The principal use for journal bearings in automotive applications is for the crankshaft bearings, piston pin joint, balancer shafts, and crankshafts. These can either be half shells in which a split is required for assembly (e.g., crankshaft bearings) or bushes that can be inserted without a split (e.g., piston pin joints). For engine bearings, the oil is normally supplied at pressure from the oil pump via a filter/ cooler assembly to a main oil gallery that runs along the side of the engine and then to the main bearing as indicated in Figure 8.1. The oil supply hole in the upper main bearing normally feeds into a groove so that a drilling in the crankshaft journal can transport the oil to the big-end bearing in the connecting rod. Oil supply to the pin joint small-end and piston pin boss is typically by “splash” from the sides of the or via internal drillings in the connecting rod or piston. During the design process, the assembly must be considered as a “system” with adequate lubricant supply and clearance shape—the bearings are often the “fuse” where any deficiencies become obvious in damage such as engine seizures. The normal design envelope for journal bearings is indicated by:
• Maximum specific load (MSL): MSL is the applied load divided by the projected area (length × diameter). Typical values are 30–70 MPa for “bimetal” bearings and 60–100 MPa for “trimetal” bearings. Overloading can result in loss of the lining by fatigue mechanisms or “swaging”/plastic flow of material near edges.
• Minimum oil film thickness: This is where the oil film is not able to completely separate the surfaces and the opposing sliding surfaces or tips of surface asperities start to contact each other with potential wear, flash temperatures, or seizure. Typical surface roughness values (Ra) are approximately 0.2 µm for crankshaft journals and 0.05 µm for gudgeon/piston pins. High temperatures can result from “mixed” or “boundary” lubrication conditions because the friction coefficient for a lubricated contact is approximately 1 or 2 orders of magnitude less than that for “dry” friction. At high engine speeds (6000–7000 r/min, gasoline engines), localized thin film areas can result in heat generation and hot spots within the oil because of high shearing.
• Shearing of oil within the clearance space: This can further increase the bearing surface temperatures by up to approximately 20–50°C higher than the supply temperature, which can result in overheating of the bearing. During engine operation, the oil sump or bearing oil supply temperatures can reach approximately 100–155°C. Under some load-speed conditions, bearing surface temperatures of 190°C or more are possible where some components within the bearing alloy such as tin can start to melt, causing engine seizures if the high temperatures are sustained. Prolonged periods of engine running at high temperatures can also lead to the degradation of oil where soot-like material or lacquers can clog up oil passages and leave a dark residue on the bearing running surface.