Fig. 11.
EFFECT ON THE VALVE-MOTION OF CONNECTING-ROD ANGULARITY.
As the opening and closing of the steam-ports by the valves are regulated by the eccentrics, which are subject to the same motion as the crank, following it at an unvarying distance, it is evident that their tendency will be to admit and cut off steam at a certain position of the crank’s movement. If the motion is planned to cut off at half stroke, it will be apparent, that, in the backward stroke, the piston will be past its mid travel before the crank-pin reaches the quarter, so that end of the cylinder will receive steam during more than half the stroke. On the forward stroke of the piston, however, the crank-pin will reach the quarter before the piston has attained half travel; the consequence being, that in this case steam is cut off too early. The disturbing effect of the angularity of the connecting rod on the steam distribution thus tends to make the cut-off later in the backward stroke than in the forward stroke, resulting in giving the forward end of the cylinder more steam than what is admitted in the back end. The link-motion provides a convenient means of correcting the inequality of valve opening due to the connecting-rod angularity, the details of which will be explained farther on.
AIDS TO THE STUDY OF VALVE-MOTION.
An engineer or machinist who wishes to study out this peculiarity of connecting-rod angularity, will find that the use of a tram or long dividers will help him to comprehend it better than any letter-type description. All through the study of the valve-motion, there are numerous difficult problems encountered. The use of a good model will be found an invaluable aid to the study of the valve-motion, and every division of engineers or firemen should make a combined effort to furnish their meeting-room with a model of a locomotive valve-motion. In no way can the spare time of the men connected with locomotive running be better employed than in the wide range for study presented by a well-devised model. Great aid can be obtained in the study of the valve-motion from good books devoted to the subject, and they will impart more information than can be obtained by mere contact with the locomotive. The valve and its movements are surrounded with so many complicated influences, that an intelligent man may work for years about a locomotive doing valve setting occasionally, and other gang boss work, yet, unless he studies the valve-motion by the aid of the drawing-board, or by models, which admit of changing sizes and dimensions, he may know less about the cause of certain movements than the bright lad who has been a couple of years in the drawing-office. The man who thinks he can study the valve-motion, and understand its philosophy, by merely running the engine, deceives himself. The engineer who never looks at a book or a paper in search of information about his engine, knows very little about any thing not visible to the eye. Yet many men of this stamp, by looking wise, and by exercising a judicious use of silence, pass among their fellows as remarkably profound. But let a fireman, in quest of locomotive knowledge, put a question to such a man, and he is immediately silenced with a “You ought to know better” answer.
Where the use of a model can not be obtained, any one beginning the study of the valve-motion can assist himself by making a cross section of the valve and its seat, similar to those published, on a strip of thin wood or thick paper. By slipping the valve on the seat, its position at different parts of the stroke can be comprehended more clearly than by a mere description. With a pair of dividers to represent the motion of the eccentric, and strips of wood to act as eccentric, and valve rod and rocker, and some tacks to fasten them together, a helpful model can be improvised on a table or board. By the time a student gets a rig of this kind going, he will see his way to contrive other methods of self-help.
EVENTS OF THE PISTON STROKE.
By the aid of [Fig. 10], we will trace the relative movements of the crank and eccentric connections. For the sake of simplicity, the eccentric is represented as connecting directly with the rocker-arm.
The crank-pin being at the point A, or the forward center, the piston must be in the front of the cylinder, or at the beginning of the backward stroke. Owing to the angular advance already referred to, the eccentric center is at F; and, being a certain distance ahead of the middle position, it has pushed the lower arm of the rocker from a to b, drawing back the top arm, which, in its turn, has moved the valve so that it is just beginning to admit steam at the forward port, i. As the crank-pin goes round, the eccentric follows it, opening the steam-port wider till the eccentric reaches the point of its travel nearest A, the limit of the throw. When the eccentric is at this point of its throw, the valve must be at the outside of its travel; and therefore the steam-port is wide open. By this time the crank-pin is getting close up towards the quarter. After passing this point, the forward eccentric begins to draw the bottom rocker-pin towards the axle, and to push the valve ahead, this being the point where the valve changes its direction of motion, just as the piston returns when the crank-pin passes the center. When F reaches the point B, the valve is in the same position it occupied at the beginning of the stroke; but, as it is traveling in the opposite direction, a very small movement more closes the port, cutting off steam. When this happens, the crank-pin has reached the point x. When F gets to g, it is on the central point of its throw; so the valve must then be on the middle point of its travel, with the exhaust cavity just covering the outside edges of the bridges, the forward edge being ready to put the steam-port, i, in communication with the exhaust cavity. This releases the steam from the forward end of the cylinder; and at the same moment the inside edge of the valve covers the back port, k, causing the piston-head to compress any steam left in the back part of the cylinder. When the piston reaches the beginning of the forward stroke, the eccentric F has got to the point f, and the valve is beginning to admit steam for the return stroke, the events of which are similar to those described.