Soft strength... Sounds paradoxical, doesn’t it? Commonly we perceive something as being strong, when it is hard, tough and rigid. But never soft.
Why can’t the engine bearings be just strong?
Is it not enough for their durable and reliable functioning?
The answer to the last question would be positive but only under one condition: if the engine bearings were ideal hydrodynamic bearings.
Unfortunately such ideal condition is unachievable in the real engines.
A loaded rotating journal is always displaced from the concentric position, forming a converging gap (wedge) between the bearing and the journal surfaces. The presence of this oil wedge is indispensable to the normal operation of a hydrodynamic bearing.
Journal rotation causes pumping of lubricant (oil) flowing around the bearing in the rotation direction.The pumping action of the journal forces oil to squeeze through the wedge shaped gap generating pressure. The force created by oil pressure (pressure force) counteracts the external force F therefore the film separating the bearing and the journal surfaces remains stable. The film thickness is determined by the values of F, rotation speed, oil viscosity, oil clearance  and the geometric parameters of the bearing (diameter and length).
It is presumed that the ideal hydrodynamic bearing is absolutely rigid (as well as its housing), its axis is parallel to the journal axis, the journal has a shape of a perfect cylinder, the surfaces of the bearing and the journal are perfectly smooth, the oil supplied to the bearing is pure and its amount is sufficient for hydrodynamic lubrication.
The internal combustion engines are characterized by cycling loading of the bearings caused by alternating pressure of combustion gases in the cylinders and inertia forces developed by accelerating parts. Oscillating loads applied to a part may cause bearing failure as a result of the material fatigue. The lubricant (oil) film prevents localized overloading providing a distribution of the applied force over a relatively wide area. However If the pressure transmitted by the oil film to the bearing is greater than the fatigue strength of the material fatigue cracks form on the bearing surface. Thus sufficient fatigue strength is the only material property required for the ideal hydrodynamic bearing.
Unfortunately the real engine bearings differ from the ideal journal bearing.
Thus the real engine bearings operate in a mixed regime of lubrication  characterized by intermittent contacts between the friction surfaces.
In contrast to the ideal hydrodynamic regime the bearings operating in a mixed lubrication should have a combination of the material properties including those attributed with softness.
The real racing bearings work under conditions of great alternating loads, high rotation speeds, and intermittent metal-to-metal contacts with the journals and in the presence of impurities transported in the oil.
Here are the main properties of materials for engine bearings:
Fatigue strength, wear resistance and cavitation resistance characterize the material strength and hardness. Whereas compatibility, conformability and embedability are attributed to the material softness. The engine bearings should combine strength and softness. Common sense tells us that this is not possible since one characteristic (strength) contradicts the other (softness).
The contradictory combination of strength and softness may be achieved if the bearing material has a composite structure. Engine bearings are composed of a steel back, onto which is applied a relatively strong base (copper or aluminum) combined with a solid lubricant in the form of either a thin overlay or small particles distributed throughout the base material.
Bearing material having a thin overlay is called tri-metal, in contrast to materials without any overlay – bi-metal. Typical tri-metal and bi-metal engine bearing construction is shown in Fig. 2:
Fig. 2 Structures of bearing materials
Microstructure of a typical tri-metal bearing is depicted in Fig. 3:
Fig. 3 Microstructure of a tri-metal bearing
The overlay provides softness of the bearing. Soft leaded alloys commonly used as the overlay material in racing bearings have excellent conformability, embedability and compatibility. The strength is provided by the intermediate layer made of a leaded bronze. Lead is added into the bronze composition in order to improve its compatibility (seizure resistance).
Tri-metal bearings have a limitation with regards to the thickness of the overlay. Overlay thickness is always the result of a compromise between the required load capacity and the bearing’s anti-friction properties. If the overlay is partially worn off this increases the threat of seizure between the crankshaft journal and the exposed bronze of the intermediate layer.
Bi-metal bearings do not have overlays therefore they are much more tolerant to the amount of wear. Fig. 4 depicts a typical microstructure of a bi-metal bearing.
Fig. 4 Microstructure of a bi-metal bearing
Aluminum alloy of a bi-metal bearing contains tin particles distributed throughout the aluminum matrix. Soft tin serves as a solid lubricant. It improves the seizure resistance of aluminum. Aluminum alloys are commonly softer than bronzes in tri-metal bearings therefore they provide good conformability and embedability properties for the material. Thickness of aluminum alloy in a bi-metal bearing is about 0.012”. They are capable to conform to greater distortions and misalignments than tri-metal racing bearings having overlays with a thickness 0.0005”.
Tri-metal bearings with soft leaded overlays are traditionally popular in racing applications. However the engines efficiency and the output have been considerably increased for last years. The specific loads applied to the bearings have also increased. In addition to the greater loading there is a trend to use the motor oils with a reduced viscosity which lower the power losses caused by friction.
Thus the bearings in modern racing engines work at higher loads and lower minimum oil film thickness. The load capacity and wear resistance of traditional tri-metal materials may be insufficient for some high performance applications. King Engine Bearings has solved the problem with the strengthened tri-metal material pMax Black™ (Fig. 5) which has been developed for highly loaded racing engines.
This development comprises an innovative technique which enables the formation of an ultra-thin hardened shield on the overlay surface. This shield of nano-scale thickness considerably reduces wear of the overlay. It also effectively suppresses the formation of fatigue cracks on the overlay surface.
pMax Black™ bearings withstand greater loads without a formation of fatigue cracks and distress. The experiments using King’s test rig demonstrated that fatigue resistance of the pMax Black™ overlay is 10,200 psi. This is 17% greater than that of conventional tri-metal bearings (8700 psi).
At the same time, the properties of excellent seizure resistance, conformability and embedability – which are characteristics of soft overlays – are preserved. All King XP series high performance bearings are manufactured with this hardened pMax Black™ overlay. These bearings are easily recognizable by their distinctive dark color.
Fig. 5 King pMax Black™ bearings
It is important to emphasize that the overlay with the nano-scale hardened layer is still much softer than steel or cast iron. Therefore it produces no damage to the journal surface.
pMax Black™ bearings have been tested as compared to identical bearings made of conventional tri-metal material by four bearing manufacturers. The tests were performed in King Test Rig .
Test conditions: Load: 10,150 psi, Duration: 24 hrs.
The conditions of the bearings after the tests are depicted in Fig. 6.
Fig. 6 Fatigue test of King pMax Black™ vs. competitive materials
The bearing pMax Black™ has survived the test without overlay fatigue and any noticeable signs of wear.
Substantial area (between 28 to 50%) of the overlays on the competitive bearings has been affected with fatigue cracks and wear.
Most of aluminum alloys used in bi-metal bearings have load capacity below 10,000 psi therefore they cannot be used in the engines combining high loads and relatively long service life. However some features of aluminum bi-metal bearings have clear advantages over tri-metal. First of all they do not have overlay. The working layer of a bi-metal bearing is relatively thick therefore bi-metal bearings are very tolerant to geometric defects of the housing and the journal and to the conditions of continuous metal-to-metal contact (characteristic for drag racing). The critical amount of wear in the bi-metal bearings is considerably greater than that in tri-metal bearings with thin overlay. King Engine Bearings has developed HP Bi-metal aluminum bearings. HP bearings are best suited for street/strip, all out drag racing, monster truck and medium loaded circle track racing engines. The aluminum alloy of HP bearings provides a balanced combination of good load capacity with good embedability and conformability - characteristics for absorbing debris and conforming to crankshaft deflection. \Compatibility of the alloy is additionally improved by the presence of silicon in the composition. Silicon helps to keep the ferrite cavities on the surface of the cast iron crankshaft rounded off to prevent seizure and excessive bearing wear.
 Dudley D. Fuller (1958), A Survey of Journal Bearing Literature, Amer. Soc. of Lubricating Engineers
 Dmitri Kopeliovich, “Optimization of Clearance Design for High Performance Engine Bearings.” SubsTech (Substances & Technologies). Retrieved from www.substech.com/dokuwiki/doku. php?id=optimization_of_clearance_ design_for_high_performance_ engine_bearings.
 Dmitri Kopeliovich, “EccentriX™ Optimal Eccentricity for High Performance Bearings” SubsTech (Substances & Technologies). Retrieved from www.substech.com/dokuwiki/doku.php?id=eccentrix_optimal_eccentricity_for_high_performance_bearings
 Dmitri Kopeliovich, “Geometry and Dimensional Tolerances of Engine Bearings.” Engine Professional October-December 2011: 70-76.
 Dmitri Kopeliovich, “Lubrication regimes” SubsTech (Substances & Technologies). Retrieved from www.substech.com/dokuwiki/doku.php?id=lubrication_regimes
 Dmitri Kopeliovich, “Engine bearing fatigue test”, SubsTech (Substances&Technologies), Available from www.substech.com/dokuwiki/doku.php?id=engine_bearing_fatigue_test