Fig. 2523.
The cord or line is set true to the cylinder bore, and coincident with the centre of the pillow block, as at a a in [Fig. 2521], and the two bottom bars are put up in line horizontally with the axial line of the crank shaft, and at a distance below the stretched line equal to one-half the height of the guides for the cross head, as in [Fig. 2522], in which a represents the stretched line, b, b the bottom bars, c c a straight-edge, and d a piece of wire whose length from point to point is equal to one-half the height or thickness of the guide blocks. The width apart of the bars is set to suit the width apart of the flanges of the guide blocks on the cross head, by means of a square. The square is applied in the following manner: On a straight-edge mark two lines a and d, [Fig. 2523], a distance apart equal to the distance between the flange edges of the cross-head guides. Midway between a and d mark the line c; place the straight-edge across the bars as shown, and when the edge of a square, placed on the straight-edge, coincides with c and the stretched line, and the marks a and d coincide with the edges of the bars, the latter are set true, and will come right for distance apart, and distance from the centre line, supposing the flange edges of the cross-head guides to be equidistant from the centre of the length of the cross-head journal. If, however, such is not the case, the width from a to c and from c to d must be made to suit, c representing the centre of the length of the cross head journal, d the flange on one guide and a the flange on the other guide. Here it may also be remarked that, if the thicknesses of the cross-head guides vary, or if they are not central to the axial line of the cross-head journal, the bars must be set for distance from a in [Fig. 2523], to suit the error, because in that figure the straight-edge is supposed to stand parallel to the axial line of the shaft, as it is also in [Fig. 2522], the aim in both cases being to so set the bars that the cross-head journal shall stand parallel with the crank shaft.
It is the liability of variation in the thickness of the guide blocks, and of their not being central to the cross-head journal, that constitutes the disadvantage of setting the bars by lines, it being obvious that the bars must be either set to suit any errors in the guides, or those errors must be eliminated before setting the bars. The top bars must be set parallel to the bottom ones, at a distance from the bottom ones equal to the thickness of the guide blocks, and parallel to one another. It is preferable to set the top ones with the cross head and guides in place, observing all the precautions as to springing them given in the case of the bottom bars.
Fig. 2524.
The bars thus set will be in line with the crank axle, but unless the piston accurately fits the cylinder bore, they will not long remain in line with the line of motion of the piston rod. For example, [Fig. 2524] shows a piston head too small for the cylinder bore, the guides fitting the bars properly, and the gland and stuffing box fitting the piston rod; the piston will be suspended in the cylinder, its overhanging weight being supported by the guides b, the gland, and packing ring. This would cause friction and rapid wear of the gland bore and guide surfaces in a direction parallel to the line c, which would gradually let the piston fall to the bottom of the cylinder bore, touching at the end of d first. In some engines provision is made to adjust the piston to take up its wear, which is, it will be seen, a great advantage.
The Heating and Pounding, or Knocking, of Engines.—The heating of any part of an engine occurs from one of two causes, viz., either the fit of the parts is too close, inducing undue friction, or the parts are not in line.
When the former is the cause the remedy is to ease the fit. If the parts are not in line, the heating may also be remedied by loosening the fit of the parts; but this will often induce a pound or knock, hence the true remedy is to properly align the parts.