Engineering Question for repairing broken excavator booms.

[Portable Welder]

When repairing a broken arm on a backhoe or excavator boom or stick, should nice heavy reinforcing plates be added to the sides or is plating the top and bottom of the boom more important.

[theweldor]

I was always led to understand that the reenforcement plate should never be thicker than the original material.

[Portable Welder]

I would agree with that.

[Meltedmetal]

Do you guys see heavier plate for the repair as detrimental to the strength of the repair or just a waste of material? Why?---Meltedmetal

[MMW]

No engineer here but just my thoughts. I think it all depends on the original crack & where it formed & how/why it cracked. Most repairs seem to be different & one fix won't fit all.

Anytime you add a plate over the top it will transfer the stress to somewhere else. It can also become a stress point itself as the boom/stick can not flex evenly throughout it's length. If it is just a crack then I gouge it out & weld it up. Maybe a really big crack I might cut a slot & insert a backer for the weld. If it is completely broken & bent then I just use my judgment. Again I don't think one type of fix is right for all situations.

I think the important thing is to get a good repair before you plate it. I don't know how many times I have fixed something that has been plated & when I cut the old plate off I find bird crap underneath.

Here is a mini I did last year. Completely broke so I welded it up & plated it. You can see why it broke as this was the weakest point right where it transitioned & the thumb base plate ended. I plated both sides & extended the thumb plate up to spread the load. A larger machine I would probably do the same way.

[MMW]

To try & answer your original question I would say this. If the crack started on the sides only & was limited to the sides I would only add side plates. If the inside/bottom plate cracked first then I would plate the inside/bottom also. I have never seen a cracked boom/stick start on the top so I don't think that would need it as it almost always under compression. I never plate the top.

Again each job may require a different approach.

[Tryagn5]

When repairing a broken arm on a backhoe or excavator boom or stick, should nice heavy reinforcing plates be added to the sides or is plating the top and bottom of the boom more important.

Where the plate goes depends on the crack..however most booms crack from the inside out. The booms have webbing inside which act as stiffeners, unless this is fixed the boom will crack again. Older cat 345's were known for this. Sometimes better to replace a section of the boom, but depends on manufactor. What kind of excavator boom is it?
Kevin

[Portable Welder]

Tragn5 and MMW, thanks for the input.

Where I was going with this question is where is the most stress on the boom at!!!

The sides or the top and bottom, Many of the owners that I deal with have it in their head that they want a big plate on the side.

The way I have fixed them is to cut the side of the boom out so I can gusset the top and bottom from the inside, Tryagn5, you are right about some of them having a hidden gusset plate on the inside, on the last one I did I cut it out and replaced it with a new one, I then weld the side plate back in.

[Arizona Joe]

Tragn5 and MMW, thanks for the input.

Where I was going with this question is where is the most stress on the boom at!!!

The sides or the top and bottom, Many of the owners that I deal with have it in their head that they want a big plate on the side.

The way I have fixed them is to cut the side of the boom out so I can gusset the top and bottom from the inside, Tryagn5, you are right about some of them having a hidden gusset plate on the inside, on the last one I did I cut it out and replaced it with a new one, I then weld the side plate back in.

Besides bending stress, isn't the boom subject to torsional stress as well? In a square tube, the maximum torsional stress occurs at the corners. Is there anything to prevent bending and torsional loads from occurring simultaneously? If not, they will add. Hence, a failure cannot conclusively be attributed to bending only.