In typical internal combustion engines, pairs of hollow semi-cylindrical bearing lining shells (also known as “half bearings”) are used to allow rotation of the crankshaft at both connecting rod bearing assemblies and main bearing assemblies.
Connecting rod bearing assemblies are provided at the “big end” of a connecting rod, where it connects onto a crankshaft pin. The bearing shell in the connecting rod bearing assembly that is closest to the piston is known as the “loaded” bearing shell, as it bears higher peak loads than the complementary “unloaded” bearing shell, due to experiencing the combustion load from firing the corresponding piston. Main bearing assemblies are provided where the engine block and engine cap form a housing to support the main (axial) crankshaft journal. In contrast to the connecting rod assembly, in the main bearing assembly, it is the bearing shell that is furthest from the piston that is known as the “loaded” bearing shell and which bears higher peak loads that the complementary “unloaded” bearing shell, also due to experiencing the combustion load from firing the corresponding piston cylinder.
The bearing surfaces of bearing shells generally have a layered construction, in which a strong backing material is coated with one or more layers having preferred tribological properties to provide a bearing surface that faces the cooperating moving part (i.e. a crankshaft journal), in use. Known bearing shells comprise: a backing, lined with a lining layer, and optionally an overlay layer. The strong backing material may be steel, having a thickness of about 1 mm or more. A lining layer is provided on the backing, for example, a copper-based material (e.g. copper-tin bronze) or an aluminium-based material (e.g. aluminium or aluminium-tin alloy), which is adhered to the backing. The thickness of the lining layer is generally in the range from about 0.05 to 0.5 mm or 0.1 to 0.5 mm (e.g. 300 μm of copper-based alloy of 8% wt Sn, 1% wt Ni, and balance of Cu, apart from incidentally impurities). The bearing surface is optionally coated with a layer (the “overlay layer”) of 6 to 25 μm of a plastic polymer-based composite layer or a metal alloy layer (e.g. a tin-based alloy overlay). For example, WO2010066396 describes a plastic polymer-based composite material for use as an overlay layer, and WO2004007809 discloses a known tin-based overlay.
Lubricating oil is distributed from an oil gallery running through the engine block or engine cap to the main bearing assemblies, with oil supply channels supplying the oil to the bearing clearance through radial oil supply holes in the bearing shells. Channels (known as “drillings”) running through the crankshaft then redistribute oil from the bearing clearances of the main bearing assemblies to the bearing clearances of the connecting rod bearing assemblies. In normal operation, the crankshaft journal and bearing shell are spaced apart by a wedge-shaped cushion-like oil film that is drawn between them, under a condition known as “hydrodynamic lubrication”.
The bearing shell can become damaged by operating under excessively high peak loads, causing mechanical fatigue, which is commonly referred to as “overloading” the bearing assembly.
High peak loads in bearing assemblies result in correspondingly high, localised peak oil pressures and oil temperatures between the crankshaft journal and the bearing shells. Despite the addition of sophisticated oil additives, when the bearing assembly runs under high temperature operating conditions, the dynamic viscosity of the lubricating oil may greatly reduce, producing a thinner oil film. High peak loads and the low dynamic oil viscosity arising from high oil temperatures increase the incidences of direct physical contact between the crankshaft journal and bearing shell through failure of the intervening oil film, leading to increased rates of abrasion of the bearing shell, in use.
High temperature operation of the bearing shells also causes thermal degradation of the bearing shells, for example by enhancing the migration of minority species to metal-alloy grain boundaries (e.g. migration of tin to grain boundaries in an aluminium-tin lining layer), which weakens the layer. Further, high temperature operation can cause metal species to migrate between layers (e.g. to diffuse out of the overlay and/or lining layer). Yet further, high temperature operation causes thermal degradation of the lubricating oil, necessitating more frequent lubricating oil changes.
In conventional crankshaft main bearing assemblies, oil is fed into the bearing clearance between the crankshaft journal and the bearing shells through a radial oil supply hole in the unloaded bearing shell, which receives high pressure lubricating oil by an arrangement of channels through the engine.
In the case of main bearing assemblies, it is known to provide a wide circumferential oil distribution groove along the middle of the bearing surface (the concave inner surface) of the unloaded bearing shells, to supply oil to a connecting rod bearing assembly through a corresponding crankshaft drilling. The oil groove is used to increase the period of each crankshaft rotation during which oil is supplied from the wide circumferential groove, through a hole in the main journal, and through the crankshaft internal drillings, to the connecting rod bearing assemblies. A wide circumferential groove is required to ensure that oil is provided to the entire crankshaft bearing assembly at high pressure. Motivated by maximising the surface area across which the peak load is distributed, a grooved bearing is not used as a lower main bearing shell. An unloaded main bearing shell having a wide circumferential oil distribution groove is illustrated in WO2012069191.
In conventional crankshaft connecting rod bearing assemblies, oil is fed into the bearing clearance between the crankshaft journal and the bearing shells through a radial oil supply hole in the unloaded bearing shell, which receives high pressure lubricating oil from main bearing clearance, through the crankshaft drillings.
Connecting rod bearing assemblies may be subjected to higher peak loads than the main bearing assemblies, and may operate particularly close to their performance limit. Conventionally, to minimise the peak pressures in connecting rod bearing assemblies, neither the loaded nor unloaded connecting rod bearing shells are provided with oil distribution grooves, in order to maximise the surface area across which the loads are spread.