Cranks

[Fingers]

So, I've been looking at why cranks fail and what we might be able to do about it. I don't see this turning into a product, so I am going to open this up to the community.

I'll open with a simplified model of the first throw of the crank and a representation of only the Max force from a single piston firing. Note the location of the concentrated stress. (red) Exactly where the cranks break.

 

[Kappa9012]

I struggle with the crank being the issue. At least from my experience with Cat iron, we always get reports of crankshaft failures and everyone that I've recalled and had metlab analysis ran on, comes back fine metallurgically, and everytime prior to the analysis being ran, I find out there was a spun bearing which annealed the crank pin.

so while my rant probably has little bearing, no pun intended, on the duramax failures, i'm just curious how many crank failures legitimately failed do to true fatigue vs. a localized heat issue from a bearing.

What size fillet is there where that stress is the highest.

 

[hondarider552]

The cranks that I have seen all had exact same beach marks in them at the break, which indicates fatigue fracture.

 

[dirty_max]

I struggle with the crank being the issue. At least from my experience with Cat iron, we always get reports of crankshaft failures and everyone that I've recalled and had metlab analysis ran on, comes back fine metallurgically, and everytime prior to the analysis being ran, I find out there was a spun bearing which annealed the crank pin.

so while my rant probably has little bearing, no pun intended, on the duramax failures, i'm just curious how many crank failures legitimately failed do to true fatigue vs. a localized heat issue from a bearing.

What size fillet is there where that stress is the highest.

ive seen quite a few cat cranks break and there were no spun bearings on any of them

 

[Fingers]

Radius is 0.125"

It appears to be a fatigue failure on all those that I have examined or seen pictures of.

 

[Burn Down]

This is a crank from a member on here. I had the NDT guys at work stick it under there microscope 1x power iirc. The first pic shows what appears to be a sub surface inclusion in the fillet. That is the worst possible spot have a defect. I have been waiting for a large engine engineer with a degree in metalurgy to have a look at it before I posted about it but there travel funds have been cut drastically. Not sure when I will get a chance to show him the crank?

Second picture shows the fillet and it is not discolored nor did it show signs of a spun bearing upon disassembly.

Another straight on shot of the inclusion.

From what Fingers posted on the stress areas of the crank being the greatest in the fillet area, it explains this failure pretty well. The inclusion gave a place for the crack to start. The fine beach marks radiating away from the crack initiation site show this as a Bending fatigue fracture. These types of cracks are slow growing.

No crank can survive with flaws in certain areas... I think alot of these failures can be traced back to a flaw or spun bearing something... It explains why some hold riduclous power and others break stock.

 

[SSchmi5519]

A characteristic of mass production.

 

[SmokeShow]

This should be awesome!! :thumb:

 

[Fingers]

Now let me throw another tid-bit at you. The initiation points are rotated from dead center by 10 - 20 degrees. Sometimes more. Always in the same direction.....

 

[delong_1]

I will throw in my bit of info on the subject, my crank snapped in the same area as most. no spun bearing. My good friend works for cat took a look at it, he said that it looked to be a mfg flaw. the outer edge has smoothed fracture lines and got more rough toward the center meaning that it had been separated/separating for a long time.

 

[nwpadmax]

Now let me throw another tid-bit at you. The initiation points are rotated from dead center by 10 - 20 degrees. Sometimes more. Always in the same direction.....

Is cylinder pressure typically a maximum in that region?

 

[McRat]

The following is Probably Incorrect:

The nose of the crank has low rotational mass and low resistance.
The rear has high rotational mass and high resistance.
The middle has high resistance.

I think it's twisting at the front from unequal load. One side has a huge amount of mass and resistance, and the other side is "easy".

But it seems some break with little abuse, and others take huge abuse for years. So it's a combo of that being the most unequal loading and a mfr defect combined, hence why they break in the same place most (but not all) the time.

 

[McRat]

So the solution is to put a huge freakin' Roots Blower on the nose to add mass and resistance.

 

[Burn Down]

The following is Probably Incorrect:

The nose of the crank has low rotational mass and low resistance.
The rear has high rotational mass and high resistance.
The middle has high resistance.

I think it's twisting at the front from unequal load. One side has a huge amount of mass and resistance, and the other side is "easy".

But it seems some break with little abuse, and others take huge abuse for years. So it's a combo of that being the most unequal loading and a mfr defect combined, hence why they break in the same place most (but not all) the time.

I disagree on the twisting Pat. A Torsional fracture looks completley different from a Bending Fatigue Fracture. I do think your on to something possibly with the different weight/inertia between the front and rear of the crank. I also think flaws in the casting are a big/biggest contributer, I think I read somewhere you had mentioned that as well?

 

[Burn Down]

Now let me throw another tid-bit at you. The initiation points are rotated from dead center by 10 - 20 degrees. Sometimes more. Always in the same direction.....

I don't follow you? What relation to tdc or bdc? I would have to look at this crank again but I thought the crack was on the side of the counterweight. Which makes sense to me looking at your stress chart. The biggest stress is at TDC cylinder firing.

Do you think empire is on to something with there different firing order cam shaft? I haven't studied it enough, might just be over my head

 

[1slowmax]

Good stuff:thumb:

 

[Pure Diesel]

I didn't stay at a "Holiday Inn" so I can't help on this.

 

[05chevy]

Dumb question but most of the cranks with failures. Where they stock, built, balanced, unbalanced or a little mixture of all?

I can understand if the crank has a weak spot or edge from manufacturing that would cause a starting point. But wouldn't a balanced rotating assembly have less of a chance of this happening. Wouldn't the machine shop or engine builder look over the crank shaft for these flaws?

Just asking because I'm curious. Great info tho

 

[smarquis]

Subscribe. My crank was bent, bearing was pretty good and had not damaged the crank.

 

[juddski88]

Magnafluxing the crank doesn't catch these casting flaws, so it really is just the luck of the draw then? I mean, there are ways to keep stress off of it obviously to prolong its life, and to me every bit (logical setup) is worth it regardless if it breaks or not, but in the end if it's meant to be, you ain't going to change fate. yippee