More on nose-fed crankshafts

Thursday, March 29, 2012

Tags :  crankshafts

crankshaftsIn the previous article on crankshafts, the use of the crankshaft as a centrifugal separator was briefly discussed. The principle of centrifugal separation/refinement is well understood, and one only has to hear of news stories about uranium enrichment to appreciate that its uses are widespread.

At the end of the previous article, I said we would further develop the idea of air and debris separation.

Air in oil is a real nuisance, especially where we want to use the oil to provide the lubricant in a hydrodynamic bearing, as is the case with the main and big-end bearings. Discussions with crank manufacturers in the past has revealed that it is common not to feed the main bearings via a nose-fed crankshaft, but only to feed the big ends. Nose-feed crankshafts are very much the domain of the specialist engine design company, and many crank manufacturers will only make such parts to designs supplied to them.

The separation of air and oil relies on the air-oil mixture having a rotational velocity applied to it. If the aim is to provide proper separation of the air and oil in a short length, then we need to ensure that the air-oil mix undergoes rapid angular acceleration. It may not be enough though to rely on the transfer of momentum from the crankshaft to the oil by shear stresses alone, especially when dealing with high flow rates. In this case, we may need a rudimentary paddle wheel or impeller to impart the required energy. This has the added effect of causing small air bubbles to coalesce. Larger air bubbles can be separated more easily as the ratio of centrifugal forces to viscous forces is increased (look up 'Stokes Flow') although this effect is probably minimal.

Once we have used the nose-feed cavity to separate the air and the oil, what do we do with the air that is now contained in a theoretical cylinder on the crankshaft axis? Common practice is to drill through into the first crankcase cavity, into which the air is then expelled; we have to accept though that some oil may also find its way through the hole. At the far end of the crankshaft we also have to consider trapped air, so we should provide a 'bleed' through which this air can escape, and this is commonly also connected to a crankcase volume.


crankshafts-spitfire

The separation of solid debris in the oil again relies on there being a difference in density. It is certainly true that our filtration should be sufficient to contain debris that could prove damaging, but I'm sure I'm not the only engineer to be caught out by faulty assembly, damaged filters or debris that has become loose after engine start-up, following lots of flushing, ultrasound and so on, especially if cast galleries are used.

In the case of solid debris, we have to provide a 'dam' to retain the debris on the outer wall of the nose-feed cavity. Such a dam can be provided by a machined step or an undercut. I expect that fewer people consider the separation of debris than consider air when designing a nose-fed crankshaft, especially compared to decades ago, where good filtration could not be relied on.

Fig. 1 - The Rolls-Royce engine in the Supermarine Spitfire used both air and debris separation in its nose-fed crankshaft

Written by Wayne Ward

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