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Thrust face design - part 2

The previous article on thrust face design of con rods was published more than a year ago. In this belated follow-up, we will look at the reasons why certain features are employed.

One problem with thrust bearings is the fact that, in their simplest form, they comprise a stationary disc in sliding contact with a moving disc. If the plates remain a set distance apart, they do not generate any oil film pressure and so cannot act as a bearing. It is only by the relative movement of the bearings relative to each other that squeeze-film lubrication operates, producing pressure which counteracts the movement of the plates towards each other.

The recent article on crankshaft rod thrust face design mentions that there are features that can be used on the rod thrust face to help generate pressure in the oil film. The article mentioned that a discontinuous face on the rod thrust bearing of the crankshaft would help generate oil film pressure between the crankpin thrust faces and the corresponding thrust faces on the con rod.

We can often see this when looking at racing con rods as a series of shallow and relatively narrow grooves in the thrust face of the con rod; where two grooves are used they are commonly placed over the split line of the rod. This not only adds a feature that can aid lubrication, it removes sharp edges at the split line from the thrust face. As we will see, having sharp edges is not necessarily a bad thing in this situation.

Where a fluid is present in the gap between two parallel plates moving relative to each other, and the gap between the plates suddenly narrows, there is an increase in oil pressure. This effect isn't restricted to liquids; the same effect can be observed in gases too, which can form an effective bearing between parallel flat surfaces when a step (or multiple steps) are introduced.

This type of arrangement is known as a Rayleigh step bearing, after the famous physicist Lord Rayleigh. While the arrangement seen on many con rods with two or more narrow grooves is common, it is possible to be much more aggressive if the rod thrust face arrangement is designed with the principles of the Rayleigh step bearing in mind. The pressure generated in the bearing is greatest if the edge of the step is sharp, and there are some general design rules that optimise design ratios for maximum generated pressure.

If the thrust face is designed as a series of small Rayleigh step bearings around the periphery of the rod bore, a lighter big end can be achieved. On a vee engine where two rods share one crankpin, this principle of 'aggressive lightening' by using a series of small step bearings can only be reliably applied to one side of the con rod; the thrust faces between the two rods are best kept as relatively plain, flat faces.

While not specifically designed as effective Rayleigh step bearings, grooves across the face of crank thrust bearings act in the same way.

Written by Wayne Ward

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