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Coatings on Con Rods

coatingsSome of the important design features of the con rod have been discussed briefly in the articles on the RET Monitor website, and there has also been a recent RET Focus article in the magazine on the con rod. In that Focus article, we mentioned coatings, but here is a good forum to expand on the material a little further.

Looking at the thrust faces of the big end of the rod, many people choose to run without any form of coating. If the lubrication regime is such that wear is not a problem, then this solution is a good one, and certainly economical. However, for those who perhaps want to run the absolute minimum thrust face width on their connecting rods, or those who need their rods to have the best endurance in terms of wear, it has been common practice to coat the thrust faces of the con rod and this has traditionally been a metal-sprayed coating of a molybdenum alloy, although other materials have been used for the purpose. Although the metal-spraying process is still in use, it has, to some extent, been overtaken by the rise in the modern hard coatings. Chromium Nitride (CrN) is a popular choice for rod thrust faces, and this is applied only a few microns thick. We should note that care has to be taken with the surface finish of hard coatings as they can, if too rough, act as very effective cutting tools, damaging the components which they slide against.

Remaining with the big end, some companies who I spoke to reported that they thought that it would be possible to dispense with the big end bearing and run using coatings applied to the bore of the con rod. DLC was mentioned for this purpose, but we don't yet know of any successful applications of this technology. It would offer some significant advantages in terms of con rod design, and would also give the fastener an easier life too, as the bending stresses associated with the off-centre loading would be reduced as the fastener moves toward the big-end axis.

Looking at the small end of the con rod, there are a number of applications where a coating is applied. Where the con rod is piston-guided as opposed to being crank-guided, the same CrN coating could be applied to the thrust faces. Where two bronze 'top-hat' bushes have been used in these circumstances, the single-piece plain bush can be used, with the coating taking the thrust loads. In some cases, bronze 'top-hat' bushes have been replaced with coated steel items. In the case where piston-guided con rods have managed to dispense with the bush altogether, the modern hard coatings are ideal for taking thrust loads.

For titanium con rods which use a pressed-in small end bush, there exists the problem of galling when the bush is pressed into place. The resulting surface damage may not be detected until the con rod has failed, and for these reasons people have tried coating both the small end bush and the small end bore in the con rod to prevent this problem.

Fig. 1 - Connecting rods are often coated on thrust faces to prevent premature wear (Courtesy Arrow Precision Ltd)

Written by Wayne Ward

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