What strength of beam do I need for a crane ?

[Martin]

Here's photos of my makeshift crane to lift boats into the attic where they rest during winter. Last time I used an aluminium ladder with an improvised crap and a pulley to double my force.

I felt pretty unsecure with above setup, as the aluminium ladder bent quite a lot under the load, the crab didn't slide nicely over the ladder and the pulley was almost too hard for me to pull. I decided I want a redesign:
A T-type aluminium beam, an electric hinge, a crab that runs on the T with wheels.
The total length of the beam is 5.8m (228 inch), 3.3m (130 inch) inside the attic, 2.5m (98 inch) outside. The load of 30kg max is suspended at the outside end, the beam is held 2.50m (98 inch) from that load end and at the inside end. So it's 2.50m (98 inch) of beam protruding without support and 30kg of load pulling down the end. Wait - the load is not static it's being pulled up slowly, so there needs to be some margin, I assume.
The load travels 7.5m vertically.

Now I would like to know what strength of beam I need for this. Obviously, I don't want it heavier and more expensive than needed, but I also don't want that it cracks.

I've asked a mechanical designer and the shop selling the aluminium, but they were not sure.
Any help greatly appreciated.

 

[dom]

Looks scary Martin
During my stay at hospital i noticed they used enclosed track type lifting apparatus -may be worth a look as well.

http://www.spanco.com/pages/wsalutrack.php

Cheers
Dom

 

[KC2IXE]

You REALLY want to talk to what would be called in the states " a Material Handling company" - depending on rated load etc, they will design it (and maybe install it)

You are PROBABLY looking at a piece of 6" aluminum I beam, the Crab (called a Trolley here in the USA) and the hoist. Your HUGE problem is there is noting to pull the trolly araw from the building

Now, the BEST bet - and probably cheapest - is if you are allowed to mount something on the outside of the building - they would use what is called a rotating Jib - let see if I can do some ASCII art


|
|----------------
| / H
| /
| /
| /
| /
| /
| /
| /
| /
|/

The line at the left is the mount to the building wall - it's NEXT to the window, and pivots to fold flat
The diagonal is a brace - the horizontal is the beam holding the Hoist (H) - and as there is only a short distance betwee the brace and the hoist, everything is smaller and lighter

Please do not consider this professional advise. I worked fro a crane company installing these systems, but I'm NOT a mechanical engineer. I would not even design a system for my OWN use, never mind someone elses - too many factors - point loads, shock loads etc
edit:
Ascii cart didn't come out too well in the viewer

Here is another way using a top brace

http://www.low-cost-cranes.com/jib-cranes-311.html

http://www.spanco.com/pages/jibwm300.php

http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200325507_200325507

 

[gadget_lover]

The Machinery's Handbook has the formulas for how much weight a beam can support. I don't have time to look it up this morning, but mybe some of our other folks can help.

Daniel

 

[Martin]

Dom, so you're back on your feet ?
I do have the enclosed track rails here, this is what started the idea. They were used for a heavy double-sliding door. But they need reinforcement, at 2.5m x 30kg. Brings me back to the original question...

KC2IXE, a roating jib, I know what you mean.
However, I want to have the beam extend 3.3m to the inside, so I can pull the hulls straight to their parking position on this 2.5 + 3.3 = 5.8 m rail.
So I need the full length beam plus the rotating jib for additional support.
I'd really like to know what size of beam I need for 2.5 m x 30 kg = 750 Nm, to go WITHOUT this additional support. I suspect it's not that very big.

Pushing out the trolley is no problem with a little wheel at the end of the beam. This lets me pull the trolley outwards with a rope.

 

[mahoney]

I don't have time to look at the math now, and it's likely that the materials available in the US are not quite the same as are available to you, so I'll start with an obvious question or two.

Are you sure the house can take the load? Sure, the ladder outfit worked and the roof is still standing, but a beam and trolley will be heavier. There is considerable downward force where the beam is supported as it leaves the house and upward force inside on the roof. Have you consulted with a structural engineer?

 

[9x23]

I believe that KC2IXE is trying to describe a davit arm, and you can see examples at www.tractelswingstage.com. These are used on many buildings for window washing, and in my experience they are firmly attached to the building structure.

Please note that there is much more to installing a properly designed crane for your described use. Just sizing the beam is only part of the formula, as you will need to make sure the it is attached properly (engineered) to the building structure. Like everyone else, I recommend that you consult a company that makes/builds/installs these system as "turnkey" to ensure that you will be able to safely and confidently operate this without fear of hazards. Good luck!

9x23

 

[FredM]

I would dump the aluminum Idea and pick up some 8 inch galvanized steel C channel beams. You would only need 2 and since they could be a light gauge and still be plenty strong. they also would be cheap and light.

 

[gadget_lover]

I just looked up the fomula for determining the beam size.... Page 260 of Macinery's Handbook has quite a section on the subject.

Unfortunately, It's all greek to me.

Sorry to be of so little help. Maybe someone else can look at it?


Daniel

 

[Martin]

The wooden beam that makes the top of the roof rests on bricks and mortar, that wall having a strength of 13 inch. The force is straight down and I'm pretty confident that the bricks don't crumble. Compared to other houses I have seen, this is overdesigned. And when I used my ladder, there was no creaking of the wood at all, it was rock-solid. So I don't worry attaching my beam that pulls downwards.

My intention is to remove the crane and close the window when not needed, as I will use it only twice per year. This is why I rather not want to heavily overdesign it.
The aluminium ladder that I have used is 3" deep and it didn't break, so I feel the 6" that KC2IXE suggested are not unreasonable. Eventually, 750 Nm is not such a lot but I really lack the confidence not knowing how to calculate this properly.

 

[precisionworks]

That is one of the most scary rigs I've ever seen:whoopin:

You really want to contact a mechanical engineer on this one, as you need a no-fail calculation.

 

[Martin]

That is one of the most scary rigs I've ever seen:whoopin:

You really want to contact a mechanical engineer on this one, as you need a no-fail calculation.

Maybe scary but I'm not afraid. It's not even 10m..

Relax. A friend of mine offered to run a mechanical simulation, I just need to define the shape of the cross section and the material. He can then scale it to the strength I need. This is a way forward.

I also learned what's the German equivalent to the "Machinery's Handbook". It's Maschinenelemente by Roloff/Matek and Taschenbuch fuer den Maschinenbau by Dubbel.

 

[precisionworks]

I'm not afraid.

I wouldn't be afraid either, unless walking underneath while you were hoisting.

 

[mahoney]

Trolleys are typically designed for I-beams. You can also use them on back-to-back C-channels, and that would be a little lighter to mount/remove as you could do it one side at a time.

Engineering advice gotten over the internet from someone is worth less than what you are paying for it, and long distance engineering is only slightly more useless, but for an idea of some of the considerations involved...

If we look at the intended loads, you have the 30Kg for the kayak. You are moving the kayak, and we have to allow for accelerating force, we'll assume you are going to buy a SLOW (say less than 1m/5 sec) winch so we'll only multiply by 1.5. This leaves NO safety factor and rigging can involve a lot of unpredictable shock loads. For example, your kayak could swing and hang up on the overhang above the door and then slip free. Depending on how far it falls, enormous force can be applied to your beam. If someone watches the kayak go up from below, the minimum safety factor should be 8, if the yard is clear, you could potentially get away with a factor of 4, with the understanding that the rig could break if you are careless or something goes very wrong.

Just something to consider here, aluminium is much lighter than steel, but performs poorly when shock loaded, just the type of load you can get when rigging goes bad.

So assuming a clear yard and no neighbors in the path of falling beams or kayaks, 30 times 1.5 times 4 is 180 Kg. Your beam, anchorages (beam mounts), winch and trolley and any other bit of hardware in the load path (shackles, spansets, ...) have to be sized to take at least this much weight plus... Your winch and trolley weigh something as well, so let's allow 30 Kg for them (and the rope you are using to pull them in and out with, the pulley at the end of the beam, the extension cord...), and we need to consider the weight of the beam as well so lets allow 30Kg/m (assuming aluminium). I'm also assuming the winch is on the trolley, and not in the attic with the cable going over a pulley on the trolley. That produces compressive load in the beam and the math becomes much more complicated.

So, some fairly theoretical math done, the downward force on the mount where the beam exits the building would be 522 Kg. Upward force inside the building at the other end of the beam would be 138 Kg. Maximum moment in the beam would be 619 Kgf*m and maximum vertical and horizontal shear are 285 Kg. To progress further you need the mechanical properties of the beams available to you and the strength of the material.

These are not small values and I encourage you to get a proper engineer involved. What you want to do can probably be done safely, but I'm not sure it is a good do-it-yourself project.

 

[Martin]

mahoney, all excellent points. These become quite impressive figures, from a 30kg kayak.
I will update the input I gave to my friend who designs gear boxes for a living. He has state-of-the-art mech design software.
Hopefully the final design that has everything factored in doesn't turn out too heavy to be mounted / dismounted by hand. Otherwise I really have to figure out a way to support the beam on its end, without being in the way of the load.

 

[FredM]

Your winch and trolley weigh something as well, so let's allow 30 Kg for them (and the rope you are using to pull them in and out with, the pulley at the end of the beam, the extension cord...), and we need to consider the weight of the beam as well so lets allow 30Kg/m (assuming aluminium). So, some fairly theoretical math done, the downward force on the mount where the beam exits the building would be 522 Kg. .

30kg/m for aluminum? How thick will it be? Your numbers are crazy.

Here is the deal. Your boat weighs 30kg, if you raise it slowly technically there is a 30kg load on the "crane" once it gets moving. So I agree with the 1.5x assumtion made above for a large safety factor.

However, if it does say fall a bit for some reason. Chain gets tangled or something:

1 meter fall means it will going about 4.4m/s when it stops it will weigh about 100kg.

so above the safety factor of 4 seems ok as well so if it can handle 400kg you are ok for a hard fall and with a nice margin.

But in the real world you ladder worked so these numbers are useless.


This is what I would do as stated above by me and others.

Use C channel beams 4 of them total like this

][ ][

put your rollers on the inside edges and put stops at the end. You can source these beams anywhere as they make car ports and covered parking with them.

_____
][ ][

Bolt them together like above with some bracing. The c channel will be expensive so get galvanized. The best part is they will have the numbers for what one can hold when you buy it then you can decide how big to buy, 4 inch, 6 inch 8 inch, whatever. 4 of them will be strong as heck because 1 will be way stronger than your ladder.

 

[dom]

Hi Martin
Yes- i'm walking again thanks

I have to agree with Fred here -amazing that the ladder didn't snap.

I was going to suggest using the ladder on its edge as it will be much stronger but that doesn't give you much room to slide the boat thru the window frame -plus you probably already thought of that LOL

Cheers
Dom

 

[mahoney]

30 kg/m might, or might not, be high depending on what type of beam is used. The beauty of going a bit high on beam weight in the calculations is not having to do the calcs over when you find your beam is heavier than you allowed for. The contribution of the beam weight is only about 1/6th of the moment value, and although calculating for a lighter beam would lower the shear values, this beam is probably most likely to fail in moment.

It's a small miracle the ladder did not fail, but it also means nothing went wrong at the moment when it was used. Things like cranes are over engineered because "stuff happens". For example, so far we have ignored potential wind load...

The light gauge C-channels suggested are a possible solution, however they are an "open" shape, the depth to material thickness ratios are high, and they have stability issues (failure can occur by rolling to the side). They work well for roofs because the flanges are held in place full length by the deck. A couple of them bolted on the outside of a rectangular tube could solve some of those problems, but the shape would need to be analyzed by an engineer.

As a real world example, I once saw a fall protection demonstration where 150 lbs of weight was dropped 6' on a cable attached to a jib crane arm and the dynamometer recorded the force of the weight stopping at over 1,200 lbs. That's an 8:1 factor. I certainly hope it would be noticed that the kayak had snagged before enough slack was out to allow a 6' fall, but in the world of overhead lifting, 4:1 is not considered much of a safety factor.

 

[FredM]

but the shape would need to be analyzed by an engineer.

As a real world example, I once saw a fall protection demonstration where 150 lbs of weight was dropped 6' on a cable attached to a jib crane arm and the dynamometer recorded the force of the weight stopping at over 1,200 lbs. That's an 8:1 factor. I certainly hope it would be noticed that the kayak had snagged before enough slack was out to allow a 6' fall, but in the world of overhead lifting, 4:1 is not considered much of a safety factor.

Well that may be true for a factory, but this is a homebrew solution to doing it the hard way.

The force that a fall produces is extremely related to if the system flexes. If that crane arm you saw moved .1" or ,01" that would produce a much higher reading than one that had 2-3" of flex in it.

At any rate to calculate the force of a falling object you just take the speed when "caught" then over what length it is caught. This adds to the acceleration due to gravity and you recalculate the weight. It adds up fast for sure but in this application I cannot see it happening. I guess when lowering the boat, it is the most likely.

 

[Martin]

One issue with the ladder is that it flexes. It flexes half the hight of the window, about 0.5 m ! So I have to pull the boat in at an angle.
The flexing also makes the boat rock a little vertically, as I pull it up. So I pull, then wait until the ladder calms down. Pull more, wait..

Initially, the flexing worried me and I thought the ladder would soon snap. I called my neighbor to look at it and tell me what he thinks. He's a practical guy, doing lots of work on his home. He had abused ladders in a similar way and said they just flex like this, they can even take more. So I proceeded. It worked.

My conclusion is, that if the calculation yields a beam that's bigger than the ladder (2 beams of 71.5 x 20 x 1 mm) I can expect less flex and it would be OK.
If however the calculation yields a beam smaller than what equals the ladder, something must be wrong.

Right now I expect that I need 4kg of aluminium per meter, using a rectangular cross-section beam. To be confirmed by my friend's computer. He will add in the factors 1.5 and 4.