Fig. 2527.
If, instead of having a Corliss frame and fixed guide bars, the engine has a flat bed and adjustable guide bars, as shown in [Fig. 2527], the operation is as follows:—
In setting up a new engine it is obvious that if the flanges of the cylinder are planed parallel with its bore and at the proper distance from its axial line, and the pillow block is made of the proper height, a line stretched axially true with the cylinder bore will pass through the centre of bore of pillow-block brasses, or be equal in height from the engine bed; but the length of the cylinder being only about one-fifth of the distance from the cylinder to the centre of pillow block, any error in the planing of the cylinder flange true to the cylinder bore becomes magnified five times at the pillow block; hence it is necessary to stretch a line through the cylinder bore and set the cylinder so that the line, being axially true with its bore, will pass the pillow block at the centre of the bore of its brasses. This is sometimes done by inserting thin pieces of sheet tin, metal, or even paper beneath the cylinder flanges and the bed, and in the requisite positions. The method of stretching the line is shown in [Fig. 2527]. f is a device for holding the line at that end. It consists of a frame in the form of a cross, with adjusting screws at the end of each arm, and a small hole at its centre to receive the line. The other end of the line a must be secured, under as much tension as the line will safely bear, to a piece of wood clamped to the engine frame at r. The adjustment of the line is made by measuring its distance from the walls of the bore of the cylinder at one end and of the bore of the gland hole at the other end, using a pair of inside calipers or a wire gauge. The latter should be bent in its length to admit of adjusting the same by straightening to increase, or still further bending to diminish, its length to suit the requirements.
The wire, when applied, should only just meet or touch the line and not bear the least hard, or it will spring the line, causing an error of adjustment that will be serious when multiplied by the length of the line to the pillow block as compared with the length of the cylinder bore.
If the pillow block is planed on its bottom face and has its brasses fitted, the latter may be marked off for boring from the line a, [Fig. 2527], when stretched to set the cylinder, thus avoiding a second adjustment of the line a a.
Suppose now that it is required to line the brasses in the pillow blocks true to be bored (the pillow blocks being bolted in position). The distance of the face p, of the brass from the stretched line a, in [Fig. 2527], must equal the distance from the centre of the length of the crank-pin journal, to the face of the large crank hub, and this distance may be shown by a line marked on the edge of the brass flange.
Place a straight-edge c, [Fig. 2527], having a line d parallel with its edge e, so that this line will be in the centre of the width of the pillow-block jaws, and at a right angle to the line a. The line d will then represent the axial line of the crank shaft, and may be used as the centre from which to mark the lines on the brasses used to set them by for boring. To test if a and d are at a right angle, or to set d to a, a large square should be used. If the side face p p of the pillow block stands parallel to a, as it should do when it is true, it will serve to chuck the pillow block by, thus boring the brasses in their places in the pillow block, with the centre line of the bore at a right angle to p. If otherwise, two flat places should be filed on the brasses, as shown in [Fig. 2528], in which c is the straight-edge, and a the stretched line as before, h and i representing the flat places whose distance from a, as shown at j j, may be made to represent the thickness of the crank from its large hub face to the centre of length of crank-pin journal; hence the depth of the flat places will show how much to take off the face of the brass to leave it of the proper thickness.
