The power impulses of an engine result in torsional vibration in the crankshaft. A vibration damper mounted on the front of the crankshaft controls this vibration (fig. 12-21). If this torsional vibration were not controlled, the crankshaft might actually break at certain speeds.

Most types of vibration dampers resemble a miniature clutch. A friction facing is mounted between the hub face and a small damper flywheel. The damper flywheel is mounted on the hub face with bolts that go through rubber cones in the flywheel. These cones permit limited circumferential movement between the crankshaft and damper flywheel. That reduces the effects of the torsional vibration in the crankshaft. Several other types of vibration dampers are used; however, they all operate in essentially the same way.

Figure 12-21.-Sectional view of a typical vibration damper.

The flywheel mounts at the rear of the crankshaft near the rear main bearing. This is usually the longest and heaviest main bearing in the engine, as it must support the weight of the flywheel.

The flywheel (fig. 12-22) stores up rotation energy during the power impulses of the engine. It releases this energy between power impulses, thus assuring less fluctuation in engine speed and smoother engine operation. The size of the flywheel will vary with the number of cylinders and the general construction of the engine. With the large number of cylinders and the consequent overlapping of power impulses, there is less need for a flywheel; consequently, the flywheel can be relatively small. The flywheel rim carries a ring gear, either integral with or shrunk on the flywheel, that meshes with the starter driving gear for cranking the engine. The rear face of the flywheel is usually machined and ground and acts as one of the pressure surfaces for the clutch, becoming a part of the clutch assembly.