196. ECCENTRIC.
An Eccentric (Fig. 196) is an ingenious piece of mechanism that answers exactly the opposite purpose of a crank, viz. to convert a rotary motion into a backward and forward movement. An eccentric is a circular iron disc, with the main crank shaft passing tightly through it, but the shaft does not pass through the centre; hence the term “eccentric.” This disc is encircled and revolves within an iron strap, which is attached to the valve rod or gear. It will be readily seen that as the disc revolves it gives a reciprocating movement to the rod, causing the slide valve to which it is connected to open and close the ports in the cylinder, and the object in attaching the eccentrics to the crank shaft is that the piston rod and valve rods may have an equal and corresponding action, which it will be understood is absolutely necessary.
197. CRANK. CRANK. 198. DISC CRANK.
The Crank (Fig. 197), which on small engines is generally attached to one end of the crank shaft, is one of Watts’ most famous inventions (but which, however, was pirated from him), and its object is to convert the backward and forward movement of the piston rod into a rotary motion at the shaft. Disc crank plates (Fig. 198) are now getting into favour as having a steadier action, and it is to all intents and purposes a crank, but of improved form.
The Flywheel is a heavy cast-iron wheel attached to the crank shaft on the opposite end to the crank itself; it serves more than one useful purpose, viz. giving great steadiness to the motion, assisting propulsion to some extent by its momentum and carrying the piston over the dead points (a dead point is the position which the piston is in when it has finished one stroke and about to return just at the time it becomes quite still for an instant, and this is called the dead point, and it happens at the end of each stroke).
The Pulleys are of two kinds, fast and loose; they are light wheels about one-sixth the diameter of the flywheel, at whose side they are attached to the extreme end of the shaft. They have broad flat faces or rims, and their object is to carry the strap or belting which transmits the power from the engine to the work. The fast or driving pulley is the one that is secured to the shaft and revolves with the flywheel; the loose pulley is the one that is not secured to the shaft, and rotates loosely upon it when occasion demands; a forked arrangement transfers the belting from the fast to the loose pulley when it is necessary to stop the machinery, or vice versâ.
199. Governor.