Steam Engines

by Anonymous

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.

If the valve is to be given a lead, as indicated in [Fig. 12], the eccentric must be turned still further on the shaft to open the valve slightly before the piston starts on its forward movement. This brings the eccentric to the position Ab shown in [Fig. 11]. The angle through which the eccentric is turned to give the necessary lead opening to the valve is called the angle of lead, and is shown by angle b´Ab. By reference to [Fig. 11], it is seen that the total angle between the crank and the eccentric is 90 degrees, plus the angular advance, plus the angle of lead. This is the total angle of advance.

The relative positions of the piston and valve at different periods of the stroke are illustrated in [Figs. 12 to 16]. [Fig. 12] shows the piston just beginning the forward stroke, the valve having uncovered the admission port an amount equal to the lead. The crank is in a horizontal position, and the eccentric has moved from the vertical an amount sufficient to move the valve toward the right a distance equal to the outside lap plus the lead. The arrows show that steam is entering the left-hand port and is being exhausted through the right-hand port.

Fig. 12. Piston just beginning Forward Stroke	Fig. 13. Steam Port fully Opened
Fig. 14. Valve has started on Backward Stroke	Fig. 15. Both Steam Ports Closed

Fig. 16. Exhaust Begins

In [Fig. 13] it is seen that the valve has traveled forward sufficiently to open the steam port to its fullest extent, and the piston has moved to the point indicated. The exhaust port is still wide open, and the relative positions of the crank and eccentric are shown in the diagram at the right. In [Fig. 14] the eccentric has passed the horizontal position and the valve has started on its backward stroke, while the piston is still moving forward. The admission port is closed, cut-off having taken place, and the steam is expanding. The exhaust port is still partially open.

In [Fig. 15] both ports are closed and compression is taking place in front of the piston while expansion continues back of it. Release occurs in [Fig. 16] just before the piston reaches the end of its stroke. The eccentric crank is now in a vertical position, pointing downward, and exhaust is just beginning to take place through the left-hand port. This completes the different stages of a single stroke, the same features being repeated upon the return of the piston to its original position. The conditions of lap, lead, angular advance, etc., pertain to practically all valves, whatever their design.