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

1. Engine 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.

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.

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.

1.3. Bearing Location

Bearing Location

Bearings must not move within the bearing bore and must have good contact within the bore for good heat transfer. Several construction features of a bearing are common to ensure these functions. One feature is the use of locating lugs on bearings to prevent movement and to properly center the bearing in the bore Figure 81. To ensure correct matching of bearing caps and lugs, bearing caps are assembled together with lugs located tab against tab. Make sure that the identification numbers match for the main bearing location in the block and on the connecting rod bearings. Another construction feature is called bearing spread . To assist bearing retention and good contact with the cap or web of the bearing bore for heat transfer, the parting face of a bearing is made wider than the diameter of the cap or web. Bearings, having a spread feature, must be snapped or pushed into place Figure 82.

FIGURE 08-81: A lug fits into a notch in the bearing cap or bearing bore to center the bearing and prevent its movement.

FIGURE 08-82: The concepts of bearing crush and spread. Bearings must seat tightly into the bearing bore to prevent movement and transfer heat.

Bearing crush is another construction feature that helps locate a bearing where it will be kept from moving and transferring heat. The diameter of both bearing shells is larger than the bearing bore; the difference in height between the bearing and the cap or web is called bearing crush . When assembled, the protruding section of parting halves of the bearing inserts above the bearing cap are forced against one another, crushing or forcing the bearing tightly into its bore. Dark spots observed on the back of a bearing indicate where the greatest heat transfer takes place in a crushed bearing. For this reason, bearings should go in dry on the backside with no grease or thread locking liquid applied to the back of the shell. Using this material would insulate the transfer of heat from the bearing.

1.4. Sputter Bearings

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.

1.5. Bearing Lubrication

Bearing Lubrication Every bearing receives lubrication through holes drilled into the crankshaft. Main bearings receive oil first from the oil pump and the main oil gallery. The oil enters the main bearing journal and drilled passages connecting to the rod bearing journals Figure 87. Most main bearing journals are cross-drilled through the journal centerline so that the oil hole aligns with a single annular groove in the top bearing shell twice for every crankshaft revolution. Without the annular groove, oil supply is otherwise cut off, so many main bearings have an annular groove down the center of both halves of the main bearing that allows oil to flow around the main bearing journal. An oil clearance around the bearing is calculated by subtracting the diameter of the bearing bore from the shaft diameter.

FIGURE 08-87: Lubrication oil travels from the main bearing journals to the rod journals through cross-drilled oil passageways in the crankshaft.

1.6. Measuring Oil Clearances with Plastigauge ®

Measuring Oil Clearances with Plastigauge ®

To evaluate the oil clearances of engine bearings, the use of Plastigauge ® is recommended. Plastigauge ® is a measuring tool consisting of a strip of oil-soluble plastic material and packaging printed with a thickness gauge.

Measuring bearing clearances with Plastigauge® is very simple when the engine is removed and turned upside down while on an engine stand. It cannot be performed in chassis when the crankshaft rests on the main bearing caps. After the main bearing cap has been removed from the inverted engine and the oil wiped from the crankshaft and bearing shell, a piece of Plastigauge ® is laid across the crankshaft journal. The bearing cap is reinstalled with a torque recommended by the engine manufacturer. When the bearing cap is tightened, the pressure causes the Plastigauge ® to be flattened. The less clearance there is, the greater the flattening and the wider it will be. When the bearing cap is removed, the width is measured by direct comparison with the graduated scale on the Plastigauge ® envelope Figure 88. The numbers on the graduated scale indicate bearing clearance in thousandths of an inch or millimeters on the reverse side of the paper gauge. Plastigauge ® is available in four different sizes and differentiated by four colors: red, green, yellow, and blue. Each color covers a particular bearing clearance range.

	A	B
	FIGURE 08-88: A. Plastigauging a main bearing journal. The Plastigauge ® is squeezed by the bearing and cap. B. Afterward, the width of the Plastigauge ® is compared against a gauge on the package.

1.7. Video: Measuring Bearing Clearance

1.8. Undersize and Oversize Bearings

Undersize and Oversize Bearings

An undersize bearing is thicker than a standard bearing. It does not describe the bearing but rather the shaft it fits. If a crankshaft is worn or ground to undersize, an undersize bearing is used to fit the journal. If a spun main bearing damages a block, the bearing bore can be rebored to an oversize dimension, and an oversize bearing can be installed.

Camshaft bushings can typically outlast two sets of main bearings because there is less loading. However, excessively worn and bleeding cam bearings can starve main bearings of oil and lead to premature crankshaft bearing failure.

1.9. Thrust Bearings

Thrust Bearings Thrust bearings are used to control end play in a crankshaft Figure 89. The forces of disengaging the clutch, helical gearing on the geartrain, and automatic torque converters can easily push or pull a crankshaft to the point where severe damage to the connecting rods and journals occur. If thrust bearings are worn out, it will be difficult to maintain a proper clutch adjustment when pressure plates, which are separated by just a few thousandths of an inch, move back and forth even a few thousandths of an inch more. Thrust washers or bearings with flanges ride against a specially machined journal on (usually) only one cheek of a main bearing journal.

FIGURE 08-89: A thrust bearing is used on a main bearing journal to control crankshaft end play.