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Crankshafts and Bearings - Keeping the Relationship Strong
Keeping the Relationship Strong
By Brendan Baker
Today's engines typically have very tight tolerances everywhere and the crankshaft bearings are no exception. The truer the crank is in its alignment with the mainline and cylinder bore, the tighter the tolerances can be.
Bearings and mainline bores must be very precise because during operation the crankshaft is not actually straight: it is elastic. Think of a jump rope spinning around and you can begin to picture what a crankshaft is like under the pressure and forces of combustion.
Arguably, it's the most important piece in the engine, because almost every moving part in the engine is in some way attached to the crankshaft. For engine builders this requires close attention to any crankshaft's condition before returning it to service.
The relationship between the crankshaft and the engine bearings is a tight one - and thanks to increased advancements in technology, getting tighter every year. Engine builders are faced with the responsibility of machining the main and rod bearing journals to allow the bearings to fit tightly yet still allow an oil film. The crankshaft does not turn directly on the bearings but on the thin film of oil trapped between the surfaces. Should the crank journals get out of round, tapered or scored, the oil film will not form properly.
However, service on a crankshaft is no walk in the park. One engine builder we spoke to for this article said that it has become difficult to make a good profit on crankshaft work once its condition gets beyond a certain point. Companies that specialize in crankshaft reconditioning may be getting more work from engine builders these days because many are sending them out to be welded or repaired instead of doing it themselves.
There are several things that must be done in order to properly recondition a crankshaft such as removing all the galley plugs along with any gears that are attached. Then the crankshaft should be thoroughly cleaned and inspected for any damage to see how far out of specification it may be. There is much more to reconditioning a crank than this, of course, but the main point is you have to know what you have before you decide to reuse it or trash it.
According to our experts, it's not always necessary to grind a crankshaft to bring it back to acceptable condition. However, journal roundness, surface finish, taper, diameter, residual magnetism and fillet radii must be checked and meet the manufacturer's specifications.
Clearance is a crutch for bad alignment, according to some experts. "If you have very 'elegant' true position on the main line, and the true position of the cylinder for the main journals and pin journals and the big end and little end of the rods and bore alignment are all 'sweet,' the less clearance you need," says Don Sitter, Clevite Engine Parts.
For the average custom engine rebuilder, typical clearance is around .001" per engine journal diameter, and for safety sake you should add .0005" just to be safe. "If you've got a 2.200" crankpin, then you'll want between .0025" - .003" with the Plastigage for clearance. If you've got a 3" main bearing, then you're going to try to fit between .0034? - .0037"," says Sitter. The rule of thumb is roughly .001? of clearance per inch of shaft diameter above .500".
"The elegant true position may fit to 75 percent of this rule or less," says Sitter. "You have to remember, your incentive for doing this is: what does the damage to the bearing? It's not the crankpin or the main journal pounding on the bearing; it's the pressure in the oil film that's supporting that connecting rod on the pin and the main bearing journals in the saddle. The more clearance you have, the higher your oil film pressure becomes and the lower your film becomes at the same time."
The oil wedge that the crank rides on is only about .00005? thick when the engine is running. Tolerances like these necessitate a properly polished crankshaft. If there are any iron nodules or burrs sticking up from the surface of the journal, it doesn't take much to abrade the bearing or drastically increase the chances of seizure.
"The process of crankshaft grinding and polishing is an art form," says Sitter. There are some good polishing and micropolishing machines out there, but they must be used properly. Material, on the other hand, is very picky - it requires a scientific approach. The ferrite caps can't poke through and can't point in the wrong direction, opposed to the rotation.
Ferrite burrs create a directional sawtooth-like finish on the surface of the crank journal, typically the opposite direction the journal was ground or polished. It is critical to know which way the crank normally rotates so that it can be polished in such a way that the ferrite caps point in a favorable direction. A relatively flat and smooth surface finish is most desirable. There aren't any nodules or ferrite caps to be concerned with on forged steel cranks, only cast cranks, so it is not necessary to grind in one direction and polish in the other.
No matter how good an artist you are, it is not always possible to reuse a crankshaft in some cases. Some crankshafts are either too far out to bring back to spec or the labor involved to do so is too costly. One rebuilder we spoke with said he doesn't weld cranks anymore because he doesn't make enough money doing them, and if a crank does need to be welded he sends it out to another shop that specializes in repairing them. This doesn't mean, however, that he doesn't do any crank work. He still grinds them and balances them and even straightens them.
If you do go the reconditioning route, it is very important to thoroughly inspect the crankshaft for any cracks because it may mean that crank cannot be reconditioned. One of the leading reasons rebuilders buy replacement crankshafts is that when they do a magnetic particle inspection (MPI) they find that it is cracked. The next biggest reason, according to manufacturers we spoke with is when rebuilders have to weld the crank because one journal is .040" or .050" out and it has to be built up.
According to one crankshaft manufacturer, many of his customers walk the fine line between reconditioning a crank and buying a new one. "If all the crank needs is a straight grind, you can do that and move on. But if you need to weld the crank at all, you may be better off buying a new crank. After all the time and labor are added together when you're done welding it, the cost may actually be less to buy a new one. And after all that, you still have a welded crank," he notes.
One of the more recent trends seen in crankshaft development is they are starting to make more cast cranks instead of forged. And the main reason for this, we've learned is because the OEMs went back to cast cranks. It's less expensive for the manufacturers to make a cast crank than a forged one. John Deere was one of the first ones to do it, according to a leading supplier of replacement crankshafts.
"They have cast crankshafts with rolled fillets even on some of their larger engines such as the 8.1L. With the rolled fillets there's no radius, it's on the inside," says a spokesperson. "Many rebuilders have gone to this type of crank. And if I send them a forged crank with a regular radius in it, then we have to explain that it's just as good or better, but to some it's not genuine John Deere if it doesn't have a rolled fillet."
Cranks with rolled fillets may reduce an engine builder's ability to recondition it. The problem with rolled fillets is that you can only grind them to .010", which takes a lot of the serviceability out of the crankshaft."
Having a well-balanced crankshaft is another important element to properly reconditioning a crank. In a running engine, there are dynamic forces that exist, and a little bit of imbalance at low rpm becomes much more at higher and higher rpm. For example, at four inches from the crankshaft's center axis and with 28 grams out of balance there's 112 lbs of force on the crank at 4,000 rpm, which then increases to a whopping 448 lbs at 8,000 rpm.
Debris is our number one cause of failure, without a doubt," says Federal-Mogul Corp.'s Jeff Richardson. "When people grind cranks or they clean a head, there's always debris somewhere."
Clevite's Sitter also agrees that debris is a major cause of bearing failure. "I'd say at least half of all failures are debris-related," says Sitter. Misassembly and misalignment are about equal as far as bearing failures go. Debris can get trapped between the journal and the bearing surface and cause all kinds of problems. Debris may abrade the surface and score a journal or wipe the oil film, which may lead to a bearing seizure.
Insufficient lubrication needs to be considered as another cause of failure, too. If you break down the oil wedge you'll have metal-to-metal contact. It'll destroy both the bearing surface and crankshaft surface.
With regard to bearing materials, the aftermarket has generally been forced to follow whatever the OEMs decide to do as far as bearing technology is concerned. However, the aftermarket has at least one thing on its side that the OEMs don't - hindsight. Aftermarket bearing manufacturers can watch what the OEMs do and study their failures and successes to see what works best. Most late model engines are now designed to use aluminum bearings.
"The biggest reason that we're finding for going to aluminum bearings is that it seems to be a better choice for late-model applications," says Jeff Richardson, Federal-Mogul Corporation. "And even going back to some of the older applications, we're finding that aluminum is just a better overall fit for light-duty replacement applications."
Clevite also offers a few aluminum bearings for some of the more popular applications, mostly for production engine rebuilders. However, tri-metal bearings are still popular choices for heavy duty and performance applications.
"We still offer our tri-metal bearings, but the aluminum gives engine builders another option," says Jeff Schaerer, Clevite. "One of the reasons we use aluminum is that it's a harder material, so the aluminum silicon actually conditions the journal after it has been reground."
Aluminum bearings are not new technology. In fact, they have been around for a long time, but today's materials are superior to yesterday's aluminum bearings. "Our old corporate material on aluminum goes back into the '60s," explains Federal Mogul's Richards. "A couple of years back we created the material we termed 'A-500.' This is what really pushed us over the edge to what was better."
A-500, the material makeup of the lining material itself, is an aluminum-silicon lining. The silicon is what bearing manufacturers have determined make aluminum a preferred material for late-model bearing applications. The small particles of silicon, finely and evenly dispersed throughout the lining material, give it a better fatigue rating and better seizure resistance. And those small particles of silicon will actually polish the crank if you have any imperfections on the crank.
Another bearing manufacturer, King Engine Bearings, uses what is called 'Alecular' bearing material in which the alloy is made up of aluminum, copper, tin and several other elements. In addition, since it's an alloy it can maintain its properties throughout its entire depth resulting in more consistent, reliable performance. King's Alecular material can be used in high-load applications as well as street applications and has been able to withstand the stress and strain of blown, nitro-burning Top Fuel dragster engines that produce up to 5,000 hp. Also the Alecualar material has a melting/fatigue point that is over 1,100° F, which is about three times higher than the babbit overlay in a typical trimetal bearing.
Because they're harder, aluminum engine bearings don't offer the embedability that tri-metal bearings do, says Clevite's Schaerer. So the particles will wash through the bearings."
Clevite's Don Sitter says that due to the material makeup and design aluminum bearings can compensate for some crankshaft irregularities. "If there are weaknesses in the crankshaft finish, then the nature of the aluminum bi-metal is forgiving to those weaknesses and helps address that."
Compensating for those irregularities, however, means that metal from the crankshaft has to go somewhere, says Sitter. "We're not convinced that it's a good thing to remove that metal and rely on the filtration system to collect it. But that's going to happen; there's nothing for free there."
The 'grinding' or surface conditioning from aluminum bearings occurs when there's less oil film present. "If you've got the total of peaks and valleys that are less than the oil film, then the oil film is going to go away from the surfaces and the surfaces are going to touch until the high points are equalized," Sitter continues.
"Polishing of a cast crank normally reduces the surface finish disparities to a point that the total roughness of the bearing and the total roughness of the shaft if you oppose those, is less than your oil film. So the oil film fills in and carries the load, which is what we always design them to do," says Sitter. "The purpose of the bearing is always to support the oil wedge, not to support the crankshaft."
Federal Mogul's Richardson agrees but says that not all of the aluminum bearing is harder than trimetal. "Pieces of the lining material are harder but not the entire thing is harder," Richardson explains. "We were getting debris, scoring cranks with a broached bearing that had a flat ID surface. So what we did was we started boring the ID to get what we term 'micro grooves on the ID. And what that helps to do is if you do get debris in there it'll go into one of those microgrooves and get flushed out of the bearing itself, so it won't embed. What happens is if you get a piece of particle embedded in the lining it will score the crank.
Once we started looking at it, we figured out that embedability isn't what we wanted. We don't want stuff to get stuck in the bearing. We want debris to flush out of the bearing. How do we get it to flush out? We put the micro-groove or bored finish on it to help flush that material out."
Trimetal vs. Bimetal
Traditional main bearing construction is based on a three-layer construction made of a steel backing, a copper-lead layer and a thin overlay of babbit material.
"When you stack the tolerances up, one bearing may measure 2? exactly and another may measure 2.001?, so it's off .001?," says Federal Mogul's Richardson. "It's difficult to keep the bearing shell consistent because all the layers and the tolerances stack up. With the aluminum there are only two layers. You've got the steel backing and the aluminum lining. So when we talk about being able to reduce tolerances, we're mainly talking about reducing the stack up of tolerances. We're only stacking two tolerances up versus 5 to 7."
"We use both aluminum bimetal and the copper-lead trimetal bearings; we're starting to get into a little of both," says Eric Grilliot, Mahle Aftermarket. "When it comes to trimetal or bimetal applications it seems as though some companies have aligned themselves to one or the other, or carry specific lines of each. We have really not established ourselves one way or the other. We look at the OE bearing and if it's a trimetal bearing we'll stick with that; if it's a bimetal bearing, we'll go ahead and develop a bimetal bearing."
In comparing bimetal to trimetal in severe load applications Grilliot says that it really boils down to trying to get more from less. For higher specific loads there are some cases where the bimetal will outperform the trimetal. Still, he says Mahle is starting to shift toward aluminum and bimetal bearings. "The last time I spoke with the people in our bearing facility they said they would like to see it move in that direction because for the OE-replacement applications they're doing, trimetal products are becoming fewer and far between," says Grilliot.
The future trend for bearings may be headed toward more aluminum, but don't write off trimetal's existence just yet. There is still plenty of demand for them and many applications where trimetal bearings are better for the job.
Aluminum and tri-metal may continue to co-exist for quite some time, according to our experts. OEMs are primarily using aluminum right now, and part of the reason is because they can get the material for less than tri-metal bearings. Aluminum bearings are also bored so they can achieve tighter tolerances on the wall sizes, which allow them to select-fit the bearings. The tri-metal surface's plated overlays provide, regardless of what thickness they may be applied at, embedability, conformability and the surface action for a dry start separate from the structure of the bearing.