The Crankshaft — Converting Motion

The crankshaft is a heavy steel shaft that runs the length of the bottom of the engine block. It converts the up-and-down motion of the pistons into rotational motion — spinning — that ultimately drives the wheels. Think of a bicycle pedal. Your legs push straight down on the pedals. The crank arm converts that push into rotation of the sprocket. The crankshaft does exactly the same thing with the pistons.

The crankshaft has offset sections called throws or crank journals. Each connecting rod attaches to one of these offset journals. Because the journals are offset from the center of the shaft, the downward push of the piston on the rod forces the shaft to rotate. Multiple cylinders firing in a carefully timed sequence keep the crankshaft turning smoothly.

The crankshaft sits in main bearing saddles in the bottom of the block. Between the crankshaft and the saddles are precision insert bearings — thin shells of soft bearing material that the crankshaft rides on. A thin film of pressurized oil separates the bearing surface from the crankshaft surface. The crankshaft never actually touches the bearing — it floats on a microscopic film of oil. When that oil film fails — low oil level, low oil pressure, wrong oil viscosity, or contaminated oil — metal contacts metal and the bearing is destroyed. This is why oil maintenance is not optional.

Bolted to the front of the crankshaft, the harmonic balancer — also called a damper — absorbs torsional vibrations in the crankshaft. Every time a cylinder fires, the crankshaft twists slightly. These repeated twisting forces at high RPM can crack the crankshaft if not dampened. The balancer uses a rubber ring between an inner hub and an outer ring to absorb these vibrations. When the rubber deteriorates with age, the outer ring can shift or separate — changing ignition timing readings and causing vibrations.