VALVE GEAR.
The mechanism by which the valve is opened and closed is somewhat complicated, as various things are accomplished by it besides simply opening and closing the valve. If an engine has a reverse lever, it works through the valve gear; and the governor which regulates the speed of the engine may also operate through the valve gear. It is therefore very important.
The simplest valve gear depends for its action on a fixed eccentric.
An eccentric consists of a central disc called the sheave, keyed to the main shaft at a point to one side of its true center, and a grooved ring or strap surrounding it and sliding loosely around it. The strap is usually made of brass or some anti-friction metal. It is in two parts, which are bolted together so that they can be tightened up as the strap wears.
The eccentric rod is either bolted to the strap or forms a single piece with it, and this rod transmits its motion to the valve.
It will be seen, therefore, that the eccentric is nothing more than a sort of disc crank, which, however, does not need to be attached to the end of a shaft in the manner of an ordinary crank.
The distance between the center of the eccentric sheave and the center of the shaft is called the throw of the eccentric or the eccentricity.
The eccentric usually conveys its force through a connecting rod to the valve stem, which moves the valve.
The first modification of the simple eccentric valve gear is