The Eccentric
Fig. 10. Diagrammatical View of Eccentric
Fig. 11. Relations of Crank and Eccentric
The slide valve is usually driven by an eccentric attached to the main shaft. A diagram of an eccentric is shown in [Fig. 10]. An eccentric is, in reality, a short crank with a crank-pin of such size that it surrounds the shaft. The arm of a crank is the distance between the center of the shaft, and the center of the crank-pin. The throw of an eccentric corresponds to this, and is the distance between the center of the shaft and the center of the eccentric disk, as shown at a in [Fig. 10]. The disk is keyed to the shaft, and as the shaft revolves, the center of the disk rotates about it as shown by the dotted line, and gives a forward and backward movement to the valve rod equal to twice the throw a.
In [Fig. 11] let A represent the center of the main shaft, B the crank-pin to which the connecting-rod is attached (see H, [ 1]), and the dotted circle through B the path of the crank-pin around the shaft. For simplicity, let the eccentric be represented in a similar manner by the crank Ab, and its path by the dotted circle through b. [Fig. 12] shows a similar diagram with the piston P and the valve in the positions corresponding to the positions of the crank and eccentric in [Fig. 11], and in the diagram at the right in [Fig. 12]. The piston is at the extreme left, ready to start on its forward stroke toward the right. The crank-pin B is at its extreme inner position. When the valve is at its mid-position, as in [Fig. 8], the eccentric arm Ab will coincide with the line AC, [Fig. 11]. If the eccentric is turned on the shaft sufficiently to bring the left-hand edge O, [Fig. 8], of the valve in line with the edge C of the port, the arm of the eccentric will have moved from its vertical position to that shown by the line Ab´ in [Fig. 11]. The angle through which the eccentric has been turned from the vertical to bring about this result is called the angular advance, and is shown by angle CAb´ in [Fig. 11]. The angular advance evidently depends upon the amount of lap.