univ&sa=X&ved=0CDcQ7AlqFQoTCMvYn468rsgCFcwcPgodSSEMvA

An A-frame has a beam held up by supports at either end.
I initially described the tripod. After you questioned that, I posted in regard to an A-frame. The A-frame requires more material, but it would be better over a cart. A tripod is easy to set up, and pound-for-pound of equipment can lift more, but it doesn't give as much space under it, so it is harder to get a truck or a cart under it.

I have also seen what you mean, but I don't know what they are called. They aren't very useful, but they are simple.

RKarina
10-07-2015, 02:26 AM
Natural fiber rope is difficult to test accurately for load rating - too much depends on the fibers themselves, manufacturing methods (how many strands, how tight the twist, etc.), processing, oils/treatments applied, etc. However...

You can find all sorts of resources for tensile strength for natural fibers - http://goo.gl/sA4P6r is a ULine listing that shows tensile strength on jute twine. Note - tensile only measures the amount of force that can be applied prior to failure. Force does not equal load capacity.

The Engineering Toolbox has a great reference for rope - http://www.engineeringtoolbox.com/manila-rope-strength-d_1512.html - specific to manilla rope, but it will give you a good idea. Contractor's Rope has a similar chart - http://www.contractorsrope.com/twisted-rope.html

By both charts, you'd need 5/8" rope to have a safe load rating for something about 330 pounds, 3/4" for 500-ish pounds, and 1 1/4" for 1,000-ish pounds.

King Neptune
10-07-2015, 02:41 AM
Yes, I forgot about that part. Look at the label on some rope the next time you're in a store that sells it, or just work from what she wrote. Ususally there would be more than one piece of rope holding something.

Bolero
10-07-2015, 12:27 PM
I think you mean a tripod hoisting frame.
https://www.google.com/search?q=tripods+hoisting+frame++image&biw=1920&bih=964&tbm=isch&tbo=u&source=univ&sa=X&ved=0CDcQ7AlqFQoTCMvYn468rsgCFcwcPgodSSEMvA

An A-frame has a beam held up by supports at either end.
I initially described the tripod. After you questioned that, I posted in regard to an A-frame. The A-frame requires more material, but it would be better over a cart. A tripod is easy to set up, and pound-for-pound of equipment can lift more, but it doesn't give as much space under it, so it is harder to get a truck or a cart under it.

I have also seen what you mean, but I don't know what they are called. They aren't very useful, but they are simple.

Definitely not a tripod - one of these A frames http://www.rescue.hastpsc.com/frames/bipod/bipod.html. (They are calling it an A frame - maybe it is a US/Europe difference on terminology)

But in wood and rope, without the fancy feet and with a cross bracer to stop the bipod doing the splits. I think delving around with searches involving "animal rescue" is going to sort it for me.

Thank you for your help.



@RKarina

Thank you for your help too. A small doh moment here - should have gone and worked that one out myself - your links are brilliant, thank you.


@Everyone else :)

If anyone has ever used a cross-braced bipod A frame in wood and rope.......... :)

RKarina
10-07-2015, 11:35 PM
One question I do have - how much detail do you really need to include in the writing itself?
I know I don't need a diagram of a block and tackle, or even a detailed description of one, to understand what's going on if something is being lifted with ropes and pulleys.

If someone is observing this, and it's the norm for them, whatever mechanism would be nothing remarkable - I would expect the character to simply indicate the carts were loaded and unloaded (whatever). Maybe a brief mention of pulling into the loading area, or setting up the lift frame, or whatever.

If they're observing and this is new and different to them, a brief description would make sense. They erected a solid-looking wooden frame over the cart, then slung a series of thick ropes around the load. In minutes, the cart was unloaded, and...

You get the idea - it doesn't need to be an explicit, technically accurate, engineering level description of whatever mechanism it is you want to create. Just enough to get the idea across to the reader.

Unless you want to write that level of technical detail... in which case, knock yourself out!

Bolero
10-08-2015, 01:20 PM
Good question on how much do I need to include in the writing. What I am up to and where I am coming from is this:

I used to do a lot of re-enactment - so I've worn period clothes, cooked over firepits, marched on a dirt track on a hot day, drawn water from the well and so on. I can write all that with just tiny hints as to how it feels. (And the challenging part of the writing is getting in enough for it to work in someone else's head without burdening the story - because it is a crystal clear picture in my head.)

But at the end of the week or the weekend, we didn't pack everything into carts and attach the horse or oxen team, what we did was back up the four wheel drive with the low load trailer and the power winch.

So what I am doing at the moment is finding out the nitty gritty details of how people in the period would have loaded heavy items on a cart. Then I will write myself an account - sizes of timber, sizes of ropes, how it was all done, roughly how many people were needed, until I can see and feel it in the same way that I can the stuff I've actually done.

Then I can put detail in the main book such as "need another three bodies over here" or "bring that thick pole, no, no, the six inch one," "for god's sake you can't control that on your own, get him to help". Also the size and weight of ropes as you pick them up.
I have one POV character who is part of the travelling group and a second who will be travelling with them - and for the packing is an observer as he is not part of the well organised team. Well mostly well organised - they've just taken on a couple of apprentices.

And what I'd really hate, is if one day in the future someone says to me "by god your skinny little apprentice was a budding Hercules - you do realise that you needed a six inch timber for that job and that kid couldn't have lifted it". Probably really only matters to me, but it matters :)

Plenty of things I gloss round - "later the same week" :) but I also enjoy research and getting the details right.

I'm moving on to a bit of googling on heavy horse teams for my next chunk of research.


Its also been useful to learn that when I say "A frame" I picture a period A frame, which was a bipod and did give you a mechanical advantage. But most people see a "swing set" thing as an A frame, with a cross beam, for which you would need pulleys to get any kind of mechanical advantage. So I can tweak my text accordingly.