Fig. 308

The wheels and axles which have been mounted with boxes, big ends, springs, etc, are now put under, the horn stays and big ends coupled up, and the cradle, Fig. 304, placed under the driving wheels. The engine is now ready to have the valves set. It has been previously mentioned that the valves have been boxed up, but their position is indicated upon the valve spindle at the point of lead, also that the reversing shaft has been only set temporarily, and that the "bump" or clearance has been marked on the slide bars.

Fig. 308.

The four centres are now marked upon the face of the driving tire in the following manner, which gives the dead centres for each crank. A man is stationed in the pit to watch the progress of the slide blocks as the driving axle is turned. When they have approached the 1 bump line within, say, 1 inch, he stops the turning of the axle, and from a convenient centre on the bottom slide, he scribes on the bar a mark of convenient length, between the centre of the block and the bump line. This position is also indicated upon the face of the tire. The axle is again turned slowly, and he notes as accurately as possible when the slide block stops, which gives the clearance between the cylinder end and piston. One leg of the dividers is then placed in the slide block centre and the axle turned until the point of the other leg covers the original scribed line, this position being also indicated upon the face of the tire, and by dividing the space between these two points on the face of the tire, the dead centre of the crank in that position is obtained accurately, and it does not depend upon the judgment of the operator in the least degree. This is repeated until all four centres are obtained. One crank is then placed at a dead centre, then the motion is reversed from full gear to full gear; if the valve spindle moves, the reversing shaft is either too high or low, and should be adjusted accordingly, reasons being given for this later on. Each crank should be tried on both centres, and at the same time the lead should be tested, which has been marked by trammel upon the spindle, this being equalised for the back and front port in both gears, by advancing or receding the reversing shaft according to requirements. This constitutes the whole mystery of setting the valves in the shop. It is simple enough; but under the surface lie reasons which make it even more simple, certainly very much clearer, for knowing the reason of doing each operation essentially causes the whole to be done more scientifically and with greater accuracy. We must therefore turn to the drawing-office; but it is with a certain amount of diffidence, because the motion has now become so familiar, owing to its application to almost every existing class of engine, to its being illustrated in several of the engineering papers, and to its forming the subject for papers read before nearly every scientific body, that it appears almost superfluous to go very deep into detail; but at the same time, to show clearly the setting of the valves, this portion of the work would not be complete without it.

The essential conditions required to be known are the relative positions of the centre lines of the piston and valve spindle (the latter being in the plane of the vibration of the connecting-rod) the full stroke of the valve, also the stroke of the piston, the lap and the lead.

Fig. 309

Lay down the centre line of the cylinder a a, and that of the valve spindle b b, at their relative distances. Draw the path of the crank pin, and the centre lines of the connecting-rod c c1, c c11, for both upper and lower positions, when the piston is at half stroke. Take a point d, on the centre line of the connecting-rod, where its vibration between dl and d2 is equal to about double the length of the full stroke of the valve, allowing more rather than less, because it then renders a less angle of the reversing shaft when in full forward or backward gear. Through d draw the vertical z z, at right angles to a a, and mark off the two points e e1, ou each side, which are the extreme positions of the point d, for front and back strokes ; from these points draw lines to a point f on the vertical, such that the angle between them shall not be more than 90°, less if there is room to allow it. Select a point f 1 either backward or forward, at a convenient distance, so that the point f, which rises and falls with the connecting-rod, is maintained by the anchor link as nearly as possible on the vertical line. This vibration is shown in the figure to be all upon one aide of the vertical, which does not make any material difference to the motion. The vertical is also indicated by the dotted lines.

Next, on the valve spindle centre line b b mark off upon each side of the vertical the amount required for lap and lead together, g - g 1 and g - g 2 , the former being lap and lead for the front port and the latter for the back port. Assume the piston to be at the front end of the cylinder, then the point d will be at e1, and the stirrup link at e1 f. From a point on this link which has at first to be assumed, say at j - which will be about one-third more than the half vibration of the connecting-rod, that is d to d1 - draw the centre line of the lever actuating the valve, that is joining j and gl; the point where this line crosses the vertical, z z, will be the centre of the fulcrum of the lever, and will also be the centre of oscillation of the curved links in the reversing shaft, in which the blocks carrying the centre of the lever slide. This centre is marked m, and stands for both centres, which must be concentric at each end of the stroke, From this construction, that is, by marking out the lap and the lead and actuating it by a lever, the steam is admitted at the commencement of each stroke by the amount of the load only, and as this depends upon the link as a lever (which does not admit of a varying length) the lead remains constant for all degrees of expansion. Also because the point m, was made the centre or fulcrum of the actuating lever and the. centre of oscillation, which coincide when the piston is at each end of the stroke; when reversed from full gear to full gear, or any intermediate point, the valve should not move. This is the test always given to ascertain the correctness to which the valves have been set, nothing being admitted beyond a movement equal to the line of a fine scriber point.