Bearings perform an awesome task

Tag : Printed Plain

The bearings supporting the crankshaft, the heart of an engine, areseldom rolling-element bearings.



They are usually plain bearings, meaning they are just plaincylindrical linings inside which the short, shaft-like sections ofthe crankshaft, called journals, rotate.
An oil film between journal and lining prevents metal-to-metalcontact.
Considering that a crankshaft is a hefty piece of iron, spinning atanything between 10 and 100 revolutions per second while beingsubjected to considerable forces from the reciprocating pistons andconnecting rods, these bearings have a daunting task.
The fact that in a modern engine they routinely last 250000km, andsometimes much longer, is testimony to the many years ofimprovement in bearing materials, crankshaft design, precisionmachining, and oil technology.

The bearings in which the main journals of the crankshaft ride,“main bearings”, as well as the bearings inside thebig-ends of the con rods, “big-end bearings”, in whichthe crank’s con rod journals rotate, are typically made inthe form of two half-round, replaceable, steel-backed shells, keyedto the bearing caps.
When the caps are bolted down, the shells join up to form acylindrical liner around each journal.
The shells have traditionally consisted of three layers, the steelbacking on the outside, then a layer of a copper-lead alloy, whichin turn is coated with a thin layer of Babbitt metal forming thebearing surface.
Babbitt metal, also called white metal, goes back to 1839 when thefirst such alloy was invented by Isaac Babbitt in America.
These are soft, easily damaged alloys, and seem at first sight anunlikely choice for a bearing surface, but this appearance isdeceptive.
Babbitt metal contains small, hard crystals, usually of antimony,dispersed in a matrix of softer metals. In service the latter canwear away slightly to provide microscopic channels for thelubricant between the high spots. Some makers have changed toaluminium alloy bearings in recent years.
These have just two layers, a steel backing on the outside, and alayer of special aluminium alloy on the inside.

The film of oil between the rotating journal and the inside surfaceof the bearing is obviously of vital importance. It’s a verythin film, typically less than half the thickness of the paper onwhich this newspaper is printed, and it seems incredible that itcan prevent metal-to-metal contact at all times while thecrankshaft is spinning. The secret lies in the fact that a wedge ofoil will form in the oil film when the journal starts to rotate,and the journal will then “ride up” on this wedge, justas a car’s tyres ride up on a wedge of water on the road whenthe car starts to aquaplane. As long as the oil pump can supplyenough oil to the bearings to maintain this effect, the journalswill always be riding on a film of oil. Most crankshaft bearingsare designed with a very small amount of eccentricity so that theoil can more easily form a wedge to support the journals.