Oil Film & GM's Spun Beariings: What Went Wrong?

[FetchFar]

Originally Posted By: Trav

I believe GM has gone to an all aluminum bearing shell.
Given the expansion of the aluminum is double that of forged cast iron its going to expand right into the crank effectively eliminating the oil film clearance and blocking the oil feed hole.


Aluminum in crank bearings have always been in an insert inside of a steel outer. Yes heat expansion is an issue, and the expanding aluminum will expand away from the crank, not toward it, making the clearance a little higher in hot conditions.

 

[Nayov]

"GM also examined cars that were still on dealership lots, awaiting shipment at the assembly plant, or in transit, by putting them through a stress test that involves hard acceleration and revving the engine to high rpm."

Not to worry, the dealer guys are putting them through a "stress test." I can picture them doing "hard acceleration" and "revving the engine to high RPM."

 

[FetchFar]

Originally Posted By: Nayov
"GM also examined cars that were still on dealership lots, awaiting shipment at the assembly plant, or in transit, by putting them through a stress test that involves hard acceleration and revving the engine to high rpm."

Not to worry, the dealer guys are putting them through a "stress test." I can picture them doing "hard acceleration" and "revving the engine to high RPM."


Nice to buy one of those thrashed cars, eh? Not new anymore!

 

[Trav]

Originally Posted By: FetchFar
Originally Posted By: Trav

I believe GM has gone to an all aluminum bearing shell.
Given the expansion of the aluminum is double that of forged cast iron its going to expand right into the crank effectively eliminating the oil film clearance and blocking the oil feed hole.


Aluminum in crank bearings have always been in an insert inside of a steel outer. Yes heat expansion is an issue, and the expanding aluminum will expand away from the crank, not toward it, making the clearance a little higher in hot conditions.


Tri metal was the most common until recently. Steel, copper, lead/babbitt. This is a bi metal, steel and aluminum only.
Let Shannow answer please, the guy is an qualified engineer and thats why i directed my question to him.

 

[FetchFar]

Originally Posted By: Trav
Originally Posted By: FetchFar
Originally Posted By: Trav

I believe GM has gone to an all aluminum bearing shell.
Given the expansion of the aluminum is double that of forged cast iron its going to expand right into the crank effectively eliminating the oil film clearance and blocking the oil feed hole.


Aluminum in crank bearings have always been in an insert inside of a steel outer. Yes heat expansion is an issue, and the expanding aluminum will expand away from the crank, not toward it, making the clearance a little higher in hot conditions.


Tri metal was the most common until recently. Steel, copper, lead/babbitt. This is a bi metal, steel and aluminum only.
Let Shannow answer please, the guy is an qualified engineer and thats why i directed my question to him.


OK, I will now allow Shannow to answer. I'm giving permission now.

 

[Ducman]

I mostly put it down to a lack of crush on the bearing shells in the majority of cases.
It's the static friction that's created by the clamping force on the bearing shells which mostly stops the bearing from spinning.
Most spun bearings are found in older engines where the bearings are considerably worn, and the big ends of the conrods need re-sizing. Most likely the main bearings need attention as well.
They have insufficient bearing crush.

I firmly believe there also has to be a balance between the tolerances of getting the bearing crush right, and bearing to journal clearances right to suit the viscosity of the oil the engine is designed to run on as well.

In the simplest of terms.
Insufficient bearing crush is a major contributor in older(high mileage) engines.
In newer engines, if the bearing tolerance is too tight and the oil is too thin and when combined with insufficient oil flow, it's a recipe for a spun bearing.

I believe this to be the case in these newer GM engines.
And it probably comes down bearing to journal clearances and production tolerances which are too tight regardless of how thin the oil is, and the oils isn't good enough to stop the bearing from picking up on the journal in the boundary lubrication state.

There is also the tolerance for bearing crush, with a minimum amount of crush to provide enough clamping force to stop the bearing shells from spinning.
And a maximum amount of crush, so as to not have too much crush on the bearing that can potentially induce an out of round condition of the bearing as in the case of main and big end bearings.
This can affect the effectiveness of the wedge effect of the oil in the hydrodynamic state of lubrication where it's potentially compromised, as well as the undesirable effect of inducing undue stress on the assembly.


Regarding cam bearings.
If I may relate my experience in one instance of an isolated and rather bizarre case.

I have experienced a situation, where a single factory installed cam bearing in a Small Block V8 has had "too much" crush which reduced the bearing clearance on that particular cam journal.
This issue wasn't actually discovered until the engine was disassembled with a view to carrying out modifications/blueprinting.

To explain further
After a long period of sitting idle, this particular engine was started and as it fired up within the first revolution of cranking(like on the first cylinder to fire on that revolution) immediately as the starter engaged.
There was absolutely no oil pressure as the engine started, and you could hear an abnormal sound from the engine for about a 1/2 a second.
Initially the sound was put down to a lifter, but on reflection the sound was emanating at a faster rate(higher frequency) than a single lifter could generate, but the sound stopped and the engine ran normally after that.

When the engine was eventually pulled down, one single cam bearing was found to have grabbed on the cam journal and spun in the block until oil pressure had come up and released its grip on the journal.
It had also walked backwards in the block until it had run up against the side of the adjacent cam lobe which subsequently halted its rearward progress.

By some miracle the slot in the bearing stoped short of completely blocking the oil gallery hole as it stopped rotating in the block and moving rearward, thereby restoring some lubrication and seemingly normal operation with no abnormal engine oil pressure or ill effects to other components in the lubricating circuit.

Upon more detailed inspection of the matter, that particular cam bearing bore in the block was found to be considerably "tighter" on that particular journal as opposed to the others.
It certainly flies in the face of the theory of a bearing being too loose with insufficient crush to hold it in place, but excessive crush can't be discounted as the major contributing factor.
This matter was subsequently corrected when the engine was blueprinted and rebuilt.

The engine continues to this day in it's modified form with an external oil priming pump to provide oil pressure throughout the engine before starting.
No more dry starts.

 

[Shannow]

Originally Posted By: FetchFar

Aluminum in crank bearings have always been in an insert inside of a steel outer. Yes heat expansion is an issue, and the expanding aluminum will expand away from the crank, not toward it, making the clearance a little higher in hot conditions.


Given that the modulus of expansion IS greater,and the con-rod big end is massively stiffer than the thin shells, the Al can't expand outwards,and away from the crank asyou suggest.

If the shell is trying to grow 0.002"(as anexample), and the rod big end is only growing 0.001", the expanded material has to go somewhere...

It will expand inwards,towards the crank, or extrude axially along the crank...worst case is that the tension of the steel causes the Al to creep, or platically deform, then when it cools down it will gointo tension, pulling the part line in towards the crank...could reduce the crush some as well...think bimetallaic strip, constrained until one yields.

Mahle reckon that the newer bimetal bearings are for cost savigs, instead of eletroplating materials on, and recommentd them for enignes that have been reconditioned to OEM specs...and not for HD applications.

Maybe GM are working out that their designs are bordering on HD, in terms of spead, viscosity, and load

 

[FetchFar]

Originally Posted By: Shannow

If the shell is trying to grow 0.002"(as anexample), and the rod big end is only growing 0.001", the expanded material has to go somewhere...


So the connecting rod is steel just like the crank journals, same coefficient of thermal expansion. The crank journal will see the same temperature and will expand slightly when hot. When the bearing shell, presumably aluminum here, presses against the connecting rod and goes into further crush-compression in the shell housing big-end when hot, it will conform to the big-end housing, just under more crush. .... Therefore, I don't see hot conditions changing the clearance that much.

 

[FetchFar]

Originally Posted By: Ducman
.....
I firmly believe there also has to be a balance between the tolerances of getting the bearing crush right, and bearing to journal clearances right to suit the viscosity of the oil the engine is designed to run on as well.......


Great analysis. I guess only the insiders at GM know what was wrong about the shells. As you said, maybe insufficient shell crush due to shorter dimensions maybe.

 

[yvon_la]

One thing is sure .honda engineer must be laughing very hard at gm and thinking out loud:NOOB!

 

[Ducman]

Originally Posted By: FetchFar
Originally Posted By: Ducman
.....
I firmly believe there also has to be a balance between the tolerances of getting the bearing crush right, and bearing to journal clearances right to suit the viscosity of the oil the engine is designed to run on as well.......


Great analysis. I guess only the insiders at GM know what was wrong about the shells. As you said, maybe insufficient shell crush due to shorter dimensions maybe.


Or incorrect rod sizing(at their maximum tolerance)????

Yeah, I think it's more like bearing clearances to the journal being too tight due to production tolerances, in combination with oil that's not capable of keeping the parts apart for long enough to transition through the boundary lubrication state and safely back to the hydrodynamic state.
Throw in insufficient bearing crush, and you'll get funny noises.

Manufacturers have been moving to tighter bearing clearances these days.

Yes, your spot on about the fact that only the insiders at GM really know.
And they're not telling.
Or at least they will have worked it out for certain by now, and well on the way to getting on top of it.

End of story really.

 

[Shannow]

Originally Posted By: FetchFar
Originally Posted By: Shannow

If the shell is trying to grow 0.002"(as anexample), and the rod big end is only growing 0.001", the expanded material has to go somewhere...


So the connecting rod is steel just like the crank journals, same coefficient of thermal expansion. The crank journal will see the same temperature and will expand slightly when hot. When the bearing shell, presumably aluminum here, presses against the connecting rod and goes into further crush-compression in the shell housing big-end when hot, it will conform to the big-end housing, just under more crush. .... Therefore, I don't see hot conditions changing the clearance that much.


So you reverse your position entierly after my comments...and I'm SILL wrong ?

Great work

When the Al goes into "further crush", if it exceeds it's yield point, it will lose crush on cooling, making it sloppy(er) than originally intended.

If (as per my original post), it's not completlely thermall connected to the rod (it isn't), it will grow faster until the heat gets to the big end and catches up on expansion...suddenly hot bearings, not yet hot big ends, clearances close up.

 

[Ducman]

True enough.

Anything is a possibility, and the physics certainly don't change.

To have a real shot at getting the hypothesis correct, one really needs more detailed information which we won't ever have access to.

I'll wager that even at GM, you could put 2 or 3 of the people responsible for making the call on the matter together, and get some variation on the solution to the problem.

 

[Shannow]

Agreed...crush, and disimilar metals (and coatings count there) between the crank and the rubbing surface are all that there is to firstly stop rotation, and secondly preent welding.

Weld, or lose crush, and it's game over.

 

[FetchFar]

Originally Posted By: Shannow

When the Al goes into "further crush", if it exceeds it's yield point, it will lose crush on cooling, making it sloppy(er) than originally intended.
Wrong. It never goes into plastic deformation. Nonsense again.

 

[OneEyeJack]

After all this discussion it comes down to quality control, nothing mysterious, just mistakes.

Years ago in amateur racing I had a oil pressure drop half way thru a race, from 60 to 40 psi. On tear down a rod bearing spun. sheared off the tabs. Now both sides of the bearing became the lubricated bearing surface. I left it alone and raced the rest of the season like that and it worked fine. I called is "double full floating bearing". It was a 1300cc Alfa in E/P.

 

[Ducman]

That's beautiful OneEyeJack.

It never ceases to amaze me just what one can get away with.

On the other hand it's always a rude shock when one finds out what one can't get away with as well.

 

[Shannow]

Ford did tht, and intentionally, on the flathead big ends


http://www.reds-headers.com/html/red_s_engine_talk_32.html

Would have been an interesting decision point.

 

[Ram01]

I thought GM did extensive real world testing on these motors

 

[OneEyeJack]

Originally Posted By: OneEyeJack
After all this discussion it comes down to quality control, nothing mysterious, just mistakes.

Years ago in amateur racing I had a oil pressure drop half way thru a race, from 60 to 40 psi. On tear down a rod bearing spun. sheared off the tabs. Now both sides of the bearing became the lubricated bearing surface. I left it alone and raced the rest of the season like that and it worked fine. I called is "double full floating bearing". It was a 1300cc Alfa in E/P.


I used to hang license plates on that racer from another Alfa for street testing. One time I got a written warning for speeding on an isolated back road. On the form the cop identified the make of my car as an "Alpha Beta".