telescoping tubing load question

I'm building an I-beam gantry. It's not an A-frame; rather, it is the kind with one vertical leg on each end. It will be adjustable for height.
I'm planning to use 4x4x.180" wall square steel tube for the outer leg, and 3-1/2x3-1/2x.180" wall square steel tube for the inner leg. To minimize rattle, I'm going to fix 16 gauge strips to the inside of the outer tube (straddling the weld seam) probably by making small holes in the outer tube and then plug welding the shims in place. I've pretty much investigated this, and I don't see why it won't work.
What I want to know is how far into the outer tube the inner tube must go for safety. Obviously a quarter of an inch is way way too little, and probably three feet is too conservative. The inner leg (the 3-1/2" stuff) pieces are going to be about 69" tall. The top eight or ten inches of each leg will be bolted to a short piece of the 4" tube which is attached to the bottom of the beam. (I want the gantry to disassemble as I can easily envision someone in my metalworking club needing to use it when they bring home a lathe or something.)
I'm planning this gantry to lift 4000 pounds. I really don't know where the safety point is, in how far I can let the inner leg piece stick out of the outer one.
Anyone?
Grant
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Grant Erwin writes:

You might just copy the Chinese shop-crane/engine hoist, with its variable telescoping positions.
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Grant Erwin wrote:

Got good insurance? It's one thing if you injure yourself with an improperly engineered hoist, quite another if someone you loan it to is injured. Be prepared to have people sign a liability release form when borrowing it.
There are plenty of properly engineered commercial units available. I'd suggest doing a lot of surfing to the various manufacturers sites to gather enough data to clone an existing unit. As it is I don't think your planned material sizes are adequate for your planned load.
I think the Harbor Freight unit uses material at least the size you're looking at and it's only 1T rated. One of the HF stores near me has the gantry on display where you can take a close look, perhaps one near you does. Other industrial supply places may have larger commercial units on display and area shops may have units they use that you might be able to spend a few minutes looking at.
At any rate you'll be much better off if you closely clone an existing unit. Heck, buy two of the cheap HF units, couple them together, add a hydraulic power pack, some wheel motors, etc. and make a mini version of one of the traveling gantry boat lifts :)
Pete C.
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You aren't going to get a good numeric/analytic answer to your question since the failure mode is going to be some weird deformation of the tube end. Example is a 2" receiver hitch on your truck that has the extra band around the end of the tube to deal with the deformation. You will probably be reduced to thinking about depth equal to some multiple of the tube size. Take a look at your receiver hitch for a good idea of what might be the right multiple.
The big problem with your design is not the 4000# vertical load, your uprights have a huge safety margin in compression. The issue is random lateral and torsional loads: what happens when you have a 3000# lathe hanging from it and hit a crack in the sidewalk? How about twisting of the beam when just one side or one caster hits the crack? The A frame gantries I've seen at HF and others are bolted together, much less flexing than your design.
Grant Erwin wrote:

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Roy is right. This is hard to model. Much harder that a simple member in tension, compression or bending. I thought about how I might model it, but I'm not going to try because the last time I tried something (simpler than this) I got it somewhat wrong. It would be hard to come out with a useful answer to this by modelling.
But you build loads of stuff, so you ought to have good intuition. If you aren't sure, err on the safe side. And after construction, test it with a heavier load than you will normally lift. If it breaks, at least it will break when you're expecting it.
How are you going to lock the telescoping section in its extended position so that it can't retract unexpectedly?
Best wishes,
Chris
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The outer leg will be drilled through in one place and the inner leg will be drilled through every 6" or so, and I'll use 1" steel pins.
GWE
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Drill every 3", you will be surprised how often you need an odd height. 1" pins are nice but consider the self locking tractor hitch pins. Hardened steel.
Grant Erwin wrote:

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    How about drilling the inner tube every six inches, and put two holes through the outer tube, separated by either 3" or 9" to let you get 3" increments in the extensions. That will reduce the total number of holes needed
    Good Luck,         DoN.
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DoN. Nichols wrote:

I don't buy the need for high resolution in overhead beam height. As long as it can go low enough to roll under my shop door, then go as high as it can go inside my shop, or max height outside, those are all I think I'll really use. Every six inches is mostly for easing height adjustment.
GWE
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    O.K. If RoyJ turns out to be right, a single extra hole in the outer tube 3" or 9" from the existing one will probably be easier to add than pulling the inner tube and adding a bunch of extra holes. Just something to keep in mind.
    Good Luck,         DoN.
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Drill inner at 3" outer at 2", more adjustment that way if i'm thinking right.
Wes
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Your design sounds more sturdy than most "landing legs" on Fifth-Wheel RVs and, for that matter, many gooseneck trailers.
You might want to consider checking around to see if you can get some "landing gear" salvaged from an OTR trailer: these have cranks for vertical adjustment and are capable of handling far more weight than your 2-ton target figure.
Good Luck with the project!
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TLAR engineering followed by testing.
If you test to a higher loading than you expect to use in service and the device doesn't deform then that would be one solution.
Of course this begs the question on what the design safety factor is on this gantry.
Wes
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