« НазадПродовжити »
causes to be exerted when the engine is on a dead centre absorbs, or nullifies, much of the energy furnished by the other source of power at its zenith-the engine on the other side, with its admission port wide open and the relation of crank-pin to driving-axle giving the maximum leverage.
If the pre-admission of steam to the cylinder, that is referred to as lead, is finally conceded as necessary only for cushioning the piston at the completion of its stroke, any argument for it is now clearly fallacious, for the reason that the exhaust may be cut off at any point toward the finish of the piston's stroke and the resultant compression will produce the cushioning effect just as well—and more economically,—as evidenced by the quoted authority, than by means of any pre-normal opening of the admission port.
Motive-power officials often try to induce engineers to run their engines with full open throttle and reverse lever hooked up to the limit, claiming that an engine would run faster and work more economically that way. That method has been tried, fairly, and also the other one of using a lighter throttle and a little longer travel of the valves, and invariably the latter way gave the best results, both in speed and fuel economy. It is not imagination, and might be attributed to the fact that the lower down the link is worked the less the lead, with the Stephenson motion, and the
less "friction on the bearings of the rods and crankpins” there will be. Of course, it may be said in answer, that there is but a slight increase in the amount of lead from the point where the engineer worked the lever up to the point at which the master mechanic wanted it worked; yet there was a difference, and an otherwise unexplained variance in the performances of the engine-favorable to the position of the reverse lever in which it is claimed by some that the steam is “wire-drawn.”
The simplest test to prove the superiority of the Walschaert valve gear, and the easiest to make, yet most convincing to the practical mind, is, where there are engines of the same general type except that some are equipped with the Stephenson link and others have the Walschaert gear, to note the boiler pressure required to move each of these engines from its stall in the roundhouse, with its cylinders “cold.” The test with big “battleship” engines has shown that eighty-five pounds on the steam gauge were required to get the engine with the old link motion on the turntable, while the engine having the Walschaert valve gear was moved out, easily, with the gauge showing thirty-five pounds of steam.
QUESTIONS AND ANSWERS
Question 1.—What is meant by “Valve Motion”?
Answer.-Valve motion, or valve gear, refers to the system of rods, levers, etc., that transmits motion to the main valye that is used to admit steam to, and exhaust it from, the cylinder of an engine; it is the action of the steam in the cylinder that empowers the piston.
Q. 2.-From where does the valve gear receive its initial motion?
A.—The valve gear receives its motion from some point, or points, in the machinery that is actuated by the piston.
Q. 3.-What kind, or style, of valve is used in connection with the Walschaert gear?
A.-Any valve that is used with any other form of valve gear. Strictly speaking, the valve is not a part of the valve gear.
Q. 4.-What is the difference, then, between the