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The overlayer

bearingsThere is nothing simple about a modern crankshaft bearing. Often referred to as 'plain' or simple shell bearings, I can assure you that even after 100-plus years of the internal combustion engine - 60 of them since the arrival of the 'thinwall' steel shell - development is still very much ongoing.

Running for most of its time under benign, hydrodynamic lubrication, such a bearing nevertheless has to cope with a conflict in characteristics. A bearing has to be strong, being able to take high cyclic loads for long periods of time with little or no wear. It needs to resist seizure and physically prevent itself from sticking to the crankshaft journal. Embeddability - the ability to retain small foreign particles from the oil - needs to be good, along with the ability to accommodate small misalignments of the crankshaft. If that wasn't enough, the material also needs to resist corrosion, cavitation and have a high enough melting point to cope with any heat that is inevitably generated. In short, a bearing needs to be strong and soft at the same time, and how this is achieved depends on the intended application.

The simplest approach for most light and medium duty applications is the bi-metal bearing. An aluminium-tin (up to 20% tin) alloy attached to the steel backing using a thin layer of pure aluminium will provide much of the above but without the strength. Adding small amounts of silicon, copper or other transition metals (chromium, manganese, nickel) will improve the strength but not to the point now demanded by most current high-performance engines. For applications such as these, a more complex approach is required - that of the tri-metal bearing.

Here the aluminium alloy is replaced by a much stronger copper-based alternative, which includes a high proportion of lead (up to 25%, where regulation allows) to act as a solid lubricant. Somewhere between 0.25 and 0.4 mm (0.01-0.015 in) thick, small amounts of tin can be added to strengthen things further. However, copper-lead-tin alloys may be strong but they lack the softness required for embeddability and that other necessity, conformability.

For these we have to introduce yet another layer - the overlayer. At around 0.01-0.02 mm (0.0005-0.0008 in) thick, this overlay will usually consist of a lead and 10% tin mixture on top of a nickel barrier. While the lead-tin mixture will absorb any dirt or fine debris in the oil, the nickel is present not necessarily to bond it to the lower layer but to prevent the tin from progressively migrating towards the copper in it.


The problem with this overlayer, however, is its lack of strength, and although small amounts of copper can be introduced, strangely the thicker the layer the less its load-carrying capacity, and so overlays are always very thin. But as in many things the thickness is very much a compromise. Too thin and any slight wear or misalignment will uncover the copper layer; too thick and the surface may start crazing, causing partial flaking as the layer parts company with the rest. Either way, bearing failure quickly follows and is why in a modern tri-metal bearing the overlay is most critical.

These days lead is now banned in most OE automotive components, so bearing manufacturers for production vehicles - probably the worst affected - have had to come up with lead-free substitutes, particularly for the overlay. Tin-based alloys with around 6% copper deposited on a nickel barrier layer is one approach. Other proprietary methods include mixtures of solid lubricants (molybdenum disulphide, graphite or PTFE) and hard, wear-resistant particles (silica, alumina or silicon carbide/nitride) all bound up in polymer resins. Sprayed onto the base bearing material using suitable solvents, these are cured at temperature. Although race engines are currently unaffected by these directives, the increasing concern over the use of lead in any manufacturing process may force race bearing manufacturers down the same route.

So do you still think bearings are simple?

Fig. 1 - A 'simple' tri-metal bearing with lead-tin overlay

Written by John Coxon

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