Nose-feed oil drillings - some further advantages?

Thursday, September 27, 2012

Tags :  crankshafts

In previous articles on oil drillings, nose-fed oil galleries have been discussed. This is not a new concept. However, the drillings used in a modern race engine with significant main bearing/crankpin overlap are far removed from those used in the comparatively long-stroke crankshafts without overlap.

For a recent article on crankshafts for Race Engine Technology magazine, crankshaft suppliers revealed that the proportion of their racing crankshaft business which has nose-fed oil galleries has increased in the past two or three years. There are some basic advantages of nose-fed crankshafts in terms of requiring less oil system pressure and, possibly, some simplification of the maze of interconnected drillings arranged between the oil pressure pump and the numerous entries to the main bearings.

Some nose-feed crankshafts feed main bearings and crankpins, while others feed crankpins alone. In the latter case, the oil drillings in the cylinder block assembly are probably not appreciably simplified by using a nose-fed crankshaft. Where the crankshaft carries the oil flow for the main bearings and crankpins, the simplification of the oil drillings might improve the durability of the cylinder block. Another issue is that drilled and grooved main bearing shells would not be required, and this might improve bearing life.

The work by Lurenbaum and Martinaglia referenced by Taylor* in his book looks at the various methods for increasing crankshaft fatigue life through the disposition of material in a crankshaft. There are several ideas used widely in production and racing crankshafts these days, but there are others that do not lend themselves to modern short-stroke race engines. In particular, the boring of main journals is problematic where there is a requirement to accommodate other oil drillings. Also, depending on the size of the main journals and crankpins - and considering overlap - the size of through-bore may not be worthwhile.

The boring of crankpins and main journals is an effective way to manipulate the load paths in the crankshaft, thereby improving fatigue life. Of course, hollow main bearings and pins have the advantage of reduced mass and inertia. Boring pins through also means that smaller counterweights can be used, which again reduces mass and inertia further.

However, where a nose-feed crankshaft is used, and where the oil galleries are of an appreciable size, the effect of the interconnected drillings forming the oil gallery in the crankshaft will tend to spread the load path and direct it away from the area that is normally the worst case for stress concentration. Although the layout of the drillings is not the same as the crankpin and main bearing bores shown in Taylor's book (they run parallel to the crankshaft axis) the overall effect will be similar.

In fact, owing to the proximity of the drillings to the main bearing and crankpin fillets, the effect may even be enhanced compared to the drilling patterns shown in Taylor. While the spreading of the load path will lead to increased stress in other locations, providing the overall maximum stress is reduced, the effect is likely to be positive in terms of fatigue life.

* Reference
Taylor, C.F., "The Internal Combustion Engine in Theory and Practice", vol 2, MIT Press, 1982, ISBN 0-2627-0016-6

Written by Wayne Ward

Leave your comment

Comments

I've read through Taylor, but I still do not agree with the notion that a single nose fed crank will have a more uniform distribution of pressure at the rod journals than one fed from each main bearing journal.  In the end, the oil feed pressure at each rod journal is dependent upon the pressure drop through the circuit and the dynamic forces acting on the fluid mass.

With grooved mains, the net dynamic CF assisting the flow to the rod journal alone is quite significant.