Cranks

[Fingers]

See how the break follows an almost perfect radius in the middle and then goes jagged at the left and right side? Where the jaggedness starts, there is a slight rub mark on the fillet.

And no, the initiation point is not square in the middle. It is offset several degrees off center. They all are.

The high stress point depends on where the loads are being applied to the cranks. And the loads are not equal or static. They also have a harmonic factor to them.

 

[Burn Down]

See how the break follows an almost perfect radius in the middle and then goes jagged at the left and right side? Where the jaggedness starts, there is a slight rub mark on the fillet.

And no, the initiation point is not square in the middle. It is offset several degrees off center. They all are.

The high stress point depends on where the loads are being applied to the cranks. And the loads are not equal or static. They also have a harmonic factor to them.

So your saying the crack starts closer to the jagged edges?

 

[jlawles2]

So do you think the rub mark is creating a stress riser? That would explain why the break creaps the way it does.

I wonder if anyone has pictures of the rod or main that was against the break. Might show what is causing the rub mark.

 

[Fingers]

So your saying the crack starts closer to the jagged edges?

No. But it is not centered TDC either.

Sent on the run from my Cell Phone

 

[BHoward6.6]

pic of mains

 

[BHoward6.6]

Another view of same crank,

 

[Burn Down]

No. But it is not centered TDC either.

Sent on the run from my Cell Phone

That is not what I meant... Centered between the the 2 jagged edges. I should have been more specific.

 

[jkholder09]

When the cranks are twist forged at the factory. Where are the cranks grabbed and pulled on to twist them and which area's would cool the quickest?

I have no idea if the are annealed afterward to get an equal hardness, or if it is possible this area could just be an area that gets brittle.

If it was something that simple, could we just anneal our cranks when doing a build, and then go back and induction harden or nitrite treat the bearing surfaces?

I am just asking because I have had some cranks welded and then annealed and they are still in tractors working perfectly.

 

[Racer]

All material has some level of nonmetallic inclusion; this imperfection has to be taken into the design of the component along with the material strength as I am sure GM did in this case taking stock loading conditions into consideration. Since the failures are obviously fatigue initiated (in bending) there is cyclic loading that is putting strain on the crank. If the MEP of the engine is increased through modifications or tuning that could be one source of the strain. If for some reason the combustion events occur to close to TDC (half mass burn should occur at ~7° ATDC) this also could be where the strain is coming from. The fix will be to back down on the tune up (spark) or look for a crankshaft with more structure in the arm. My 2 cents

 

[PACougar]

Yes my statments are bold, but true. Nothing i have presented can be challenged with anything other than a theory. Here is a homework lesson for you. Tell me the stock Fillet*Angle, then check with a few Racing Crank manufacture's Fillet*Angle... Write it down.

Then tell your software to turn EACH v-8 crankshaft with properly phased 90* and 180* jurnals under load instead of just vertical bending...then plug in the Fillet #'s

Take a picture of your face when done. :roflmao:

You should start building and selling us unbreakable cranks, you clearly know what needs to be done.

 

[Fingers]

Removed

 

[Dave c]

Here is a few good articles. http://www.precisionenginetech.com/tech-explained/2009/05/22/top-fuel-rings-bearings/

http://www.carcraft.com/techarticles/ccrp_1009_8000hp_top_fuel_engine/

http://www.epi-eng.com/piston_engine_technology/crankshaft_design_issues.htm

SAE papers

http://www.recycle-steel.org/~/medi...ams/longproducts/cranksfaft_full_report_1.pdf

 

[Fingers]

removed

 

[Fingers]

As of late, most of the failures are actually behind #4 main. Early failures were mostly behind #1. I've yet to see one behind #3.
I believe harmonics are to blame. That and a lack of bearing support due to the mains not being large enough.

Sent from my C6906 using Tapatalk

Most common is still in front of #2, but right behind #4 is common. They share similar stress patters with the one behind #4 benefiting from the support from the thrust bearings.

 

[Fingers]

Apologies for not cleaning this up earlier.

Sent on the run from my Cell Phone

 

[03spoolindmax]

A lot of good info here. Keep up the good work Jon :thumb:

 

[BHoward6.6]

Need a little help. I could use some more pictures of the breaks and in particular, I want to see if the witness marks shown in the attachment line up with the break.

These marks are outside where the bearing should be riding and are on the fillet radius itself. They are typified by the unworn or polished band between them and the contact area of the bearing itself.

It would be great if you can post a picture the shows the intersection of the witness mark and the initiation point of the break.

ok

 

[matt78]

Does oil breakdown have anything to due with broken cranks. My socal motor has my stock crank, I guess anything can happen. Just wondering.

 

[Mikey]

Does oil breakdown have anything to due with broken cranks. My socal motor has my stock crank, I guess anything can happen. Just wondering.

I have read somewhere that contamination does cause crank failure. I'll try to find te article

 

[smarquis]

lI seriously doubt mine was related to oil. I had never run anything but synthetic, and always had regular changes. Ii will try to snap a picture of mine this afternoon.