ADJUSTMENT OF LINK.
A very important feature of the link is its property of adjustability, which serves to neutralize the distorting effect of the connecting rod’s angularity. As has already been explained, the angularity of the main rod tends to delay the cut-off during the backward stroke, while it is accelerated in the forward stroke. With the ordinary length of connections, this irregularity would seriously affect the working of the engine. But it is almost entirely overcome by the link, which can be suspended in a way that will produce equality for the period of admission and point of cut-off for both strokes in one gear. Perfect equalization of admission and cut-off for both gears has been found impossible with the link-motion; and designers are generally satisfied to adjust the forward motion, and permit the back motion to remain untrue. The point about the link which exercises the most potent influence on adjusting the cut-off, is the position of the saddle, or of its stud for connecting the hanger. This stud is called the point of suspension. Raising the saddle away from the center of the link will effect adjustment of steam admission; but in locomotive practice the saddle is nearly always located in the middle of the link, there being practical objections against raising it. Equalization of steam distribution is produced by placing the hanger-stud or point of suspension some distance back of the center line of the link-slot, the distance varying from ⅛ inch to ⅞ inch.
Moving the hanger-stud affects the link’s movement in a way that is equivalent to temporarily lengthening the eccentric-rod during a portion of the piston-stroke. The length of the tumbling-shaft arms, the length of hanger, the location of the rockers and tumbling-shaft, the radius of link, and length of rods, all exercise influence on the accurate adjustment of the valve-motion.