Flexibility and friction

Friday, April 05, 2013

Tags :  crankshafts

Anyone involved in the development of a race engine understands the basics of performance improvement. We want to trap as much air as possible in the combustion chamber, mix with it the amount of fuel that will reliably give best power, burn it as efficiently as possible and turn as much of the resulting energy as we can into mechanical power. Much of the energy released as a result of combustion is lost to the cooling system or out of the exhaust, but there are also substantial mechanical losses through friction.

The crankshaft is one component where we can make substantial gains in performance by reducing friction. The losses in bearings due to oil shear are substantial, and any reduction in shear losses are worth chasing. Within the limits of the bearing materials available to us, we can reduce the bearing area in order to reduce friction. Although a loaded bearing is a more complex case, we can learn something from studying a plain shaft running concentrically within a plain collar with oil between the two; this example was used in the Race Engine Technology article on crankshafts in issue 65. The torque required to turn such a shaft is proportional to the third power of the shaft diameter, but directly proportional to the bearing width. There is therefore a much greater benefit to be gained from reducing bearing diameters than width where friction is concerned.

The strategy of friction reduction through decreased bearing diameters is not without risk though, as any useful decrease in the diameters will cause a significant reduction in the torsional and bending stiffness. A large change in torsional stiffness may also serve to bring a torsional resonance within the operating range of the engine, leading to reliability problems. A usually ‘bullet-proof’ engine may therefore become prone to crankshaft failure, or other problems may be introduced if the natural frequency of the cranktrain is changed markedly.

Even if torsional problems are avoided, there is a limit to the useful reduction in crankshaft bearing diameters, owing to a lack of stiffness in bending. Just as torsional stiffness is proportional to the fourth power of diameter, so is bending stiffness, as both are directly proportional to the moment of inertia (second moment of area). Excessive bending in the crankshaft will lead to loading of the edge of the bearing rather than presenting a relatively even load across the face. When a bearing is loaded in the ideal sense – that is, with the axis of the shaft parallel to the axis of the bearing – the maximum pressure is at the centre of the bearing. When edge loading occurs, the bearings are quickly damaged and friction tends to rise again.

One possible solution to limited amounts of bending in the crankshaft is to have ‘barrelled’ crank journals running in cylindrical bearings, or cylindrical journals running in barrelled bearings. The crankshaft may also be damaged by the high contact stresses due to the edge-loading condition, so as we reduce bearing diameters in an attempt to minimise friction, there will be a situation of diminishing returns and eventually no further friction decrease will be seen. In fact going further is likely to increase friction.

Written by Wayne Ward

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