What exactly happens when you spin a bearing?
09-01-2014, 06:44 PM
#2
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The crankshaft and bearing no longer have a protective wedge of oil between them. The bearing starts to gaul or stick to the crank. After it continues it eventually spins with the crank.
09-01-2014, 07:10 PM
#4
So basically that's where a ported and shimmed oil pump comes in? So higher oil pressure is a good thing, but say with a the high volume high pressure melling pump that could theoretically suck the pan dry, would cause a spun bearing? Or am I missing something?
09-01-2014, 07:25 PM
#6
So with adequate pressure (nothing super high, I know) you should theoretically never spin a bearing? I see cars with 60k miles spin bearing and heads cam cars with 120k miles still driving just fine and get beat on daily, just wondering what the cause was is all.
09-01-2014, 07:34 PM
#7
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Contaminated oil not changed frequently enough will cause bearings to wear. I'm sure there is no one reason. The ls engine is actually very stout and if maintained it will last a long time.
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09-01-2014, 07:57 PM
#8
Some cranks flex under power and you even have to consider harmonics.
Running a belt to tight can cause bearing failure. Anyone running a blower/supercharger has to balance possible belt slip versus wiping out main bearing if they run the belt to tight.
09-02-2014, 01:30 AM
#10
Many things can lead to a spun bearing, Poor oil control would be at the top of the list but other factors can come into play like detonation or turning to many rpms or anything that may throw the rotating assembly off balance.
Some cranks flex under power and you even have to consider harmonics.
Running a belt to tight can cause bearing failure. Anyone running a blower/supercharger has to balance possible belt slip versus wiping out main bearing if they run the belt to tight.
Some cranks flex under power and you even have to consider harmonics.
Running a belt to tight can cause bearing failure. Anyone running a blower/supercharger has to balance possible belt slip versus wiping out main bearing if they run the belt to tight.
09-02-2014, 08:19 AM
#11
Spinning a rod bearing means your big end of the rod, where it connects to the crank, was out of round, improperly oiled, or simply pushed too hard and failed. Rod bolts could still be holding, but the crank and rod would be damaged. If the damage is catastrophic, the actual metal of the rod could fail.
09-02-2014, 12:40 PM
#12
In the case of a rod bearing, you can cause the stock bolts to stretch and distort the ID of the rod. A bolt acts very much like spring, as you torque the bolt, you're preloading the bolt with a force that holds the rod together. When the force of the piston at TDC is greater than the force of the bolts holding the cap together, the bolts will stretch. When this happens, the parting lines typically pinch in, reducing that reservoir or even closing up all the clearance altogether. Higher strength bolts can be preloaded to a higher force and resist stretching.
As mentioned already, poor maintenance can lead to spun bearings, as well as manufacturing tolerances. Bearing clearances can be less than .001" in an OEM LS engine, which is pretty tight for a performance application by most standards. Clevite, for example, recommends .001" per 1" of journal diameter as a general rule of thumb. That would mean clearances should be closer to .002"-.0025" for an LS engine.
Good machining, adequate clearances, quality bearings, and frequent maintenance should keep a performance engine heathy for a long time.
09-02-2014, 08:00 PM
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The oil pressure you see on the gauge doesn't really do much for the bearing or the oil film that protects it. The bearing ID is elliptical, so the ID actually gets larger near the parting lines than it is at 12 and 6 o'clock. This creates a reservoir at the parting lines that gets picked up by the spinning rod or main journal, squeezing it into the smaller ID at 12 and 6 to create what's called a "hydrodynamic wedge". The eccentricity, or rate at which the clearance decreases, has a large effect on this wedge that keeps the bearing and journal from meeting.
In the case of a rod bearing, you can cause the stock bolts to stretch and distort the ID of the rod. A bolt acts very much like spring, as you torque the bolt, you're preloading the bolt with a force that holds the rod together. When the force of the piston at TDC is greater than the force of the bolts holding the cap together, the bolts will stretch. When this happens, the parting lines typically pinch in, reducing that reservoir or even closing up all the clearance altogether. Higher strength bolts can be preloaded to a higher force and resist stretching.
As mentioned already, poor maintenance can lead to spun bearings, as well as manufacturing tolerances. Bearing clearances can be less than .001" in an OEM LS engine, which is pretty tight for a performance application by most standards. Clevite, for example, recommends .001" per 1" of journal diameter as a general rule of thumb. That would mean clearances should be closer to .002"-.0025" for an LS engine.
Good machining, adequate clearances, quality bearings, and frequent maintenance should keep a performance engine heathy for a long time.
In the case of a rod bearing, you can cause the stock bolts to stretch and distort the ID of the rod. A bolt acts very much like spring, as you torque the bolt, you're preloading the bolt with a force that holds the rod together. When the force of the piston at TDC is greater than the force of the bolts holding the cap together, the bolts will stretch. When this happens, the parting lines typically pinch in, reducing that reservoir or even closing up all the clearance altogether. Higher strength bolts can be preloaded to a higher force and resist stretching.
As mentioned already, poor maintenance can lead to spun bearings, as well as manufacturing tolerances. Bearing clearances can be less than .001" in an OEM LS engine, which is pretty tight for a performance application by most standards. Clevite, for example, recommends .001" per 1" of journal diameter as a general rule of thumb. That would mean clearances should be closer to .002"-.0025" for an LS engine.
Good machining, adequate clearances, quality bearings, and frequent maintenance should keep a performance engine heathy for a long time.
09-02-2014, 08:02 PM
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Also another thing that can occur to compromise the hydrodynamic oil wedge is oil aeration. Which is not hard to get into a situation where this can occur with the OE style 1:1 crank driven LS oil pumps. Or in a high G handling or drag car launch/quick 60' situation when the pickup becomes uncovered.
09-02-2014, 08:06 PM
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The oil pressure you see on the gauge doesn't really do much for the bearing or the oil film that protects it. The bearing ID is elliptical, so the ID actually gets larger near the parting lines than it is at 12 and 6 o'clock. This creates a reservoir at the parting lines that gets picked up by the spinning rod or main journal, squeezing it into the smaller ID at 12 and 6 to create what's called a "hydrodynamic wedge". The eccentricity, or rate at which the clearance decreases, has a large effect on this wedge that keeps the bearing and journal from meeting. In the case of a rod bearing, you can cause the stock bolts to stretch and distort the ID of the rod. A bolt acts very much like spring, as you torque the bolt, you're preloading the bolt with a force that holds the rod together. When the force of the piston at TDC is greater than the force of the bolts holding the cap together, the bolts will stretch. When this happens, the parting lines typically pinch in, reducing that reservoir or even closing up all the clearance altogether. Higher strength bolts can be preloaded to a higher force and resist stretching. As mentioned already, poor maintenance can lead to spun bearings, as well as manufacturing tolerances. Bearing clearances can be less than .001" in an OEM LS engine, which is pretty tight for a performance application by most standards. Clevite, for example, recommends .001" per 1" of journal diameter as a general rule of thumb. That would mean clearances should be closer to .002"-.0025" for an LS engine. Good machining, adequate clearances, quality bearings, and frequent maintenance should keep a performance engine heathy for a long time.
09-02-2014, 09:04 PM
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Well, that may be. But bearings were not designed as they are because rod bolts stretch. The big end distorts. Do you run perfectly round bearings with ARP2000 bolts? No.
Last edited by treybadd; 09-02-2014 at 11:09 PM.
09-02-2014, 09:21 PM
#19
I think you are missing KCS's point Trey. If I understand him correctly, he is talking about at higher RPM. As the RPMs increase, it increases the force against the connecting rod cap trying to pull it apart when at or near top dead center. The connecting rod bolts job is to have enough pre-load or clamping force to be able to counter-act this force. If you were to exceed the connecting rod bolts tensile strength, it starts to stretch. As it stretches, the rod journal can distort from the additional force as well as loose its clamping force on the bearing. This is where excessive wear and/or bearing damage or failure could occur.
I don't know if you are aware of this or not, but rod bolts and most fasteners stretch when torqued. This is how they maintain tension. Any good engine builder will not only measures torque when tightening rod bolts, but they also measure how far each stretched when being torqued. This is a secondary safeguard to assure a fastener is performing according to design.
I don't know if you are aware of this or not, but rod bolts and most fasteners stretch when torqued. This is how they maintain tension. Any good engine builder will not only measures torque when tightening rod bolts, but they also measure how far each stretched when being torqued. This is a secondary safeguard to assure a fastener is performing according to design.
Last edited by speedtigger; 09-02-2014 at 09:27 PM.
09-02-2014, 09:38 PM
#20
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Yeah...don't need educated on bolt stretch. I know all about it. That's how I torqued my rods... Found the torque that gave me desired stretch. The rod ID distorts is the reason bearings are made several thousandths bigger (thinner) at the parting line. On any engine. Revving the **** out of an engine with shock rod bolts is a totally different issue. Yes that would also affect the shape of the rod ID. You will distort the housing ID on a normal engine running at high rpm. The bolt is not necessarily stretching.
Last edited by treybadd; 09-02-2014 at 11:10 PM.