Excavator Boom Weld Inspection at the Foot: How to Identify a Repaired Crack Before Purchase

“Seventy-two hundred,” he said, repeated it as though for emphasis, “seventy-two hundred.” It is bad practice and I am in a car park with a paper cup on the bonnet of my truck going cold as warm body heat moves up from my ear with the phone being hot against it and I have already decided I am not a fan of this creature before he finishes the sentence with, “My bad. What nobody noticed in that call, is that he wasn’t selling an excavator. He was touting a thumb — the watery part on the stick, and he spouted on and on about it, until I realized that out of all the palterings I heard in response to my priced questions, maybe raising my offer by ten bucks was the safest bet considering that this was the “one thing” he was bragging about, the one thing that would certainly have “broken the boom foot” if it hadn’t been watery. I consumed the tea without its being hot.

The boom foot is not too difficult. It is brutal. Because boom axis creates a horizontal digging force, in a boom vertical lift load enters via the cylinder geometry, and at that pin cluster the two become one and deal the highest combined stress anywhere on the assembly; which is why nobody at the counter when they’re shopping for a boom, says, “Kids, remember to check the boom foot stress concentration”. I knew about load path thanks to a structural engineer who used a dry marker and white board to explain it to me. In nine words I was told by a fitter with a grease gun, and I believe him/her.

Fatigue never comes along at an appropriate time. It begins at the weld toe because it is a “geomatrix discontinuity” and geometry “focuses stress. The report the multiplier corresponds to academic. A yard reads something like, “One, Two, Three, No, No, No,” You may not realise but you surely can feel it. I crouched and lay down next to a machine at idle and the vibrations appeared as hard and crisp a buzz as it did soft and the stiffness was more telling about joint loading than any hour meter.

Then the thumb goes on and the mathematics becomes different. The mechanism of the raise in stress in the foot due to mounting gusset in the thumb is more important than the 20-40% in the band based on the geometry. This gusset weld termination is normally within a hand’s access from the boom foot toe and it becomes a new fatigue initiation point, a fresh stress riser being bolted onto the existing and already most highly-stressed point in the machine.

It’s a different animal when an OEM thumb is blended under developed conditions and stress-relieved as per the design of the factory. Mostly fine. Flame-cut gusset edges have been hand scraped ragged with thumb, because no one was going to get a coupler on that fitting in there again after the paint went on and that elbow plug of grease that clears the gusset now that the pin is installed hasn’t been found in years, which is why the pin is seizing, wearing oval, and why the inside out crack develops in the boss at a load no one ever accesses. It’s the lattermost portion that obtains individuals.

He requested a pricing quote for repairing. I told him it was too early to tell how much it would cost to repair and that what he really was getting was one of three futures which only one could be seen from his position crouching and holding a torch in his teeth. Without magnetic particle inspection there is no way to tell how far down that indication is and depth is the only variable which determines if that indication is a cosmetic toe defect, or a point where a third of the ligament is now lost. Then you begin pricing a spread of the outcomes and that just isn’t nearly as warm and fuzzy an experience in a yard at 4pm with the seller’s cousin standing at the office door.

This repair documented is outcome one. On that machine I have been remarkably hard-boiled in resisting the impulse of the owner to leave the machine in some kind of primitively intact condition, for some human being has discovered the crack, gouged it to sound metal, welded, dressed the toe, and agreed stop holes and left the ticket — and I have argued that that machine, in its gouged to sound metal and welded and superficially reconstructed condition, is a more solid piece of machinery than one left untouched with a potential toe defect lurking ready for identification. Repeat of a repair, which is already known. The other one (untouched) is a question mark, in factory paint.

What most of them are, is the undocumented repair (outcome two) What you see: The weld is the wrong place and time, the grind marks are as well, the record orality is a text message “boom was fixed” — no date, no welder, no procedure, no consumable spec and the discount isn’t the cost of a weld either, it’s the cost of a hypothetical full boom-foot replacement because the joint gave when it was time, and not at 60 hours. No one takes a positive view of that discount. They come out with an as-welder day rate and they are hoping you’re punuscient.

Outcome 3 provides a conclusion to the conversation. But not to be the drippiest of all flaws, the paint film. A hand’s feel for film thickness is a true tool and it is above approx. thirty-five degrees, when the film would start to soften and the tip of the finger would drag rather than slide a thick “rattle-can” film layer starts to read smooth “factory coat” just when you need to catch it the most. One time it got the better of me in July and still gives me flashbacks of me moving my thumb all the way across this bead.

Let him know that you are looking for a patch of 30mms at the toe, and give him two seconds to wire-brush it. The difference between that is the data! If he does say yes and he then marches off for a brush, then you have discovered that the machine will likely be sincere and he will be as well. If he doesn’t want to do it because it will get dirty, that’s a comment on the presentation for Resale — that does not disqualify. If he once more refuses KOR will immediately drop the price to two thousand, and that is your clue that he would rather groosen around two thousand bucks than bring a few gallons of paint, for the information is well below him saying, “Well, I’d rather move money than move paint. If he refuses, and immediately offers two thousand off instead, that is the tipoff that a man who would rather move money than move paint has just told you what he thinks is underneath it, without telling you a single honest sentence.

What’s more difficult is the quick to agree seller with a pre-moistened solvent in the brush. That person has chipped that area – likely this week. Bead on bead, toe unwrapped, the original crack profile still evident through the new metal – somebody had to have found the crack, mended and then painted it prior to the arrival of this machine. That isn’t a maintenance record, it’s a choice that someone made about you.

Bead-on-bead reading is the last stage of the negotiation, at this point there is no more talking about money, only a consideration of good etiquette for the walk to your truck.

My rule is short. If the light test results in ghost when performed through a fresh bead and the seller will NOT allow the brush near the bead within 30 millimetres of the bead I walk away. When machine is equipped with a gusset and is ready for the yard, two spotless pockets and two gloss levels are available in a single square, I am outta here. If they don’t have a document indicating who is doing what at a drilled hole at the toe, then I leave too, irrespective of the hole that is being drilled — a form of silence is wrong.

After that it was all agreed. This explanation might be helpful when reconsidering why this fails to read the same way on a 305-Class machine. But now as I write it I can’t help but think of a ridiculously stupid incident years ago of a man buying a boat for more than its worth because the boat’s trailer had new tyres, and he never even touched the teak after he bought it. Value added by people for the accessory. Tremendously few individuals put any value on the structure that it is attached on. That is all the disease in one anecdote!

Last spring, I caught sight of three of these machines that came through our yard, all of which were “boom footached”. I can’t do any statistics to explain that. Actually this class appears this way because most often this class is purchased by a landscaper or small contractor with a fencing/drainage operation and is used in the field and thumb inserted the first year, then services are provided for two years and then the machine is sold — thus putting the majority of the work on this class and almost none of that work on the invoice.

I did have one last April which I’m still using for the teaching example. Ninety-four hundred, on the job, thumb fresh out from the construction, foot scrubbed up and sprayed within the last two weeks, the solvent fumes in the recess could be noticed — that thin and sweet tang that shouldn’t be on a working machine. I measured the boom plate in 4 places with a caliper and found it to be between nineteen and twenty-two millimetres, the sixteen-to-twenty-five margin for which booms come depending on the style indicating the extent to which ones would be higher having been through the sight of my eyes. If it’s got this much section behind it, it will be a quiet crack, working month after month with no complaints, until the time for it to be dealt with is right here and someone is pushing on it, and he or she depends on this machine leaving their yard to never have to pay somebody to chase around a slow problem.

While frequently paint was left on a bead, I dragged a file across and it did not give a harder dry drag than virgin plate and that meant it was fresh under the paint. The hydraulic oil was light brown and there were a few bubbles with the dipstick – these were gone after about 1 second and it should look like this in a healthy machine. Out of the drive up I had a hardtack biscuit in my jacket’s pocket. It was stale. During the time I was eating it I made up my mind. Eleven minutes later, the decision was made and it was ‘No’.

What’s more complicated, however, is that the 305 class is so common — and flipped so many times — that the boom foot isn’t a footnote in the table within the hydraulics section of the flipping write-up — it is actually the main buying document in the cat 305 excavator section — and I no longer carry a checklist in the truck, I carry a caliper instead.

I’m asked about things in yards. Typically when I’m still on my hands and knees, typically when the seller is able to hear. Answers must be brief, as it is brief the situation affords.

Please walk the foot first. Not the hour meter. Not the tracks. Not the cab and certainly not wherever the seller wants to stand as that’s normally in the best lighting.