It is obvious that instead of using plumb lines a square may be employed to mark the lines c d, and to set the eccentric to the lines g h, the square being applied as at s and s′, in [Fig. 2558].
Fig. 2559.
Fig. 2560.
In this example it has been assumed that the direction of crank rotation was to be as denoted by the arrow; but, suppose the crank rotation required to be in the opposite direction, then the marks on the straight-edge would require to be located precisely the same, but the position of the eccentric throw-line would require to be as in [Fig. 2559], the perimeter of the eccentric being set to the lines g h as before. The eccentric rod being supposed to connect direct to the valve spindle, without the intervention of a rock shaft, for if there is no rock shaft the eccentric leads in the direction of rotation, while if the engine has a rock shaft the eccentric follows the crank-pin in the direction of rotation, and f must be marked on the crank-pin side of e, as in [Fig. 2560].
