Roller bearing crankshafts

Thursday, May 16, 2013

Tags :  crankshafts

The 1960s must have been a fantastic time to be around race engines. The incredible diversity of engines in a single class is one reason I’m drawn to events where racing motorcycles from this era are wheeled out, started and ridden as they should be, at great risk to their owners’ bank accounts. If you have a passion for race engines, you should treat your eyes and ears to something special.

Many of these engines used roller bearing crankshafts, where either the main bearings, con rod bearings or both used rolling element rather than plain bearings; these days it is far more common to find four-stroke engines using plain bearing crankshafts. Two-stroke engines use roller bearing crankshafts owing to their need to lubricate the bottom end of the engine with a mixture of fuel and oil. Quite often with two-strokes, the big-end bearing runs directly on the crankpin, and the main bearings have a bearing inner race pressed onto the crankshaft.

That is not to say that, for four-stroke engines, roller bearing crankshafts slipped into the mists of time with the likes of the 50 cc twin-cylinder Honda, its bigger six-cylinder 250 cc cousin, or their contemporaries. Ducati has used roller bearing crankshafts on its Superbike engines for many years, right up until the recent release of the 1199 cc Panigale, which uses plain bearings.

There was also a brief flirtation with roller-bearing crankshafts in Formula One in the V10 era. The reason for this was the promise of lower friction, but while some people found this to be true, with significant reductions in friction, others found the gain to be negligible. According to one leading engineer, this variance was thought to be due to the differences in the amount of oil flow through the plain bearings that the roller bearing crankshaft was being compared to. As the roller bearing requires little oil to provide lubrication then some engines which, as a plain bearing engine, required lower flow rates would see less benefit compared to one which required higher flow rates.

The complexity of a roller bearing crankshaft, and the extra space occupied radially by the bearing arrangement, means it has to offer a very useful increase in performance in order to offset the disadvantages. The FIA effectively closed this avenue of development by outlawing ceramic rolling elements on which the Formula One crankshaft bearings relied. Since then though there have been advances in bearing element steels that might have made such a crankshaft an attractive proposition, had performance development been allowed.

In terms of the crankshaft itself, for the most compact installation of a roller bearing the rolling elements run directly on the crankshaft journals. We have to take account of contact stresses between the bearing rollers and the crankshaft, and particularly subsurface stresses. In this case, the depth of hardening and the variation of strength with depth needs to be carefully considered in order to avoid subsurface fatigue failures.

Written by Wayne Ward

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