Fig. 2528.
A straight-edge placed across these flat places, or true to the lines h i, must stand at a right angle to the line d, so that by setting the brasses by the flat places they will be bored to stand at a right angle to a. To set the brasses the other way a circle is struck from d, as a centre, upon the faces of the brasses as in the end view, [Fig. 2528], in which the straight-edge c is shown wedged in the bore of the brasses, which is the most convenient way when it can be done.
The line d is carried down on the end face of the straight-edge, and the latter is used as a support for the compass points while striking the circle m, which is defined more clearly by indenting it with fine centre-punch marks. The height of the centre for bore of brasses may be carried from the centre line of the cylinder a a to the end of the straight-edge c, by placing another straight-edge across the engine bed and measuring from the end of c to a.
Suppose now that the brasses are bored, and the position of the pillow block is to be set, and the process is the same, the line d being marked true from the bore of the brasses, and the pillow blocks adjusted until d is at a right angle to line a a.
Though in a new engine every part may be made as true as possible in the details of manufacture, yet when the parts come to be put together errors of alignment will generally be found to exist. These errors may be too minute for discovery in the separate piece, and yet form important defects in the finished engine.
In rough practice these defects are left to remove themselves by abrasion and wear, the process being to allow the parts to be somewhat loose (wherever possible) in their adjustment, and adjust them closer as the abrasion proceeds.
This is termed letting the parts wear down to a bearing. But the very process of wearing down to a bearing attests that the parts have not been properly fitted to a bearing, whereas to attain the best possible results the parts should be fitted to a bearing, because in wearing down to a bearing, undue abrasion, and to some extent or in some degree, roughness of the wearing surfaces, must ensue, because the strain intended to be distributed over the whole intended bearing area is limited to the actual bearing area. It is necessary, therefore, that, in putting an engine together, each part be properly fitted to its place, and that it be subsequently adjusted in its fit and position with relation to the other parts to which it is connected.
The fitting of the single piece is a test of its individual or disconnected truth; the subsequent or second adjustment is a test of its truth with relation to the others. Thus a pair of brasses may fit a journal perfectly, but that is no assurance that the brasses are so bored as to bring the rod holding them in proper line to enable connection at the other end without springing or bending the rod.
Furthermore, it often happens that the frame work of an engine does not form a base for the whole of the parts, thus in a large stationary engine, the end of the main shaft or crank shaft farthest from the crank (generally called the outboard bearing) is generally supported by a bearing having an independent foundation, and as this foundation does not exist until the engine comes to be permanently fixed for operation, its alignment must be performed when setting the engine. In an old engine this foundation may settle, or the wear itself may throw the engine out of line, so that the lining of an engine becomes periodically a necessity.
As a general rule a want of alignment induced by wear or incurred from repairs to the parts principally affects the main shaft, the cross head remaining more nearly true; and, with the exception of the crank pin, the same holds good with reference to a new engine.