How Does A Crankshaft Work?

Kevin Cameron explains how plain journal bearings support the fast-spinning crankshaft in a motorcycle engine

How is the crankshaft supported in a motorcycle engine? In this particular example—removed for this purpose from a late-model Honda CBR600RR—the crankshaft spins at up to 14,000 or possibly 15,000 rpm at maximum performance.

The crankshaft has a connecting rod attached to it at each of the four crankpins. Each rod has a piston rising and falling in its respective cylinder. That rising and falling produces a shaking force, which, to some extent, is counterbalanced by weights on the crankshaft.

The crankshaft is not just a simple zigzag line like my uncle drew for me when I was five. It has the added complication of these counterweights, which are there to reduce vibration and also the load on the main bearings.

Classic motorcycles of yesteryear had roller- and ball-bearing crankshafts. But roller and ball bearings don’t last as long as people would like their motorcycles to last these days. This engine has plain journal bearings.

Smooth cylindrical journals are supported by skinny little shells—a pair of them, split at the centerline, just as the crankcase halves are split—which do the work of great big ball bearings. And they do a better job.

A thin film of oil between the smooth crankshaft journals and the bearing shells acts not only as a lubricant but a damper. So the crankshaft supported in five places by plain bearings and clamped between the upper and lower cases is actually very well supported.

That allows this particular four-cylinder engine to operate routinely at speeds up to 15,000 rpm. That is not an extreme. In fact, that rpm represents the design point for the manufacturer—in this case, Honda. It is normality.