A main bearing is a bearing in a

piston engine A reciprocating engine, more often known as a piston engine, is a heat engine that uses one or more Reciprocating motion, reciprocating pistons to convert high temperature and high pressure into a Circular motion, rotating motion. This article ...

which holds the

crankshaft A crankshaft is a mechanical component used in a reciprocating engine, piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating Shaft (mechanical engineering), shaft containing one or more crankpins, ...

in place and allows it to rotate within the

engine block In an internal combustion engine, the engine block is the structure that contains the cylinders and other components. The engine block in an early automotive engine consisted of just the cylinder block, to which a separate crankcase was attach ...

. The number of main bearings per engine varies between engines, often in accordance with the forces produced by the operation of the engine. Main bearings are usually

plain bearing file:NYC 100-driving-axle-friction-bearing.jpg, Plain bearing on a 1906 S-Motor locomotive showing the axle, bearing, oil supply and oiling pad file:Linear-table with detail numbered.png, A sliding table with four cylindrical bearings file:GWR Spo ...

s or

journal bearing Plain bearing on a 1906 S-Motor locomotive showing the axle, bearing, oil supply and oiling pad A sliding table with four cylindrical bearings A wheelset from a Great Western Railway (GWR) wagon showing a plain, or journal, bearing end A pl ...

s, held in place by the engine block and bearing caps.

Number of main bearings

The number of main bearings is primarily determined by the overall load factor and maximum engine speed. Increasing the number of bearings in an engine will generally increase the size and cost of the engine, but also reduces bending stress and deflection caused by the distance from the crank pins to the nearest bearings. Most engines have at least two main bearings— one at each end of the crankshaft. Additional bearings may be located along the crankshaft, sometimes as many as one bearing per crank pin, as used on many modern diesel engines and petrol engines designed for high RPM. Some small single-cylinder engines have only one main bearing, in which case it must withstand the

bending moment In solid mechanics, a bending moment is the Reaction (physics), reaction induced in a structural element when an external force or Moment of force, moment is applied to the element, causing the element to bending, bend. The most common or simplest ...

created by the offset distance from the connecting rod to the main bearing. When describing a crankshaft design, the number of main bearings is generally quoted, as the number of crank pins is determined by the cylinder layout. For example, the Toyota VZ V6 engine is described as having a "four bearing crankshaft" and the Jaguar XK6 straight-six engine has a "seven bearing crankshaft".

Bearing caps

The lower half of the main bearings are typically held in place by 'bearing caps' which are secured to the engine block using bolts. The basic arrangement is for each bearing cap to have two bolts, but some engines may have four or six bolts per bearing cap (often referred to as "four-bolt mains" or "six-bolt mains" engines). The additional bolts result in increased strength, allowing the engine to withstand higher power output or RPM.

4 bolt main

The first car engine to use four-bolt main bearings was the V12

Maybach Zeppelin The Maybach Zeppelin was the Maybach company's ''Repräsentationswagen'' model from 1928 to 1938. Named for the company's famous production of Zeppelin engines prior to and during World War I, it was an enormous luxury vehicle which weighed approxi ...

of 1928, that used them on three of its eight main bearings.

6 bolt main

The typical design for a six-bolt main bearing is four vertical bolts (two on each side of the crankshaft) from the bottom extending into the block and two lateral cross-bolts coming from the left and right side pan rails into the side of the main caps to provide additional lateral strength.

References

{{Automotive engine , state=expanded Engine technology Bearings (mechanical)

Number of main bearings

Bearing caps

4 bolt main

6 bolt main

See also

References