Chapter XXIX.—ERECTING ENGINES AND MACHINERY.
In engines having suspended guide bars, it is not uncommon to set those bars by the working parts of the engine, instead of by lines. This is an advantage when the parts of the engines are not taken down, and, if care is taken, will make a true and smooth working job; but otherwise, it is likely to introduce errors in the lining of the engine, which throw it out of proper line, and cause a great deal of friction.
The proper method of setting the bars depends upon the condition of the engine as to wear. Suppose, for example, that a new piston head has been put in, then, if the gland is new also, or is a good fit to both the piston rod and the bar of the stuffing box, the bars may be set as follows:—
Fig. 2518.
Place the piston at the back end of the cylinder, and put the cross head and guide blocks in proper place on the rod. Put up the bottom guide bars so that they just touch the cross-head guides. Now, in adjusting these bottom bars there are two essential points: first, that the plane of their upper surfaces shall stand parallel with the axial line of the main shaft, and secondly, to place the upper surface parallel with the axial line of the cylinder (it being of course assumed that the cylinder and crank shaft are in proper line). The first of these essential points will be attained when a spirit-level, placed truly along the bore of the cylinder, shows the bubble to stand in the same position in the tube, as it does when placed upon and along the bar. The second will be attained when a spirit-level, placed across the bars, as in [Fig. 2518], at a, shows the bubble to stand the same as it does when the level is placed on a parallel part of the shaft, as in the figure at b. When the bars are thus temporarily set, the liners, or pieces of iron, may be fitted to the proper thickness, so that the gland will pass in and out of the stuffing box easily by hand, no matter in what position the piston may be in the cylinder.
To get the thickness of these liners, take wedges made of iron, wood, or lead, and insert the thin end between the faces of the bars and those of the supports, forcing the wedges in sufficiently to leave a mark upon them. By chalking the faces of the wedges they will exhibit the marks more plainly. The wedges should be inserted at each end and on both sides of the bar, for one bar at a time, the liners being got out a trifle too thick so as to allow some for fitting.
If the liners require to be very thin and are difficult to hold in the vice without springing, take a piece of soft wood faced true, and grip it in the vice, and fasten the liner on it by means of brads driven in around the edge of the liner.
When the four liners are ready place them in position between the bars and their seatings. Bolt the bars firmly in position, wipe them clean and test them lengthwise with the spirit-level to ascertain if they are parallel with the cylinder bore, and place the level across the bars at each end to test parallelism with the engine shaft, as in [Fig. 2518], and, having noted where further adjustment is necessary, put marking upon the bars and move the cross head back and forth to ascertain how much the respective liners require reducing. If the gland is a fit upon the piston rod and in the stuffing box, moving the gland in and out of the stuffing box will be an admirable test of the guide-bar adjustment.