Fig. 2561.
If two eccentrics are used, as in a link motion, the lines for setting one eccentric are equally applicable to both; the lap and lead line f being located on the crank-pin side of e when there is a rock shaft, as is supposed to be the case in [Fig. 2561]; and on the other side of e when there is no rock shaft; and in this case the eccentric that is to operate the valve to make the engine run forward must have its throw-line following the crank pin, as at j, in [Fig. 2561]; the eccentric k operating the valve for running backward. Conversely, in the absence of a rock shaft, the throw-line of the forward eccentric leads, while that of the backward eccentric follows the crank pin.
When the line of connection of the eccentric rod is not parallel to the axial line of the cylinder bore, the crank must be placed horizontally level (or if it be a vertical engine, on the dead centre), but instead of the straight-edge being placed parallel to the throw-line of the crank, it must be placed at a right angle to the line of connection of the eccentric rod.
Fig. 2562.
Thus in [Fig. 2562] the engine is supposed to be a vertical one, and the crank is, therefore, placed on its dead centre, its throw-line being vertical instead of horizontal as in our previous examples (which were supposed to be for a horizontal engine). It is also supposed to have a rock shaft a; hence the straight-edge is set at a right angle to the line of connection of the eccentric rod which is denoted by b.
It is obvious that to set the crank throw-line vertical the circle b in [Fig. 2509] may be used, the spirit-level being resorted to to discover when the crank stands vertical.
