For this purpose take a chisel with the cutting end ground to the form of a fiddle drill, one cutting edge being at a right angle to the other. The chisel must be held so that while one edge rests upon the axle, the other edge will bear against the radial face of the eccentric. A sharp blow with a hammer upon the chisel head will make a clean indented cut upon the axle and the eccentric, the two cuts exactly meeting in a point where the eccentric bore meets the axle circumference, so that when they coincide the eccentric is in its proper position.
If the eccentrics of a locomotive should slip when the engine is upon the road, and there are no marks whereby to readjust them, it may be done approximately as follows:—Put the reverse lever in the end notch of the forward gear, then place the crank as nearly on a dead centre as the eye will direct, and open both the cylinder cocks, then disconnect the slide-valve spindle from the rocker arm, and move the valve spindle until the opening of the port corresponding to the dead centre on which the crank stands will be shown by steam blowing through the cylinder cock, the throttle valve being opened a trifle. The position of the valve being thus determined, the eccentric must be moved upon the shaft until the valve spindle will connect with the rocker arm without being moved at all. The throttle valve should be very slightly opened, otherwise so much steam will be admitted into the cylinder that it will pass through any leak in the piston and blow through both cylinder cocks before there is time to ascertain which cock first gives exit to the steam.
Fig. 2555.
Instead of finding when the crank pin is on the dead centre by means of the process shown in [Fig. 2551], it may be found as in [Fig. 2555], which is for a vertical engine. On the face of the crank and from the centre of the crank shaft as a centre, draw a circle b equal in diameter to the diameter of the crank pin. Then take a spirit-level c and apply it to the cylinder bore and note where its bubble stands. Then apply the spirit-level to the perimeter of the crank pin a and circle b, and move the crank until the spirit-level bubble stands in the same position as it occupies when applied to the cylinder bore. If the cylinder bore stands truly vertical the bubble will in both cases stand in the middle of the spirit tube; but in any event, the bubble must stand in the same position when applied to the crank as when applied to the cylinder bore, in which case the crank will be on its dead centre whether the cylinder bore be horizontal, vertical, or at an angle, the dotted line e passing through the centre of the crank and the axis of the cylinder bore.
When an engine has two eccentrics, so as to enable the engine to run in either direction, as in the case of a locomotive, it is necessary to consider which eccentric is to be set for the forward, and which for the backward motion. In American locomotive practice it is usual to let the eccentric nearest to the wheel, and, therefore, the most difficult to get at, be for the backward motion, which is the least used, and therefore the least liable to get loose upon the axle.
The eccentric that connects to the top of the link is usually that for the forward motion, and hence that which connects with the eccentric farthest from the wheel.
In testing the lengths of the eccentric rods, work may be saved after the engine is first placed on its dead centre by putting the reverse-lever in the forward notch of the link, and adjusting the forward eccentric until the valve has the proper lead. Then set the reverse-lever in the back notch and move the backing eccentric (in both cases moving them in the direction in which they will run), until the proper amount of lead appears. The engine may then be placed on the other dead centre, and the lead both for forward and backward gear measured, so that if there are any errors both the rods may be corrected for length; but for the final trial the crank pin must be set on its dead centre for each direction of motion separately, so as to take up any lost motion in the connecting-rod brasses.