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In a piston engine, the main bearings are the bearings on which the crankshaft rotates, usually plain or journal bearings. The bearings hold the crankshaft in place and prevent the forces created by the piston and transmitted to the crankshaft by the connecting rods from dislodging the crankshaft, instead forcing the crank to convert the reciprocating movement into rotation.

While some small single-cylinder engines have only one main bearing, designed to deal with the bending moment exerted by the crank as a result of the force from the connecting rod, most engines have at least two main bearings, one at each end of the crank shaft and may have as many as one more than the number of crank pins. Increasing the number of bearings in an engine will generally increase the size and cost of the engine, but also provides stability to the crankshaft, which would otherwise have to endure greater bending moments from having the crank pin further from a bearing. The combination of bearing number and crankshaft design can also have an effect on engine balance; a more stable crankshaft will better avoid unbalance from flexing, but increasing the crankshaft length or girth to accommodate an additional bearing increases the rotational inertia of the crankshaft, which can have the opposite effect.

Most modern automotive engines have one main bearing at each end of the crankshaft and another in between each adjacent pair of connecting rod journals, but not all engines conform to this generalization. Notable exceptions include the Ford flathead V-8, which often had only three main bearings, and the Chevrolet straight-6 engine, which had in various iterations, between 3 and 7 main bearings.

When describing a crankshaft design, the number of main bearings is generally quoted, as the number of crank pins is determined by the engine configuration. For example, a crankshaft for an inline six engine will be described as three bearing or four bearing depending on its number of main bearings; The crank pins are not counted in this description. Similarly, when speaking of a crankshaft, the journals are the main bearing journals only. The crank pins are not normally called journals although they form the centre shafts of the big end bearings and are therefore journals in the more general sense.

Original courtesy of Wikipedia: http://en.wikipedia.org/wiki/Main_bearing — Please support Wikipedia.
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 22 news items
 Lamar Utilities Board Reviews Insurance Contracts, Board Member Application The Prowers Journal Wed, 27 Aug 2014 07:39:59 -0700 Repair work on the plant's T-3 wind turbine is being completed as the gearbox, shaft and main bearing have been installed, but is still off-line until the batteries needed for the primary breaking system arrive. The light plant's July financial report ... PenBayPilot.com Volvo S70-Defective O-Ring Seals on Oil Pump Pick Up PenBayPilot.com Mon, 18 Aug 2014 09:07:30 -0700 As the 850 and S/V70 series engines use a two piece crankcase, you can not easily remove the main bearing caps for inspection, nor are the rod bearings readily accessible. The best test of the engine condition after this service is to use the ... Crosscut Bertha at rest: How much will the delays cost? Crosscut Fri, 01 Aug 2014 05:01:10 -0700 The tunnel diggers have to rescue her by digging an enormous pit so they can access her front end and remove the cutterhead and repair the main bearing that turns her cutters. It cost \$80 million to build Bertha; it will cost at least \$125 million to ... Bartlesville Radio Carousel Needs More Work Bartlesville Radio Thu, 07 Aug 2014 08:45:00 -0700 Many of the parts are being manufactured for the carousel in Wichita, KS. Those parts are expected to be delivered in 8 to ten weeks time. While it was believed that only the main bearing needed replacement, upon dismantling the carousel it was ... North American Palladium Announces Second Quarter 2014 Results and ... CNNMoney Wed, 30 Jul 2014 03:07:30 -0700 During the quarter the Company experienced a main bearing failure at its surface crusher and appropriate repairs were made to the surface crusher to help ensure reliable production going forward. Lac des Iles Operations. Q2 2014 Production. In the ... Sentient Science Strengthens Company with Former Kaydon Bearing ... DigitalJournal.com Fri, 01 Aug 2014 05:05:42 -0700 Jim will be a key contributor during this time of growth and introduction of new products, including main bearing, pitch bearing, blade solutions, plus new transactional and risk products,“ commented Edward Wagner, Vice President of Sentient Science. Mustang's 50th Birthday - Nifty Fifty Mustangs and Fords Magazine Thu, 31 Jul 2014 06:30:00 -0700 Frankly, we would have been surprised if this immense celebration spanning the entire U.S. ran like an oiled main bearing. It was a huge enterprise, and one that only need run once each generation. The vibe throughout the speedway grounds was unhurried. New York Daily News 2015 Range Rover Sport SVR unveiled, costs \$110475 WorldCarFans.com Sun, 10 Aug 2014 17:23:03 -0700 Land Rover has officially unveiled the 2015 Range Rover Sport SVR, ahead of its debut at Pebble Beach on August 14th. Described as the "most dynamically focused" Land Rover ever produced, the high-performance model is distinguished by a new front ...
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