Engine Bearings Engine bearings are known by a number of terms. Precision inserts, plain bearings, tri-metal bearings etc.. Bearings used around the crankshaft are split type insert bearings because the bearing is made in two pieces. Manufacturing the bearing into halves allows the bearing to be easily serviced and the use of a one-piece crankshaft. Camshaft bushings are examples of plain bearings that are one piece. Plain bearings also are lighter and more compact for its load bearing capacity than a roller type. Bearings perform a number of important functions inside the engine. Reduce friction. Support moving parts under load Serve as replaceable wear surfaces Reducing friction is one of the most obvious function bearings perform in conjunction with the lubrication system. Bearing surfaces should minimize friction and heat generation. It should be noted that plain bearings in conjunction with a pressurized oil film have less high-speed friction than an antifriction type (Roller, ball bearing etc.). Antifriction bearings may be used in gasoline fuelled two-stroke engines such as in outboards, motorcycles etc. because they rely on the oil-fuel mixture to lubricate the bearings. To achieve the engine life that bearings have today, plain bearings are best used. Today a typical B-50 bearing life is approaching 750,000 miles. That means that at that mileage there is still 50% bearing life remaining. To achieve the long durability the characteristics of the lubrication film between the rotating shaft and bearing are critical. The term Hydrodynamic Wedge is the explanation given to understand why bearings can last so long but wear so little. Essentially the oil, bearing and rotating shaft are separated from one another by hydraulic pressure. Engine bearings depend on a film of oil to keep shaft and bearing surfaces separated. Bearings fail when the oil film breaks down or when the bearing is overloaded. The oil film pressure is generated by shaft rotation. Above: Plain Bearings Bushings and Split Insert.
Examine the chart of Federal Mogul. At rest, the shaft and bearing are in contact. On start up the shaft rubs the bearing briefly. Running, the shaft pulls oil from the clearance space into the wedge shape area between the shaft and bearing. Oil is literally pulled in because of the attraction or affinity oil has to the metal shaft. The oil wedge lifts the shaft off its bearing and supports it during engine operation. With normal operating conditions and a continuous supply of clean oil the shaft and bearing surfaces will remain separated. Bearing Construction In early automotive history, bearings were made of a lead tin alloy called Babbitt. This alloy was effective because of its low frictional characteristics, the ability to distort under severe load and embed dirt into its shell so the crankshaft would not be damaged. Bearings like this were serviced by pouring in the molten hot alloy into the block/connecting rod areas. When cooled the bearing caps were parted, shimmed and excess Babbitt flashing removed with a pocket-knife. However, one characteristic these bearings did not have was fatigue strength the ability to withstand high loading for long periods of time. A summary of the preceding important bearing characteristics is the following: Compatibility: The ability of a bearing to allow friction without excessive wear or friction. Dissimilar metals have better compatibility. Copper to steel and bronze or lead to steel have good compatibility or lower wear when used against one another. A bronze crankshaft with steel bearings could be combined to minimize wear except the crank isn t as strong as steel/cast. 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: Ability of a bearing to conform to irregularities on of a journal surface. (Works against fatigue strength) Embedability: Ability of a bearing to absorb particle contamination. Dirt particles will scratch a shaft and ruin it. A variety of construction techniques are used to achieve different balances among the characteristics that will adapt to each engines unique operating conditions. Bi-metal bearings are used by OEM s in automotive engines at manufacturing level. They have a lower cost factor but give good service if assembled only in very clean working conditions and in new engines where construction tolerances are limited.
Aluminium alloy bearings may also be used at the aftermarket or OEM level in automotive engines. They are not commonly used on diesel engines. More commonly tri-metal bearings with copper alloys for the intermediate layer are used. A steel back with a copper alloy intermediate layer gives the strength, fatigue resistance and conformability characteristics required of a diesel engine bearing. The lead tin Babbitt layer provides good wear and compatibility features. Above: Tri-metal Bearing Construction Above Tri-metal Bearing Dimensions (Navistar) Below: Example of Dirt Embedded Into Bearing A nickel barrier plating prevents galvanic reactions between the Babbitt and copper/lead that would lead to corrosion. Flash lead/tin plating protects and provides a cosmetically more attractive finish. This finish can be easily removed when new by touching the bearing or wiping it with a shop rag.
to maintain pressure to the rod journal. This may only be in just the top shell since any groove subtracts from bearing area. Oil holes are drilled offset from the thrust load surface of the journal to improve the quantity of oil for film strength. Above: Too thick a Babbit reduces bearing life since Babbitt does not have good fatigue strength federal Mogul Bearing Lubrication Every bearing receives lubrication through holes drilled into the crankshaft. Main bearings receive oil first from the oil pump and main oil gallery. The oil enters the main bearing journal and connecting drilled passages to the rod bearing journals. Some journals are crossed drilled so the oil holes align twice for every crankshaft revolution. Oil is otherwise cut off so many main bearings have a groove down the centre of the main bearing that allows oil to flow around the main bearing journal Above: Note the miss-assembly problem between the bearing and the oil hole for lubrication oil! The amount of oil clearance is important to maintaining the hydraulic pressure of the oil wedge separating the rotating shaft and bearing. Too little clearance and inadequate oil will flow into the bearing for lubrication and cooling. Too much clearance and the oil wedge will collapse resulting in shaft to bearing contact. Excessive bearing clearance is a primary reason to have low oil pressure. If the oil pump is required to pump extra volume to fill the clearances while even greater quantities of oil are thrown of a shaft, loos of pressure will result. (Pumps produce volume not pressure lubrication systems require restrictions to produce pressure). Higher shaft speeds also produce greater oil wedge pressure. To evaluate the oil clearances of engine bearings the use of plasti-gauge is recommended. Plasti-gauging Plastigage consists of two essential parts: the Plastigage proper, which is a platic material soluble
in oil, and a graduated scale printed on the envelope enclosing the Plastigage. Measuring bearing clearances with Plastigage is very simple. After the bearing cap has been removed and the oil wiped from the crankshaft and bearing shell, a piece of Plastigage is laid across the bearing insert. The bearing cap is reinstalled with a torque recommended by the engine manufacturer. When the bearing cap is tightened, the pressure causes the Plastigage 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 Plastigage envelope. The numbers on the graduated scale indicate bearing clearance in thousandths of an inch or millimetre on the reverse side of the paper gauge. Plastigage is available in four different sizes, each covering a particular clearance range: Bearing Location Bearings must not move within the bearing bore and have good contact within the bore for good heat transfer. Several construction features of a bearing are common to ensure these functions. 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 wider than the diameter of the cap or web. Bearings require snapping or pushing into place. Bearing Crush is the distance that a bearing is higher than the cap or web. In fact, the diameter of the bearing shells is larger than the bearing bore. This feature is to assist retention and heat transfer. The dark spots observed on the back of a bearing indicate where greatest heat transfer takes place. For this reason bearings should go in dry on the backside with no grease or thread-lockers applied to the back of the shell. Using these things would impeded the transfer of heat from the bearing.
Above: Bearing Crush Above: Mis-assembled locating lugs. To ensure incorrect matching of bearing caps and lugs does not occur assemble caps together locating tab to tab. Make sure identification numbers match for main bearing location in block and on connecting rod bearings. Locating lugs are used on bearings to prevent movement and to properly center the bearing in the bore. Not all bearings are manufactured to be round when operating in the bore. Thicker shell material is located on the highly loaded surfaces. Larger oil clearances are found at 90 degrees to the loaded surfaces. This Delta Wall bearing enhances bearing life through better lubrication properties and less fatigue.
Above: Delta Wall bearing Undersize & Oversize Bearings An undersize bearing is thicker than a standard bearing. It does not describe the bearing but the shaft it fits. If a crankshaft is worn or is ground to an undersize an undersize bearing is used to fit the journal. 0.001", 0.010" & 0.030" are common undersized bearings for rods and mains. If a spun main bearing damages a block, the bearing bore can be re-bored to an oversize and an oversized bearing can be installed. Cam bearings typically can outlast 2 sets of mains since there is less loading however excessively worn and bleeding cam bearings can starve mains. Thrust Bearings Main bearings are those that support the crankshaft in the engine block. Rod bearings support the connecting rods on the crankpin journals. Thrust bearings control end-play in a crankshaft. The forces of disengaging the clutch, helical gearing on the gear-train, and automatic torque converters can easily push/pull a crankshaft to the point where severe damage to the connecting rods and journals occur. Thrust bearings are used to control end-play. Washers or bearings with flanges ride against a specially machined journal on usually only one cheek of a main bearing journal. Bearing Failures Bearing failures are indicated by the following: A drop in lubricating oil pressure Excessive oil consumption (Excessive oil thrown onto the cylinder walls overwhelming the rings. Rhythmic engine noises. Main bearings thump usually on acceleration only. Rod bearings rattle while light load cruise when oil is hot.