« НазадПродовжити »
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 valve 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
Walschaert gear and the Stephenson link motion so generally used, until recently, in America?
A.-The difference is in the manner in which the initial source of motion from the turning of the drivingwheels is secured, in the way the gear is reversed to change the direction in which the engine shall run, and the method of securing lead.
Q. 5.—Do not both the Stephenson ard Walschaert gears secure their initial motion through eccentrics actuated by the turning of the main pair of drivingwheels?
A.-Yes; the Stephenson link is given its motion by two eccentrics keyed upon the main-axle, one of which is thrown into gear to so actuate the valve that the engine will run forward, while throwing the other eccentric into gear will cause the engine to run backward. In the Walschaert motion the link receives the main, or initial, valve-actuating power through a single eccentric, not on the axle but in the form of a “return crank” fixed upon the main crank-pin, and this eccentric is in full gear and works the link at its full throw always, and with the engine running in either direction.
Q. 6.-How is the Walschaert motion reversed ?
A.-The position of the radius rod that directly actuates the valve-stem is shifted from one to the other end of the link to cause reversion; the radius rod is
raised or lowered for this purpose by the regular reversing gear, the link being suspended from a bracket by a fulcrum pin, or trunnion, at its exact centre.
Q. 7.—What is the position of the valve in relation to the piston?
A.-Theoretically, the valve is always one-fourth of a complete stroke, or cycle of motion, ahead of the piston, when the engine is running in either direction.
Q. 8.-What is meant by Lead?
A.-If the valve was exactly one-fourth of a cycle of motion in advance of the piston, the eccentric that actuates it would necessarily be placed at a point on the axle just 90 degrees ahead of the main crank-pin in respect to the direction in which that eccentric should cause the engine to run, but, if so placed, when the piston was at the beginning of its stroke—at one end of the cylinder-the valve would be exactly centred on its seat, with both ports, or steam passages, to the cylinder covered and closed. In actual practice the valve is advanced slightly, but far enough, in the direction of its required travel, to open, by a very short distance, the admission port at the piston end of the cylinder; this gives a pre-admission of steam against the piston just before its stroke has been completed, and has the double effect of cushioning the piston and hastening the full opening of the steam
admission port. This preliminary port opening to the cylinder is termed the Lead.
Q. 9.—What further effects are produced in advancing the valve to create the lead opening, that cannot be regarded as beneficial?
A.—The advance of the valve causes an earlier closing of the exhaust of the used steam from the cylinder, thereby creating higher compression ahead of the piston-between the piston and the cylinder head toward which it is travelling; the cut-off of live steam that is being admitted to the cylinder occurs sooner during the piston's stroke, and this detracts from the turning power of the main crank when the engine is working in full gear with a heavy load; and, as giving lead hastens all of the valve events, the steam that is driving the piston is retained in the cylinder during a shorter part of its stroke, and this earlier steam release is a loss of a certain per cent of the steam's expansive force.
Q. 10.—After the cut-off of the exhaust has occurred, will not the compression between the piston and cylinder head provide the cushioning effect that may be necessary as the working parts approach the reversing point in their motion?
A.-Yes, the very slight amount of compressible resistance that is deemed necessary may be so obtained, by a suitable design of valve motion.
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 leverand the valve-stem connected intermediately.
Q. 13.-Does the lead, as derived by the Stephenson method, remain constant?