3rd. It may, when viewed from the crank-pin end of the engine on about a horizontal position, be too high or too low at the crank-pin end only.
4th. It may be too high or too low at the outboard end only.
5th. It may be too high or too low at both ends, although parallel to the cross-head journals.
It will be found on consideration that with the exception of the last-named case, the connecting rod forms the best test whereby to discover an error in any of these directions, because it magnifies the error and makes it more plainly discernible. It will further be found upon careful observation, that although a combination of these errors may exist, the connecting rod will serve to discover each error separately, as well as the collective error, because, although in some respects two distinct errors may have the same general result, yet the result will be different if taken in detail, and it follows, therefore, that the testing must be taken or made in detail first.
Fig. 2529.
To test the parallelism of the axial line of the crank shaft with that of the cross-head journal, when viewed vertically: In [Fig. 2529], let a a represent a line true axially with the bore of the cylinder, and b b a line at a right angle to a a, and passing through the centre of the pillow block or bearing spaces. If the engine were in line, b b would be coincident with the axial line of the crank. Suppose, however, that line b c represents the actual centre line of the crank, not then being at right angles to a, the end e of the connecting rod, if connected to the crank pin as shown, and made a good working fit so that there is no play of the pin in the brasses, will not come fair laterally with the bearing in the cross head. The amount of the error is the amount it is out of true in the length of the crank-pin journal, multiplied by the product of the length of the connecting rod (from centre to centre of the bores of the brasses) divided by the length of the crank-pin journal. It is apparent, however, that if the crank shaft be set to have its axial line at b b instead of at b c, the error at e d will be corrected, and thus we may employ the connecting rod to set the crank shaft in line.
It is, however, not sufficient to try the crank on one dead centre only (as will be seen presently), hence we place it on the other, and move the cross head to the other end of its stroke, and again try the end e of the connecting rod with the cross-head journal, and if it falls to one side, and on the same side as before, but to a less amount, it demonstrates that the axial line of the crank forms with the line a a an acute angle. If, however, instead of falling too much laterally towards the side f of the cross head, it fell too much towards d, but more so when tried with the crank on the dead centre nearest to the cylinder than when tried with the crank on the other dead centre, then it is proof that the axial line of the crank shaft forms with a a an obtuse angle.