« НазадПродовжити »
ufacturers direct particular attention to the squareness of the cut-off, in back-up gear as well as the go-ahead. And they claim the longest release for given cut-off and lap condition of any locomotive valve gear in practical use to-day.
Whenever certain persons who are familiar with the “Stephenson” link motion are brought into association with a modern, radial valve-gear their first inquiry usually is as to the directions for “valve setting”; they cannot get clear of the idea that every valve motion requires constant tinkering with “to keep it square.” All engines with radial gear should have adjustable valve stems, however. Before the gear is taken apart, tram the valve rod and the reach rod; put them back in the same place, test the lead and cut-off in one position of the reverse lever, equalizing the travel of the valve—if this should be necessary—by adjusting the valve stem, and the engine is ready for the road.
QUESTIONS AND ANSWERS ON BREAK-
ON THE ROAD
Question 1.-In case of breakdown with the engine disabled on one side, how may the valve be blocked centrally on its seat so as to cover all ports—is it necessary in the use of this gear, to carry a valve-stem clamp for the purpose?
Answer.—It is not necessary to carry a valve-stem clamp on an engine with the Baker-Pilliod improved valve gear. The valve-stem crosshead is provided with a set-screw by which it can be clamped to its guide, after the valve has been centred; and it holds the valve immovable.
Q. 2.-In case the eccentric crank or eccentric rod should break, what should be done?
A.—When the gear was first erected, after the valves had been set the reverse lever was placed in mid-gear and a hole drilled through the gear frame and bellcrank arm (see a and a', Figs. 49 and 50). You now remove the broken parts-if a broken eccentric crank remove the eccentric rod, of course; move the bell crank until holes a and a' are in register, insert a bolt through
the holes, and you are ready to go. With the gear bolted in this manner the valve will get the lap-and-lead movement, and will permit a port opening equal to the lead for all cut-offs, from the motion received through the combination lever from the crosshead.
Q. 3.-With what other breaks should the bell crank be bolted to the frame and the valve be able to receive its lap-and-lead movement, as directed above?
A.-In cases of breakages of the gear-connecting rod, radius bars, reverse yoke, short reach-rod, or horizontal arm of bell crank, remove the eccentric rod, and the lower arm of the bell crank can be bolted to the gear frame; and as previously stated, the engine will not have entirely failed on that side, the lap-and-lead movement of the valve supplying enough steam to the cylinder to give a certain amount of turning effort to the main pin.
Q. 4.-What should be done where the union link or crosshead arm has failed?
A.-In such cases it will be necessary to block the valve over the ports, by tightening the set-screw that clamps the valve stem to its guide, and to disconnect the valve rod, proceeding with the engine working on one side. Or, if it is possible to do so, chain the lower end of the combination lever in the position in which its pin hole F will be on the same vertical line as the fulcrum pin E of the bell crank; it will not then be necessary to disconnect the valve stem nor to clamp the valve; the valve will get the eccentric movement, but the port opening will be reduced and be closed in any cut-off shorter than about 50%.
Q. 5.-Suppose the combination lever breaks very close up to the bell crank, how should one arrange?
A.-In that case remove the union link and the broken piece of the combination lever, secure the valve over the ports as has been described, disconnect the valve rod and tie up the loose parts to keep them from doing damage. Proceed under power of the other side of the engine.
Q. 6.—If the valve rod should break, what should be done?
A.-Block the valve by tightening the set-screw on the valve-stem crosshead as already understood, and either disconnect the broken valve rod from the combination lever, or tie it up in such a manner that it can swing to the motion of the combination lever without striking anything, and go on with the engine working on one side.
Q. 7.-If the vertical arms of the bell crank should get broken, what would be the best thing to do?
A.—The valve rod should be disconnected from the combination lever, and as the latter is carried by the vertical arms of the bell crank, the union link and combination lever must be taken down; the valve should
be centred and secured in the prescribed way, and you can then proceed-on one side.
Q. 8.-In these cases of failure of certain parts of the gear such as the eccentric rod or union link, etc., what are the advantages in being able to still secure the short movement of the valve and a limited port opening when the piston is at the beginning of its stroke?
A.-On the big up-to-date engines it is impractical to try to remove the main rod in cases of breakdowns, and when it is left up and the valve is blocked at the centre of its travel, the piston will meet with high resistance toward the finish of either stroke, on account of the air compression ahead of the piston and a partial vacuum behind it; this added to the braking power at applications of the air brake may cause the drivers to slide and flatten. The port opening provided as referred to, gives relief from this effect; further, it permits oil from the lubricator to reach the cylinder and prevent the walls from becoming dry and being cut by the piston—a condition that must be particularly guarded against when the main rod is left up, in cases of breakdowns.