All About Engines

Fig. 7.—Eccentric 10 All About Engines of the valve is large enough to cover the middle or “ exhaust ” port, and one of the steam ports, but not both. As the valve moves backwards and for- wards it not only uncovers each steam port in turn, so that steam can enter the cylinder, but it also places the covered port in communication with the exhaust port, through which the steam escapes into the open air or into a condenser. If this valve, then, moves backwards and forwards, always in the opposite direction to the piston, steam is admitted to each end of the cylinder alternately, at the right moment for con- tinuous working. The usual device for securing this motion of the slide valve is called an eccen- tric (Fig. 7). An eccentric consists of a cast-iron disc or “ sheave,” keyed on to the shaft, with a strap fitting closely round it and connected by a jointed rod to the slide valve. Now if the hole in the sheave through which the shaft passes were in the centre the sheave would merely turn with the shaft like a wheel. But this hole is out of the centre—hence the term eccentric— and as the sheave rotates with the shaft the valve is moved backwards and forwards over the cylinder ports. The distance from the centre of the sheave