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Q. 11.-- How is the advance of the valve to create lead opening secured with the Stephenson link motion?
A.-By advancing both the “go-ahead” and the “back-up” eccentrics in the proper directions, on the axle.
Q. 12.-How is lead obtained with the Walschaert
A.-In the Walschaert valve gear lead is obtained from the straight-line motion of the piston, instead of from the circular motion of the axle with its errors incidental to the angularity of the conveying rods. Walschaert's vertical combination lever has its lower end connected to the crosshead and the upper end to the valve-stem where an outside admission valve is used; intermediately, the radius-rod connection with the combination lever forms its fulcrum, and the motion derived from the crosshead so modifies the motion imparted by the single eccentric, through the radius rod, as to produce the required lead in either forward or back gear. With the use of inside admission valves the points of connection of the radius rod and valve-stem to the combination lever are reversed, the radius rod being connected to the extreme upper end of the lever and the valve-stem connected intermediately.
Q. 13.—Does the lead, as derived by the Stephenson method, remain constant?
A.-No; each eccentric is separately adjusted, or advanced, to give the decided amount of lead when in full gear, but as the link is hooked up the lead automatically increases.
Q. 14.—Is this increase of lead to be desired?
Q. 15.—Does the lead vary with the Walschaert gear?
A.—No; with the Walschaert method lead, if present, remains unvaryingly the same at all points of the cut-off.
Q. 16.—Is the position of the valve advanced for any other purpose than that of securing lead?
A.—Yes. The steam edges of the valve are widened beyond the outside edges of the admission ports, and the first advance of the valve is for the purpose of overcoming the delay of port opening in consequence of this overlap. The distance the edges of the valve extend beyond the port openings is termed the lap, and the valve must be moved far enough to overcome the lap before the opening for lead can begin.
Q. 17.—Is the further advance of the valve to secure lead necessary?
A.—Whether lead is a necessity or not is a matter of argument, but it is being conceded by many expert men that lead, as a port opening, is not necessary, and by some it is considered a detriment.
Q. 18.-Explain the manner in which lead is derived through the action of the Walschaert combination lever.
A.-The Walschaert eccentric is set exactly 90 degrees, in effect, from the main crank-pin, and if the combination lever were not present the valve would always be a “quarter” from the piston, and would be centred on the seat with both admission ports covered when the piston was at either end of the cylinder. Now we introduce the combination lever: The connection of radius rod near, or at, the upper end of the combination lever represents its fulcrum; the lower end of the lever is connected with the crosshead which is now at one end of its travel in the guides. Now, if the valve-stem is connected to the combination lever above the fulcrum (radius-rod connection) it is plain that the angle of the short (upper) end of the combination lever will throw the valve, slightly, in a direction opposite to the position of the piston in the cylinder, and this is the correct movement to be given the valve with outside admission to secure the port opening for lead. If, however, the valve-stem is connected to the combination lever below the fulcrum, or radiusrod pin, the valve will be drawn in the direction of the piston's position in the cylinder by the angle of the short end of the combination lever, and this is the proper direction for the valve of inside admission to
be advanced in order to permit the pre-admission of steam against the piston, called lead.
Q. 19.-Suppose that an engine should be designed to use the Walschaert valve gear, and lead was not desired: That is, with the crank-pin on a dead centre and piston at an end of its stroke the admission port would not be open any,-steam edges of the valve and port line-and-line--are we to infer that the combination lever would not be necessary—that it could be dispensed with?
A.—Not at all. The combination lever must be included in the Walschaert gear on any engine and with any style of valve. As explained, an advance of the valve is necessary to move it the distance of the lap, so that the steam edge of the valve and the outside edge of the admission port will be line-and-line when the piston is at the start of its stroke and lead not required. The amount of advance of the valve derived from the action of the combination lever is equal to the distance of lap, or lap plus lead, according as the design of the engine calls for.
Q. 20.—Is there not a great deal of confusion as to the actual meaning of lead?
A.—Yes. The angular advance of the Stephenson eccentric upon the axle and the angular inclination of the Walschaert combination lever are often referred to as the lead, because it is known that they cause an
advanced motion of the valve, when, as just shown, an advance of the valve is highly necessary even although it gives no actual lead.
Q. 21.–Assuming an engine to be running forward, is the Walschaert eccentric placed a quarter ahead, or a quarter behind, the main crank-pin?
A.-This is a variable matter, depending upon the construction of the reversing gear; if, with the reverse lever in the forward gear the radius rod is worked by the lower end of the link, with outside admission valves the eccentric is placed a quarter ahead of the main-pin and with valves of inside admission the eccentric is just opposite, or a quarter behind the main crank-pin.
Q. 22.-Is the Walschaert valve gear of the type referred to as of " direct motion”?
A.—The Walschaert motion is either direct or indirect, according as to which direction the engine is running. When the radius rod is working below the centre, or fulcrum, of the link there is a single direction of motion from the eccentric to the valve and the motion is direct; but with the radius rod working above the centre of the link, the motion is indirect, for the reason that the link then acts as a double rocker-arm, and when the eccentric throws the lower end of the link in one direction the upper end of the link moves the valve the opposite way.
Q. 23.-Reversing the engine, then, changes the