In equalizing the valve-motion, and overcoming the discrepancy of steam admission, due to the angularity of the connecting rod by moving the link-hanger stud away from the center of the slot, a new distortion is introduced. The link-block being securely fastened to the bottom of the rocker-pin, moves in the fixed arc traversed by that pin, which is nearly horizontal. The action of the eccentric-rods on the link, on the other hand, forces the latter to move with a sort of vertical motion at certain parts of the stroke, making it slip on the block. Moving the hanger-stud back tends to increase this slip, which will become excessive enough to seriously impair the efficiency of the motion if not kept within bounds by the designer. Where the slip is very great, the motion will not be serviceable, a consideration which can never be overlooked; for the block will wear rapidly, producing lost motion, a very undesirable defect about any part of a link-gear. With the long rods which prevail in locomotive practice, designers have no difficulty in keeping the slip within practical bounds; but with marine engines it is sometimes necessary to sacrifice equality of steam admission to the reduction of the slip. The greatest amount of slip is in full gear, and it diminishes as the link-block is moved towards the center.
Placing the eccentric-rod pins back of the link-arc, as is almost universally done in this country, has a tendency to make the link slip on the block; and care has to be taken not to locate these pins farther back than is actually necessary for other requirements of the link-motion’s adjustment. Auchincloss, who is a recognized authority for designing of link-motion, gives four varieties of alterations capable of reducing the slip when it is found too great for a practicable motion. His resorts are, either to increase the angular advance, reduce the travel, increase the length of link, or shorten the eccentric-rods. One, or a combination, of these methods may be adopted, as the designer finds most convenient.
RADIUS OF LINK.
Among the constructing engineers who plan link-motion, there is considerable diversity of opinion about what radius of link helps to produce the best valve-motion. The distance between the center of axle and center of lower rocker-pin may be accepted as approximately correct, although some designers slightly increase beyond these points. On the other hand, the locomotives sent out from a leading building establishment have the radius of link drawn ¾ inch per foot short of the distance between the axle and rocker; and the claim has been made, that the arrangement produces an excellent motion.
A committee of the American Master Mechanics’ Association have placed themselves on record on this subject by asserting that the distance between the centers of axle and rocker-pin is the proper radius for the link. That same committee recommended that the link-motion should be planned to give as long a link-radius as possible, subject to the first-mentioned conditions.
It must be noted that the middle of the link-slot is the radius arc. I knew of a case where the links for an altered locomotive were finished out of the true radius through the edge of the slot being taken as the radius-curve.
INCREASE OF LEAD.
Most of the men who are at all familiar with the valve-motion are aware of the fact, that, with the shifting link, the lead increases as the link is notched towards the center. Where the valve has 1/16 inch lead in full gear, it is no unusual thing to find it increase to ⅜ inch lead opening at mid gear. The phenomenon is better known than its cause is understood.
