INFLUENCE OF ECCENTRIC THROW ON THE VALVE.
As reducing the travel of the valve diminishes the port opening, a point is reached in cutting off early in the stroke where the port opening is hardly any more than the port opening due to the lead. This is what makes long steam-ports essential for a successful high-speed locomotive. The best-designed engines give an exceedingly limited port opening at short cut-offs, and badly planned motion sometimes seriously detracts from the efficiency of the engine, by curtailing the opening at the point where a very brief time is given for the admission of steam. The magnitude of the eccentric throw exerts a direct influence on the port opening when cutting off early. A long throw tends to increase the opening, while a short throw reduces it. The long-throw eccentric will draw the valve farther away from the edge of the steam-port, when admitting steam for the same point of cut-off, than a short-throw eccentric will move its valve. For an ordinary 17 × 24 inch locomotive, the throw of eccentric should not be less than five inches, unless the engine is intended entirely for slow running. There are many engines running with eccentric throw less than five inches, but they are invariably slow unless the valve-lap is very short. With an ordinary lap, an engine having an eccentric throw of 4½ inches needs so much angular advance to overcome the lap, and provide lead, that the rectilineal motion of the eccentric is very meager at the beginning of the stroke. That is, the center of the eccentric is traveling downward in its circular path, which gives little motion to the valve, just as the crank gives decreased motion to the cross-head when near the centers.
HARMONY OF WORKING-PARTS.
Hitherto we have regarded the link as merely performing the functions of transmitting the motion of the eccentrics to the valves, with the additional capability of reducing the travel at the will of the engineer. Otherwise, the motion of the link is intensely complex; and its movements are susceptible to a multitude of influences, which improve or disturb its action on the valve. A good valve-motion is planned according to certain dimensions of all the working-parts; and any change in their arrangement will almost invariably entail irregularities upon the link’s movement, which will radically affect the distribution of steam. A link-motion schemed for an eccentric throw of 4½ inches will not work properly if the throw be increased to five inches: a link with a radius of 57 inches can not be changed with impunity for one of 60 inches. Any change in the position of the tumbling-shaft or rocker-arms distorts the whole motion, and any alteration in the length of the rods or hangers has a similar effect. That the link may perform its functions properly, all its connections must remain in harmony.
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.