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Thrust-face design and finishing

con-rodsThe forces acting on the con rod are considerable, and the components to which they are attached are surprisingly flexible, although they may not appear so when you pick up a component such as crankshaft. The deflections of the crankshaft (bending and torsion) and the con rod (torsion) give rise to a tendency for there to be some axial force on the con rod trying to force it one way or the other parallel to the crankshaft axis.

In providing adequate lubrication (and therefore very low friction) for the big end, we aid the con rod in its desire to move parallel to the crankshaft. We therefore need to provide a component for it to react (or thrust) against. Con rods are often described as being 'crank-guided' or 'piston-guided', and these descriptions refer to the component that provides the restraint to the con rod's ambition to move axially.

Where a rod is piston-guided, a surface on the rod reacts against a machined surface on the piston. In order to prevent wear of the piston, the surface of the con rod which is used to thrust against the piston needs to have a reasonable amount of surface area, and may be equipped with features that encourage an oil film to form. This is important given the small relative sliding velocity between surfaces at the little end and the reciprocating nature of the sliding contact.

The thrust surface on the rod can be in the form of the small-end bush being split into two 'top hat' bushes, thus being of the same material (commonly a copper alloy) as the small end bush. An engineering coating can be used on the thrust faces, and in doing so this dispenses with the need for multiple top-hat bushes. Chromium nitride (CrN) is a good candidate for this application, and while some people contend that no coating is really necessary, it might be a desirable option.

The big end, on a crank-guided rod, has similar needs in terms of reacting to a load, but with greater potential surface area and higher sliding velocities, the matter of lubrication is a much easier problem to solve. When the engineer has control of both the crankshaft and the con rod designs, there is scope to design an optimal solution here in terms of assembly mass and lubrication. Given the sliding velocity in the thrust contact, there is very little requirement in terms of thrust bearing area.

Where two crank-guided con rods sit on a single crankpin, as is common in vee engines, there is a thrust contact between the adjacent rods, and this is a different consideration to the crank contact. In common with the thrust contact of a piston-guided rod, the thrust contact between rods is one of reciprocating rotation over a relatively small angle and with low sliding velocities, and the thrust-bearing faces need to be considered in light of this fact. Consequently, rod-to-rod contact faces commonly have larger contact faces than do rod-to-crank faces.

In both circumstances, thrust-face coatings are common, although by no means universal, and the wear resistance of the substrate material is one consideration when assessing the need for a thrust-face coating. Sprayed metallic coatings (especially molybdenum) are commonly used, although there are a number of others based on different alloys or ceramics which could be used. Again chromium nitride (CrN) has found use for this application.

Fig. 1 - Con rod thrust faces are commonly coated to prevent wear

Written by Wayne Ward