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

1.2. Bearing Construction

Bearing Construction

The materials and construction methods used to make bearings are chosen to extend the life of the engine by reducing friction between the moving surfaces. Optimal bearing design for the longest service life is best formulated according to application; LD versus HD or stop-and-go urban driving versus on-highway driving. Experimentation with various types of bearing materials and construction techniques has helped diesel engines evolve to the point where they can travel more than a million miles (>1,609,344 km) in on-highway service and last up to 50,000 hours in off-road machines.

In early automotive history, bearings were made with a lead-tin alloy called babbitt . This alloy was effective because of its low frictional characteristics and its ability to distort under severe load and embed dirt into its shell. Embedding dirt, rather than having a particle remain in an oil clearance and scratch the shaft, is the preferable alternative to a damaged crankshaft. One characteristic these bearings did not have was fatigue strength: the ability to withstand high loading for long periods of time.

To summarize, the most important bearing characteristics include the following:

• Compatibility: The ability of a bearing to allow friction without excessive wear or friction. Dissimilar metals have better compatibility. Copper and steel have good compatibility and lower wear, as do bronze or lead and steel. A bronze crankshaft with steel bearings could be combined to minimize wear, except bronze is not as strong as steel or cast iron.
• Fatigue strength: The ability of a bearing to carry a load. A bearing will crack or be wiped away if overloaded by shaft pressure.
• Conformability: The ability of a bearing to conform to irregularities on a journal surface. Because crankshafts bend and connecting rod ends distort, mechanical properties of bearings achieved by the selection of materials and construction technique are critical. Bearings with poor conformability will work against fatigue strength and limit load-bearing ability.
• Embedability: The ability of a bearing to absorb particle contamination. Dirt particles will scratch a shaft and ruin it Figure 79.

A variety of construction techniques are used to achieve different balances among the characteristics that will adapt to each engine’s unique operating conditions. Layered bearings, which are made from different metals in each layer, are combined to produce a plain bearing with trade-offs between each desirable characteristic for the longest service life. Tri-metal bearings, with copper alloys for the intermediate layer, are traditionally used in diesels Figure 80. A steel foundation layer makes up the bearing back, giving the bearing strength properties to prevent cracking or splitting. On top of the steel backing is a copper or bronze alloy intermediate layer that provides some strength, fatigue resistance, and conformability characteristics required of an average diesel engine bearing. The lead-tin babbitt layer provides good low-friction wear characteristics as well as embedability and compatibility features. To prevent galvanic corrosion between each metal layer, a nickel barrier plating is added between the babbitt and the copper or lead. A final top layer called lead flash or tin plating protects and provides a finish for the engine’s break-in period. This finish can be easily removed when new by touching the bearing or wiping it with a shop rag.

FIGURE 08-79: Embedability refers to the ability of a bearing to absorb dirt particles and embed them into the soft lead babbitt layer to prevent scoring the crank journal.

FIGURE 08-80: Construction of tri-metal composite bearings used in diesel engines. A tri-metal plain insert bearing is made up of three layers of metal with a nickel layer between each.