Machine Hoist Build/Questions

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Hey weldingweb! Bostick here with a few questions and a story to tell. I recently came across a "new" 3 ton chain fall for $50 bucks at a yard sale. Couldn't pass this item up. Now that being said I am a rig welder and I carry my machine in the back of my truck and seeing how I do not have a shop or a large tree lying around, I am planning on building a H-frame or an A frame hoist in use with the chain fall. Now my question is: What is better? Square tubing, 2x2 3x3 or 4x4 or 2" carbon steel pipe. Ive been a welder for a little while and know a little about metallurgy and such but not sure as to the exact amount of weight it would take to bend the pipe or tubing. The welding machine in question is roughly 1300lbs with fuel and fluids. Any help on a design or info would be greatly appreciated. ~1987 Lincoln Sa-200~~1978 Miller Big 40~ (restored) ~and everything inbetween~
Reply:Are you asking about the legs or the beam? No way I would use any of those for the beam. I'd want to use a wideflange/H beam or I beam/S shape for the beam. Size of the beam will be dictated by span and load.Wallace lists their A frames at 6" beams for 1 ton cap, 10" beams for 2 ton  and 3 ton capacity units, assuming 10' spans. They don't list anything but beam height, so I don't know what the weight of the beam is for sizing. This would make a big difference when you start talking about the 2 and 3 ton units. I'm betting the 3 ton beams are heavier than the 2 ton units even though the height is both listed at 10".http://www.wallacecranes.com/When in doubt, go heavier. Over head lifting things are not where you want to cheap out. If you must go cheap, go taller, not lighter. Taller lighter beams tend to hold more load..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:I concur with you but how does this work then? I used one of these before to load my machine too and from my truck. It was a smaller truck but same design, and the 2" pipes supported the load.~1987 Lincoln Sa-200~~1978 Miller Big 40~ (restored) ~and everything inbetween~
Reply:Pipes in that are not in bending. They are strictly compression members even though they are on an angle. The tension is done with the cables going back by the cab. What you have there would be a type of truss.The legs on a crane don't need to be as heavy as the beam, even though they are supporting the same weight. It's HOW those pieces are used to support the weight that makes a big difference. Try and stand on a 2" piece of pipe spanning 10 foot and watch how much it deflects in bending. Then stand that same 2" piece of pipe vertical and load it only in compression and it will now take a lot more load.H beams are the simplest way to resist bending, but not the only way. Bar joists use light weight members jet can resist quite a bit of load. The reason is just like the poles on that jib boom, they work very efficiently, putting the steel in the best location based on it's design. Problem is to design an I bar truss to hold your load, you'd need to be an engineer.I'm also assuming you want to get the truck under the gantry ( which makes the most sense. Shorten the span, and you reduce the bending forces given the same weight. I have seen short beam cranes built where the legs are designed to splay in. However it complicates the design issues vs straight legs on each side using a wider beam. I think Kevin Morin has a splayed legged hoist he posted up for boat work in the past..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan