bending 4x4x.250 sqtube

jb1390 · May 5, 2015

Sodo said:
OK solidworks. I thought that was about $5,000 program & $thousands per year fees, am I right? I've looked into that and wished for it but can't justify it.

I read somewhere that garden-variety Sq tube in the USA is generally A513, ~74,000 yield strength. I've built stuff and tested deflection and found that (the rect tube I have access to) does not bend permanently at ~74,000 psi stress. I'm inclined to believe my supplier is providing A513. YMMV of course.

Can you run that deflection test with the main boom made with doubled 2 x 4 x.25 in? How about a 4 x 4 x .25 with a 4" x 3/16 flatbar doubler welded ontop covering 3 feet of the high stress area?

I'm not sure what my supplier provides for box tubing - but I have some at home, I'll try to remember to see if it's marked.

I'll run the analyses at some point. Solidworks is expensive yes, but I use it for work so I have access to it for a personal project or two. Back in the day they didn't require the licenses to run, so if you could get ahold of older installation discs you might be able to use it at home on the cheap. You wouldn't be able to open more current model files, but that won't matter unless you're file sharing with other people.

The drawback with the analysis is really the inputs. If I have an idea of what size pistons, etc, I can get much better numbers for the OP. The analyses I ran don't really say if a design will fail per se, they just compare design a and design b using arbitrary inputs.

muddstopper · May 5, 2015

JB, this pic that sodo posted,

423122d1430524691-bending-4x4x-250-sqtube-z9182-jpg

is pretty darn close to what I am trying to build. My goal is to keep it as light as possible, but dont want to sacrifice strength just for the sake of keeping it light. There are 2 sections of the boom, both sections are supposed to be 8 ft long. I may reconsider the extended reach to 14ft instead of 16ft if thats what it takes to keep the boom light. At the end of the outer boom(dipper boom), I will be building a light weight grapple. Since the grapple hasnt been built, I am just guessing it will weight around 200lbs. I am building this boom out of whatever parts I can source at the scrap yard. The cylinders I will be using are 3.5in bore, rod of 1.5in retracted length is 34.5 inches. Extended 58.5in to center of pins for a 24inch stroke. My base mount is 20in dia 1in flat plate. The side supports for my boom are 1/2in flat plate which will be doubled in thickness, but its 20"x20" square so thats what I have to work with. The max estimated weight I intend to pick up, not including boom and grapple weight, is about 2000lbs. My base swivel is a 48000lb rear truck hub assembly. I will be turning the boom using a 72tooth double #60 roller sprocket and chain assembly and hydraulic stator rotor motor.

I have been figureing on this for weeks now and still am not any closer to actually welding anything together. I have figured out all the mounting points using 4in sq tube. If I go with some other size of tube, it will mean refigureing the mounting points, but now that I have done it once, I believe I can do it again. My main concern is using to light of tubing and having the boom bend under load. Any help you can give would be greatly appreciated.

FarmWrench · May 5, 2015

I get the copy an existing thing idea but maybe it is not the right thing TO copy. A gin pole with cable grapple (old school shovel crane with a clamshell but simplify). You could save a lot of weight.

Before you doubt the ability of the design Google "rock hawk" a small jib and grapple for the front of your BIG tractor to pickup BIG rocks.

Xfaxman · May 6, 2015

FarmWrench said:
I get the copy an existing thing idea but maybe it is not the right thing TO copy. A gin pole with cable grapple (old school shovel crane with a clamshell but simplify). You could save a lot of weight.

Before you doubt the ability of the design Google "rock hawk" a small jib and grapple for the front of your BIG tractor to pickup BIG rocks.

Is this it? Rock Hawk - Ag Storm Equipment

LD1 · May 6, 2015

Looks like an interesting project.

My 2 cents is you either need to use larger steel, or truss the top as mentioned. 4x4 with 2k is too close. I would want at least a 2x safety factor. And not for safety reasons. But dynamic loads. Picture this.....You have 2k hanging on the end at some elevation. Bump the boom lever down a bit and sharply let off the lever. As it comes to a quick stop, that 2k hanging on the end causes a big spike. Without doublechecking the numbers, if sodo is right and 2400 is a permanent bend, and you have 2k on there and do this, you will have a permanent bend.

You mentioned going to 5x5, but think 4" width is going to be plenty stiff for lateral loading. Why not consider a 4x6 tube.

Sodo · May 6, 2015

muddstopper said:
My goal is to keep it as light as possible, but dont want to sacrifice strength just for the sake of keeping it light.

If you want it light you have to truss it to get the strength. There's a reason you see trusses in engineered structures. A bridge, for example has to carry its own weight plus its payload, and minimize material costs too. Aircraft. Race cars. A truss is very very easy cheap, fast, effective.

A truss ontop retains the 4" width. I'd use the 'bowed' one for the dipper stick part, and position it in the "happy" direction. I'm imagining it's bowed very slightly.

You can commence to build this whole thing without the truss. Then carefully lift 1,000 lbs, 1,500 lbs, 2,000 lbs. Stretch a string (along centerline of beams) & measure the deflection. If not satisfactory a truss can be easily added after testing and it will solve the problem, for sure.

That side plate on the LEMCO is efficient manufacturing, and probably the beam is oversized for use by lunk-heads, and to discourage lawyering, but it costs payload. I like trusses, for the fact that they don't create areas of concentrated stress like that side plate will on a beam that will flex like the 4". Just my preference,,,,,,

LD1 · May 6, 2015

Not sure if you already have cylinders or not, but I would be very careful about selection.

You want the framework to be able to handle whatever the cylinders can deliver. So if you already have cylinders, build strong enough to handle what you got. If you dont have cylinders yet, size them for the rated load and do more. That way if you try to lift something too heavy, it will just fail to lift rather than destroy the boom.

muddstopper · May 6, 2015

Well the engineers at work claim my 4x4 tube will support 3800lb overhung braced in the middle of 8ft. I explained that 8ft is just the root boom and the dipper boom would add 8 more ft to the overhang, right off said then it was going to bend, but he would do the math and get back to me. Well he never got back to me so i still dont know what I need to know.

Looking at the Lemco and where the cylinders are mounted, well thats almost exactly what I had in mine. I feel the plating for the cyl mounts should add some strenght, but dont know how much. I am not opposed to the truss ideal, but would prefer not to use trusses simply because of eye appeal. I would rather it look ugly than fail, so the final build will probably include some sort of truss. The ideal of build it then add weight until it gives, then add a truss if needed, I had considered. Right now the only way I see not to use a truss is One, use bigger tube. Two, add plenty of plate in the middle of the boom for the cyl mounts and let that be the extra strenght. Three, use 2x4 tube and double it along with the extra plating. Right now I think all three methods will work, but will have to see what I can scrounge before actually building anything.

muddstopper · May 6, 2015

LD1 said:
Not sure if you already have cylinders or not, but I would be very careful about selection.

You want the framework to be able to handle whatever the cylinders can deliver. So if you already have cylinders, build strong enough to handle what you got. If you dont have cylinders yet, size them for the rated load and do more. That way if you try to lift something too heavy, it will just fail to lift rather than destroy the boom.

Cyl power can be handled with pressure reliefs. I already have 2 3.5in bore x 24in stroke cyl. Got the cylinders first and then decided to build a loader with them. Thats the way it is when scrounging, find something that looks useful and then figure out something useful to use them for.