bending 4x4x.250 sqtube

[Sodo]

JB, I think that 24,000 lb rated hub might be "right in the hunt" for this project. Plus it's built to take a cyclic load for tens of thousands of miles, with a significant safety factor (DOT approval).

This log loader's gonna cook Mudd out of his house before it even scrounged a hundred 2000 lb logs at 16ft extension, I don't see the cycles adding up to anything significant.

Truck hub widened to 16 feet and runs over a rock, lifting inside tire off the ground. 1500 lbs at 16 feet creates the same moment on the axle stub as 24,000 lbs on just one wheel. 1500 x 16ft = 24,000 x 1ft

The FBD is not really correct, it's a simplification. The load actually comes in where the hub is affixed thus it can support MORE than 1500 lbs at 16 feet. This is just to make the point is I think that hub is OK. Can you look at my assumptions and give your opinion?

 

[jb1390]

JB, I think that 24,000 lb rated hub might be "right in the hunt" for this project. Plus it's built to take a cyclic load for tens of thousands of miles, with a significant safety factor (DOT approval).

This log loader's gonna cook Mudd out of his house before it even scrounged a hundred 2000 lb logs at 16ft extension, I don't see the cycles adding up to anything significant.

Can you look at my assumption here and give your opinion if it applies? Truck hub widened to 16 feet and runs over a rock, lifting inside tire off the ground. 1500 lbs creates the same moment on the axle stub as 24,000 lbs on just one wheel.

The drawing looks mostly correct. One minor correction in attached image. However, I'm not sure the assumptions are accurate. I'm not positive on the construction of all truck axles, but from what I've seen, the tire is often directly over the bearing, not a foot out like you have drawn. Also with dual tires, it takes quite a bit to get the 2nd (inner tire) completely unloaded like you have shown.

Regardless, the addition of a second support would make it much stronger, and remove the hub from a significant bending load.

My original concerns still exist however, that this kind of a design and build takes some engineering effort, especially to make it safe. An understanding of moments and resulting forces are critical to a safe design. This is not a log splitter build, and a failure due to something not considered could have serious consequences.

 

[Sodo]

Also with dual tires, it takes quite a bit to get the 2nd (inner tire) completely unloaded like you have shown.

Happens all the time. https://www.youtube.com/watch?v=Rtx3JxbL7tI

 

[jb1390]

I thought about the 4x6, but what I am running into is not having enough base metal to make the base mount. Right now as I have it drawn out, the boom mount is in the center on above my hub. The base pin for the cylinder is just inside the outer edge of the 20in wide base. I can slide the side mounts forward and mount the boom off center of the hub, probably wouldnt have to move it more than about a inch, but the more I move it off center, the more side load I place on the hub. Ideally, the boom would be mounted back of center and the cyl mounted forward of center to even out the load, I think?? I could always look for more metal to make the base out of, but used flat plate around here is hard to come by and expensive to buy new. Just going from 4x4 to 5x5 moved my top boom mount up almost 2 inches. I would have thought only 1/2in, the difference between the center of tube to outer edge, but it didnt work out that way. I havent said this yet, but the hub extends 6 inches up and is 8.5 inches wide in between the upright side mounts. Just doesnt give me a lot to play with. I'll take a pic of the setup to show what I mean.

Dont know if you can tell what the first pic is, its where I drew everything out on the garage floor. Piss on Autocad.

Mudd can you make some measurements on your spindle for the hub? I can run a quick calculation that would take many of the assumptions out of the equation.

Also I am not trying to rain on your parade whatsoever. I just know there are things that I have built that have failed in ways I had not considered until it happened. It's tough to consider all the resulting forces etc. I can try to help as I'm able to moving forward as you get dimensions worked out.

 

[Sodo]

Another way is to build the thing, crank all the relief valves down tight, slowly proof it on 3,000 lbs. Watching it carefully, how & where it flexes, maybe measure it and come back to the forum. Then dial the relief valves back to 2,000 lbs and you have sort of a 1.5 safety factor (for some conditions). That doesn't proof it for off-axis, dynamic loads, stabilizers sinking, etc,,, but maybe you can think of ways to get confidence there too.

4x6 is a good way to prevent a truss, but I don't see the reason, especially if it adds weight and causes trouble too.

 

[LD1]

4x6 is no heavier than 5x5, but alot stronger in the direction he needs it

 

[Sodo]

4x6 is no heavier than 5x5, but alot stronger in the direction he needs it

You were going to add " and in the direction that a truss could solve more elegantly with less cost and less weight." ????

Sorry ( not paid by the truss lobby, honest! )

 

[Shield Arc]

One good thing about trusses, they are fun to build. :thumbsup:

 

[LD1]

You were going to add " and in the direction that a truss could solve more elegantly with less cost and less weight." ????

Sorry ( not paid by the truss lobby, honest! )

Yes, trusses can gain alot of strength with little weight. Thats why we use them over our heads. But in his case of wanting to use a 5x5 and NO truss, the 4x6 is stronger and NOT heavier.

So, if a 5x5 NEEDS a truss when all said and done, it will be heavier than the 4x6.

 

[Sodo]

5x5 is probably strong enough with no truss, certainly with a flat plate doubler ontop of the 5x5. And of course the 4x6 is the same lbs/foot as the bare 5x5 and strongest. If a truss was approved then the 4x4 works fine.

I got these modulus numbers from http://www.cim.mcgill.ca/~paul/HollowStruct.pdf page 10 and 16. From this .pdf you can assume the relative bending strengths, and choose from the shapes your steel supplier has in stock.

4x4x1/4: 4.69 modulus see Page 16 (Z column)
5x5x1/4: 7.61 modulus page 16 Z
4x6x1/4: 8.53 modulus page 10 X-X axis, (Zx column) (stiffer in the x-x axis than the y-y axis, see diagram at top right of the rectangular)

Here's how the "bending strengths" work out. 5x5 (7.61) is 1.6 times the strength of the 4x4 (4.69). The 4x6 has a modulus of 8.53, almost twice as strong as the 4x4. The 5x5x5/16 at 9.16 is fully DOUBLE (but I doubt you can get 8 feet of that shape). These have the full steel weight along the entire beam (where its not needed). Because this "design" has (sort of) a point load at center from the 2 cylinders it lends itself to the efficiency of a truss design, where steel is added only where needed. Increase the truss offset and it can easily exceed the 8.53 in3 of the 4x6 for minimal extra weight. I'd guess that a truss with just a 4x4" block of sq tube at center will match the strength of the 4x6, but would have to get out a pencil.

I'm not really arguing the truss design just trying to impart some basic design skills for comparison during design. These are things the fabricator just sort of "feels" but often doesn't know how to put numbers to it. You may not know the "tons" the beam can support, but you can see from the numbers that one is twice as strong as the other in resisting a permanent bend). This can be very much different than the "flex comparison". Of course engineers cringe, but seat-of-the-pants design is done all day, every day, all over the world. This .pdf is a good reference if you know what to look at. Keep in mind it can be used wrong too.

======== attempt to show a simple example how this .pdf can be used ===========

There are two numbers you look at in these tables to compare materials, I and Z. I is related to the stiffness (amount of flex) and Z is related to its resistance to a permanent "bend". Example using these to important table numbers.
4x4x 1/4: 7.80 and 4.69.
5x5x 5/16: 19.0 and 9.16

Understood the 5/16 was not really discussed, I just chose it for the example because it's 2x stronger.

The 5x5 (x 5/16 !) is will take 9.16/4.69=1.95 times more load (lbs or tons) before taking a permanent 'set' (permanent bend)
The 4x4 is will flex 19.0/7.80=2.43 times farther than the 5x5 under the same load (also called displacement, movement)

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Mudd has a look that he wants though so there ya go, it's only a few $$ and a few lbs per foot to get that look he wants, & sounds like the cyls can handle it. Almost every project I do - the look I want is more important than what some very well informed & experienced forum guys suggest, and I can almost see them shaking their heads, but I do it the way I want. I'm just arguing like engineers do, wearing efficiency expert's hat, but its fully understood this is a man's PROJECT, there's one person to be satisfied!