To get the oil to these bearings, oil passageways run inside the crankshaft - through the main journal, diagonally through the web, and out through holes in the rod journals. A groove in the main rod bearing allows oil to be continually forced down the passageway to the rod journals, assisted by being flung outwards by the centrifugal force of the rotating crankshaft.
The clearances between the journals and bearings are the main source of oil pressure in the engine. If the clearances are too high then oil flows out freely, and pressure is not maintained. Clearances that are too low will cause high oil pressure and risk metal-to-metal contact.
It is therefore essential that the clearance between the bearings and journals are measured when an engine is rebuilt. The crankshaft is subject to strong rotational forces, and the mass of the connecting rod and piston moving up and down exerts a significant force. Counterweights are cast as part of the crankshaft to balance out these forces. These counterweights allow for a smoother running engine and higher RPM. A crankshaft will be balanced at the factory.
In this process, the flywheel is attached and the entire assembly is spun on a machine which measures where it is out of balance. Balance holes are drilled in the counterweights to reduce weight. If weight needs to be added, a hole is drilled and then filled with heavy metal or mallory.
This is repeated until the crankshaft is balanced. At some point along its length, two or more thrust washers will be installed to prevent the crankshaft moving lengthways. On the illustrated crankshaft, there are thrust washers on both sides of the central journal.
These thrust washers sit between machined surfaces in the web and the crankshaft saddle - maintaining a specified small gap and minimizing the amount of lateral movement available to the crankshaft.
The distance that the crankshaft can move from end-to-end is called its endplay and an acceptable range will be specified in the service manuals. Some engines have these thrust washers formed as part of the main bearings, others, generally older types, use separate washers.
Both ends of the crankshaft extend beyond the crankcase, so some method of preventing oil escaping these openings must be provided. This is the job of the two main oil seals, one at the front and one at the rear. The rear main seal is installed between the rear main journal and the flywheel. It is commonly a synthetic rubber lip-seal.
They are made of other materials as well, depending on the application and load requirements. Other bushings may be used for alignment jigs in drilling operations. Image credit: Thomas A. A common application of journal bearings is in supporting an engine crankshaft such as the one shown at right. The highly polished surfaces of the forging shown are the journals of both the main bearings and the connecting rod bearings.
The main bearings themselves seat in the crankcase. The main bearings are made as lower and upper shells. They are installed into machined portions of the crankcase casting, as is being done in the photo at left. Oil is pumped into the bearing through feed holes that distribute oil to the main and connecting rod bearings. Journal bearings are used in large industrial turbo-machines such as compressors and turbines.
Many bearings in this service are hydrostatic, meaning that the shaft can be supported by an oil film even when not rotating. Sometimes the bearings are segmented, as shown at right, and sometimes the bearings can tilt to suppress a phenomena know as shaft whirl or whip.
A common form of the tilting pad bearing is used as a thrust bearing on large turbomachines. Generally, the surfaces of such bearings are babbitt-lined. Babbitt is a relatively soft white metal which supports fluid film lubrication while providing a forgiving surface if contacted by the hardened shaft journal.
Automotive engine bearings are often bronze lined. Journal bearings in these large sizes are often split designs similar to engine main bearings which permit removal of large rotors for maintenance. Although similar in principle to hydrodynamic and babbitted journal bearings, sleeve bearings are also used for linear motion with nearly the frequency they are used for rotary motion. And while hydrodynamic and babbitted journal bearings often connote a fairly elaborate system with lubrication systems and so forth, sleeve bearings can be relatively simple pressed-in devices used for a host of applications from guidepost bushings to caster bearings.
Sleeve bearings are often made of bearing bronze either sintered or cast and sometimes filled with plugs of lubricant such as graphite as with the bearings at left. Various plastics are also popular for sleeve bearings. These can be tapered roller bearings such as those used on automotive wheels, cup type ball bearings working in unison such as those on small ball bearing wheels on wagons and carts.
They can also be deep groove caged ball bearings. Combination bearings control the rotational motion around the shaft, and carry the weight of the vehicle. They also limit side to side movement along the shaft, such as when cornering hard in a sports car.
In this capacity, they function as thrust bearings. Combination bearings are used in applications where the thrust loading might be coincidental or relatively small compared to the radial loading. Pure thrust bearings are used in applications where thrust loads are the predominant forces transmitted by the rotating components into their stationary containment. Sliding thrust bearings such as Oilite washers are used in some electric devices. The four major parts of this type of bearing are the shaft journal; the removable bearing shell halves, usually steel with a soft alloy lining; the bearing shell support halves; and the oil that actually makes up the bearing action.
Since most crankshafts are either cast or forged, they tend to be all one piece, and the bearing journals are machined into the rough shape that comes from the casting or forging process. The shells and supports are split exactly in half at the bottom of the engine block to allow the crankshaft to be inserted into top half-rounds in the block. The caps that make up the bottom half rounds of each bearing are then bolted into place under the crankshaft so that each crankshaft main bearing and connecting rod journal is completely surrounded by a bearing surface that conforms tightly.
The resulting bearing clearances are ideally in the realm of ten thousandths to thousandths of an inch thousandths to hundredths of a millimeter and the journals are virtually perfectly round. Holes and grooves in each main bearing shell allow pressurized motor oil coming from the oil galleys in the engine block to flood each bearing with oil, which continually runs out the side of the bearings and returns to the oil pan.
Besides providing a thin slippery film that prevents metal-to-metal contact, the oil performs several other functions, too.
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