Engine builders turn into medical examiners when this happens; getting the story behind the story is what you must do to understand what went wrong and what to do to avoid it in the future.
According to engine bearing experts, a failed bearing is not usually the fault of the bearing but rather the fault of something else happening inside the engine. Things such as dirt or debris, oil starvation, overloading, low viscosity oil, rough journal surface finish and misalignment can be some of the main suspects of a bearing failure.
It’s important to remember that, by and large, the original equipment bearings do a good job up to a certain point in many mild performance applications.
But it all depends on bearings riding on a clean film of oil to create what bearing engineers call a “hydrodynamic wedge,” where the oil provides a cushion between the moving metal parts.
“As long as the bearings have an adequate supply of oil, they can last almost indefinitely,” says MAHLE Clevite’s Bill McKnight. “It’s not uncommon for diesel bearings to last 1 million miles and to see them last well over 200,000 miles in passenger car applications.”
According McKnight, 40% of bearings are damaged by foreign particle abrasion. In a bearing with normal wear, the pattern of wear should be uniform over approximately 2/3 of the bearing’s surface. Wear near the parting lines should become less noticeable, and the wear pattern should extend uniformly across the bearing in the axial direction.
Bob Sturk from Federal-Mogul’s Sealed Power and Speed-Pro brands agrees. “Debris in the oil is still the number one cause of bearing distress. This is related to oil maintenance, which is also responsible for marginal-lubrication related failures.”
Sturk says most failures due to bearing design and material selection are revealed and addressed during OE engine development, with the possible exception of cold-start issues in northern climates. “Of course, older engines that are re-fitted with new bearings are dependent on the cleanliness of the assembly, the condition of the refurbished crank in terms of surface finish and geometry, and maintaining proper oil clearances.”
King Engine Bearing’s Research and Development Manager Dr. Dmitri Kopeliovich says that metal-to-metal contact (a mixed regime of lubrication) may result from several factors such as insufficient oil supply (oil starvation), disruption of the oil film caused by bearing material fatigue, misalignment (e.g. hour glass journal, distorted connecting rod), poor journal surface finish, foreign particles embedded into the bearing surface, or low viscosity (diluted or overheated) oil.
Dr. Kopeliovich says that bearing damage due to metal-to-metal contact may appear in the following forms. Accelerated wear: the bearing is not overheated (shiny appearance); wiping: the bearing is heavily worn (overlay is partially removed), there are signs of overheating (blackening), partial melting of the overlay. or a hot short: severe wear, torn surface, heavy overheating, melted overlay and lining material.
Overlay fatigue itself does not cause engine failure, says Dr. Kopeliovich. However, running a bearing with a fatigued overlay for an extended period may lead to flaking of the overlay and a loss of the oil film thickness. The conditions for boundary lubrication (momentary dry contact) occur at low oil film thickness. This leads to excessive wear and localized loading, which may result in lining (intermediate layer) failure.
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.
The cracks then propagate throughout the lining thickness, meet the steel back surface and continue to advance along the steel-copper boundary. As a result, parts of the intermediate layer detach (flake) from the steel surface.
The appearance of fatigue in an aluminum based lining is similar to that of a copper based lining. It is also a result of overloading caused by running the engine under high loads (torque) at low rotation speeds for an extended period, localized loading of the bearing due to a misalignment, fuel detonation or other factors.
Fatigue cracks form on the surface and propagate inside the lining, reaching the steel back. The cracks then progress along the bond line between the lining and the steel. Ultimately, pieces of the lining may then separate or flake off from the steel back.
Selecting the proper bearing material based on the specific bearing loads is the most important consideration, according to Federal-Mogul’s Sturk. Then, clearance selection based on the operating speed range is an important criterion. Finally, some consideration should be given to the crankshaft material (NCI or steel) and the bearing material in order to achieve the optimal combination.
MAHLE Clevite’s McKnight says in order to select the correct bearings for your application you should know the answers to the following questions: How good is the geometry of the shaft and the housing bore? How good is the surface finish on the crank? Do you plan a significant increase in power output? What came in the OE engine is usually pretty good to begin with so you need to be sure there are reasons to upgrade.
Bearing experts say that leaded bearing materials in passenger car applications are on the way out due to environmental concerns. “In today’s world of lead-free materials, in many cases the absence of lead has affected the sliding characteristics of these new alloys,” says Sturk. “Therefore, to improve the seizure resistance of lead-free materials, Federal-Mogul, for example, has developed a new polymeric coating to help enhance these characteristics.
Known as ‘IROX,’ this coating has a unique combination of solid lubricants and hard particles that not only improve sliding performance, but also wear resistance while enhancing the fatigue strength of the substrate material.” Sturk says Federal-Mogul is just now starting production of this coating for the OE market.
The bearing load capacity (fatigue strength) should be higher than the maximum specific load, according to Dr. Kopeliovich. A safety factor of at least 10-15% should be taken into account.
Crankshaft material such as nodular (ductile) cast iron shafts have a rough surface resulting from the cast iron microstructure. Such rough surfaces cause increased wear to the soft overlays of tri-metal bearings when metal-to-metal contact occurs. Aluminum Silicon (AlSi) bearings are more compatible with nodular cast iron crankshafts. Since tri-metal bearings do not have the surface conditioning properties of AlSi bearings, they are better suited for use with steel crankshafts.
Aluminum bearings are more tolerant of misalignments and distortions due to the greater thickness of the bearing layer (.010?). Tri-metal bearings with babbitt overlay (thickness .0005?- .0008?) are more sensitive to geometric defects, says Dr. Kopeliovich. However, other arguments are made that tri-metal bearings are actually more forgiving to imperfect crankshaft conditions and that the babbit overlay helps it conform to irregularities easier.
Minimum oil film thickness is not always known, but it is important for the proper selection of bearing material. If the minimum oil film thickness is .000060? or lower, mixed lubrication regime (momentary dry contact) occurs frequently. Tri-metal bearings with soft thin overlays are less suitable than aluminum-silicon bearings for these applications, according to Dr. Kopeliovich.
What can you do to help your bearings survive and live a long, healthy life? “You need clean crankshafts and engine blocks,” says McKnight. “Make sure you always have clean oil and in the proper grade, and hold tight tolerances for crankshaft geometry, finish and vertical oil bearing clearance.
Sturk agrees: “Pay attention to cleanliness, surface finish and geometry and proper build clearances. These three areas are where engine builders can positively impact the service life of the bearing.”
While there are many variables that contribute to bearing failure than can be discussed here, thinking like a doctor will help lead you to a cure.
To download a chart of what the most common bearing failures look like, click here.