Click on a thumbnail to see the full-size image
According to one crankshaft manufacturer, the fir...
Engine Bearings and Crankshafts: Best Friends Forever (BFFs)
Crankshafts and the engine bearings that support them in the engine block share a very special relationship. In a stock engine this is important for long engine life, but in a high performance or race engine it becomes absolutely critical to both power and durability.
By Doug Kaufman
Bearings appear to be such simple engine components. On the surface, they’re just half-circles of metal whose only purpose is to reduce friction between and support moving parts. The engine bearing has to establish and maintain a film of oil between moving and stationary parts to prevent wear to expensive engine components.
They look like they just lie there, yet the reality is that the engine bearing is a complex part of the internal combustion engine and manufacturers continue to refine the technology to keep up with the latest original equipment and racing demands.
After several lengthy discussions with bearing manufacturers, crankshaft suppliers and fastener companies recently, it is clear that the information available to the engine builder is impressive. There is so much detailed information on selection and use of bearings in catalogs and on the internet that it can be intimidating or overpowering. Yet suppliers insist that asking questions is the only way to be sure that your engine build situation is addressed correctly don’t assume that one solution will be right every time.
This article consists of information provided by several sources. Thanks to Raymond King, Bob Sturk and Matt Barkhaus from Federal-Mogul; Ron Sledge from King Engine Bearings; Bill McKnight from MAHLE Clevite; David Leach from Lunati; Randy Madden from Quality Power Products; and Tom Lieb from Scat Crankshafts. Complete contact information is available at the end of this article to allow you to continue your research.
According to one crankshaft manufacturer, the first factor to be considered in choosing the correct engine bearings is the actual material the crankshaft is made of. The crankshaft manufacturer will provide you with reliable information as to which bearings they consider to be best suited for their product, and you should heed this information closely.
For high performance and mild race engines high quality, cast iron or steel alloy is a popular material. Due to its metallurgical composition, crankshaft suppliers often recommend using a classic bi-metal design bearing with any cast crankshaft. A forged or billet steel crankshaft considerably expands your choices for bearing designs.
Perhaps the best advice given here is again to check with the crankshaft supplier and choose main, thrust and rod bearings recommended by the crank and bearing manufacturers. It’s also imperative to carefully check that each bearing will clear the radius in the journals and still provide the right amount of clearance required for the application. Again, information you obtain from the crankshaft and bearing manufacturer should be your guide for bearing material and design, and recommended clearances.
Bearing manufacturers’ catalogs include much more than just part numbers and interchange information. If you’re unfamiliar with bearing terminology and selection criteria, the front of the catalogs can be a great place to start your education.
One bearing manufacturer says selection based purely on horsepower can be a dicey proposition. A more accurate calculation can be a load based on cylinder pressure to determine the available bearing loads that those bearings are supposed to absorb. “It is not quite so easy to say I am making 500 horsepower and I need this type of a bearing,” explains one engineer. “It’s more of a cylinder pressure calculation over the available area of the bearing and that kind of directs us towards the material we would recommend.”
This can be calculated if you know what the cylinder pressure is and you know what the piston diameter is, say experts. You’ve got the force that will be working on the bearing below it. It’s a simple bearing length times crankpin diameter calculation that can give you that number.
One supplier says OE customers understand the importance of this calculation, but in the aftermarket it may not be so apparent. Does the typical engine builder have any comprehension or understanding of what unit load numbers might represent as far as the kind of work he would do?
Suppliers say that, among all the other variables he must take into consideration when specifying the components for his engine, regardless of its application an engine builder must consider his peak cylinder loads, rpm range and operating temperatures. Is the application short duration, medium to high loads or long duration with extreme loads? Does the bearing need to have embedabilty and conformability characteristics built in for smooth operation? Will the operating environment of the engine be clean or dirty? What type of crank will be used, cast nodular or forged steel?
Of course, application cannot be overlooked. Most engine builders and bearing manufacturers choose specific bearing designs and compositions that are based on a number of factors. One of the most important is matching the bearings to the engine’s usage. Bearing manufacturers have untold amounts of data from which to make their recommendations. This data will include any possible application your engine may encounter.
Particular emphasis should be placed on the exact type of racing the engine will see, and track surface. Paved circle track racers and drag racers operate in an environment that has considerably fewer airborne contaminates, tire dust and fine grit than do dirt track and sand drags racers. For obvious reasons, these ambient factors must be considered when choosing engine bearings. Finally, bearings must be chosen to match the correct journal sizes needed, and to obtain the desired journal clearances.
Horsepower should be matched to the correct bearing material. Higher loads require bearing materials to have higher load capacities to be resist fatigue (tiny cracks). Lubrication choices (viscosities) are usually based on oil pressure, oil clearances and operating temperatures. Today’s high performance, multi-viscosity racing oils provide excellent shear strength, high temperature thresholds and contain friction fighting additives.
The performance and racing industry continues to experience an amazing evolution of technology rapidly thrusting forward, solving old problems and creating new opportunities. Bearing manufacturers are no different. Due to the extremely critical role bearings play in both power output and longevity, the search for newer, better bearings is ongoing.
Although basic bearing design continues to reflect the technology of the last century, manufacturing techniques and materials reflect an ongoing desire to improve the product and its performance. Major engine bearing manufacturers are continuously testing many different configurations of materials as well as new materials. Their research programs are ongoing in seeking ways to make bearings that have a higher load capacity, can withstand higher operating temperatures, are compatible with current racing oils and can reduce parasitic operating loss by reducing friction.
The relationship between bearings and engine oil cannot be overlooked, of course. Important factors include the type of oil used (synthetic or mineral-based), viscosity weight of the lubricating oil and the basic oil system design of the engine. Wet-sump oil systems versus dry-sump systems and the design of the oil pan can be critical to correct engine lubrication needs for a race engine.
A more recent development concerns coatings for bearings. Some engine builders swear by the results they get with coated bearings, others claim they get the same results with careful engine assembly, proper bearing clearances and careful attention to the entire engine lower-end assembly.
For their part, some manufacturers are investing heavily in coatings research. One bearing supplier explains that it offers two very good and durable coatings right now in the aftermarket. “One is just for alcohol and nitromethane fueled engines, the other works everywhere else. At the OE level, we’re working on coating technology that may someday soon, replace the overlay on a trimetal bearing or allow a bimetal aluminum bearing to work more like a conventional trimetal bearing.”
These developments are an ongoing part of the business and most of the time are driven by the original equipment manufacturer. Many of the coatings bearing manufacturers are looking at are for solving specific problems in terms of where engine design is going in the future, says a leading manufacturer. It isn’t so far fetched to envision a time in which internal combustion engines of all sizes are built similar to a golf cart.
“You take your foot off the gas, the engine completely stops, then you put your foot back on and the engine restarts. That has some very serious consequences on bearing wear and all these frequent restarts that happen without pressurized oil. So the coatings help us beat a lot of the wear concerns that we have with all these multiple starts and stops.”
Even today, hybrid vehicles are presenting challenges that coated bearings are called on to address. The coatings can also help bearings suppliers address the trend of very low viscosity oils. As the industry goes to thinner and thinner oils, the operating oil film thickness can get very thin at times coatings can help when those films are so thin that the surfaces come in contact.
The complexities of the crankshaft and engine bearing relationship are greater than we can discuss in these pages, but you can find significantly more information on the rest of our website. You can search for past articles on crankshaft design and selection and you can read the complete discussion from the Engine Bearing Summit we held with leading bearing manufacturers (and read the side bar at the end of this article below).
THE FASTENER FACTOR
It’s important to note that the fasteners you employ, and the manner in which the rod and main caps are attached, can have a significant effect on bearing loading and clearances. Noted fastener manufacturer ARP has conducted extensive research into this area and the firm’s Director of Research & Development, Jeff Kibler, shared some important information with us.
The majority of ARP’s main studs and bolts are manufactured from heat-treated 8740 chrome moly and have a nominal tensile strength of 200,000 psi. They are designed for high performance use and can exert some 25-35% more clamping force than OEM fasteners they replace. The use of stronger fasteners and additional clamping load can prevent main cap “walk” and bearing fretting.
One problem, notes Kibler, is that in rebuilds where the mains are not align honed the extra clamping force can cause the cap to “crush” the bearing and reduce the oil clearance by .0005˝ to .002˝. With a number of import engines running very tight main bearing clearances, this can lead to serious problems. ARP strongly recommends align honing whenever high performance fasteners are employed.
During the align honing process, or final assembly for that matter, another problem can rear its ugly head; preload scatter. This phenomenon occurs when torqueing the fastener does not achieve the desired preload and is especially problematic with different loads on adjacent fasteners, which can affect bearing clearances.
Most of the preload scatter problems can be attributed to the fastener lubricant. Oil, moly, EPL and other substances have demonstrated inconsistencies in their friction characteristics. You set your torque wrench to the desired load, hear a “click” and assume all is well.
Unfortunately this is often not the case, as the friction in the threads and lubricant consumes a portion of the wrench’s torsional energy and it “clicks” without the fastener having been stretched sufficiently to generate the desired preload, or clamping force. And it can be off by as much as 30%. In the past, the only way to mitigate preload scatter was to loosen the fastener and retighten it for up to a half-dozen cycles until the friction was equalized. This, obviously, is a tedious and time-consuming process.
To provide a solution to the preload scatter problem the R&D team at ARP spent years testing all manner of substances using a sophisticated computer-controlled device that can measure actual preload in cycles. This data enabled them to develop ARP Ultra-Torque fastener lubricant, which has proven to deliver 95-100% of the desired preload on the first pull, and all subsequent pulls.
Torque wrench accuracy is another area of concern. ARP has been offering free torque wrench testing services at all NHRA national events for several years and discovered that a large percentage of the wrenches tested were inaccurate; some off by as much as 25-30%. Needless to say, it is important to continually monitor this critical tool.
Another factor can be the “quality” of the align hone itself, as an operator can linger at one main and remove slightly more material. One way to counteract this is to carefully measure each bearing bore (typically from top to bottom or vertical axis, and just above and below the parting line of the cap), then check the bearings themselves with a ball micrometer. Often there’s a tenth or two variation in thicknesses, so the bearings can be installed in the block to even out the bearing oil clearance as much as possible.
Connecting rod resizing is also important when installing higher preload bolts in connecting rods, as the caps can be affected by the added compression, Dealing with rod bearings is much the same as the mains, however rod bolt preloading can be accurately validated through use of a rod bolt stretch gauge. Because a fastener must be stretched a given amount to attain a specific preload regardless of the lubricant it’s an absolute.
For example, a typical small block Chevy rod bolt will require some 50 ft. lbs. of torque (using ARP Ultra-Torque) to attain a preload of approximately 10,000 psi. The bolt will have stretched .0055 - .0060˝. A different lubricant may require more torque, but the stretch will be the same.
A rod bolt stretch gauge can also be used to monitor the condition of the rod bolts in racing applications where there are systematic rebuilds. The length of each bolt should be charted prior to initial assembly, and re-checked when the engine comes apart. If a rod bolt has permanently stretched .001” or more it should be replaced.
In the final analysis, bearing effectiveness and service life can be improved with the correct use of high performance fasteners.
Automotive Racing Products, Inc. (ARP)
Engine bearings continue to be one of the most hotly contested topics in the engine parts aftermarket. We can only scratch the surface in an article such as this, so in the spirit of open discussion, Engine Builder
magazine recently invited participants from the leading bearing manufacturers to participate in a roundtable discussion on pre-determined topics. We asked eight questions of our participants, including:
What are the primary differences in hardness, strength and embedability between today’s aluminum and tri-metal bearings?
How does the alloy and construction of a bearing impact its durability?
What other engine factors, including the parts do you have to take into consideration when you are designing bearings and when selecting a bearing?
What effect have low viscosity, low-ZDDP motor oils, and the changes in emissions had on bearing construction and selection?
From your perspective on the installer side, what are the biggest installer-focused issues and concerns regarding bearing installations?
Are there specific problem vehicles that you’ve had to develop fixes for?
Looking at coatings on engine bearings, it is a new technology that is getting more and more interest. What processes do you recommend for bearings and when?
What type of bearing do you recommend for the following applications: stock engine, street performance, race engine and diesel?
A complete transcript of each question in this extremely educational and interesting discussion is available to download here.
Contributors to this article included:
King Engine Bearings
Quality Power Products
Scat Enterprises Inc.