Can we manage without big-end bearings?
The big-end bearing shells that we find in almost all four-stroke racing engines are an item made with great precision. They are generally 'graded' and as such are available in thickness grades which have a range of only a few microns. Moreover, their length is critical in ensuring that the correct pre-load is achieved when fitting. The continuing development of various platings and coatings applied to the bearing shells allow higher pressures than were possible a few years ago.
Despite the fact that these high-precision parts are widely available at quite reasonable cost, there have been a number of attempts to dispense with the bearing shell in recent times, and some of these have been reported in both Race Engine Technology magazine and RET-Monitor. One materials article in Monitor discusses the fact that one company offers a product line in metal matrix composite (MMC) rods for four-stroke applications which run successfully without any form of bearing. In this application the original bearing was a needle roller, but the fact that the engine has run successfully with no bearing shows the potential to do so on a wider basis.
For the motocross application detailed in the article above, there has been no attempt to substitute the bearing with anything else, and it is in this respect that it differs from other attempts. I have spoken with a number of industry experts on the possibilities for running without a bearing, and many have made the point that the success of this approach would rely on having excellent oil filtration. An important aspect of conventional shell-bearing technology is the ability of the soft metal platings to trap small, hard particles so that they don't cause damage to the crankshaft or the big-end bore. In dispensing with any form of bearing shell, we run the risk of hard particles passing through the journal bearing and causing abrasive wear damage.
An obvious solution, although technically difficult, is to apply the soft metal platings directly to the bore of the con rod. This approach is being developed by one European coating supplier, and its solution seems advanced. It claims to be able to coat many different materials, with some others not being tried as yet. But it can successfully coat both steel and titanium.
There are various advantages to being able to dispense with the bearing shells. Clearly there is an advantage in terms of reduced mass, but there are other significant advantages. By being able to place the rod bolts closer to the applied load means the bending stresses in the bolts will be lower. Moving the bolts closer to the crankpin axis also has positive implications in terms of a smaller rod locus. And the tensile stresses in the rod due to the interference of the bearing shell are no longer present.
While this technology is far from maturity, it will surely begin to make an impact on the design of both series production and racing rods in the near future. For the time being, only a very few rods will be able to run successfully without some sort of big-end bearing.
Fig. 1 - In future will racing rods such as this feature a coated big-end bore, rather than using a bearing shell? (Courtesy of Carrillo)
Written by Wayne Ward