What causes a rod or main bearing to spin? (not lacking oil)
10-19-2008, 06:42 PM
#1
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What causes a rod or main bearing to spin? (not lacking oil) What would make a Rod bearing spin ? what would make a main bearing spin? Looking for all reason but i ran low on oil.
10-19-2008, 08:30 PM
#5
Think about the abuse rod bearings take compared to the mains. On every power stroke a single rod bearing absorbs the shock which is then shared by all the mains. Friction due to lack of lubrication causes heat resulting in expansion. The result is not good. Oil is cheap. A tear down is not.
10-19-2008, 08:55 PM
#6
High RPMs on a cold engine greatly increase your chances of spinning a bearing, because the engine has not warmed and therefore expanded to the proper clearances prior to being run at the limits of normal operation.
10-19-2008, 09:01 PM
#7
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well another problem is the factory design of the engine. if im not mistaken the oiling priority is cam first crank second which is why bearings get turned. stock cranks have one oiling point for each rotation and most aftermarket cranks have 2 oiling points per revolution. so a little is design but like these folks have said. engine temperature and oil condition is number 1 problem
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10-19-2008, 09:07 PM
#8
DING DING DING! Over revving stretches the rod bolts that lets the bearing start to spin, once it starts your done, to answer your question. But as others have said, cold oil can do it ( oil dosent properly lubricate till is hot ) worn oil that has broken down can definately do it.
Main cause, again, dufus's that think they can spin a bottom end to the moon, you cant. Look into "tensile load" its the load that is against the rod and bolts when it tries to yank the piston back down, at high rpm it can get really extreme, and its not only the weight, its changing the direction of travel of the piston effectively increases the weight, the vacuum the piston is pulling etc. will stretch a rod bolt and loose the "crush fit" so to speak of the rod cap on the bearing.
10-20-2008, 12:04 PM
#10
Excessive boost pressure can "crush or squish" the bearing(s). I ran 150hp NX for 3 years with no problem on a stock bottom end. 200hp squished #6. King Alecular bearings claim 5 times the crush strength, (which I now use), in conjuntion with XPR Royal Purple, also claiming incredible crush strength, should give insurance from failure. I guess I'll find out the next time I take the motor apart. I'll try to find the bearing at my shop and put it on here for everyone to see what I mean about squish. There was only a knock, no rod or bolt failure. The rest of the answers above are also correct in regards to each individual situation.
10-20-2008, 12:36 PM
#11
The idea that "letting the bearing start to spin" causes spun bearing failures doesn't hold up. There has to be a great deal of force acting on the bearing to compel it to spin, which your theory doesn't account for.
Every spun bearing we've seen at our shop traced back to oil film failure.
Whether it's assembly error (bearing clearance insufficient) or oiling system failure (pump died or low oil level in pan or solvent contamination of the oil) or systemic pressure loss (bearing clearances opened up due to wear) the failure mode is the same.
The engine loses the "wedge" of oil between crank and bearing, and the two come into direct contact. The resulting heat welds the bearing shell to the crank, which then starts the bearing spinning inside its saddle.
10-20-2008, 10:14 PM
#12
Can you post any links to credible tech supporting this, or is it just a theory?
The idea that "letting the bearing start to spin" causes spun bearing failures doesn't hold up. There has to be a great deal of force acting on the bearing to compel it to spin, which your theory doesn't account for.
Every spun bearing we've seen at our shop traced back to oil film failure.
Whether it's assembly error (bearing clearance insufficient) or oiling system failure (pump died or low oil level in pan or solvent contamination of the oil) or systemic pressure loss (bearing clearances opened up due to wear) the failure mode is the same.
The engine loses the "wedge" of oil between crank and bearing, and the two come into direct contact. The resulting heat welds the bearing shell to the crank, which then starts the bearing spinning inside its saddle.
The idea that "letting the bearing start to spin" causes spun bearing failures doesn't hold up. There has to be a great deal of force acting on the bearing to compel it to spin, which your theory doesn't account for.
Every spun bearing we've seen at our shop traced back to oil film failure.
Whether it's assembly error (bearing clearance insufficient) or oiling system failure (pump died or low oil level in pan or solvent contamination of the oil) or systemic pressure loss (bearing clearances opened up due to wear) the failure mode is the same.
The engine loses the "wedge" of oil between crank and bearing, and the two come into direct contact. The resulting heat welds the bearing shell to the crank, which then starts the bearing spinning inside its saddle.
Are you serious, ok put in a set of bearings a tad small for the rod end, they will get spun, thats what happens when the bolts stretch, there is tons of info out there on bolt stretch resulting in spun bearings and failure, have done it myself to an engine, sorry I dont have any research saved that I can post up, and im not gonna go search it out, its a well known fact, if you dont believe me contact an engine builder. Furthermore if you really dont think it is an issue why do you think people go and chage rod bolts ONLY, numerous threads on this very board about that, and the point of that is to prevent bolt stretch and bearing failure.
My theory dosent account for the bearing starting to spin?.... If the bolts stretch, how tight do you think that bearing is in there at that point? You really think it takes "a great deal of force" to spin a bearing in a hole larger than its self? Bearings are crush fit to keep them from spinning, thats what you are reffering to, but if the bolts stretch the caps lift and where is your crush fit now?
edit: this isnt a theory either, its researched info, and by the way how did you trace bearing failure back to oil film failure? That would mean metal to metal contact is what you found, that could be from multiple things, and egg shaping a rod end by stretching it will cause metal to metal contact also, when the cap gets loose and you start banging on the top and bottom of the bearing it can bring the sides inward and touch, then it spins then you are done. Again an experienced engine builder/racer should know this also. I didnt come up with this stuff, its stuff I have been told by builders, racer/fabricators of professional race shops and other old timers too, and have read about it too.
Last edited by 00pooterSS; 10-20-2008 at 10:26 PM.
10-20-2008, 10:23 PM
#13
Can you post any links to credible tech supporting this, or is it just a theory?
The idea that "letting the bearing start to spin" causes spun bearing failures doesn't hold up. There has to be a great deal of force acting on the bearing to compel it to spin, which your theory doesn't account for.
Every spun bearing we've seen at our shop traced back to oil film failure.
Whether it's assembly error (bearing clearance insufficient) or oiling system failure (pump died or low oil level in pan or solvent contamination of the oil) or systemic pressure loss (bearing clearances opened up due to wear) the failure mode is the same.
The engine loses the "wedge" of oil between crank and bearing, and the two come into direct contact. The resulting heat welds the bearing shell to the crank, which then starts the bearing spinning inside its saddle.
The idea that "letting the bearing start to spin" causes spun bearing failures doesn't hold up. There has to be a great deal of force acting on the bearing to compel it to spin, which your theory doesn't account for.
Every spun bearing we've seen at our shop traced back to oil film failure.
Whether it's assembly error (bearing clearance insufficient) or oiling system failure (pump died or low oil level in pan or solvent contamination of the oil) or systemic pressure loss (bearing clearances opened up due to wear) the failure mode is the same.
The engine loses the "wedge" of oil between crank and bearing, and the two come into direct contact. The resulting heat welds the bearing shell to the crank, which then starts the bearing spinning inside its saddle.
wait wait just noticed this part lol, what do you think happens when the bolts stretch from over rev? this is a much shorter summary of all the crap I was trying to type
10-21-2008, 12:05 AM
#15
Thanks man, hadn't heard of the aerated oil but makes sense, I believe bolt stretch took out my last blown motor, not LSx motor, the bottom end was factory redline 6800, I was taking it to 8k daily since the factory redline in the ECU was 8K and the head was built to take it, and lastly because the bottom ends were a dime a dozen so **** it. lasted 2 years on short spurts to 8k, until I did a 5th gear pull racing a bike and took it to 8k in 5th ( top gear in that tranny )thats a long time above 6800, then I had razor thin bearings on 2 cylinders. It also seized I broke it loose coasting on the highway by letting out the clutch lol, got me home and to the shop the next day.
then there were the bike motors, one spun bearings cause I got pissed off ( stupid ) and pounded on it with the motor cold, took out the bearings right away, next time was riding extended wheelies and lack of oil killed it, so yeah im good at killing bottom end bearings
.
10-21-2008, 12:17 PM
#16
10-21-2008, 05:51 PM
#18
mine spun 2 shells after a track day when the oil was hot and i had been caning it. i had also upped my rev limit to 6500rpm from 6200rpm that day, coincidence or not. only mod to engine as such was arp rod bolts and a cam (220/220) with valvesprings and pushrods. oil pump was original but had relief spring mod done. car had already done 2 trackdays and about 200 miles on track but as i had used a 2nd hand ls1 it wasnt a major disaster.
10-21-2008, 06:17 PM
#19
10-21-2008, 08:06 PM
#20
His "edit" was spot on. Darin Morgan, formerly of Reher Morrison, would hone the rod big ends out of round (larger at the parting line, similar to how a bearing is eccentic) to prevent the parting lines from pinching in at high inertia loads/RPM. This has been a well known trick among the stock/comp eliminator crowds for decades.
