In diesel engines, friction bearings serve to support the crankshaft, connecting rod, camshaft, and gear train. In some engine applications, friction bearings also support the rocker arm shaft as well as various pumps.

A type of friction bearing that is representative of most of the bearings used in Navy diesel engines is the PRECISION BEARING. Precision-type bearings that act as supports for the crankshaft are referred to as MAIN JOURNAL BEAR-INGS. In our discussion of friction bearings, we will use the main journal bearing as a representative sample.

Main journal bearings are of the sliding contact, or plain, type consisting of two half shells. (See fig. 4-25.) The location of main engine bearings in one type of block is shown in figure 4-26. The main journal bearings of most marine engines used by the Navy are of aluminum, aluminum alloy, or trimetal construction. In the trimetal type of construction, the bearing has a steel back bonded with an intermediate layer of bronze to which is bonded a layer of bearing material. The bearing material is either lead-based babbitt or tin-based babbitt. Regardless of the construction materials, the function and performance of main journal bearings are basically the same, with the exception of bearings, which are constructed not only to support the crankshaft but also to hold the crankshaft in position axially. This is done by flanges, which are part of the bearings, as shown in figure 4-26. Such flanges are on both halves of the bearing.

Figure 4-26.Main bearings in a cylinder block.

These types of bearings are called THRUST bearings. Some engines use separate flat thrust washers, as shown in figure 3-8 of chapter 3, on each side of one main bearing to control the crankshaft thrust (back and forth movement). Main bearings and their housing and caps are precision machined with a tolerance sufficiently close that, when properly installed, the bearings are in alignment with the journals and fit with a predetermined clearance. The clearance provides space for the thin film of lubricating oil which is forced, under pressure, between the journals and the bearing surfaces. Under normal operating conditions, the film of oil surrounds the journals at all engine load pressures. Lubricating oil enters the bearing shells from the engine lubricating system, through oil grooves in the bearing shells. (See figs. 4-25 and 4-26.) These inlets and grooves are located in the low-pressure area of the bearing.

Main bearings are subjected to a fluctuating load, as are the connecting rod bearings and the piston pin bearings. However, the manner in which main journal bearings are loaded depends on the type of engine in which they are used. In a 2-stroke cycle engine, a load is always placed on the lower half of the main bearings and the upper half of the piston pin bearings. In the connecting rod, the load is placed upon the upper half of the connecting rod bearings at the crankshaft end of the rod. This is true because the forces of combustion are greater than the inertial forces created by the moving parts.

In a 4-stroke cycle engine, the load is applied first on one bearing shell and then on the other. The reversal of pressure is the result of the large forces of inertia imposed during the intake and exhaust strokes. In other words, inertia tends to lift the crankshaft in its bearings during the intake stroke and exhaust stroke. Additional information on bearings can be found in Naval Ships Technical Manual, chapter 244.