« НазадПродовжити »
When this motion is used with the Allfree cylinders a separate and independent valve rod is attached to the combination lever to drive the compression valve; this compression-valve rod P is shown in Fig. 45. And where the Allfree cylinders are used with the Walschaert gear, the compression-valve rod is connected with the combination lever in similar manner.
As will be noticed in Figs. 41 and 45, the connection of eccentric rod with transmission bar is on the centre line of motion, and the movement of the engine on its springs can have no effect on the cut-off. And there is no resetting of the valves after the motion has been erected, unless the engine should have its frame and motion work bent in a wreck; and in which contingency, if repairs are made with regards to blueprints, there will be no “valve setting” except to equalize the travel of the valve, possibly, by correcting the length of the valve stem. In fact, "once set, always square' can be quoted with reference to any of the radial valvegears that are taking the place of the old, doubleeccentric link motion on our modern American locomotives.
A valve-motion report is shown in Fig. 47 which gives the complete valve events in both forward and backward motions.
QUESTIONS AND ANSWERS ON
With the Hobart-Allfree Gear
Question 1.-If the main-valve rod should break, how can the engine be put in condition to work on the other side alone?
Answer.—Clamps for main-valve stem and compression-valve stem should be carried on every engine. In this case, remove the parts of the main-valve rod, set the valve centrally to cover all ports, and clamp the stem. Figs. 43 and 44 show two plugged holes in the steam-chest, one near each end of the cylinder, that lead to either admission port; remove both plugs and oil the cylinder through the holes. Proceed, but give the cylinder a little more oil every now and then, to keep the piston from cutting the walls of the cylinder.
Q. 2.-If the main-valve rod and the compressionvalve rod both fail, how should one proceed to remedy the trouble?
A.–After removing all the pieces of both rods, proceed the same as in answer to Question 1.
Q. 3. Suppose the compression-valve rod only should become broken, what then?
A.—The compression valve can be put out of service without disabling the engine on that side, though there will be a certain loss of power there and a limit to the engine's speed. As to repair, it will only be necessary to centre the compression valve and clamp its stem, after removing the broken pieces. There will be a blow while each admission port is getting steam, due to the opened “exhaust clearance” at both ends of the compression valve when clamped in the central position. This exhaust opening amounts to about oneeighth of an inch.
Q. 4.-If the reverse link or reverse arm should break, what should be done?
A.-Move the radial block to the position on the radial guide in which the valve will give the longest cut-off that will be demanded during the balance of the trip, and block it there securely; remove the broken parts and go on, carrying the reverse lever in the notch that will cause an equal cut-off on the uninjured side
of the engine.
Q. 5. Suppose that the reach rod from the reverse lever, or the reach-rod arm, or the reverse shaft should break, what then?
A.-If the reach rod or reach-rod arm should break, secure the radial block in the proper working position as previously explained, on one side only. If the reverse shaft were broken, however, the radial
block on each side should be blocked in the proper position.
Q. 6.- In case of eccentric rod or eccentric crank failure, how can you overcome the trouble?
A.-Remove the broken parts, and if it is the crank that is broken take down the eccentric rod; push the transmission bar into a strictly vertical position and tie it there from each end of the radial guide, and then proceed on your way. There will not be a great deal of strain on the lashings that secure the transmission bar, and by this method the valve will have its lap-andlead movement derived from the crosshead, through the combination lever; the admission port will be slightly open when the piston is at the beginning of either stroke, thus admitting of oil from the lubricator reaching the cylinder; also relieving compression.
Q. 7.—If the transmission bar fails, what must be done?
A.-If the short, lower portion should break between the two connection pins, arrange the same as explained in answer to the previous question; but if the long, main portion is broken, remove the detached piece and the eccentric rod, lash the remnant of the transmission bar and the radius bar so they will not swing in the way of anything else—or remove them—and block the bell crank with the vertical arm in a plumb perpendicular position (as shown in Fig. 45), by blocking
through, or lashing the bell-crank arms to the gear frame. Then move on, with the valve having the short lap-and-lead travel as heretofore explained. If the rocker arm E of the bell crank should break off, proceed in the same manner, after removing broken parts and the transmission bar and eccentric rod.
Q. 8.—If the bell crank should break, how do you fix things?
A.-If the upper, or rocker arm E of the bell crank is broken, get the answer in the preceding question. If the vertical arm fails, the main-valve rod and the compression-valve rod must be disconnected from the combination lever and tied up to clear other parts, and the combination lever and crosshead link must come down; then block the main and compression valves, and proceed on your way—on one side.
Q. 9.-How can you provide for getting along with a broken combination lever?
A.-About the only thing to do is to remove the crosshead link and both pieces of the combination lever, disconnecting the main-valve rod and compression-valve rod, of course, and tie their ends up to clear the lower arm of the bell crank; block both valves at the centre of their travel, and proceed on one side.
Q. 10.--How could you adapt the valve gear to a working condition when the crosshead link should be broken?