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above the centre line of the axle; and the erratic actions so introduced cannot be overcome. When this connection from eccentric rod to link is normally located, the rise and fall of the engine caused by the alternate yielding and exertion of the driving-wheel springs has no material effect on the motion delivered to the valve, but when that point of connection is raised above the normal, the interference to correct action is in proportion to the distance the link-foot pin is placed above the centre line of the axle. And if we imagine a case in which this connection point is raised to a position perpendicular to the axle centre, it is plain that while the engine is running and rolling from side to side over uneven track, for each inch of rise or fall of the link support there will be a full inch in the deflection of the connecting point between the eccentric rod and link extension, and this deflection will be in the direction of motion, causing alternate quickening and halting in the valve's action.
Fig. 37 represents the half-way between the case of extreme link elevation just considered and the technically correct position of the link in which the link-foot pin is on the same level with the centre of the axle; and while the former design would be practically impossible in locomotive practice it would work very well in stationary service. By reference to Fig. 37, and bearing in mind that the distance between the link fulcrum and axle centre is not invariable but greater or less when the engine is running, it can be understood why the link-foot pin should be as close to its theoretically established location as possible; also, why the eccentric in actual practice is not usually exactly 90 degrees from the main pin.
The folder diagrams, Figs. 35 and 36, show that by being properly designed the Walschaert gear in connection with piston valves of inside admission will produce valve events corresponding closely to those obtained from the use of the D-slide valve and outside admission. It will be noted that the action is practically identical in both cases, the steam admission taking place at the point 7, and the exhaust closure occurring and compression beginning at the point 8, with the main-pin at the same point in its revolution in each
ADVANTAGES OF WALSCHAERT VALVE
And Arguments for its Use as Against the Stephenson
ADVANTAGES OF WALSCHAERT VALVE GEAR
And Arguments for its Use as against the Stephenson
(1) Accessibility. There is not room enough for the Stephenson gear under a very large passenger or freight locomotive. The eccentrics are crowded, and proper inspection, not to speak of proper care, is difficult, except over a pit. Valve gear to be properly maintained must be accessible for inspection and lubrication. The accessibility of Walschaert gear should reduce engine failures.
(2) Weight. A saving of 1,745 pounds is possible by using the Walschaert gear, in the case of a very heavy passenger engine. The Stephenson gear, weighing as much as two tons, is far too heavy to be satisfactorily reversed twice in every revolution on fast running locomotives.
(3) Directness. Walschaert gear transmits the moving force to the valve in very nearly straight lines, avoiding the springing and yielding of the rocker arms, rocker shafts, and transmission bars, which