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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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Inspection of Main Bearing

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How to Check Bearing Clearances Plastigage - Engine Rebuild Do It Yourself (Part One)

DIY engine rebuild. See Part Two: http://www.youtube.com/watch?v=C2aHnlHYW7A How to check main and rod bearing clearances. This is to prevent low oil pressur...

How to measure main bearing clearances

How to measure main journal bearing clearances in an engine using a dial bore gauge and micrometers.

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A demonstration in the SIUC Automotive Technology AUT-150 Internal Combustion Engine Principles class of using Plastigauge to measure crankshaft main journal...

Blueprint 105 - Main Bearing Oil Clearances

In this episode we measure the bores for the crankshaft and calculate the oil clearances based off of information gathered in the previous video. If you subt...

Part 2: Stihl chainsaw main bearing replacement .026 / MS260. Fitting the bearings to the crank.

Replace the main bearings in the crank cases and get ready to refit the crank and cases. My books are available here: http://www.lowimpact.org/books_LILI_pub...

Measure Main Bearing Bore Diameter

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440917 videos foundNext > 

38 news items

Crosscut

Crosscut
Mon, 28 Jul 2014 18:01:16 -0700

The machine began to experience problems early last December. Subsequent inspections revealed that a set of seals that protects the machine's main bearing had failed. The main bearing allows Bertha's 57.5-foot cutter-head to spin and mine through the ...
 
The Seattle Times (blog)
Mon, 28 Jul 2014 14:30:28 -0700

The main bearing and leaky seals will be replaced, by parts being assembled in Japan by Hitachi-Zosen. The more than 57-foot-diameter tunnel machine, which set out from Sodo last July, has advanced only 4 feet since Dec. 6, after the front end ...
 
MyNorthwest.com
Mon, 28 Jul 2014 07:26:15 -0700

The tunneling machine only made it about 1,000 feet before breaking-down in December. The main bearing that drives the machine was damaged, and the seals that protect it were leaking. The state had two options to fix the world's largest drill, either ...

GTspirit

GTspirit
Mon, 28 Jul 2014 06:18:45 -0700

The lightweight aluminium block has then been stiffened with cast-iron liners and cross-bolted main bearing caps. Just like all other F-Type Coupé and Convertible variants, the engine is mated to a ZF Quickshift eight-speed transmission driving the ...

Lewiston Sun Journal

Lewiston Sun Journal
Mon, 21 Jul 2014 21:03:58 -0700

On March 22, during a daily inspection of the resort's machinery, employees discovered that the main bearing on the return bullwheel at the bottom of the Wayback Chair was making strange noises and showed signs of wear-and-tear. The bullwheel is the ...
 
The Seattle Times (blog)
Sat, 26 Jul 2014 17:02:04 -0700

Workers will replace the main bearing and its leaky seals, add steel plates to reinforce Bertha's interior and carve out larger gaps in the disc-shaped cutterhead for dirt to pass through. Despite any delay, the state says Seattle Tunnel Partners is ...
 
Cycleworld
Fri, 25 Jul 2014 11:01:20 -0700

Many engines are built today using the concept of “torque-to-yield” on fasteners such as head and main bearing cap bolts. This is done to get maximum performance from fasteners, and is often done by tightening angle. This may be programmed into torque ...

Kelley Blue Book

Kelley Blue Book
Thu, 24 Jul 2014 13:44:08 -0700

The engine block is composed compacted graphite iron, the same material employed in Ford's 6.7-liter turbodiesel V8, and the bottom end of the engine is diesel-tough: forged crankshaft, six-bolt main bearing caps, and heavy duty Federal Mogul bearings ...
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