Rod thrust face design

Tuesday, July 03, 2012

Tags :  crankshafts

In terms of controlling the axial position and axial float (in a direction parallel to the crankshaft axis) of the con rod, there are two main methods of achieving this - crank-guided rods and piston-guided rods. As you may discern from the descriptions, the first type thrusts against the crankshaft, the second type against the piston. Crank-guided rods remain the most popular type for many kinds of race engine. Commonly, the contact between the crank and rod thrust faces is one of annular contact defined by two annular faces.

Where plain annular contacts are used, we are only able to generate pressure in an oil film due to 'squeeze film' operation - that is, where there is a decreasing clearance between the two annular surfaces. Otherwise, we rely on enough lubricant being forced through the contact to prevent metal-to-metal contact. There are a number of design features that can be employed on the thrust face of the con rod to allow hydrodynamic lubrication to become established. Very little thrust area is required as the forces, and thus stresses, involved are only very small. A full annulus is generally used because people are comfortable with this solution, especially given the difficulties in generating pressure in the oil film.

However, we are not limited to producing a full annulus on the crankshaft to bear against the con rod thrust face, and there are a number of reasons to consider something other than a complete annulus. Most obviously, there is a weight saving to be made, especially if a portion of the thrust face is removed at the furthest distance from the crankshaft axis. Not only can this weight saving be realised, but weight on the opposite side of the crankshaft axis can also be saved as there is less mass to balance. Fig. 1 shows the sort of material removal that might be considered if weight is to be removed from the outer part of the rod thrust face on the crankshaft.


Less obvious is the fact that equally significant mass savings, accompanied by very useful reductions in stress concentration factor, can sometimes be made by removing part of the rod thrust face on the crankshaft at the point closest to the crankshaft axis. By removing the innermost part of the thrust face, we can create a more generous fillet radius, especially where we undercut the crank web. Undercutting and removing material from the crank web, when done judiciously, is a proven method to reduce stress concentration. I would refer the reader to Taylor*, which neatly summarises earlier work by other engineers on the subject of crankshaft stress concentration.

Care must be taken when interrupting the rod thrust face on the crankshaft. Careful detail of the leading edge of the thrust face is required to prevent wear. The edge should not be left sharp.

Another advantage of not having a complete annular thrust area is that such a design is actually likely to provide the hydrodynamic oil film which is impossible to create between parallel flat surfaces.

* Taylor, C.F., "The Internal Combustion Engine in Theory and Practice", vol 2 (Combustion Fuels, Materials, Design), MIT Press, 1985, ISBN 0-2627-0027-1

Fig. 1 - Reducing the thrust area between the crank and rod can bring a number of benefits, including lower crankshaft mass and inertia. Compare the left side, having reduced thrust area, with the right

Written by Wayne Ward

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The load capacity of the hydrodynamic oil film at the rod bearing/crank journal interface is unaffected by the rod/crank thrust face shape or fit.  The bearing oil film is generated by the relative surface geometries, motions, and fluid properties.  The only effect the rod side clearance and shape has on rod bearing performance is due to its impact on oil mass flow past the bearing.  More clearance and/or less contact overlap will increase oil flow past the radial bearing gap, and increase heat transfer.

Your points are valid, although the point you question in this article is about hydrodynamic lubrication of the thrust faces, not of the journal.

Went back and re-read the piece, and I stand corrected.  But I would still question the actual concern over the lube film conditions between the outside conrod face and the crank cheek.  As you noted, there is little load at this interface so there would naturally be little friction loss even if the contact was boundary conditions.  With paired rods on a common crank pin there is only slight oscillatory contact motion between the inner conrod faces.  Wouldn't lack of hydrodynamic contact conditions at this interface be of similar concern?

The minute metallurgical and geometry details of the transition area between the crankpin and cheek are of great importance with regards to crank fatigue life.  The actual shape of this "fillet" is usually not a true radius.  Instead it is typically some sort of ellipse or conic shape.  This area of the crank also typically receives various types of mechanical cold working to compressively pre-stress the surface material.  And as for the crankpin itself, it is usually ground with a slight crown profile to compensate for bending.