A metal beam design question

I am looking at designing a metal backhoe boom, looking for strength and minimal weight. Rather than using a structural steel beam, I am designing a
honeycomb metal box. As only three sides can be welded with the honeycomb structure, the fourth side being welded to close the box, I have a question about which three sides should be welded with the honeycomb structure. Obviously the two sides (left and right), but should the third side be the top which tends to operate in compression or the bottom which tends to operate in tension. If it makes any difference, one boom has a slight taper from end to end (side view), the other a bit of a curve, curving up in the center of the length (once again the side view). Anyone know?
Rick
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I think that if you do the analysis, a honeycomb structure is not the best approach here. A hollow box structure puts 100% of the metal as far as possible from the centroid (center in cross section) of the boom. A hollow box is also relatively easy to fabricate. Given the taper, one would need to bend the plate a little before you weld it, but heating it red hot would make that easier.
Honeycomb structures make sense for aircraft wings or boat hulls, which have relatively thin skins which need the internal support to prevent oil canning or excessive flex. 1/4 or 1/2 inch steel plate does not need the internal support, especially if the width of the boom is only a foot or so. If you had a boom that was 5 feet wide with 1/16 inch sheet, a honeycomb might make more sense.
Richard
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RJ wrote:

Have you actually calculated the saving in weight? By going to a lighter gauge of material in your boom, high stress points such as pivots and ram mounts may require reinforcement of a size that may negate any saving in the actual boom construction.
Tom
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With simple box beams such as for overhead cranes the tension side is last to go on and is not welded to internal baffles/stiffeners. The stiffeners are stitched to the side walls and top plate. In the case of a backhoe stick I doubt that internal stiffeners are as important as a smooth transition to the bucket mounts and hydraulic cylinder attachments. You might note the shape of outside doubler plates on a commercial boom. Randy
I am looking at designing a metal backhoe boom, looking for strength and minimal weight. Rather than using a structural steel beam, I am designing a honeycomb metal box. As only three sides can be welded with the honeycomb structure, the fourth side being welded to close the box, I have a question about which three sides should be welded with the honeycomb structure. Obviously the two sides (left and right), but should the third side be the top which tends to operate in compression or the bottom which tends to operate in tension. If it makes any difference, one boom has a slight taper from end to end (side view), the other a bit of a curve, curving up in the center of the length (once again the side view). Anyone know?
Rick
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R. Zimmerman wrote:

Is there such a thing as a tension side on a backhoe? They dig/pull, but they also lift. Unless I am missing something about the "anatomy" (which piece is the stick, where/how the cylinders attach, etc.), either side could be in tension or compression depending on the task. Is one or the other set of loads severely limited such that it does not matter?
Bill
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last
stiffeners
as
cylinder
For all practical purposes, lift is not much more then the weight of the booms and the material in the bucket. There will be times that objects will be lifted rather than dug from the ground. The tension side can be assumed to be the underside of the boom the majority of the time. Rick
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Wouldn't lift often far exceed that? I know there are times (such as stump pulling) where I have levered the entire tractor off the ground, only contact points being the stabilzer pads and the backhoe bucket. Total machine weight is in the 14k lb range.
It's equipped with a thumb, and I routinely move logs 16 feet x 16 in with it (pine).
It never fails to amaze me the amount of force that can be generated at the bucket once you start to understand the leverage of the hoe in different positions.
Leo
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A hollow box structure is indeed the most common form for a backhoe boom. JCB made a few with a lattice construction in the 1950s, but these are the only exception I've seen. I know you're probably building it for your own use, but make it strong as backhoes lead a hard life! Have you seen an operator lift the rear of the tractor using the backhoe, then shift it a foot or two to the side? With a strong backhoe it's a tempting thing to do, and of course it reverses the loading on the boom. If a boom or dipper fails, it will usually crack through the central pivot. The pivots are the places to add extra plates for reinforcement. The best advice I can give you is to look at some backhoes closely and see how they're built.
Best wishes,
Chris
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