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

Engine Bearings

Engine bearings are replaceable wear surfaces used on the rotating journals of the crankshaft. To enable the engine’s connecting rods and main bearings to be easily replaced, the bearings are made as split halves (as opposed to a crankshaft made of multiple pieces using a one-piece bearing). They are also called plain bearings because they have smooth, flat bearing surfaces and do not rely on roller barrels or balls to reduce sliding friction between the journal and the bearing. Split plain bearings, used on the main crankshaft journals, are referred to as main bearing inserts or bearing shells . The connecting rod journals also use plain bearings. Both rod and main bearing inserts remain locked in their installed positions and slide over the journal surfaces. The upper shell of a main bearing always has a hole to receive lubrication from the main oil gallery in the engine block. A circumferential groove, in at least the upper half of the main bearing, provides a channel from the oil supply hole to distribute oil lubricant over the bearing surfaces Figure 76.

Bearings perform important functions inside the engine, including: Reduce friction Support moving parts under load Serve as replaceable wear surfaces Reducing friction is one of the most obvious functions that bearings perform, along with the lubrication system.

FIGURE 08-76: Rod and main bearings are constructed differently. An oil hole with a circumferential groove to distribute oil is required for the upper main bearing shell. An oil hole may be present in the upper rod bearing if pressurized oil is used to lubricate the piston pin.