LO 8-12 Identify and describe the purpose, types, composition, and failures of plain split insert bearings
1. Engine Bearings
1.4. Sputter Bearings
LO 8-12: Identify and describe the purpose, types, composition, and failures of plain split insert bearings. |
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Sputter Bearings One of the methods to reduce fuel consumption caused by friction between the engine’s crankshaft and the bearings is to use low-viscosity oils. Thinner oils are less resistant to shaft rotation and require less energy to pump. The problem with low- viscosity oils is that the oil film breaks down when high loads are applied to the bearings, resulting in metal-to-metal contact between the bearings and the shaft. This problem is further aggravated by crankshaft bending and distorting of the big end of the connecting rod due to the inertia forces at high engine speed. Increasingly more compact engine sizes (pushing more power from smaller displacements) and start-stop technology stress bearings even further. A solution to the problem of using low-viscosity oils is to use polymer-coated bearings with very tough surface coatings and better antifriction properties Figure 83. The primary purpose of the polymer coatings is to provide some protection for the top surface overlay material when operating with very thin lubricant or to prevent bearing wiping (loss of the top bearing layer material) when cold or when starting. The coatings prolong bearing life when the oil film breaks down and the bearing provides better compatibility properties. Solid lubricants are incorporated in the form of fine particles within a polymer coating. |
FIGURE 08-83: Sputter bearings have a hard polymer coating enabling them to use thinner oil and operate with smaller oil clearances. |
Layers of new, harder polymer coatings are applied slowly using a process called physical vapor deposition (PVD). This top layer is called a sputter coating and is much thinner and more resistant to wear than on conventional bearing shells. The polymer coating is reported to withstand a 50% greater stress than conventional bearing shells. Only one side, the more heavily loaded surface, of the main or rod bearing inserts uses a polymer-coated bearing Figure 84. Smaller oil clearances are used with lower-viscosity oil, which further reduces the energy required to pressurize and pump oil because less oil is needed to fill the bearing clearances. These bearings were first used in light automotive diesels, but now more HD engines are using them. One example is Detroit Diesel’s DD series engines that have used sputter bearings in its GHG engines Figure 85. The hardened sputter overlay is much harder than lead-tin alloys Figure 86. Operating clearances on these bearings are also much smaller, typically just over 0.001" (0.025 mm) of oil film.
FIGURE 08-84: Only the high-pressure side of the bearing insert for rods and mains uses a sputter bearing. |
FIGURE 08-85: This set of sputter bearings from a Detroit Diesel DD15 has more than 497,097 miles (800,000 km). The hardened half of the bearing is darker. |
FIGURE 08-86: Sputter bearings in this DD15 have a darker, harder overlay material and operate with smaller oil clearances than conventional bearings. |
| Technicians Tip | ||
| Sputter-coated engine bearings with PVD top layers use lower- viscosity oils. These specially classified oils have unique properties and are designated “F” series for fuel economy. Most importantly, oils used in engines with sputter bearings are not backward compatible with engines using conventional bearings and diesel engine oils. Using thicker-viscosity conventional oils in engines with sputter bearings can lead to oil starvation of the bearings or damage to oil lubricating properties due to high temperatures and molecular shearing of oil molecules. The result will be catastrophic engine failure. |
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