Fig. 3331.
In [Fig. 3331] the link motion is shown in mid gear, in which position the engine is at its least power, the cut off occurring at its earliest point, and in [Fig. 3332] it is shown in full gear for the backward motion.
Fig. 3332.
Referring to [Fig. 3329] for the full gear forward, the reversing gear proper consists of the reversing lever, the segment, the reach rod, the tumbling shaft, and its counterbalance rod and spring; while the link motion proper consists of the eccentrics and their rods, the link, the link block or die, the suspension link s, the rock shaft and the rod p p. These, however, are terms applied for shop purposes, so as to distribute the work in sections to different men, it being obvious that a complete link motion includes the reversing gear, the eccentrics, the link and its block, the rock shaft, the rod p p, and the valve and its spindle or stem. This mechanism, as a whole, may also be called, and is sometimes called, the valve gear, because it is the mechanism or gear that operates the slide valve.
The link motion may be moved from full gear forward to full gear backward or to any intermediate position, whether the engine is running or at rest, but is, when the engine is running, harder to move from full gear forward toward back gear, and easier to move from full gear backward toward mid and forward gears, which occurs because of the friction of the eccentrics in the straps, and it follows that this will be the case to a greater extent in proportion as the revolutions of the eccentrics are increased.
If in a properly constructed link motion we move the link from full gear forward to mid gear when the engine is standing still, and watch the valve, we shall find that the lead or opening at f gradually increases; and if we then move it from mid gear to full gear backward, the lead will gradually decrease and finally become the same as it was in full gear forward. The construction of the parts is as follows:
Referring to [Fig. 3329] (full gear forward), the segment is fixed in position and the reversing lever is pivoted at its lower end. r r is a bell crank, which is pivoted to the reversing lever and to which the latch rod is pivoted at its upper end. The spring acts on the end of r r, and thus forces the tongue of the latch into the notches on the sector as soon as the tongue comes fair with the notch and r r is released from the hand pressure. As the reversing lever is moved over from full gear forward, the reach rod moves the tumbling shaft, whose lower arm i (through the medium of the suspension link s) lifts the link and brings the centre of the saddle pin nearer to the centre of the pin in the link block, which reduces the amount of motion given to the lower arm (b, [Fig. 3331]) of the rock shaft, and therefore reduces the amount of valve travel, thus causing the point of cut-off to occur earlier in the piston stroke.