Piston Engines - General — Page 29, Lesson 34

Let's start with the crankshaft — the heart of the engine's mechanical output. I want to walk you through what it does, how it's built, and why it behaves the way it does. The crankshaft, sometimes called the cranked-shaft, has one central job: it converts the reciprocating, or linear, motion of the pistons — that's the straight up-and-down movement — into rotary motion, which is spinning. That spinning motion is what turns the propeller, and it also provides the drive for the accessories, like the alternator, fuel pump, and oil pump. Now, the key geometric feature here is the offset crank throw. The crank throw is the offset portion of the shaft, and its offset distance is what determines the piston stroke — that's the total distance the piston travels from top to bottom. So the geometry of the crank throw literally sets how far the piston moves. Let me walk you through the parts. The journals are the main part of the shaft — they're the straight sections that are supported by the main bearings in the crankcase. The pistons are attached to the shaft through the connecting rods, and the connecting rods connect to the crank-pin. The crank-pin is the journal that the connecting rod actually rotates around. A crankshaft often has as many crank throws as there are pistons — so a four-cylinder engine has four throws. Now, inside the shaft, there are oil-ways drilled through it. These are internal passages that transfer lubricating oil onto the bearing surfaces. And importantly, those same oil-ways can also carry oil for the operation of a variable pitch propeller — that's the mechanism that lets the propeller blade angle change in flight. For bearings, plain bearings are used. A plain bearing is just a simple cylindrical surface — no rolling elements like balls or rollers — and it's used here because it can carry the high reciprocating loads that come from the pistons moving up and down. Now, here's where it gets interesting — the balance and vibration story. The crankshaft is accurately balanced to minimize vibration. But here's the physical reality: when a shaft has to transmit a torque, or twisting moment, it must flex to some extent and spring back when released. That's just elasticity. The problem is, if the shaft has a lot of kinks in it — those bends that create the crank throws — then the twisting moments are hard to resist, and you get perceptible deflection. The shaft actually twists and untwists slightly with each power impulse. Let me give you the radial and opposed engine variations. In a radial engine, several cylinders may be connected to a single throw — so one crank throw serves multiple pistons arranged around it. In a horizontally opposed engine — that's the flat engine where cylinders sit opposite each other — you may have only two pistons connected to one crank-pin. Now, the repeated applications of force can set up oscillations as the shaft recovers its original shape between power impulses. Here's the critical part: at certain speeds, the impulses may coincide with the natural vibration period of the shaft. When that happens, you get very rough running — even in an engine that is in good mechanical balance. This is a resonance condition. So for these reasons, the shafts should be as short as possible, adequately supported, and counter-weighted to minimize these torsional effects. Counter-weights are added to the shaft to offset the forces. And here's the practical consequence for you as a pilot: many engines have rpm ranges which are prohibited for prolonged use. These are called critical rpm ranges — the speeds where that resonance happens. This is indicated by a Red Arc on the rpm indicator. So when you see that red arc on the tachometer, that's the range you must not linger in, because that's where the torsional vibration is at its worst. Let me make sure you've got the full picture. The crankshaft takes linear piston motion, turns it into rotary motion, drives the propeller and accessories. The crank throw offset sets the stroke. Journals ride in main bearings, connecting rods attach at the crank-pin. Oil-ways feed the bearings and can feed a variable pitch propeller. Plain bearings carry the heavy loads. And because the shaft must flex under torque, it can resonate at certain speeds — so we have critical rpm ranges marked by a red arc, and we design the shaft short, supported, and counter-weighted to keep those torsional effects down. That's the crankshaft in full.