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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 case.
THIRD DIVISION ADVANTAGES OF WALSCHAERT VALVE
GEAR 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
cannot be avoided in these parts of the Stephenson motion, even if they are made very heavy.
(4) Permanence of Adjustment. The advantage of permanence of adjustment lies with the valve gear which has no large eccentrics. This is proved by the statement of the Superintendent of Motive Power of one of the great trunk lines—a comparative statement covering the performances and condition of the valve gear of engines that differ only in having Walschaert vs. Stephenson gears. This statement appears in this book later on. All connections in the Walschaert gear are made with pins and bushings, which are designed specially to resist wear.
(5) Wear. Large eccentrics, besides occupying too large space, wear unevenly, and lubrication is difficult. with the high surface velocities of the largest sizes. With hardened pins and hardened bushings the Walschaert gear has an important advantage in maintenance.
(6) Smooth O peration. Stephenson links, under the influence of two eccentrics, move through wide angles, resulting in a wedging action of the link-block, which strains the gear when working hard, and produces lost motion. Walschaert links oscillate through smaller angles, producing less lost motion. The effect of this angularity of the links is plainly discernible on the testing plant.