Sat Oct 22 10:56:52 CAT 2016

Bearings perform an awesome task

By unknown | Jul 30, 2008 | COMMENTS [ 0 ]

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

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

They are usually plain bearings, meaning they are just plain cylindrical linings inside which the short, shaft-like sections of the crankshaft, called journals, rotate.

An oil film between journal and lining prevents metal-to-metal contact.

Considering that a crankshaft is a hefty piece of iron, spinning at anything between 10 and 100 revolutions per second while being subjected to considerable forces from the reciprocating pistons and connecting rods, these bearings have a daunting task.

The fact that in a modern engine they routinely last 250000km, and sometimes much longer, is testimony to the many years of improvement in bearing materials, crankshaft design, precision machining, and oil technology.

The bearings in which the main journals of the crankshaft ride, "main bearings", as well as the bearings inside the big-ends of the con rods, "big-end bearings", in which the crank's con rod journals rotate, are typically made in the form of two half-round, replaceable, steel-backed shells, keyed to the bearing caps.

When the caps are bolted down, the shells join up to form a cylindrical liner around each journal.

The shells have traditionally consisted of three layers, the steel backing on the outside, then a layer of a copper-lead alloy, which in turn is coated with a thin layer of Babbitt metal forming the bearing surface.

Babbitt metal, also called white metal, goes back to 1839 when the first such alloy was invented by Isaac Babbitt in America.

These are soft, easily damaged alloys, and seem at first sight an unlikely choice for a bearing surface, but this appearance is deceptive.

Babbitt metal contains small, hard crystals, usually of antimony, dispersed in a matrix of softer metals. In service the latter can wear away slightly to provide microscopic channels for the lubricant between the high spots. Some makers have changed to aluminium alloy bearings in recent years.

These have just two layers, a steel backing on the outside, and a layer of special aluminium alloy on the inside.

The film of oil between the rotating journal and the inside surface of the bearing is obviously of vital importance. It's a very thin film, typically less than half the thickness of the paper on which this newspaper is printed, and it seems incredible that it can prevent metal-to-metal contact at all times while the crankshaft is spinning. The secret lies in the fact that a wedge of oil will form in the oil film when the journal starts to rotate, and the journal will then "ride up" on this wedge, just as a car's tyres ride up on a wedge of water on the road when the car starts to aquaplane. As long as the oil pump can supply enough oil to the bearings to maintain this effect, the journals will always be riding on a film of oil. Most crankshaft bearings are designed with a very small amount of eccentricity so that the oil can more easily form a wedge to support the journals.


Login OR Join up TO COMMENT