LO 8-12 Identify and describe the purpose, types, composition, and failures of plain split insert bearings

1.1. Hydrodynamic Wedge and Plain Bearings

Hydrodynamic Wedge and Plain Bearings

Plain bearings have the advantage of being lighter and more compact (considering their load-bearing capacity) than roller bearings. It is important to note that plain bearings with a pressurized oil film have less high-speed friction, last much longer, and support heavier loads than antifriction bearings (e.g., rollers, ball bearings).

Plain bearings are the best type of bearing to achieve long engine life. Today, a typical B-50 bearing life approaches 1,500,000 miles (2,414,016 km) for HD diesels. B-10 and B-50 are industry terms and are used to statistically qualify the average service life of a bearing. For example, a B-50 bearing life means that 50% of engines will likely require overhaul or replacement of bearings at a specified distance traveled. It does not mean that it has 50% service life remaining.

Plain split bearings can wear so little over a long travel distance due to the operation of friction-reducing properties of the hydrodynamic oil wedge. Essentially, the oil, bearing, and rotating shaft are separated from one another by hydraulic pressure. At rest, the shaft and bearing are in contact. On start-up, the shaft may briefly contact the bearing. But as the engine runs and the shaft turns, the shaft pulls oil from the clearance space surrounding the bearing into the wedge-shaped area between the shaft and the bearing. Because of the adhesive attraction the oil has to the metal shaft, oil is literally pulled into the wedge ahead of the direction of the shaft. The oil wedge lifts the shaft off its bearing and supports it with a film of oil, separating the metal parts during engine operation. The oil film pressure is generated by speed shaft rotation Figure 77 and is a function of the oil viscosity and bearing clearance Figure 78. During normal operating conditions, a continuous supply of clean oil will keep the shaft and bearing surfaces separated. Without enough clearance, inadequate oil will flow into the bearing for lubrication and cooling. With too much clearance, the oil wedge will collapse, resulting in contact between the shaft and the bearing. Excessive bearing clearance can result in low oil pressure. If the oil pump is required to pump extra oil volume to fill the clearances, while even greater quantities of oil are thrown off a shaft, the result is loss of pressure. Bearings fail when the oil film breaks down or when the oil film is overloaded.

FIGURE 08-77: Formation of the hydrodynamic wedge.

FIGURE 08-78: A large oil clearance around the bearing will allow more oil side leakage and a smaller oil wedge support area, as compared with a narrower oil clearance.