Counterweight Knife Edging

Thursday, January 03, 2013

Tags :  crankshafts

Crankshaft counterweights are required on certain engine configurations to allow the engine to run without excessive rocking couples; cruciform V8 crankshafts are an example of this. Other engine configurations can run happily without counterweighting of any type to eliminate rocking couples, but they often require some counterweighting in order to maintain bearing loads within reasonable limits. Bearing loads not only have affect engine reliability, but also on friction. Some engineers prefer to control maximum load by counterweighting, while others look for a level of counterweighting that will give the lowest average bearing load at a given operating speed.

It is very likely that you will encounter a crankshaft with counterweights, and you might notice that the edges are chamfered or bevelled. Some people design and manufacture counterweights with more extreme bevelling, sometimes referred to as ‘knife-edging’.

The aim is to reduce windage losses in the crankcase, allowing the crankshaft to cut through the mixture of oil, air and blow-by gases with minimum disturbance. However, there are two schools of thought on the significance of this effect.

If we imagine the air-oil mist to be a stationary dense ‘fog’ through which the crankshaft counterweights must pass, it seems logical that we should make some real effort to shape the crankshaft (and possibly other moving components on the crankcase) such that the amount of energy lost through ‘aerodynamic’ effects is minimised. The knife-edging will have its greatest effect at the largest radius on the crankshaft. Removing material here while maintaining the same balance factor means increasing the breadth/width of the counterweight, assuming that we don’t make its radius larger. This knife-edging generally means that the counterweight centre of mass moves slightly back towards the crankshaft axis, with the larger counterweight mass compensating for this. So, we have larger crankshaft mass and inertia for an expected aerodynamic gain.

The second school of thought says that the oil-air mist in the crankcase is anything but still, and circulates rapidly in the direction in which the crankshaft is travelling. Thus, the difference in speed between the crankshaft and the oil-air mist is not as high as we might first assume, and any aerodynamic losses are consequently not as high. This school of though leads us to spend less effort on knife-edging of crank counterweights. It can’t be argued that anything other than crankshaft motion has imparted significant angular motion on the crankcase oil-air mixture, but we have to note that its effects are likely to be small and most noticeable when the engine is being asked to accelerate.

Lots of people offer knife-edge crankshafts, and one has to take a view on how significant this effect is. It is likely to have a very small effect on steady-state output, and if the output can be demonstrated to have improved then it might well be for reasons other than the intended aerodynamic drag decrease. If we look hard at the other effects in play here, we might find that there are other clues as to where we might minimise cranktrain friction losses.

Written by Wayne Ward

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