Bearing overload


by 351tom
Tags: bearing, overload
351tom
351tom is offline
#19
Nov1-13, 08:30 PM
P: 14
A lot of good points made. I found some real world data on a similar size Ford engine I was troubleshooting a while back. Along with the torque characteristics assumptions of 256bits & the observed pressures by Baluncore, I put together the following plot;
the cranking speed was 185 rpm so if I use the 15.3:1 starter/flywheel ratio, & 90 ft lb starter, that puts the starter rpm at 2831. The engine had a 10.4 compression ratio, so, at 65 degrees it works out to 5.226 ft lbs load at the starter vs. 6.044 ft lbs for a 14:1 compression. The starter rpm at 14:1 compression works out to 2804. That works out to a engine rpm difference of only 2 rpm. If I look at using a 200 ft lb starter, the engine rpm difference is only 6 or 7 rpm. We have not been considering other resistance to turning the engine like drag of the rings or compressing valve springs, but regardless, this shows the difference in starter load due to compression between a 10.4cr engine & a 14cr engine is minimal.
Chronos
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#20
Nov10-13, 01:32 AM
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Shock is a concern with every load cycle. A typical bearing endures an enormous number of shock loads over its life cycle. The number of shock loads can be enormous [as in many millions] in a typical reciprocating application. Lubrication is the usual suspect. You need an exotic lubricant to reliably provide efficient lubrication under such conditions. Even the best lubricants can only perform reliably for a few months under heavy loads.
351tom
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#21
Nov11-13, 08:17 PM
P: 14
Yes, shock loads-this seems to bring us back to the original premise; that it's not high compression that requires high torque starters, but rather ignition occurring well befofe top dead center.
Back in the 60's & early 70's before electronic ignitions were available, there was no provision to retard the spark at start-up, but the stock engines didn't require all that much advance to begin with, so they were able to get by. Today, however, circle track racers for example, often run 42 degree locked out distributors, but they usually push start the cars & don't flip the ignition on until the engine rpm is well above typical starting speeds. Others, however, who don't have the option of push starting use starters to start their engines. While aftermarket ignition systems often have a high degree of starting retard circuitry built in them, many do not, & it seems there are an ample number of high torque starter motor manufacturers ready to sell them high torque starters to get the job done.
So in the case of ignition well before top dead center, the original calculation of nearly 62,000 PSI (which is probably about 10 times the load normally seen at full power) on the rod bearings could (depending on the exact time the ignition fires) be true!
Baluncore
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#22
Nov11-13, 11:13 PM
P: 1,282
Shock is not really a problem with big end bearings. The bearing has a huge contact area with a pressurised oil supply. It is much more likely that a shock will bend the connecting rod than the bearing will be damaged or it's oil film lost.

Gone are the days when cars had a spark advance adjustment on the steering column. If you had to turn the motor with a crank handle you would definitely want it to fire closer to 2° past TDC.

Spraying ether into the air intake of an old diesel makes for an easier cold start. But in modern diesels, ether spray only incites them to run backwards.

These new fangled self starters aren't all they are cranked up to be.
351tom
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#23
Nov12-13, 05:45 AM
P: 14
[QUOTE=Baluncore;4569610]Shock is not really a problem with big end bearings. The bearing has a huge contact area with a pressurised oil supply. It is much more likely that a shock will bend the connecting rod than the bearing will be damaged or it's oil film lost.

Not during cranking, because no oil pressure is yet developed, and the soft babbit layer gets squeezed like a mashed potato.
Baluncore
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#24
Nov12-13, 11:44 AM
P: 1,282
“squeezed like a mashed potato” is an emotional exaggeration. “the soft babbit layer”, as you refer to it, is so thin that it can not move sideways. It is only a surface coating on the steel shell. Pressures in the system all come down to bearing area or member cross section. The cross section of the rod is the bottle neck for plastic deformation at low RPM.

The big end bearing has a greater area than the section of the rod. That is so because when running, the high surface velocity of the big end bearing requires a large area oil film. When starting the surface velocity is very low so lubrication is non-critical. The oil pump does not have to provide bearing pressure because the film that supports the load will build up during cranking or immediately the engine starts. The film is the tail of an oil wedge. Oil is deliberately circulated to keep the oil and bearing cool.

Babbitt bearings were replaced with thin shell bearings over half a century ago. That was done to reduce weight and to facilitate maintenance. It also meant that only the inner surface needed to be plated with the much more expensive corrosion resistant low friction bearing material. Extrusion of thin shell bearing material is not an issue.
351tom
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#25
Dec18-13, 04:28 PM
P: 14
Quote Quote by Baluncore View Post
“squeezed like a mashed potato” is an emotional exaggeration. “the soft babbit layer”, as you refer to it, is so thin that it can not move sideways........................................Extrusion of thin shell bearing material is not an issue.
From the Clevite Engine Bearing failure analysis guide;
For many years, nearly all camshaft bearings were manufactured with a lining of babbitt. Babbitt is a soft
slippery material made up primarily of lead and tin and is quite similar to solder. As a bearing surface layer,
babbitt possesses the desirable properties necessary to survive under adverse conditions such as foreign
particle contamination, misalignment and marginal lubrication on start up.
The trend in modern engines has been toward higher operating temperatures and higher valvetrain loads. Babbitt
is limited in its ability to survive under these conditions due to its relatively low strength. When babbitt cam bearings
are installed under these demanding conditions, the lining may extrude or fatigue. Fatigue can be identified by
craters in the bearing surface where sections of lining material have flaked out.

While this is speaking specifically of cam bearings, another section about rod & main bearings titled "Foreign Particles in Lining" describes the same effects;

DAMAGING ACTION
Dust, dirt, abrasives and/or metallic particles present in the
oil supply embed in the soft babbitt bearing lining, displacing
metal and creating a high-spot.

Quote Quote by Baluncore View Post
Pressures in the system all come down to bearing area or member cross section. The cross section of the rod is the bottle neck for plastic deformation at low RPM.
ok, so the rod has a smaller cross section, but the bearing lining material has much less compressive strength
Quote Quote by Baluncore View Post
When starting the surface velocity is very low so lubrication is non-critical.
This would only be true in an extremely lightly loaded situation. Now add the overload condition as In the case of using the starter to force the piston up against the expanding combustion gases (as when a high amount of static timing is used).
From;
http://machinedesign.com/bearings/de...earing-failure
Generally, boundary lubrication occurs when the shaft is running at speeds less than the minimum speed required for a full oil-film development. This can happen, for example, during starts and stops. Boundary lubrication permits metal-to-metal contact and this is when smaller particulates will embed into the babbitt or abrade the babbitt material.

Quote Quote by Baluncore View Post
The oil pump does not have to provide bearing pressure because the film that supports the load will build up during cranking or immediately the engine starts.
Again, this would only be true if the ignition were retarded enough at start up to prevent the types of loads that tax the stock starter motors, otherwise, the load has already occurred, then later during cranking, or ever later after start up the film will eventually build up.
Baluncore
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#26
Dec19-13, 02:29 PM
P: 1,282
As you appear to be more convinced by marketing rhetoric than by pragmatic reality, any attempt to correct your misconceptions would clearly be a waste of my time.
351tom
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#27
Dec20-13, 08:57 AM
P: 14
Usually marketing rhetoric is used to over state the merits of a product, not to depreciate the integrity of a product, but none the less, I found a third manufacturer (King Engine Bearings) that uses terminology that may be more to your liking. They describe fatigue of the overlay (due to among other things, advanced ignition timing);
The fatigue of a copper based lining starts with fatigue of the overlay. The overlay flakes off from the underlying layer, disturbs the oil film, and changes the lubrication regime from hydrodynamic to boundary. The load localizes at the contact area causing the formation of small cracks on the lining surface.
So whether the bearing overlay is displaced, extruded, abraded, or flaked, the point is the bearing is damaged. As to the cause of damage, if ignition timing is advanced enough while the starter is cranking the engine, eventually a point is reached where the ignited burning expanding gases push down with enough force to stall the starter motor. We have already calculated the forces involved- and sure, the rods can be damaged, but that doesn't prevent the rod bearings from being damaged also. If you ever seen rod bearings with copper showing only on the top bearing shell, one of the causes of this is what I am talking about.
Baluncore
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#28
Dec20-13, 03:58 PM
P: 1,282
Quote Quote by 351tom
If you ever seen rod bearings with copper showing only on the top bearing shell, one of the causes of this is what I am talking about.
What you appear to be saying is that; advanced timing during starting has the same effect as operating an overheating engine, with advanced timing, under full power, at high RPM, for more than an hour. That is simply not the case.

I have rebuilt hundreds of broken engines and seen many different modes of failure.
I have seen plenty of bent and broken connecting rods.
I have never ever seen an extruded low friction film.

You are clearly living in a different universe or reality to me.
This Physics Forum must be a wormhole between our different universes.
351tom
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#29
Dec20-13, 06:45 PM
P: 14
I think the different types of engines we are considering may account for the "different universes". Your earlier comments;
During starting, the ignition timing should prevent an advanced spark occurring, so we can ignore pre-ignition.
indicate you have in mind either stock production engines made since electronic ignitions have been used, or engines that have been upgraded in this regard. This leaves out precisely the engines I am referring to. Take the chevy 350 for example, by far the most widely used small block engine of all time across many forms of racing , vintage vehicles, custom builds, etc. When these engines are built for performance, the stock electronic ignitions (if there were any as in pre early 1970's versions) no longer apply because the characteristics of the engine change so greatly. For example the HEI module may have had only 6 degrees retard at start. That was perfect for the engine it was designed for, but is just about useless for the racing engine with a 42 degree locked out distributor. Many times aftermarket ignition products that facilitate ignition retard at cranking are utilized, but for various reasons, many times they are not. There are a lot of variables involved, but depending on the specifics of the engine in question, you may not need to go anywhere near as far as 42 degrees to completely stall a starter motor. What I was trying to do is quantify the forces involved when this happens.
Now as far as comparing bearing loads of an engine at full power vs at cranking, consider this; the load on the engine at full power is determined by the combustion gases expanding into chamber that is increasing in size as the piston goes down the bore, whereas, the load on the cranking engine is determined by the combustion gases expanding (no, make that trying to expand) into a chamber that is decreasing in size as the piston is being forced up the bore.
You indicate that in your wealth of experience you have never seen the top rod bearing damage I mentioned. I don't doubt you at all, in the realm of the types of engines you deal with, it may be likely you never will. But take the comparative novice speedfreek who went through 2 engine rebuilds in 5000 miles with rod bearings failing showing copper (locked distributor at 36 degrees) then went to a mechanical advance with 18 degrees initial & had no problems. He is just dealing with a different kind of an engine.
Baluncore
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#30
Dec20-13, 08:22 PM
P: 1,282
A “comparative novice speedfreek” or a computer chip “over-clocker” is NOT an engineer.

They are individuals abusing the technology.
They have no concept of reliability engineering.
They are pushing beyond the SOE to destruction.

Economics and reliability are critical to engineering.
This is supposed to be an engineering forum.
351tom
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#31
Dec21-13, 03:25 PM
P: 14
A “comparative novice speedfreek” or a computer chip “over-clocker” is NOT an engineer.

They are individuals abusing the technology.
They have no concept of reliability engineering.
They are pushing beyond the SOE to destruction.

Economics and reliability are critical to engineering.
I agree with you 100%
My aim here is to quantify the possible bearing loads in the aforementioned scenario to point out the folly of such engine modifications. Knowing the torque of the starter motor, I suspect it will depend on on 3 more things; location of spark BTC, cranking speed, & the time interval from spark to combustion. If anyone has insight into that last item, (or the characteristics of the combustion process with regard to this) please contribute.


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