Fig. 3382a.
This is shown in [Fig. 3382a], in which it is seen that the weights a are adjustable along the arms a, a. The points of attachment d, d of the springs to the weight arms are also adjustable.
When reversing is done, by shifting the eccentric across the shaft, the lead cannot be kept equal, but will, if the eccentric is swung from a pivot that is on the line of centres, when the crank is on a dead centre, be greater at the head end than at the crank end of the cylinder. The discrepancy may, however, be equalized by swinging the eccentric from a pivot that is not on the line of centres at a time the crank is on a dead centre.
But this equalization will only exist at some one point in the eccentric position, or in other words, if the eccentric is shifted across the crank shaft, simply to reverse the engine, and not to vary the point of cut-off, it will naturally be moved, in reversing the engine across the shaft, to a given and constant amount, and in this case, the pivot on which its hanger is hung may be so located with reference to the line of centres and the crank (the latter being on a dead centre when the point of suspension of the eccentric hanger is found) that the lead is equal for both the backward and forward gears.
But if the eccentric is shifted across the shaft to vary the point of cut off as well as to reverse the direction of engine revolution, the lead cannot be kept equal.
It is better, in this case, to so locate the point of eccentric hanger suspension as to let the lead be the most at the head end cylinder port, because the piston travels fastest at that end of the cylinder, and therefore requires more lead, in order to cushion the piston.
Fig. 3383.
A construction for shifting the eccentric across the shaft is shown in [Fig. 3383], in which d, d is a disc, having at b a pivot for the eccentric hanger. The amount the throw line of the eccentric must be shifted to reverse from full gear forwards to full gear backwards is from the line b x to line b x′, and the shifting is done by two racks f and j, having teeth at an angle of 45° to their lengths. f is fast to the eccentric, and j is carried in a sleeve that slides along the shaft, and sliding it moves the eccentric across the shaft by reason of the teeth of one rack being at a right angle to those of the other.