Cummins 5.9 rod separation - root cause?
Cummins 5.9 rod separation - root cause?
(OP)
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See this link for photos of a Cummins B5.9 connecting rod that failed by separation of the cap from the rod. It is pretty self evident that the failure mode was one of the capscrews backed out over a period of engine operation. The question is, what was the root cause?
This is from a fairly seasoned engine, approximately 225,000 mi, so it is not a case of infant mortality. There is no evidence or reason to think that the bottom end of the engine was touched since its manufacture, or that the engine was subjected to any untoward abuse such as extreme over-revving. None of the other connecting rods show any sign of distress. The other connecting rods were disassembled with an air wrench, so break-away torque data are not available, but the mechanic reported that the break-away torque seemed normal for the remaining rods.
Has anyone seen this before? Any theories for the root cause?
See this link for photos of a Cummins B5.9 connecting rod that failed by separation of the cap from the rod. It is pretty self evident that the failure mode was one of the capscrews backed out over a period of engine operation. The question is, what was the root cause?
This is from a fairly seasoned engine, approximately 225,000 mi, so it is not a case of infant mortality. There is no evidence or reason to think that the bottom end of the engine was touched since its manufacture, or that the engine was subjected to any untoward abuse such as extreme over-revving. None of the other connecting rods show any sign of distress. The other connecting rods were disassembled with an air wrench, so break-away torque data are not available, but the mechanic reported that the break-away torque seemed normal for the remaining rods.
Has anyone seen this before? Any theories for the root cause?
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RE: Cummins 5.9 rod separation - root cause?
Over torqued at factory.
Faulty bolt.
Any of the above may have been only to a minor extent so it took years to crack then fail.
Regards
Pat
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RE: Cummins 5.9 rod separation - root cause?
I forgot what I was going to say
RE: Cummins 5.9 rod separation - root cause?
ISZ
RE: Cummins 5.9 rod separation - root cause?
Regards
Pat
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RE: Cummins 5.9 rod separation - root cause?
I forgot what I was going to say
RE: Cummins 5.9 rod separation - root cause?
The surviving rod bolt sure gave it the good fight.
Dan T
RE: Cummins 5.9 rod separation - root cause?
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In the center of the respective photos of the cap and the capscrew flange, there is a tiny gouge, in the 3 O'clock position (deliberately oriented for the photo; it is not conclusive that these marks are related to each other.
One interpretation of these marks is that a hard foreign particle was trapped between the capscrew flange and the cap when the capscrew was installed and torqued, which as Pat suggested, may have falsified the torque and hence the preload.
A difficulty with this theory is that one would expect more of a ring-shaped scratch on the surfaces, created during the final torquing of the capscrew, rather than just a single pit.
Any other theories?
Tmoose, I fully agree with your comments!
I forgot what I was going to say
RE: Cummins 5.9 rod separation - root cause?
I'd like to see some better pictures of the bolt head face, there appears to be some signs of fretting and would be nice to have a better look.
The bolt seat on the cap shows an uneven contact pattern, subtle but there, there are a couple of smaller scratches that run radial, the elongated pit you see may have been caused by a larger piece of debris under the bolt head. The machine marks in this area indicate a rougher surface finish, could be the piece of debris caught a slight edge and the bolt head forced it into the rod.
The parting face of the con rod also appears to have a fairly rough surface finish, the machine marks running parallel to the rod bore seem fairly pronounced. Take a look at that area with some magnification, about 10X, see if you can determine signs of longer term fretting. I know its hard now since the cap got knocked around a bit.
I have seen similar failures, all higher time, most of which we found signs of either a poor surface finish that yeilded in long term use and reduced joint tension. We did have a group of bolts that had poor surface finish on the bolt head face area, causing a similar problem, all near the 8-10k hour mark. We also had a batch of REMAN rods that had similar failures we traced back to a surface finish problem on the parting face, was hard to tell visually, but depending on engine load factor can make a difference.
Hope that helps.
RE: Cummins 5.9 rod separation - root cause?
RE: Cummins 5.9 rod separation - root cause?
I forgot what I was going to say
RE: Cummins 5.9 rod separation - root cause?
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They are tagged to indicate the specific location being depicted.
The machining quality of the rod/cap parting surface does not look that great. The machining marks indicate tearing rather than a clean even toolmark. The last 4 photos are of one of the non-failed rods. The same issue is apparent, but to my examination not as bad as the failed rod. It's hard to be sure that the difference is not merely due to the different history of the failed rod once the separation began to occur.
As a check, I looked at one of the main caps and the mating surface is about the same as the rods'.
So, is this machining quality typical, or below par? Is it a tool issue, material issue, or procedure (e.g. cutting speed/depth) issue?
I forgot what I was going to say
RE: Cummins 5.9 rod separation - root cause?
While it's hard to firmly declare a root cause based on what we can see here, so far there are some things we can say don't appear to be the root cause.
The bolt that backed out does not appear to have been over torqued on assembly, the threads don't looked rolled like you'd normally see with an over torque, and there is no necking or plastic deformation. The polishing on the threads is typical of a loose bolt, at least based on the reference material and experience I have.
Nothing in the pictures I can see indicates a fatigue type failure, the metal surfaces aren't discolored so it looks like there was plenty of lube present while in operation. Because this was limited to a single rod and the other rods don't seem to show early signs of a similar problem it doesn't look like there was an overloading, poor driving habit like excessive lugging, or some other overall engine problem.
I'm not sure if these parting faces are machined or ground, but they sure look not so great to me. A rod sure takes a lot of abuse in a modern engine, and a lot of forces are transmitted thru them, doesn't take much of a problem to eventually cause a failure. It looks like either the cutter was getting dull or the grinding wheel was loaded.
If the rod parting faces were moving in relation to each other they would likely either show signs of fretting or polishing. I can't think of any failure mode that would make machine marks appear more pronounced.
My two cents worth, hope it helps. Mike L.
RE: Cummins 5.9 rod separation - root cause?
I forgot what I was going to say
RE: Cummins 5.9 rod separation - root cause?
OEM passenger car rods varied Enormously in quality of parting face finish. For a time Some SB Chevies were a torn up mess. the accuracy of the parting face geometry and other features could also be completely haphazard. Some FORD rods (consistently) had the OEM parting face generated at an angle to the bolt holes and parting face such that when the newly machined parts were assembled , before full torque, there might be a 0.015-0.030 inch gap on one edge. IN retrospect the way the face "cleaned up" was a clue there was trouble, because the first Sunnen cut was only on one edge, but much heavier than expected because the height above the grinding wheel of the cap-to-be-machined was referenced on a center line. Pretty quick we learned to back the wheel down, for the first cut, just in case. Also, those angled rodz-n-capz were shimmed to cut and maintain their original "mostake."
RE: Cummins 5.9 rod separation - root cause?
RE: Cummins 5.9 rod separation - root cause?
I forgot what I was going to say
RE: Cummins 5.9 rod separation - root cause?
While it looks like the bearing spun in the bore, the rod bearing bore surfaces from the pictures don't look like it did spin for long and as the rod opened up things got bad rapidly. Also looking at the crank pin can be helpful, as it can give some clues as to what happened, but sometimes the follow-on damage is too severe.
Getting the parts in hand and doing a really good lookover is the best way. When I did this stuff all the time many times it was a tiny detail that lead you to the exact root cause. And a lot of times the actual root cause couldn't be determined, but we did have a pretty good list of what we didn't think caused the failure.
Back when we were serious about applied failure analysis, a single rod bearing failure would typically generate about 40-50 pictures and a 12-20 page report. The failed rod and bearing (both shells front and back) both adjacent rods and their bearings, failed crank pin and both adjacent crank pins, and adjacent mains if determined they may have evidence to contribute, top of piston, piston pin and rod small end bearing, and if the bolts were suspected, a good number of pictures of them. But do that much anymore.
Hope that helps,
Mike L
RE: Cummins 5.9 rod separation - root cause?
8D investigations, a good methodology. It always seemed to me that the last D, "Recognition for the Team", was only window dressing and always just got lip service, so to speak.
I forgot what I was going to say
RE: Cummins 5.9 rod separation - root cause?
It was an interesting exercise, thanks.