Fig. 3333.
In [Fig. 3333] it is supposed that the piston is to be at the head end of the cylinder when the crank is on its corresponding dead centre.
The first thing to do is to put the reversing gear in full gear forward, so as to set the forward eccentric, and see if its rod is of proper length.
The next thing to do is to move the wheel so that the crank pin is nearly on the dead centre, and then take a tram (such as shown in the figure), pointed at each end, and mark on the splash plate, or any other convenient place, a centre punch dot in which the point b of the tram can rest. Next, from the centre of the axle as a centre, mark arcs or portions of circles a, a. This being done, point b of the tram is rested in the centre punch dot before referred to, and with the other end a line c is marked, a straight edge is then rested against the ends e e of the cross head, and a line d is marked on the guide bar, this line being exactly even or fair with the end e e of the cross head.
We then move the wheel in the direction of the arrow, and as soon as we begin to do so, the cross head will move to the left and away from the line d on the guide bar. But as soon as the crank pin has passed its dead centre, the cross head will begin to move to the right, and as soon as the end e e comes again exactly in line with the line d marked on the guide bar, we must stop moving the wheel, and again resting the point b of the tram in the centre punch mark before mentioned, we move its other end so as to mark a second line, which will be the line or arc f.
The next thing to do is to mark a fine centre punch dot, where c and f cross the arc or line a, and then find the point g midway between f and c, and mark a fine centre punch mark there. This being done, we must move the wheel back into the position it occupies in the figure, and then slowly move it in the direction of the arrow, until with the end b of the tram resting in the centre punch dot, the other end of the tram will fall dead into the centre punch dot at g, at which time the crank pin will be exactly on the dead centre.
During this part of the process we have nothing to do with anything except getting the crank pin on the dead centre, but there is one point that requires further explanation, as follows:
In this operation we have first put the crank on one side of the dead centre and then put it to the same amount on the other side of the dead centre, both being improper positions; but by finding the mean or mid position between the two, we have found the proper position. In doing so, however, we have moved the wheel, the wheel has moved the connecting rod, and the connecting rod has moved the piston. But in the actual running of the engine, this order of things will be reversed; for the steam will move the piston, the piston will move the connecting rod, and the connecting rod will move the crank and therefore the wheel.
The difference between the two operations is this: Suppose there is lost motion or play between the connecting rod brasses and the crank pin, or between the connecting rod brasses and the cross head pin, and then if we move the wheel in the direction denoted by the arrow, we take up this lost motion, so that if the tram was fair with the centre punch at g and steam was admitted to the piston, then there would be no lost motion to take up, and as soon as the piston moved the crank pin would move. But if we moved the wheel in the opposite direction to that denoted by the arrow, then we are placing any lost motion there may be in the opposite direction, and if steam were turned on, the piston and connecting rod might move before the crank and wheel moved.
In which direction the wheel should be moved while placing the crank on the dead centre depends upon the condition of the engine, as will be explained presently, the assumption being at present that the engine is in thorough good order, in which case the wheel should (while placing the crank on the dead centre) be moved in the direction of the arrow in the figure.