Optimal length of lag screws

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Through bolts, BIG washers or second plate.
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Look here: http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/fplgtr113.htm and specifically here: http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/ch07.pdf for application guidance regarding use of lag screws.
This (or any) design idea can't be meaningfully analyzed without at least a drawing (clear sketch)("worth 1000 words") of the configuration and loading directions envisioned. I suspect that each of your respondants has visualized something different. Generally, I too tend to favor bolts with nuts and washers. When all is said and done, the orientation and integrity of the proposed weld in the ring could still prove to be the weak link.
David Merrill

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I do not have a picture at the moment. The cart is made of three 2x12 6 ft long boards (lengthwise boards), and three 36" long 2x12 boards running perpendicularly to the 6 ft boards, under them.
=============================###### ###### ######
You are viewing these boards, such that you are seeing the long side of the top boards (depicted by ========), and only the cross section of the crossmember board, depicted by ######. If you now turned that T by 90 degrees, you would see
====== ====== =====####################
The weldment comprised of a 1/4" plate and eyebolt looks like this
!! !! ======== or
/~\ \ / =~~~
or
====!!=====o=!!=o=====!!=====o=!!=o=====!!=== The steel plate is roughly 6x2x1/4. The eyebolt is roughly 2" in diameter, 5/8" thick. The holes in the 1/4" plate are depicted by letters "o" on the last picture. They are 5/16" diameter and accommodate 5/16 by 5" long lag screws.
The top two lag screws are parallel to the grain of the top board. The bottom two lag screws are perpendicular to the grain of the bottom board.
I am least concerned about the weld.
i

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Ignoramus15296 wrote: ...

... Those two are in end grain and they're essentially worthless in tension.
You don't have nearly enough "beef" in the attachment. As long as you're only dragging it around on the ground all that will happen is it stops following, but you definitely have a serious hazard when/if you try to get that off the ground.
That isn't exactly the orientation I had envisioned as you're dragging crossways of the "runners" below instead of with them which will be the most friction you can have. I'd suggest two things...
1. Turn it around and pull from the 90-deg position from where you currently are. An additional "runner" underneath might be advantageous to cut down the friction depending on the surface.
2. I'd find or make a square 'U' bracket to go over the two pieces at the front edge and bolt thru the top/bottom w/ carriage bolts for a much more secure/stout connection. A heavy wraparound fence hinge strap for 3" thick might work well or just bend some flat.
Using your artwork,
---| ====== ====== ======|O <-- attachment point ring ####################| ----
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dpb wrote: ...

Actually, something that would work nearly as well and let you even use the lags (where I'd not complain too much :) ) would be
> > ====== ====== ======|O <-- attachment point ring > ####################| > ----
using an angle. Then the lags at both the top and the bottom are out of the end grain and the primary loading is lateral as opposed to in the withdrawal mode, a much more secure arrangement.
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